2007

1. [Alexeev et al. 2007] url = https://agupubs.onlinelibrary.wiley.com/doi/abs/10.1029/2007GL029536, eprint = https://agupubs.onlinelibrary.wiley.com/doi/pdf/10.1029/2007GL029536, abstract = A thin layer of soil used in many coupled global climate models does not resolve the heat reservoir represented by underlying ground material. Under representation of this feature leads to unrealistic simulation of temperature dynamics in the active layer and permafrost. Using the Community Land Model (CLM3) and its modifications we estimate a required thickness of soil layers to calculate temperature dynamics within certain errors. Our results show that to compute the annual cycle of temperature dynamics for cold permafrost, the soil thickness should be at least 30 meters. Decadal-to-century time scales require significantly deeper soil layers, e.g. hundreds of meters. We also tested a new geometrical configuration of the soil layer geometry which is called slab permafrost. This configuration is represented by a thick soil layer underneath the traditional resolved soil layer. The model configuration with 30 m deep resolved soil layer and a 30 to 100 m thick slab shows results that favorably compare with our benchmark model which has a fully resolved 300 m-deep soil layer.
   permafrost, climate modeling, land surface scheme

2. [Ammann et al. 2007] The potential role of solar variations in modulating recent climate has been debated for many decades and recent papers suggest that solar forcing may be less than previously believed. Because solar variability before the satellite period must be scaled from proxy data, large uncertainty exists about phase and magnitude of the forcing. We used a coupled climate system model to determine whether proxy-based irradiance series are capable of inducing climatic variations that resemble variations found in climate re-constructions, and if part of the previously estimated large range of past solar irradiance changes could be excluded. Transient simulations, covering the published range of solar irradiance esti-mates, were integrated from 850 AD to the present. Solar forcing as well as volcanic and anthropogenic forcing are detectable in the model results despite internal variability. The resulting climates are generally consistent with temperature reconstructions. Smaller, rather than larger, long-term trends in solar irradiance appear more plausible and produced modeled climates in better agree-ment with the range of Northern Hemisphere temperature proxy records both with respect to phase and magnitude. Despite the direct response of the model to solar forcing, even large solar irradiance change combined with realistic volcanic forcing over past centuries could not explain the late 20th century warming without inclusion of greenhouse gas forcing. Although solar and volcanic effects appear to dominate most of the slow climate variations within the past thousand years, the impacts of green-house gases have dominated since the second half of the last century.
   CCSM, solar variability, last millennium, model simulations

3. [Ammann and Wahl 2007] Abstract A portion of the debate about climate reconstructions of the past millennium, and in particular about the well-known Mann-Bradley-Hughes ( MBH  1998, 1999) reconstructions, has become disconnected from the goal of understanding natural climate variability. Here, we reflect on what can be learned from recent scientific exchanges and identify important challenges that remain to be addressed openly and productively by the community. One challenge arises from the real, underlying trend in temperatures during the instrumental period. This trend can affect regression-based reconstruction performance in cases where the calibration period does not appropriately cover the range of conditions encountered during the reconstruction. However, because it is tied to a unique spatial pattern driven by change in radiative balance, the trend cannot simply be removed in the method of climate field reconstruction used by MBH on the statistical argument of preserving degrees of freedom. More appropriately, the influence from the trend can be taken into account in some methods of significance testing. We illustrate these considerations as they apply to the MBH reconstruction and show that it remains robust back to AD 1450, and given other empirical information also back to AD 1000. However, there is now a need to move beyond hemispheric average temperatures and to focus instead on resolving climate variability at the socially more relevant regional scale.
   Climate reconstructions, last millennium

4. [Balmaceda et al. 2007] We present a reconstruction of total solar irradiance since 1610 to the present based on variations of the surface distribution of the solar magnetic field. The latter is calculated from the historical record of the Group sunspot number using a simple but consistent physical model. Our model successfully reproduces three independent data sets: total solar irradiance measurements available since 1978, total photospheric magnetic flux from 1974 and the open magnetic flux since 1868 (as empirically reconstructed from the geomagnetic aaindex). The model predicts an increase in the total solar irradiance since the Maunder Minimum of about 1.3 Wm-2.
   TSI reconstructions

5. [Braconnot et al. 2007] Abstract. A set of coupled ocean-atmosphere simulations using state of the art climate models is now available for the Last Glacial Maximum and the Mid-Holocene through the second phase of the Paleoclimate Modeling Intercomparison Project (PMIP2). This study presents the large-scale features of the simulated climates and compares the new model results to those of the atmospheric models from the first phase of the PMIP, for which sea surface temperature was prescribed or computed using simple slab ocean formulations. We consider the large-scale features of the climate change, pointing out some of the major differences between the different sets of experiments. We show in particular that systematic differences between PMIP1 and PMIP2 simulations are due to the interactive ocean, such as the amplification of the African monsoon at the Mid-Holocene or the change in precipitation in mid-latitudes at the LGM. Also the PMIP2 simulations are in general in better agreement with data than PMIP1 simulations.
   Holocene simulations, pmip2

6. [Brunet et al. 2007] Abstract. We analyze temporal and spatial patterns of temperature change over Spain during the period 1850–2005, using daily maximum (Tmax), minimum (Tmin), and mean (Tmean) temperatures from the 22 longest and most reliable Spanish records. Over mainland Spain, a significant (at 0.01 level) warming of 0.10°C/decade is found for the annual average of Tmean. Autumn and winter contributed slightly more than spring and summer to the annual warming over the 1850–2005 period. The overall warming is also associated with higher rates of change for Tmax than Tmin (0.11° versus 0.08°C/decade for 1850–2005). This asymmetric diurnal warming increased in the twentieth century (0.17° versus 0.09°C/decade during 1901–2005). Nevertheless, at many (few) individual stations, the difference between Tmax and Tmin is not statistically significant over 1850–2005 (1901–2005). Principal Component Analysis has been carried out to identify spatial modes of Spanish long-term temperature variability (1901–2005). Three principal spatial patterns are found, Northern Spain, Southeastern and Eastern Spain, and Southwestern Spain. All three patterns show similar significant warming trends. The overall warming has been more associated with reductions in cold extremes, as opposed to increases in warm extremes. Estimated trends in the number of moderately extreme cold days (Tmax < 10th percentile) and moderately extreme cold nights (Tmin < 10th percentile) show significant reductions of 0.74 and 0.54 days/decade, respectively, over 1850–2005. Moderately extreme warm days and nights (Tmax and Tmin > 90th percentile) increased significantly but at lower rates of 0.53 and 0.49 days/decade.
   Temperature trends in Spain

7. [Bodri and Cermak 2007] Offers a description of useful alternative paleoclimate reconstruction method. This title is a suitable source of information for those wishing to learn more about climate change. It includes: material for lecturing and use in the classroom; practical examples of borehole temperature inversions worldwide; and, illustrations and a reference list.
   Borehole climatology, climate reconstructions

8. [Davin et al. 2007] ABSTRACT: We use the IPSL climate model to investigate biophysical impacts of Anthropogenic Land Cover Change (ALCC) on surface climate. Including both the changes in surface albedo and evapotranspiration, we find that ALCC represents a radiative forcing of 0.29 W/m 2 from 1860 to 1992 and of 0.7 W/m 2 from 1992 to 2100. The simulated surface temperature response to ALCC indicates a historical cooling of 0.05 K and an additional cooling due to future changes of 0.14 K, which is consistent with the sign of the radiative forcing. However, this cooling is substantially lower than the one we would have obtained if it was caused by a radiatively equivalent change in CO2 concentration. These results thus question the relevance of the radiative forcing framework in the context of land use change, since the radiative forcing due to ALCC may not be comparable to the one exerted by other anthropogenic perturbations.
   Land cover, climate change.

9. [Delaygue and Bard 2011] Abstract Beryllium-10 in ice provides a valuable proxy of solar activity. However, complex production pathways, atmospheric transport, and deposition processes impede its quantitative interpretation. Here, we examine the influence of deposition processes on two Be-10 ice core records from Central Antarctica (South Pole and Dome Fuji stations), covering the last millennium. We try to quantify how Be-10 variations in ice relate to variations in Be-10 production, and the bias associated to this relationship. An independent bias estimation is provided by comparing atmospheric radiocarbon variations reconstructed from tree rings and deduced from Be-10 variations. Both techniques suggest an uncertainty of the order of 10production. This uncertainty estimate does not account for the geographical origin of Be-10, which remains a major issue. Because both Be-10 records are so similar, we propose to average them as a means to decrease the unshared (non solar) variability. This average record provides a new reconstruction of solar modulation parameter U and total solar irradiance over the last *1,300 years. The lowest solar activity is found during the so-called Spo¨rer Minimum (around AD 1450). The highest activities are found during the 8th century and over the last decades: as shown in previous studies, our results suggest that the recent solar activity is not exceptionally high for the last millennium.
   TSI reconstructions

10. [Drijfhout and Hazeleger 2007] ABSTRACT: Signal-to-noise patterns for the meridional overturning circulation (MOC) have been calculated for an ensemble of greenhouse scenario runs. The greenhouse-forced signal has been defined as the linear trend in ensemble-mean MOC, after year 2000. It consists of an overall decrease and shoaling of the MOC, with maximum amplitudes of 10 Sv (Sv =106 m3 s-1) per century. In each member the internal variability is defined as the anomaly with respect to the ensemble-mean signal. The interannual variability of the MOC is dominated by a monopole with a maximum amplitude of 2 Sv at 40°N. This variability appears to be driven by the North Atlantic Oscillation (NAO), mainly through NAO-induced variations in the wind field. The signal-to-noise ratio was estimated for various time spans, all starting in 1950 or later. Different noise estimates were made, both with and without intra-annual variability, relevant for episodic and continuous monitoring, respectively, and with and without an estimate of the observational error. Detection of a greenhouse-forced MOC signal on the basis of episodic measurements is impossible before 2055. With continuous monitoring, detection becomes possible after 35 years of observation. The main motivation for calculating signal-to-noise ratios and detection times is their usefulness for local monitoring strategies and detection methods. The two-dimensional pattern of detection times of a MOC change supports the rationale for deploying a sustained monitoring array on at 26°N.
    AMOC,thc.

11. [Dyer and Mote 2007] ABSTRACT:A substantial decrease in snow cover extent (SCE) and snow depth over North America has been observed over the 1960 2000 period. One explanation for the changes in North American snow cover is a change in the frequency and/or intensity of snow ablation. This study uses a gridded dataset of United States and Canadian surface observations from 1960 to 2000 to examine patterns of snow ablation over North America. An ablation event is defined as an interdiurnal snow depth change exceeding a critical value. Results show a significant positive trend in the frequency of ablation events during March (0 05) and a significant negative trend in May (0 05), indicating an earlier onset of ablation. This pattern is consistent for ablation of varying intensity. Surface energy budget components and air mass frequencies are examined in relation to the observed trends in snow ablation. Changes in March ablation frequency were shown to be dominated by increases in the sensible heat flux. A higher frequency of dry moderate instead of moist polar air masses during high ablation years may explain the increase in sensible heat flux and ablation over the study period.
    Snow cover, snow depth, ablation

12. [Easterling et al. 2007] The contiguous United States has experienced both warming temperatures and a general increase in precipitation during the period 1950 2006. During that time drought has been a recurring phenomenon with a number of large droughts occurring, starting with the major drought in the 1950s in the Central United States and culminating with the persistent drought in the western portion of the country that started in the late 1990s. Here we examine the influence of the multi-decadal warming trend on drought coverage and the possibility that the general increase in regional and contiguous U.S. precipitation since about 1980 has masked the tendency for increasing drought driven largely by increasing temperature. Results indicate that without the increase in precipitation, severe to extreme drought would have affected as much as 50$\%$ more of the U.S. during some months in the most recent drought period.
    Temperature precipitation, climate change USA

13. [Emile-Geay et al. 2007] [1] Using a climate model of intermediate complexity, we simulate the response of the El Nin o Southern Oscillation (ENSO) system to solar and orbital forcing over the Holocene. Solar forcing is reconstructed from radiocarbon production rate data, using various scaling factors to account for the conflicting estimates of solar irradiance variability. As estimates of the difference since the Maunder Minimum range from 0.05% to 0.5% of the solar  constant,   we consider these two extreme scenarios, along with the intermediate case of 0.2%. We show that for large or moderate forcings, the low-pass-filtered east-west sea surface temperature gradient along the equator responds almost linearly to irradiance forcing, with a short phase lag (about a decade). Wavelet analysis shows a statistically significant enhancement of the century-to-millennial-scale ENSO variability for even a moderate irradiance forcing. In contrast, the 0.05insolation forcing is found to produce a long-term increase of ENSO variability from the early Holocene onward, in accordance with previous findings. When both forcings are combined, the superposition is approximately linear in the strong scaling case. Overall, the sea surface temperature response is of the magnitude required, and is persistent enough, to induce important climatic perturbations worldwide. The results suggest that ENSO may plausibly have acted as a mediator between the Sun and the Earth s climate. A comparison to key Holocene climate records, from the Northern Hemisphere subtropics and midlatitudes, shows support for this hypothesis.
    Enso modulation, irradiance, teleconnections, mca

14. [Esper et al. 2007] Cedrus atlantica ring width data are used to reconstruct long-term changes in the Palmer Drought Severity Index (PDSI) over the past 953 years in Morocco, NW Africa. The reconstruction captures the dry conditions since the 1980s well and places this extreme period within a millennium-long context. PDSI values were above average for most of the 1450 1980 period, which let recent drought appear exceptional. However, our results also indicate that this pluvial episode of the past millennium was preceded by generally drier conditions back to 1049. Comparison of PDSI estimates with large-scale pressure field reconstructions revealed steady synoptic patterns for drought conditions over the past 350 years. The long-term changes from initially dry to pluvial to recent dry conditions are similar to PDSI trends reported from N America, and we suggest that they are related to long-term temperature changes, potentially teleconnected with ENSO variability and forced by solar irradiance changes.
    Paleoclimate, temperature reconstructions, drought, Morocco.

15. [Fischer et al. 2007b] We analyse the winter and summer climatic signal following 15 major tropical volcanic eruptions over the last half millennium based on multi-proxy reconstructions for Europe. During the first and second post-eruption years we find significant continental scale summer cooling and somewhat drier conditions over Central Europe. In the Northern Hemispheric winter the volcanic forcing induces an atmospheric circulation response that significantly follows a positive NAO state connected with a significant overall warm anomaly and wetter conditions over Northern Europe. Our findings compare well with GCM studies as well as observational studies, which mainly cover the substantially shorter instrumental period and thus include a limited set of major eruptions.
    ECHOg, last millennium, volcanic eruptions

16. [Fischer et al. 2007a] Most of the recent European summer heat waves have been preceded by a pronounced spring precipitation deficit. The lack of precipitation and the associated depletion of soil moisture result in reduced latent cooling and thereby amplify the summer temperature extremes. In order to quantify the contribution of land-atmosphere interactions, we conduct regional climate simulations with and without land-atmosphere coupling for four selected major summer heat waves in 1976, 1994, 2003, and 2005. The coupled simulation uses a fully coupled land-surface model, while in the uncoupled simulation the mean seasonal cycle of soil moisture is prescribed. The experiments reveal that land-atmosphere coupling plays an important role for the evolution of the investigated heat waves both through local and remote effects. During all simulated events soil moisture-temperature interactions increase the heat wave duration and account for typically 50 80pc of the number of hot summer days. The largest impact is found for daily maximum temperatures during heat wave episodes.
    Land air coupling, extremes

17. [Frank et al. 2007] [1] Proxy records may display fluctuations in climate variability that are artifacts of changing replication and interseries correlation of constituent time-series and also from methodological considerations. These biases obscure the understanding of past climatic variability, including estimation of extremes, differentiation between natural and anthropogenic forcing, and climate model validation. Herein, we evaluate as a case-study, the Esper et al. (2002) extra-tropical millennial-length temperature reconstruction that shows increasing variability back in time. We provide adjustments considering biases at both the site and hemispheric scales. The variance adjusted record shows greatest differences before 1200 when sample replication is quite low. A reduced amplitude of peak warmth during Medieval Times by about 0.4C (0.2C) at annual (40-year) timescales slightly re-draws the longerterm evolution of past temperatures. Many other regional and large-scale reconstructions appear to contain variancerelated biases. Citation: Frank, D., J. Esper, and E. R. Cook (2007), Adjustment for proxy number and coherence in a largescale temperature reconstruction, Geophys
    Millenium reconstructions

18. [Gouiran et al. 2007b] The North Atlantic sea level pressure anomalies (SLPA) associated with El NinÜo Southern Oscillation (ENSO) have been analyzed in a quasi-millennial (1000 1990) simulation with the ECHO-G model. In November December, the ENSO-related SLPA over the North Atlantic area are weak, while a realistic pattern already appears over the North Pacific and North America. In January March, the SLPA over North Atlantic are stronger and realistic from the North Pacific to Europe: the Aleutian low is strengthened (weakened), SLPA are positive (negative) over the north-central and north-eastern parts of North America, and SLPA display a negative (positive) NAO-like pattern over North-Atlantic during warm (cold) ENSO events, as in observations. The results also confirm the existence of a strong inter-event SLPA associated with warm and cold ENSO events, especially over the North Atlantic, while the relationship is stationary at multidecadal timescales. It seems that neither the intensity nor geographical longitude of the equatorial Pacific sea surface temperature anomaly (SSTA) and intensity of tropical Atlantic SSTA, nor the volcanic forcing, simply introduced here as a decrease of the solar constant, significantly induce an inter-event variability, which seems, in this run, mostly of atmospheric origin.
    ECHOg, last millennium, ENSO and north Atlantic

19. [Gouiran et al. 2007a] ABSTRACT:The atmosphere-ocean model ECHO-G, run with solar, volcanic and greenhouse gas forcing for the past millennium, is used to analyse winter and summer temperature variability in Scandinavia. Relationships with atmospheric circulation, North Atlantic SSTs and Northern Hemisphere (NH) temperatures are investigated at timescales longer and shorter than 10 yr. The simulated response to volcanic forcing is also analysed. Realistic relationships with the atmospheric circulation, with some deficiencies in summer, are found. High-frequency co-variations with SSTs and NH temperatures are too weak, but low-frequency co-variations with NH temperatures in winter are apparently too strong. The summer cooling response to volcanic forcing is realistic, but the expected winter warming is absent. The simulated long-term temperature evolution agrees broadly with proxy data. Combinations of several forcing factors can lead to decadal and multidecadal anomalies from the centennial trends. Decreased solar forcing can account for cold intervals in both summer and winter. A systematic negative North Atlantic Oscillation (NAO) phase can explain the coldest winter temperatures during 1590 1650. Several strong volcanic forcing events can have contributed to a simultaneous summer cooling. Proxy data also indicate cold summers and winters, and a negative NAO in winter, in the same period.
    ECHOg, last millennium, Scandinavia

20. [Graham et al. 2007] Abstract Terrestrial and marine late Holocene proxy records from the western and central US suggest that climate between approximately 500 and 1350 A.D. was marked by generally arid conditions with episodes of severe centennial-scale drought, elevated incidence of wild fire, cool sea surface temperatures (SSTs) along the California coast, and dune mobilization in the western plains. This Medieval Climate Anomaly (MCA) was followed by wetter conditions and warming coastal SSTs during the transition into the Little Ice Age  (LIA). Proxy records from the tropical Pacific Ocean show contemporaneous changes indicating cool central and eastern tropical Pacific SSTs during the MCA, with warmer than modern temperatures in the western equatorial Pacific. This pattern of mid-latitude and tropical climate conditions is consistent with the hypothesis that the dry MCA in the western US resulted (at least in part) from tropically forced changes in winter NH circulation patterns like those associated with modern La Niña episodes. We examine this hypothesis, and present other analyses showing that the imprint of MCA climate change appears in proxy records from widely distributed regions around the planet, and in many cases is consistent with a cool medieval tropical Pacific. One example, explored with numerical model results, is the suggestion of increased westerlies and warmer winter temperatures over northern Europe during medieval times. An analog technique for the combined use of proxy records and model results, Proxy Surrogate Reconstruction (PSR), is introduced.
    MCA drought western USA, La Niña, Pacific influence on Atlantic

21. [Hannachi et al. 2007]
    ABSTRACT: Climate and weather constitute a typical example where high dimensional and complex phenomena meet. The atmospheric system is the result of highly complex interactions between many degrees of freedom or modes. In order to gain insight in understanding the dynamical/physical behaviour involved it is useful to attempt to understand their interactions in terms of a much smaller number of prominent modes of variability. This has led to the development by atmospheric researchers of methods that give a space display and a time display of large space-time atmospheric data. Empirical orthogonal functions (EOFs) were first used in meteorology in the late 1940s. The method, which decomposes a space-time field into spatial patterns and associated time indices, contributed much in advancing our knowledge of the atmosphere. However, since the atmosphere contains all sorts of features, e.g. stationary and propagating, EOFs are unable to provide a full picture. For example, EOFs tend, in general, to be difficult to interpret because of their geometric properties, such as their global feature, and their orthogonality in space and time. To obtain more localised features, modifications, e.g. rotated EOFs (REOFs), have been introduced. At the same time, because these methods cannot deal with propagating features, since they only use spatial correlation of the field, it was necessary to use both spatial and time information in order to identify such features. Extended and complex EOFs were introduced to serve that purpose. Because of the importance of EOFs and closely related methods in atmospheric science, and because the existing reviews of the subject are slightly out of date, there seems to be a need to update our knowledge by including new developments that could not be presented in previous reviews. This review proposes to achieve precisely this goal. The basic theory of the main types of EOFs is reviewed, and a wide range of applications using various data sets are also provided.
    EOFs

22. [Harris 2007] ABSTRACT:The POM-SAT model for comparing air and ground temperatures is based on the supposition that sur-face air temperature (SAT) records provide a good prediction of thermal transients in the shallow subsurface of the Earth. This model consists of two components, the forcing function and an initial condition, termed the pre-observational mean (POM). I explore the sensitivity of this model as a func-tion of forcing periods at time scales appropriate for climate reconstructions. Synthetic models are designed to replicate comparisons between borehole temperatures contained in the global database of temperature profiles for climate recon-structions and gridded SAT data. I find that the root mean square (RMS) misfit between forcing functions and transient temperature profiles in the subsurface are sensitive to peri-ods longer than about 50 years, are a maximum when the period and the 150-year time series are equal and then de-creases for longer periods. The magnitude of the POM is a robust parameter for periods equal to or shorter than the length of this time series. At longer periods there is a trade-off between the amplitude of the forcing function and the POM. These tests provide guidelines for assessing compar-isons between air and ground temperatures at periods appro-priate for climate reconstructions. The sensitivity of com-parisons between the average Northern Hemisphere gridded SAT record and subsurface temperature-depth profile as a function of forcing period is assessed. This analysis indicates that the Northern Hemisphere extratropical average SAT and reduced temperature-depth profile are in good agreement. By adding modest heat to the subsurface at intermediate periods some improvement in misfit can be made, but this extra heat has negligible influence on the POM. The joint analysis of borehole temperatures and SAT records indicate warming of about 1.1 C over the last 500 years, consistent with previous studies.
    Paleoclimate, boreholes, POM

23. [Hegerl et al. 2007a]
    ABSTRACT: Climate records over the last millennium place the 20 th -century warming in a longer historical context. Reconstructions of millennial temperatures show a wide range of variability, raising questions about the reliability of currently available reconstruction techniques and the uniqueness of late 20 th - century warming. We suggest a calibration method that avoids the loss of low-frequency variance. A new reconstruction using this method shows substantial variability over the last 1500 years. This record is consistent with independent temperature change estimates from borehole geothermal records, compared over the same spatial and temporal domain. The record is also broadly consistent with other recent reconstructions that attempt to fully recover low-frequency climate variability in their central estimate. High variability in reconstructions does not hamper the detection of greenhouse gas induced climate change, since a substantial fraction of the variance in these reconstructions from the beginning of the analysis in the late 13 th century to the end of the records can be attributed to external forcing. Results from a detection and attribution analysis show that greenhouse warming is detectable in all analyzed high-variance reconstructions (with the possible exception of one ending in 1925), and that about a third of the warming in the first half of the 20 th century can be attributed to anthropogenic greenhouse gas emissions. The estimated magnitude of the anthropogenic signal is consistent with most of the warming in the second half of the 20 th century being anthropogenic.
    Last millennium, climate reconstructions, simulations.

24. [Herweijer et al. 2007]
    ABSTRACT: Drought is the most economically expensive recurring natural disaster to strike North America in modern times. Recently available gridded drought reconstructions have been developed for most of North America from a network of drought-sensitive tree-ring chronologies, many of which span the last 1000 yr. These reconstructions enable the authors to put the famous droughts of the instrumental record (i.e., the 1930s Dust Bowl and the 1950s Southwest droughts) into the context of 1000 yr of natural drought variability on the continent. We can now, with this remarkable new record, examine the severity, persistence, spatial signatures, and frequencies of drought variability over the past milllennium, and how these have changed with time. The gridded drought reconstructions reveal the existence of successive “megadroughts,” unprecedented in persistence (20–40 yr), yet similar in year-to-year severity and spatial distribution to the major droughts experienced in today’s North America. These megadroughts occurred during a 400-yr-long period in the early to middle second millennium A.D., with a climate varying as today’s, but around a drier mean. The implication is that the mechanism forcing persistent drought in the West and the Plains in the instrumental era is analagous to that underlying the megadroughts of the medieval period. The leading spatial mode of drought variability in the recontructions resembles the North American ENSO pattern: widespread drought across the United States, centered on the Southwest, with a hint of the opposite phase in the Pacific Northwest. Recently, climate models forced by the observed history of tropical Pacific SSTs have been able to successfully simulate all of the major North American droughts of the last 150 yr. In each case, cool La Nina-like conditions in the tropical Pacific are consistent with North American drought. With ENSO showing a pronounced signal in the gridded drought recontructions of the last millennium, both in terms of its link to the leading spatial mode, and the leading time scales of drought variability (revealed by multitaper spectral analysis and wavelet analysis), it is postulated that, as for the modern day, the medieval mega- droughts were forced by protracted La Nina-like tropical Pacific SSTs. Further evidence for this comes from the global hydroclimatic “footprint” of the medieval era revealed by existing paleoclimatic archives from the tropical Pacific and ENSO-sensitive tropical and extratropical land regions. In general, this global pattern matches that observed for modern-day persistent North American drought, whereby a La Nina-like tropical Pacific is accompanied by hemispheric, and in the midlatitudes, zonal, symmetry of hydroclimatic anomalies.
    Last millennium, climate reconstructions, simulations, drought

25. [Hopcroft et al. 2007]
    ABSTRACT: Estimates of past climate derived from borehole temperatures are assuming a greater impor-tance in context of the millennial temperature variation debate. However, recovery of these signals is usually performed with regularization which can potentially lead to underestima-tion of past variation when noise is present. In this work Bayesian inference is applied to this problem with no explicit regularization. To achieve this Reversible Jump Markov chain Monte Carlo is employed, and this allows models of varying complexity (i.e. variable dimensions) to be sampled so that it is possible to infer the level of ground surface temperature (GST) history resolution appropriate to the data. Using synthetic examples, we show that the inference of the GST signal back to more than 500 yr is robust given boreholes of 500 m depth and moderate noise levels and discuss the associated uncertainties. We compare the prior information we have used with the inferred posterior distribution to show which parts of the GST reconstruc-tions are independent of this prior information. We demonstrate the application of the method to real data using five boreholes from southern England. These are modelled both individually and jointly, and appear to indicate a spatial trend of warming over 500 yr across the south of the country.
    Borehole reconstructions, methods, bayesian.

26. [Solomon et al. 2007] Chapter IPCC07

27. [Forster et al. 2007] Chapter 2, IPCC07

28. [Trenberth et al. 2007] Chapter 3, IPCC07

29. [Lemke et al. 2007] Chapter 4, IPCC07

30. [Bindoff et al. 2007] Chapter 5, IPCC07

31. [Jansen et al. 2007] Chapter 6, IPCC07

32. [Denman et al. 2007] Chapter 7, IPCC07

33. [Randall et al. 2007] Chapter 8, IPCC07

34. [Hegerl et al. 2007b] Chapter 9, IPCC07

35. [Meehl et al. 2007] Chapter 10, IPCC07

36. [Christensen et al. 2007] Chapter 11, IPCC07

37. [Jacob et al. 2007]
    Abstract The analysis of possible regional climate changes over Europe as simulated by 10 regional climate models within the context of PRUDENCE requires a careful investigation of possible systematic biases in the models. The purpose of this paper is to identify how the main model systematic biases vary across the different models. Two fundamental aspects of model validation are addressed here: the ability to simulate (1) the long-term (30 or 40 years) mean climate and (2) the inter-annual variability. The analysis concentrates on near-surface air temperature and precipitation over land and focuses mainly on winter and summer. In general, there is a warm bias with respect to the CRU data set in these extreme seasons and a tendency to cold biases in the transition seasons. In winter the typical spread (standard deviation) between the models is 1 K. During summer there is generally a better agreement between observed and simulated values of inter-annual variability although there is a relatively clear signal that the modeled temperature variability is larger than suggested by observations, while precipitation variability is closer to observations. The areas with warm (cold) bias in winter generally exhibit wet (dry) biases, whereas the relationship is the reverse during summer (though much less clear, coupling warm (cold) biases with dry (wet) ones). When comparing the RCMs with their driving GCM, they generally reproduce the large-scale circulation of the GCM though in some cases there are substantial differences between regional biases in surface temperature and precipitation.
    RCM prudence ensemble cordex

38. [Juckes et al. 2007]
    ABSTRACT: There has been considerable recent interest in paleoclimate reconstructions of the temperature history of the last millennium. A wide variety of techniques have been used. The interrelation between the techniques is sometimes unclear, as different studies often use distinct data sources as well as distinct methodologies. Recent work is reviewed with an aim to clarifying the import of the different approaches. A range of proxy data collections used by different authors are passed through two reconstruction algorithms: firstly, inverse regression and, secondly, compositing followed by variance matching. It is found that the first method tends to give large weighting to a small number of proxies and that the second approach is more robust to varying proxy input. A reconstruction using 18 proxy records extending back to AD 1000 shows a maximum pre-industrial temperature of 0.25 K (relative to the 1866 to 1970 mean). The standard error on this estimate, based on the residual in the calibration period is 0.149 K. Two recent years (1998 and 2005) have exceeded the estimated pre-industrial maximum by more than 4 standard errors.
    Climate reconstructions, last millennium, temperature

39. [Kanzow et al. 2007]
    ABSTRACT: The Atlantic meridional overturning circulation (MOC), which provides one-quarter of the global meridional heat transport, is composed of a number of separate flow components. How changes in the strength of each of those components may affect that of the others has been unclear because of a lack of adequate data. We continuously observed the MOC at 26.5°N for 1 year using end-point measurements of density, bottom pressure, and ocean currents; cable measurements across the Straits of Florida; and wind stress. The different transport components largely compensate for each other, thus confirming the validity of our monitoring approach. The MOC varied over the period of observation by ±5.7 × 106 cubic meters per second, with density-inferred and wind-driven transports contributing equally to it. We find evidence for depth-independent compensation for the wind-driven surface flow.
    North Atlantic ocean ciirculation

40. [Kausrud et al. 2007]
    ABSTRACT: In central Asia, the great gerbil (Rhombomys opimus) is the main host for the bacterium Yersinia pestis, the cause of bubonic plague. In order to prevent plague outbreaks, monitoring of the great gerbil has been carried out in Kazakhstan since the late 1940s. We use the resulting data to demonstrate that climate forcing synchronizes the dynamics of gerbils over large geographical areas. As it is known that gerbil densities need to exceed a threshold level for plague to persist, synchrony in gerbil abundance across large geographical areas is likely to be a condition for plague outbreaks at similar large scales. Here, we substantiate this proposition through autoregressive modelling involving the normalized differentiated vegetation index as a forcing covariate. Based upon predicted climate changes, our study suggests that during the next century, plague epizootics may become more frequent in central Asia.
    Plague, climate change

41. [Kilbourne et al. 2007]
    Water that forms the Florida Current, and eventually the Gulf Stream, coalesces in the Caribbean from both subtropical and equatorial sources. The equatorial sources are made up of, in part, South Atlantic water moving northward and compensating for southward flow at depth related to meridional overturning circulation. Subtropical surface water contains relatively high amounts of radiocarbon ( 14 C), whereas equatorial waters are influenced by the upwelling of low 14 C water and have relatively low concentrations of 14 C. We use a 250 year record of D 14 C in a coral from southwestern Puerto Rico along with previously published coral D 14 C records as tracers of subtropical and equatorial water mixing in the northern Caribbean. Data generated in this study and from other studies indicate that the influence of either of the two water masses can change considerably on interannual to interdecadal time scales. Variability due to ocean dynamics in this region is large relative to variability caused by atmospheric 14 C changes, thus masking the Suess effect at this site. A mixing model produced using coral D 14 C illustrates the time varying proportion of equatorial versus subtropical waters in the northern Caribbean between 1963 and 1983. The results of the model are consistent with linkages be-tween multidecadal thermal variability in the North Atlantic and meridional overturning circulation. Ek-man transport changes related to tradewind variability are proposed as a possible mechanism to explain the observed switches between relatively low and high D 14 C values in the coral radiocarbon records.
    Paleoclimate reconstructions, coral.

42. [Krivova et al. 2007]
    Context. Total solar irradiance changes by about 0.1this quantity are only available since 1978 and do not provide information on longer-term secular trends. Aims. In order to reliably evaluate the Sun s role in recent global climate change, longer time series are, however, needed. They can only be assessed with the help of suitable models. Methods. The total solar irradiance is reconstructed from the end of the Maunder minimum to the present based on variations of the surface distribution of the solar magnetic field. The latter is calculated from the historical record of the sunspot number using a simple but consistent physical model. Results. Our model successfully reproduces three independent data sets: total solar irradiance measurements available since 1978, total photospheric magnetic flux since 1974 and the open magnetic flux since 1868 empirically reconstructed using the geomagnetic aa-index. The model predicts an increase in the solar total irradiance since the Maunder minimum of 1.3+0.2  0.4 Wm 2.
    TSI reconstruction

43. [Kueppers et al. 2007]
    ABSTRACT: Regional detection of a greenhouse warming signal relies on extensive, long-term measurements of temperature. The potentially confounding impact of land-cover and land-use change on trends in temperature records has mostly focused on the influence of urban heat islands. Here we use a regional climate model to show that a regional irrigation cooling effect (ICE) exists, opposite in sign to urban heat island effects. The magnitude of the ICE has strong seasonal variability, causing large dry-season decreases in monthly mean and maximum temperatures, but little change in rainy-season temperatures. Our model produced a negligible effect on monthly minimum temperature. In California, the modeled regional ICE is of similar magnitude, but opposite sign, to predictions for future regional warming from greenhouse gases. Given our results for California and the global importance of irrigated agriculture, past expansion of irrigated land has likely affected observations of surface temperature, potentially masking the full warming signal caused by greenhouse gas increases.
    Land use, irrigation, regional climate forcing

44. [Laird and Bell 2007]
    ABSTRACT: Publication is a critical component of modern science.By publishing their find- ings,scientists can ensure that their results are disseminated and substantiated.This brief report analyzes the publication and citation histories of American Geophysical Union (AGU)Fellows to elucidate different styles of productivity in the geoscience community.
    Publication patterns

45. [Li et al. 2007a]
    ABSTRACT: AGU Fellows are arguably the most eminent Earth scientists,recognized by their peers for their leadership within and outside the community. March 2006.AGU reports a membership of 48,000 scientists from around the world. According to the AGU Web site,there are more than 900 Fellows,of which fewer than 10$\%$ are women.Our sample group was composed of 69 men and 46 women.The overrepresentation of women was a deliber- ate effort to compile numbers that were sig- nificant enough to evaluate gender differ- ences.The Fellows ranged in age from 15 to 48 years post-Ph.D.We built our publication four categories based on citation and publica- tion patterns.Prolific scientists publish often ig. 1 AGU Fellows: different types of scholars.
    Uncertainty in millennial temperature reconstructions

46. [Li et al. 2007b]
    ABSTRACT: Soil moisture trends, particularly during the growing season, are an important possible consequence of global warming. Climate model simulations of future soil moisture changes should be made with models that can produce reliable simulations of soil moisture for past climate changes. In this paper, we compare soil moisture simulations from Intergovernmental Panel on Climate Change Fourth Assessment climate models forced with observed climate forcings for the past century, and evaluate them using in situ soil moisture measurements from over 140 stations or districts in midlatitudes of the Northern Hemisphere. To account for the observed spatial scale of soil moisture variations, we used regionally averaged soil moisture for six regions. The models showed realistic seasonal cycles for Ukraine, Russia, and Illinois, but generally poor seasonal cycles for Mongolia and China. To explore the summer drying issue for the second half of the 20th century, we analyzed the linear trend of soil moisture for Ukraine and Russia. Observations from both regions show increases in summer for the period from 1958 1999 that were larger than most trends in the model simulations. Only two out of 25 model realizations show trends comparable to those of observations. These two trends, however, are due to internal model variability rather than a result of external forcing. Changes in precipitation and temperature cannot fully explain soil moisture increases for Ukraine and Russia, which indicates that other factors might have played a dominant role on the observed patterns for soil moisture. We suggest that changes in solar irradiance (the dimming effect) and resultant changes in evaporative demand explain most of the observed soil moisture trends. To understand such sensitivity, we analyzed soil moisture outputs in a special version of the ECHAM5 model that was capable of capturing the observed radiation pattern as a result of incorporating a sophisticated aerosol scheme. Results suggest that both radiation and precipitation patterns are required to be adequately simulated to reproduce the observed soil moisture trends realistically.
    Soil moisture, ipcc

47. [Lloyd and Bunn 2007]
    We examined relationships between tree ring-width and climate at 232 sites around the circumpolar boreal forest to explore variability in two types of response to temperature: a browning response characterized by inverse correlations between growth and temperature, and a greening response characterized by positive correlations between growth and temperature. We used moving-window correlation analysis for eight 30-year time windows, lagged by 10 years, to characterize the climate response at each site from 1902 to 2002. Inverse growth responses to temperature were widespread, occurring in all species, all time periods, and in nearly all geographic areas. The frequency of the browning response increased after 1942, while the frequency of the greening response declined. Browning was concentrated in five species (Picea abies, Picea glauca, Picea mariana, Picea obovata and Pinus banksiana), and occurred more frequently in the warmer parts of species  ranges, suggesting that direct temperature stress might be a factor. In some species, dry sites were also more likely to experience browning; moisture stress might thus be an additional explanation in some cases. As inverse responses to temperature are widespread, and occur in a broad array of species, there is unlikely to be any single explanation for their occurrence.
    Divergence problem

48. [Loehle 2007]
    ABSTRACT: Historical data provide a baseline for judging how anomalous recent temperature changes are and for assessing the degree to which organisms are likely to be adversely affected by current or future warming.Climate histories are commonly reconstructed from a variety of sources,including ice cores,tree rings,and sediment.Tree-ring data,being the most abundant for recent centuries,tend to dominate reconstructions.There are reasons to believe that tree ring data may not properly capture long-term climate changes.In this study,eighteen 2000-year-long series were obtained that were not based on tree ring data.Data in each series were smoothed with a 30-year running mean.All data were then converted to anomalies by subtracting the mean of each series from that series.The overall mean series was then computed by simple averaging.The mean time series shows quite coherent structure.The mean series shows the Medieval Warm Period (MWP)and Little Ice Age (LIA)quite clearly,with the MWP being approximately 0.3 °C warmer than 20th century values at these eighteen sites.
    Soil moisture, ipcc

49. [Mann 2007]
    To assess the signi cance of modern climate change,it is essential to place recent observed changes in a longer-term context. This review assesses the evidence from both proxy climate data and theoretical climate model simulations with regard to the nature and causes of climate variability over a time interval spanning roughly the past two millennia.Evidence is reviewed for changes in temperature,drought, and atmospheric circulation over this timescale.Methods for recon- structing past climate from proxy data are reviewed and comparisons with the results of climate modeling studies are provided.The as- sessment provided af rms the role of natural (solar and volcanic) radiative forcing in past changes in large-scale mean temperature changes and in dynamical modes of climate variability such as the North Atlantic Oscillation (NAO)and El Ni Üno/Southern Oscilla- tion (ENSO) influencing large-scale climate.At hemispheric scales, late twentieth century warmth appears unprecedented in the context of at least the past 2000 years.This anomalous warmth can only be explained by modern anthropogenic forcing.
    Paleoclimate reconstructions, proxy data, last millennia

50. [Mann et al. 2007b]
    The comment by Zorita et al (henceforth ZVS06 ) is troubling for several reasons. Firstly, it does not constitute an actual comment on the Mann et al (2005) study (henceforth MRWA05; see also highlights of the paper in the Nov. 25, 2005 Science, editor s choice , and the Feb., 2006 Bulletin of the American Meteorological Society, Papers of Note ). ZVS06 have not put forward a single testable criticism of the method (Regularized Expectation Maximization or RegEM ), data (CSM 1.4 model simulations results), or analyses that MRWA05 used to reach their conclusions. Instead, ZVS06 simply assert that the study s conclusions must be incorrect because they conflict with their own purported findings (originally put forward by Von Storch et al, 2004 henceforth VS04 ). Yet, those latter results are now subject to considerable doubt for reasons outlined below. Rather than presenting testable criticisms of MRWA05, ZV06 instead have taken advantage of the comment/reply process of Journal of Climate to further advance specious criticisms with respect to other past work by Mann and colleagues (Mann et al, 1998 henceforth MBH98 and Mann et al, 1999-henceforth MBH99 ).
    Paleoclimate reconstructions, proxy data, pseudoreality.

51. [Mann et al. 2007a]
    Reply to comment on 'Testing the Fidelity of Methods Used in Proxy-based Reconstructions of Past Climate' by Smerdon and Kaplan
    Paleoclimate reconstructions, proxy data, pseudoreality.

52. [Mann et al. 2007c]
    We present results from continued investigations into the fidelity of covariance-based climate field reconstruction (CFR) approaches used in proxy-based climate reconstruction. Our experiments employ synthetic pseudoproxy data derived from simulations of forced climate changes over the past millennium. Using networks of these pseudoproxy data, we investigate the sensitivity of CFR performance to signal-to-noise ratios, the noise spectrum, the spatial sampling of pseudoproxy locations, the statistical representation of predictors used, and the diagnostic used to quantify reconstruction skill. Our results reinforce previous conclusions that CFR methods, correctly implemented and applied to suitable networks of proxy data, should yield reliable reconstructions of past climate histories within estimated uncertainties. Our results also demonstrate the deleterious impact of a linear detrending procedure performed recently in certain CFR studies and illustrate flaws in some previously proposed metrics of reconstruction skill.
    Paleoclimate reconstructions, proxy data, pseudoreality.

53. [Meier et al. 2007]
    We present an annually resolved record of grape harvest dates for Switzerland. The strong negative relationship between grape harvest dates and April August temperatures allowed a new reconstruction, with associated uncertainties, to be derived back to 1480. Calibration (1928 1979) was performed with monthly data from the Basel and Geneva stations and verified over 1980 2006. Twelve days of grape harvest difference correspond to around 1 C April August temperature difference. Periods cooler (late grape harvest dates) than the 1961 1990 mean are reconstructed during the 17th century and at the beginning of the 19th century. Warmer conditions were experienced in the early 18th century and during the recent decades, in agreement with grape harvest temperature reconstructions from France and other independent temperature estimates. On decadal (annual) time-scales the earliest harvests were in the 1580s (2003) and the latest vintages in the 1740s (1816). Large tropical volcanic eruptions led to significantly later grape harvest dates (cooling) one to two years after the event.
    Paleoclimate reconstructions, proxy data, documentary, grape harvest

    [Muscheler et al. 2007]
    Abstract Identification of the causes of past climate change requires detailed knowledge of one of the most important natural factors solar forcing. Prior to the period of direct solar observations, radionuclide abundances in natural archives provide the best-known proxies for changes in solar activity. Here we present two independent reconstructions of changes in solar activity during the last 1000 yr, which are inferred from 10Be and 14C records. We analyse the tree-ring 14C data (SHCal, IntCal04 from 1000 to 1510 AD and annual data from 1511 to 1950 AD) and four 10Be records from Greenland ice cores (Camp Century, GRIP, Milcent and Dye3) together with two 10Be records from Antarctic ice cores (Dome Concordia and South Pole). In general, the 10Be and 14C records exhibit good agreement that allows us to obtain reliable estimates of past solar magnetic modulation of the radionuclide production rates. Differences between 10Be records from Antarctica and Greenland indicate that climatic changes have influenced the deposition of 10Be during some periods of the last 1000 yr. The radionuclide-based reconstructions of past changes in solar activity do not always agree with the sunspot record, which indicates that the coupling between those proxies is not as close as has been sometimes assumed. The tree-ring 14C record and 10Be from Antarctica indicate that recent solar activity is high but not exceptional with respect to the last 1000 yr. r 2006 Elsevier Ltd. All rights reserved.
    be10, c14, dansgaard/oeschger, solar irradiance

54. [Raddatz et al. 2007]
    Abstract Global warming caused by anthropogenic CO2 emissions is expected to reduce the capability of the ocean and the land biosphere to take up carbon. This will enlarge the fraction of the CO2 emissions remaining in the atmo- sphere, which in turn will reinforce future climate change. Recent model studies agree in the existence of such a positive climate–carbon cycle feedback, but the estimates of its amplitude differ by an order of magnitude, which con- siderably increases the uncertainty in future climate pro- jections. Therefore we discuss, in how far a particular process or component of the carbon cycle can be identified, that potentially contributes most to the positive feedback. The discussion is based on simulations with a carbon cycle model, which is embedded in the atmosphere/ocean general circulation model ECHAM5/MPI-OM. Two simulations covering the period 1860–2100 are conducted to determine the impact of global warming on the carbon cycle. Forced by historical and future carbon dioxide emissions (following the scenario A2 of the Intergovernmental Panel on Climate Change), they reveal a noticeable positive climate–carbon cycle feedback, which is mainly driven by the tropical land biosphere. The oceans contribute much less to the positivefeedback and the temperate/boreal terrestrial biosphere induces a minor negative feedback. The contrasting behavior of the tropical and temperate/boreal land biosphere is mostly attributed to opposite trends in their net primary productivity (NPP) under global warming conditions. As these findings depend on the model employed they are compared with results derived from other climate–carbon cycle models, which participated in the Coupled Climate– Carbon Cycle Model Intercomparison Project (C4MIP).
    JSBACH

55. [Rahmstorf 2007b]
    Abstract:A semi-empirical relation is presented that connects global sea-level rise to global mean surface temperature. It is proposed that, for time scales relevant to anthropogenic warming, the rate of sea-level rise is roughly proportional to the magnitude of warming above the temperatures of the pre Industrial Age. This holds to good approximation for temperature and sea-level changes during the 20th century, with a proportionality constant of 3.4 millimeters/year per °C. When applied to future warming scenarios of the Intergovernmental Panel on Climate Change, this relationship results in a projected sea-level rise in 2100 of 0.5 to 1.4 meters above the 1990 level.
    Global sea level, global temperature, climate change

56. [Rahmstorf 2007a]
    Additional analysis performed in response to Holgate et al. and Schmith et al. shows that the semi-empirical method for projecting future sea-level rise passes the test of predicting one half of the data set based on the other half. It further shows that the conclusions are robust with respect to choices of data binning, smoothing, and detrending.
    Global sea level, global temperature, climate change

57. [Rahmstorf et al. 2007]
    Recent climate observations compared to projections
    Global sea level, global temperature, climate change,observations

58. [Raible 2007]
    Abstract: Analyzing ERA40 data there is evidence that extreme intensified midlatitude cyclones are related to the large-scale atmospheric circulation in winter and to a minor degree for spring and autumn. Regionally different circulation patterns are related to extreme intensified cyclones, e.g., cyclones in northern Europe are linked to a slightly rotated NAO-like pattern whereas for southern Europe a blocking-like pattern over central to northern Europe is observed. In the Pacific a north-south dipole pattern is related to extremes in cyclone intensity. In summer these relationships, however, collapse. In winter, depending on the considered region, changes in the meridional temperature gradient, the land-sea temperature contrast, and to some extent changes in static stability modulate the lower to middle tropospheric baroclinicity, being important in the intensification process.
    Storms

59. [Raisanen 2007]
    Abstract:How much can we trust model-based projections of future anthropogenic climate change? This review attempts to give an overview of this important but difficult topic by using three main lines of evidence: the skill of models in simulating present-day climate, intermodel agreement on future climate changes, and the ability of models to simulate climate changes that have already occurred. A comparison of simulated and observed present-day climates shows good agreement for many basic variables, particularly at large horizontal scales, and a tendency for biases to vary in sign between different models, but there is a risk that these features might be partly a result of tuning. Overall, the connection between model skill in simulating present-day climate and the skill in simulating future climate changes is poorly known. An intercomparison of future climate changes between models shows a better agreement for changes in temperature than that for precipitation and sea level pressure, but some aspects of change in the latter two variables are also quite consistent between models. A comparison of simulations with observed climate changes is, in principle, a good test for the models, but there are several complications. Nonetheless, models have skilfully simulated many large-scale aspects of observed climate changes, including but not limited to the evolution of the global mean surface air temperature in the 20th century. Furthermore, although there is no detailed agreement between the simulated and observed geographical patterns of change, the grid box scale temperature, precipitation and pressure changes observed during the past half-century generally fall within the range of model results. Considering the difficulties associated with other sources of information, the variation of climate changes between different models is probably the most meaningful measure of uncertainty that is presently available. In general, however, this measure is more likely to underestimate than overestimate the actual uncertainty.
    Climate models GCMs, climate change

60. [Riveros-Iregui et al. 2007]
    Recent years have seen a growing interest in measuring and modeling soil CO2 efflux, as this flux represents a large component of ecosystem respiration and is a key determinant of ecosystem carbon balance. Process-based models of soil CO2 production and efflux, commonly based on soil temperature, are limited by nonlinearities such as the observed diurnal hysteresis between soil CO2 concentration ([CO2 ]) and temperature. Here we quantify the degree to which hysteresis between soil [CO2 ] and soil temperature is controlled by soil water content in a montane conifer forest, and how this nonlinearity impacts estimates of soil CO2 efflux. A representative model that does not consider hysteresis overestimated soil CO2 efflux for the entire growing season by 19$\%$. At high levels of soil water content, hysteresis imposes organized, daily variability in the relationship between soil [CO2 ] and soil temperature, and at low levels of soil water content, hysteresis is minimized.
    Carbon cycle

61. [Schoener and Boehm 2007]
    ABSTRACT. Stepwise linear regression models were calibrated against the measured mass balance of glaciers in the Austrian Alps for the prediction of specific annual net balance and summer balance from climatological and topographical input data. For estimation of winter mass balance, a simple ratio between the amount of winter precipitation and the measured winter balance was used. A ratio with a mean value of 2.0 and a standard deviation of 0.44 was derived from the sample of measured winter balances. Climate input data were taken from the HISTALP database which offers a homogenized data source that is outstanding in terms of its spatial and temporal coverage. Data from the Austrian glacier inventory were used as topographical input data. From the group of possible predictors summer air temperature, winter precipitation, summer snow precipitation and continentality (as defined from seasonal temperature variation) were selected as climatological driving forces in addition to lowest glacier elevation and area-weighted mean glacier elevation as topographical driving forces. Summer temperature explains 60$\%$ of the variance of summer mass balance and 39$\%$ of the variance of annual mass balance. Additional factors increase the explained variance by 22$\%$ for summer and 31$\%$ for annual net balance. The calibrated mass-balance model was used to reconstruct the mass balance of Hintereisferner and Vernagtferner back to 1800. Whereas the model performs well for Hintereisferner, it fails for some sub-periods for Vernagtferner due to the complicated flow dynamics of the glacier.
    Glaciar mass balance, early instrumental data, histalp

62. [Seager et al. 2007]
    According to tree ring and other records, a series of severe droughts that lasted for decades afflicted western North America during the Medieval period resulting in a more arid climate than in subsequent centuries. A review of proxy evidence from around the world indicates that North American megadroughts were part of a global pattern of Medieval hydroclimate that was distinct from that of today. In particular, the Medieval hydroclimate was wet in northern South America, dry in mid-latitude South America, dry in eastern Africa but with strong Nile River floods and a strong Indian monsoon. This pattern is similar to that accompanying persistent North American droughts in the instrumental era. This pattern is compared to that associated with familiar climate phenomena. The best fit comes from a persistently La Nin  a-like tropical Pacific and the warm phase of the so-called Atlantic Multidecadal Oscillation. A positive North Atlantic Oscillation (NAO) also helps to explain the Medieval hydroclimate pattern. Limited sea surface temperature reconstructions support the contention that the tropical Pacific was cold and the subtropical North Atlantic was warm, ideal conditions for North American drought. Tentative modeling results indicate that a multi-century La Nin  a-like state could have arisen as a coupled atmosphere ocean response to high irradiance and weak volcanism during the Medieval period and that this could in turn have induced a persistently positive NAO state. A La Nin  a-like state could also induce a strengthening of the North Atlantic meridional overturning circulation, and hence warming of the North Atlantic Ocean, by (i) the ocean response to the positive NAO and by shifting the southern mid-latitude westerlies poleward which (ii) will increase the salt flux from the Indian Ocean into the South Atlantic and (iii) drive stronger Southern Ocean upwelling.
    Millennium climate, hidroclimate, MCA, western USA drought, La Niña

63. [Sheffield and Wood 2007]
    Drought occurrence is analyzed over global land areas for 1950-2000 using soil moisture data from a simulation of the terrestrial water cycle with the Variable Infiltration Capacity (VIC) land surface model, which is forced by an observation based meteorological data set. A monthly drought index based on percentile soil moisture values relative to the 50-year climatology is analyzed in terms of duration, intensity and severity at global and regional scales. Short-term droughts (<= 6 months) are prevalent in the Tropics and midlatitudes, where inter-annual climate variability is highest. Medium term droughts (7 12 months) are more frequent in mid- to high-latitudes. Long term (12+ months) droughts are generally restricted to sub-Saharan Africa and higher northern latitudes. The Sahel region stands out for having experienced long-term and severe drought conditions. Severe regional drought events are systematically identified in terms of spatial coverage, based on different thresholds of duration and intensity. For example, in northern Europe, 1996 and 1975 were the years of most extensive 3- and 12-month duration drought, respectively. In northern Asia, severe drought events are characterized by persistent soil moisture anomalies over the wintertime. The drought index identifies several well-known events, including the 1988 US, 1982/83 Australian, 1983/4 Sahel and 1965/66 Indian droughts which are generally ranked as the severest and most spatially extensive in the record. Comparison with the PDSI shows general agreement at global scales and for these major events but they diverge considerably in cooler regions and seasons, and especially in latter years when the PDSI shows a larger drying trend.
    Soil moisture,drought.

64. [Smerdon and Kaplan 2007]
    Mann et al cheated with the standardization interval in [Mann et al. 2005]
    Paleoclimate reconstructions, proxy data, pseudoreality.

    tem [Safanda et al. 2007]
    Update of boreholes from Check Republic, Portugal and Slovenia
    Borehole temperaure reconstructions

65. [Stendel et al. 2007]
    Even though we can estimate the zonation of present-day permafrost from deep-soil temperatures obtained from global coupled atmosphere-ocean general circulation models (GCMs) by accounting for heat conduction in the frozen soil, it is impossible to explicitly resolve soil properties, vegetation cover and ice contents in great details. On the local scale, descriptions of the heterogeneous soil structure in the Arctic exist only for limited areas. Semi-empirical approaches, e.g. based on the Stefan [Stefan, J., 1891. Über die Theorie der Eisbildung, insbesondere über Eisbildung im Polarmeere. Ann. Phys. 42, 269liss286] formula, give a more realistic depiction of permafrost temperatures and active layer thicknesses while at the same time avoiding problems inevitably associated with the explicit treatment of soil freezing and thawing. The coarse resolution of contemporary GCMs models that prevents a realistic description of soil characteristics, vegetation, and topography within a model grid box is the major limitation for use in permafrost modelling.
    We propose to narrow the gap between typical GCMs on one hand and local permafrost models on the other by introducing as an intermediate step a high resolution regional climate model (RCM) to downscale surface climate characteristics to a scale comparable to that of a detailed permafrost model. Forcing the permafrost model with RCM output results in a more realistic depiction of present-day mean annual ground temperature and active layer depth, in particular in mountainous regions. By using global climate change scenarios as driving fields, one can obtain permafrost dynamics in high temporal resolution on the order of years. For the 21st century under the IPCC SRES scenarios A2 and B2, we find an increase of mean annual ground temperature by up to 6 K and of active layer depth by up to 2 m within the East Siberian transect. According to these simulations, a significant part of the transect will suffer from permafrost degradation by the end of the century.
    Paleoclimate reconstructions, Downscaling, permafrost,borehole climatology

66. [von Storch and Weisse 2007]
    Storms represent a major environmental threat. They are associated with abundant rainfall and excessive wind force. Wind storms cause different types of damages on land and on sea; on land, houses and other constructions may be damaged; also trees may break in larger numbers in forests. In the sea, wind pushes water masses towards the coasts, where the water levels may become dangerously high, overwhelm coastal defense and inundate low-lying coastal areas; also the surface of the sea is affected wind waves are created, which eventually transform into swell. Obviously, ocean waves represent a major threat for shipping, off-shore activities and coastal defense.
    Storms

67. [Stieglitz and Smerdon 2007]
    Abstract: The objective of this work is to develop a Simple Land-Interface Model (SLIM) that captures the seasonal and interannual behavior of land atmosphere coupling, as well as the subsequent subsurface temperature evolution. The model employs the one-dimensional thermal diffusion equation driven by a surface flux boundary condition. While the underlying physics is straightforward, the SLIM framework allows a qualitative understanding of the first-order controls that govern the seasonal coupling between the land and atmosphere by implicitly representing the dominant processes at the land surface. The model is used to perform a suite of experiments that demonstrate how changes in surface air temperature and coupling conditions control subsurface temperature evolution. The work presented here suggests that a collective approach employing both complex and simple models, when joined with analyses of observational data, has the potential to increase understanding of land atmosphere coupling and the subsequent evolution of subsurface temperatures.
    Storms

68. [Taylor et al. 2007]
    Abstract: Numerical and theoretical studies have shown that mesoscale gradients in land surface properties can induce circulations in the atmosphere. This study provides the first well-resolved observations of such flows induced by soil moisture from recent rainfall, and is based on aircraft data in the Sahel. Satellite imagery was used to identify fine scale soil moisture features within a wet zone spanning 160 km. Above the wet soil, the planetary boundary layer was up to 3 K cooler, 3 gkg 1 moister and extended to only half the depth of nearby drier areas. Mesoscale perturbations to the background flow were found, consistent with low level divergence over wet soil and convergence in drier areas. The soil moisture and atmospheric wind patterns were statistically coherent on wavelengths down to 20 km. These results suggest that mesoscale convergence lines forced by soil moisture may play a significant role in the meteorology of the Sahel.
    soil moisture

69. [Tett et al. 2007]
    ABSTRACT: A climate simulation of an ocean/atmo-sphere general circulation model driven with natural forcings alone (constant pre-industrial land-cover and well-mixed greenhouse gases, changing orbital, solar and volcanic forcing) has been carried out from 1492 to 2000. Another simulation driven with natural and anthropogenic forcings (changes in greenhouse gases, ozone, the direct and first indirect effect of anthropogenic sulphate aerosol and land-cover) from 1750 to 2000 has also been carried out. These simu-lations suggest that since 1550, in the absence of anthropogenic forcings, climate would have warmed by about 0.1 K. Simulated response is not in equi-librium with the external forcings suggesting that both climate sensitivity and the rate at which the ocean takes up heat determine the magnitude of the response to forcings since 1550. In the simulation with natural forcings climate sensitivity is similar to other simulations of HadCM3 driven with CO2 alone. Climate sensitivity increases when anthropogenic forcings are included. The natural forcing used in our experiment increases decadal centennial time-scale and large spatial scale climate variability, relative to internal variability, as diagnosed from a control sim-ulation. Mean conditions in the natural simulation are cooler than in our control simulation reflecting the reduction in forcing. However, over certain re-gions there is significant warming, relative to control, due to an increase in forest cover. Comparing the simulation driven by anthropogenic and natural forcings with the natural-only simulation suggests that anthropogenic forcings have had a significant impact on, particularly tropical, climate since the early nineteenth century. Thus the entire instrumental temperature record may be contaminated by anthropogenic influences. Both the hydrological cycle and cryosphere are also affected by anthropogenic forcings. Changes in tree-cover appear to be responsible for some of the local and hydrological changes as well as an increase in northern hemi-sphere spring snow cover.
    Paleoclimate simulations, reconstructions

70. [Wagner et al. 2007] Abstract This study investigates the atmospheric circulation in transient climate simulations with a coupled atmosphere ocean general circulation model (GCM) for the mid-Holocene (MH) period 7 4.5 ka BP driven with combinations of orbital, solar and greenhouse gas forcings. The focus is on southern South America. Statistical downscaling models are derived from observational data and applied to the simulations to estimate precipitation in south-eastern Patagonia during the MH. These estimates are compared with lake level estimates for Laguna Potrok Aike (LPA) from sediments. Relative to pre-industrial conditions (i.e. 1550 1850), which show extraordinarily high lake levels, the proxy-based reconstructed lake levels during the MH are lower. The downscaled simulated circulation differences indicate higher LPA precipitation during the MH from March to August, higher annual means, and reduced precipitation from September to February. Thus the reconstructed lower LPA lake levels can not be explained solely by the simulated precipitation changes. Possible reasons for this discrepancy are discussed. Based on proxy data from southern South America hypotheses have also been proposed on the latitudinal position of the southern hemispheric westerlies (SHWs). In agreement with some of these hypotheses our simulations show an increased seasonal cycle of the latitudinal position of the SHWs during the MH, which can be explained by the orbital forcing. The simulations also show stronger SHWs over southern Patagonia during austral summer and weaker SHWs during winter. The downscaling model associates weaker SHWs with increased precipitation in the LPA region. However, this relationship is only moderate, and therefore the downscaling model does not support the assumption of a strong link between mean SHWs and precipitation over south-eastern Patagonia, which is the basis of many proxy-based hypotheses about the SHWs.
    Mid Holocene, south America

71. [Ward et al. 2007]
    Abstract: We have coupled a climate model (ECBilt-CLIO-VECODE) and a hydrological model (STREAM) offline to simulate palaeodischarge of nineteen rivers (Amazon, Congo, Danube, Ganges, Krishna, Lena, Mackenzie, Mekong, Meuse, Mississippi, MurraylissDarling, Nile, Oder, Rhine, SacramentolissSan Joaquin, Syr Darya, Volga, Volta, Zambezi) for three time-slices: Early Holocene (9000liss8650 BP), Mid-Holocene (6200liss5850 BP) and Recent (1750liss2000 AD). To evaluate the model's skill in retrodicting broad changes in mean palaeodischarge we have compared the model results with palaeodischarge estimates from multi-proxy records. We have compared the general trends inferred from the proxy data with statistical differences in modelled discharge between the three periods, thereby developing a technique to assess the level of agreement between the model and proxy data. The quality of the proxy data for each basin has been classed as good, reasonable or low. Of the model runs for which the proxy data were good or reasonable, 72$\%$ were in good agreement with the proxy data, and 92$\%$ were in at least reasonable agreement. We conclude that the coupled climate-hydrological model performs well in simulating mean discharge in the time-slices studied. The discharge trends inferred from the proxy and model data closely follow latitudinal and seasonal variations in insolation over the Holocene. For a number of basins for which agreement was not good we have identified specific mechanisms which could be responsible for the discrepancy, primarily the absence of the Laurentide ice sheet in our model. In order to use the model in an operational sense within water management studies it would be useful to use a higher spatial resolution and a daily time-step.
    Storms

72. [Willet et al. 2007]
    ABSTRACT: Water vapour is the most important contributor to the natural greenhouse effect, and the amount of water vapour in the atmo-sphere is expected to increase under conditions of greenhouse-gas-induced warming, leading to a significant feedback on anthro-pogenic climate change 1 3 . Theoretical and modelling studies predict that relative humidity will remain approximately constant at the global scale as the climate warms, leading to an increase in specific humidity 1,4,5 . Although significant increases in surface specific humidity have been identified in several regions 6 9 , and on the global scale in non-homogenized data 10 , it has not been shown whether these changes are due to natural or human influ-ences on climate. Here we use a new quality-controlled and homo-genized gridded observational data set of surface humidity, with output from a coupled climate model, to identify and explore the causes of changes in surface specific humidity over the late twen-tieth century. We identify a significant global-scale increase in surface specific humidity that is attributable mainly to human influence. Specific humidity is found to have increased in response to rising temperatures, with relative humidity remaining approxi-mately constant. These changes may have important implications, because atmospheric humidity is a key variable in determining the geographical distribution 11 13 and maximum intensity 14 of precipi-tation, the potential maximum intensity of tropical cyclones 15 , and human heat stress 16 , and has important effects on the biosphere 17 and surface hydrology 17,18 .
    Humidity, global change

73. [Wilson et al. 2007]
    No current tree ring (TR) based reconstruction of extratropical Northern Hemisphere (ENH) temperatures that extends into the 1990s captures the full range of late 20th century warming observed in the instrumental record. Over recent decades, a divergence between cooler reconstructed and warmer instrumental large-scale temperatures is observed. We hypothesize that this problem is partly related to the fact that some of the constituent chronologies used for previous reconstructions show divergence against local temperatures in the recent period. In this study, we compiled TR data and published local/regional reconstructions that show no divergence against local temperatures. These data have not been included in other large-scale temperature reconstructions. Utilizing this data set, we developed a new, completely independent reconstruction of ENH annual temperatures (1750 2000). This record is not meant to replace existing reconstructions but allows some degree of independent validation of these earlier studies as well as demonstrating that TR data can better model recent warming at large scales when careful selection of constituent chronologies is made at the local scale. Although the new series tracks the increase in ENH annual temperatures over the last few decades better than any existing reconstruction, it still slightly under predicts values in the post-1988 period. We finally discuss possible reasons why it is so difficult to model post-mid-1980s warming, provide some possible alternative approaches with regards to the instrumental target and detail several recommendations that should be followed in future large-scale reconstruction attempts that may result in more robust temperature estimates.
    Divergence problem, treering reconstructions, NH reconstructions

74. [Xu et al. 2007]
    ABSTRACT: Land surface changes affect the regional climate due to the complex coupling of land-atmosphere interac- tions.From 1995 to 2000,a decrease in the vegetation density and an increase in ground-level thermodynamic activity has been documented by multiple data sources in Northwest China,including meteorological,re- analysis from European Centre for Medium-Range Weather Forecasts (ECMWF),National Oceanic and Atmospheric Administration s (NOAA)Advanced Very High Resolution Radiometer (AVHRR)and TIROS Operational Vertical Sounder (TOVS)satellite remote sensing data.As the ground-level thermodynamic activity increases,humid air from the surrounding regions converge toward desert (and semi-desert)regions, causing areas with high vegetation cover to become gradually more arid.Furthermore,land surface changes in Northwest China are responsible for a decrease in total cloud cover,a decline in the fraction of low and middle clouds,an increase in high cloud cover (due to thermodynamic activity)and other regional climatic adaptations.It is proposed that,beginning in 1995,these cloud cover changes contributed to a green- house effect leading to the rapid air temperature increases and other regional climate impacts that have been observed over Northwest China.
    Land use, soil moisture, climate change

75. [Zorita et al. 2007]
    ABSTRACT: Mann et el. found that a version of the Regularized Expectation Maximization (RegEM) method to reconstruct the temperatures of the last millennium showed similar results to previous reconstructions in one of their earlier papers. They also tested the RegEM method in the surrogate climate of a simulation with the Climate System Model (CSM) and found no attenuation of the pseudoreconstructed centennial variability of the Northern Hemisphere mean temperature compared to the one simulated by the model. This is in contrast with the results by von Storch et al., who found, in a simulation with ECHO-G model, that the earlier Mann et al. method underestimates the centennial temperature variability of the Northern Hemisphere temperature. The newer paper by Mann et al. explains that this difference is in part due to the unrealistic character of the ECHO-G simulation. However, it is shown here that similar results to those of von Storch et al. are also found in an ECHO-G simulation that closely resembles the CSM simulation used by Mann et al. Therefore, it is argued here that this discrepancy could be related to other factors, probably to the use of a longer calibration period and to the difference between RegEM and the original method by Mann et al.
    Paleoclimate model simulations