2013

1. [Anderson et al. 2013] The thermometer-based global surface temperature time series (GST) commands a prominent role in the evidence for global warming, yet this record has considerable uncertainty. An independent record, longer and with better geographic coverage, would be valuable in understanding recent change in the context of natural variability. We compiled the paleo index from 170 temperature-sensitive proxy time series (corals, ice cores, speleothems, lake and ocean sediments, historical documents). Each series was normalized to produce index values of change relative to a 1901-2000 base period; the index values were then averaged. From 1880 to 1995, the index trends significantly upward, similar to the GST. Smaller-scale aspects of the GST including two warming trends and a warm interval during the 1940s are also observed in the paleo index. The paleo index continuously extends back to 1730 with 66 records. The upward trend appears to begin in the early 19th century but the year-to-year variability is large and the 1730-1929 trend is not significant at the p<0.05 level. In addition to its value in vetting the thermometer-based record, our approach shows the potential of the un-calibrated paleo archive in understanding environmental change; this approach can be applied to aspects of environmental change where the instrumental record is even shorter (ocean pH, sea ice, hydrologic extremes).
   Instrumental period, proxy data, trends

2. [Coats et al. 2013a] Simulated hydroclimate variability in millennium-length forced transient and control simulations from the ECHAM and the global Hamburg Ocean Primitive Equation (ECHO-G) coupled atmosphere–ocean general circulation model (AOGCM) is analyzed and compared to 1000 years of reconstructed Palmer drought severity index (PDSI) variability from the North American Drought Atlas (NADA). The ability of the model to simulate megadroughts in the North American southwest is evaluated. (NASW: 258–42.58N, 1258–1058W). Megadroughts in the ECHO-G AOGCM are found to be similar in duration and magnitude to those estimated from the NADA. The droughts in the forced simulation are not, however, temporally synchronous with those in the paleoclimate record, nor are there significant differences between the drought features simulated in the forced and control runs. These results indicate that model-simulated megadroughts can result from internal variability of the modeled climate system rather than as a response to changes in exogenous forcings. Although the ECHO-G AOGCM is capable of simulating megadroughts through persistent La Nin a–like conditions in the tropical Pacific, other mechanisms can produce similarly extreme NASW moisture anomalies in the model. In particular, the lack of low-frequency coherence be- tween NASW soil moisture and simulated modes of climate variability like the El Nin o–Southern Oscil- lation, Pacific decadal oscillation, and Atlantic multidecadal oscillation during identified drought periods suggests that stochastic atmospheric variability can contribute significantly to the occurrence of simulated megadroughts in the NASW. These findings indicate that either an expanded paradigm is needed to un- derstand multidecadal hydroclimate variability in the NASW or AOGCMs may incorrectly simulate the strength and/or dynamics of the connection between NASW hydroclimate variability and the tropical Pacific.
   Pseudoreality, drought

3. [Coats et al. 2013b] The temporal stationarity of the teleconnection between the tropical Pacific Ocean and North America (NA) is analyzed in atmosphere-only, and coupled last-millennium, historical, and control runs from the Coupled Model Intercomparison Project Phase 5 data archive. The teleconnection, defined as the correlation between December- January-February (DJF) tropical Pacific sea surface temperatures (SSTs) and DJF 200 mb geopotential height, is found to be nonstationary on multidecadal timescales. There are significant changes in the spatial features of the teleconnection over NA in continuous 56-year segments of the last millennium and control simulations. Analysis of atmosphere-only simulations forced with observed SSTs indicates that atmospheric noise cannot account for the temporal variability of the teleconnection, which instead is likely explained by the strength of, and multidecadal changes in, tropical Pacific Ocean variability. These results have implications for teleconnection-based analyses of model fidelity in simulating precipitation, as well as any reconstruction and forecasting efforts that assume stationarity of the observed teleconnection. Citation: Coats, S., J. E. Smerdon, B. I. Cook, and R. Seager (2013), Stationarity of the tropical pacific teleconnection to North America in CMIP5/PMIP3 model simulations
   Tropical Pacific, North America teleconnection

4. [Cobb et al. 2013] The El Niño Southern Oscillation (ENSO) drives large changes in global climate patterns from year to year, yet its sensitivity to continued anthropogenic greenhouse forcing is uncertain. We analyzed fossil coral reconstructions of ENSO spanning the past 7000 years from the Northern Line Islands, located in the center of action for ENSO. The corals document highly variable ENSO activity, with no evidence for a systematic trend in ENSO variance, which is contrary to some models that exhibit a response to insolation forcing over this same period. Twentieth-century ENSO variance is significantly higher than average fossil coral ENSO variance but is not unprecedented. Our results suggest that forced changes in ENSO, whether natural or anthropogenic, may be difficult to detect against a background of large internal variability.
   ENSO, holocene

5. [Durán et al. 2013] Precipitación en Guadarrama
   Guadarrama, precipitación

6. [Emile-Geay et al. 2013a] Constraining the low-frequency (LF) behavior of general circulation models (GCMs) requires reliable observational estimates of LF variability. This two-part paper presents multiproxy reconstructions of Nin  o- 3.4 sea surface temperature over the last millennium, applying two techniques [composite plus scale (CPS) and hybrid regularized expectation maximization (RegEM) truncated total least squares (TTLS)] to a network of tropical, high-resolution proxy records. This first part presents the data and methodology before evaluating their predictive skill using frozen network analysis (FNA) and pseudoproxy experiments. The FNA results suggest that about half of the Nin  o-3.4 variance can be reconstructed back to A.D. 1000, but they show little LF skill during certain intervals. More variance can be reconstructed in the interannual band where climate signals are strongest, but this band is affected by dating uncertainties (which are not formally addressed here). The CPS reliably estimates interannual variability, while LF fluctuations are more faithfully reconstructed with RegEM, albeit with inevitable variance loss. The RegEM approach is also tested on representative pseudoproxy networks derived from two millennium-long integrations of a coupled GCM. The pseudoproxy study confirms that reconstruction skill is significant in both the interannual and LF bands, provided that sufficient variance is exhibited in the target Nin  o-3.4 index. It also suggests that FNA severely underestimates LF skill, even when LF variability is strong, resulting in overly pessimistic performance assessments. The centennial-scale variance of the historical Nin  o-3.4 index falls somewhere between the two model simulations, suggesting that the network and methodology presented here would be able to capture the leading LF variations in Nin  o-3.4 for much of the past millennium, with the caveats noted above.
   ENSO reconstructions

7. [Evans et al. 2013] Abstract: A proxy system model may be defined as the complete set of forward and mechanistic processes by which the response of a sensor to environmental forcing is recorded and subsequently observed in a material archive. Proxy system modeling complements and sharpens signal interpretations based solely on statistical analyses and transformations; provides the basis for observing network optimization, hypothesis testing, and data-model comparisons for uncertainty estimation; and may be incorporated as weak but mechanistically-plausible constraints into paleoclimatic reconstruction algorithms. Following a review illustrating these applications, we recommend future research pathways, including development of intermediate proxy system models for important sensors, archives, and observations; linking proxy system models to climate system models; hypothesis development and evaluation; more realistic multi-archive, multi-observation network design; examination of proxy system behavior under extreme conditions; and generalized modeling of the total uncertainty in paleoclimate reconstructions derived from paleo-observations.
   Proxy system modelling

8. [Emile-Geay et al. 2013b] Reducing the uncertainties surrounding the impacts of anthropogenic climate change requires vetting general circulation models (GCMs) against long records of past natural climate variability. This is particularly challenging in the tropical Pacific Ocean, where short, sparse instrumental data preclude GCM validation on multidecadal to centennial time scales. This two-part paper demonstrates the application of two statistical methodologies to a network of accurately dated tropical climate records to reconstruct sea surface temperature (SST) variability in the Ni no-3.4 region over the past millennium. While Part I described the methods and established their validity and limitations, this paper presents several reconstructions of Ni no-3.4, analyzes their sensitivity to procedural choices and input data, and compares them to climate forcing time series and previously published tropical Pacific SST reconstructions. The reconstructions herein show remarkably similar behavior at decadal to multidecadal scales, but diverge markedly on centennial scales. The amplitude of centennial variability in each reconstruction scales with the magnitude of the A.D. 1860 1995 trend in the target dataset s Ni no-3.4 index, with Extended Reconstructed SST, version 3 (ERSSTv3) . the Second Hadley Centre SST dataset (HadSST2) . Kaplan SST; these discrepancies constitute a major source of uncertainty in reconstructing preinstrumental Ni no-3.4 SST. Despite inevitable variance losses, the reconstructed multidecadal variability exceeds that simulated by a state-of-the-art GCM (forced and unforced) over the past millennium, while reconstructed centennial variability is incompatible with constant boundary conditions. Wavelet coherence analysis reveals a robust antiphasing between solar forcing and Ni no-3.4 SST on bicentennial time scales, but not on shorter time scales. Implications for GCM representations of the tropical Pacific climate response to radiative forcing are then discussed.
   ENSO reconstructions

9. [Fernández-Donado et al. 2013] Abstract. Understanding natural climate variability and its driving factors is crucial to assessing future climate change. Therefore, comparing proxy-based climate reconstructions with forcing factors as well as comparing these with paleoclimate model simulations is key to gaining insights into the relative roles of internal versus forced variability. A review of the state of modelling of the climate of the last millennium prior to the CMIP5 PMIP3 (Coupled Model Intercomparison Project Phase 5 Paleoclimate Modelling Intercomparison Project Phase 3) coordinated effort is presented and compared to the available temperature reconstructions. Simulations and reconstructions broadly agree on reproducing the major temperature changes and suggest an overall linear response to external forcing on multidecadal or longer timescales. Internal variability is found to have an important influence at hemispheric and global scales. The spatial distribution of simulated temperature changes during the transition from the Medieval Climate Anomaly to the Little Ice Age disagrees with that found in the reconstructions. Thus, either internal variability is a possible major player in shaping temperature changes through the millennium or the model simulations have problems realistically representing the response pattern to external forcing. A last millennium transient climate response (LMTCR) is defined to provide a quantitative framework for analysing the consistency between simulated and reconstructed climate. Beyond an overall agreement between simulated and reconstructed LMTCR ranges, this analysis is able to single out specific discrepancies between some reconstructions and the ensemble of simulations. The disagreement is found in the cases where the reconstructions show reduced covariability with external forcings or when they present high rates of temperature change
   Last millennium simulations and reconstructions

10. [García-Bustamante et al. 2013] Abstract: The wind power generated during winter months 1999–2003 at several wind farms in the north- eastern Iberian Peninsula is investigated through the application of a statistical downscaling. This allows for an improved understanding of the wind power variability and its relationship to the large scale atmospheric circulation. It is found that 97 % of the variability of this non-climatic variable is connected to changes in the atmospheric cir- culation. The methodological uncertainty associated with multiple configurations of the statistical downscaling method replicates well the observed variability of the wind power, an indication of the robustness of the methodology to changes in the model set up. In addition, the use of the statistical model is extended out of the observational period providing an estimation of the long-term variability of wind power throughout the twentieth century. The exten- ded wind power reconstruction shows large inter-annual and multidecadal variability. Alternative approaches to calibrate the empirical downscaling model using actual wind power observations have also been investigated. They involve the estimation of wind power changes from downscaled wind values and make use of several transfer functions based on the linearity between wind and wind energy. The performance of the latter approaches is similar to the direct downscaling of wind power and may allow wind power production estimations even in the absence of historical wind turbine records. These results can be of great interest for deriving medium/long term impact-ori- ented energy assessments, especially when wind power observations are missing as well as in the context of cli- mate change scenarios.
    Downscaling wind

11. [Ilyina et al. 2013] Abstract:Ocean biogeochemistry is a novel standard component of fifth phase of the Coupled Model Intercomparison Project (CMIP5) experiments which project future climate change caused by anthropogenic emissions of greenhouse gases. Of particular interest here is the evolution of the oceanic sink of carbon and the oceanic contribution to the climate-carbon cycle feedback loop. The Hamburg ocean carbon cycle model (HAMOCC), a component of the Max Planck Institute for Meteorology Earth system model (MPI-ESM), is employed to address these challenges. In this paper we describe the version of HAMOCC used in the CMIP5 experiments (HAMOCC 5.2) and its implementation in the MPI-ESM to provide a documentation and basis for future CMIP5-related studies. Modeled present day distributions of biogeochemical variables calculated in two different horizontal resolutions compare fairly well with observations. Statistical metrics indicate that the model performs better at the ocean surface and worse in the ocean interior. There is a tendency for improvements in the higher resolution model configuration in representing deeper ocean variables; however, there is little to no improvement at the ocean surface. An experiment with interactive carbon cycle driven by emissions of CO2 produces a 25% higher variability in the oceanic carbon uptake over the historical period than the same model forced by prescribed atmospheric CO2 concentrations. Furthermore, a climate warming of 3.5 K projected at atmospheric CO2 concentration of four times the preindustrial value, reduced the atmosphere-ocean CO2 flux by 1 GtC yr−1. Overall, the model shows consistent results in different configurations, being suitable for the type of simulations required within the CMIP5 experimental design.
    HAMOCC

12. [Giorgetta et al. 2013] Abstract. The new Max-Planck-Institute Earth System Model (MPI-ESM) is used in the Coupled Model Intercomparison Project phase 5 (CMIP5) in a series of climate change experiments for either idealized CO2-only forcing or forcings based on observations and the Representative Concentration Pathway (RCP) scenarios. The paper gives an overview of the model configurations, experiments related forcings, and initialization procedures and presents results for the simulated changes in climate and carbon cycle. It is found that the climate feedback depends on the global warming and possibly the forcing history. The global warming from climatological 1850 conditions to 2080-2100 ranges from 1. C under the RCP2.6 scenario to 4.4 C under the RCP8.5 scenario. Over this range, the pat- terns of temperature and precipitation change are nearly independent of the global warm- ing. The model shows a tendency to reduce the ocean heat uptake efficiency toward a warmer climate, and hence acceleration in warming in the later years. The precipitation sensitivity can be as high as 2.5 K21 if the CO2 concentration is constant, or as small as 1.6 K21, if the CO2 concentration is increasing. The oceanic uptake of anthropogenic carbon increases over time in all scenarios, being smallest in the experiment forced by RCP2.6 and largest in that for RCP8.5. The land also serves as a net carbon sink in all sce- narios, predominantly in boreal regions. The strong tropical carbon sources found in the RCP2.6 and RCP8.5 experiments are almost absent in the RCP4.5 experiment, which can be explained by reforestation in the RCP4.5 scenario.
    Last millennium simulations and reconstructions

13. [Hagemann et al. 2013] To assess the robustness of projected changes of the hydrological cycle simulated by an Earth system model (ESM), it is fundamental to validate the ESM and to characterize its major deficits. As the hydrological cycle is closely coupled to the energy cycle, a common large-scale evaluation of these fundamental components of the Earth system is highly beneficial, even though this has been rarely done up to now. Consequently, the purpose of the present study is the combined evaluation of land surface water and energy fluxes from the newest ESM version of the Max Planck Institute for Meteorology (MPI-ESM), which was used to produce an ensemble of Coupled Model Intercomparison Project Phase 5 (CMIP5) simulations. With regard to energy fluxes, we especially make use of recent satellite data sets. Additionally, MPI-ESM results are compared with CMIP3 results from the predecessor of MPI-ESM, ECHAM5/MPIOM, as well as to results from the atmosphere/land part of MPI-ESM (ECHAM6/JSBACH) forced by observed sea surface temperature (SST). Analyses focus on regions where notable differences occur between the two ESM versions as well as between the fully coupled and the uncoupled SST-driven simulations. In general, our results show a considerable improvement of MPI-ESM in simulating surface shortwave radiation fluxes. The precipitation of the fully coupled simulations notably differs from those of the SST-forced simulations over a few river catchments. Over the Amazon catchment, the coupling to the ocean leads to a large negative precipitation bias, while for the Ganges/Brahmaputra, the coupling significantly improves the simulated precipitation.
    Water and energy fluxes

14. [Stocker et al. 2013] SPM
    ipcc

15. [Cubasch et al. 2013] Capitulo introduccion
    ipcc

16. [Hartmann et al. 2013] Capitulo observaciones
    ipcc

17. [Masson-Delmotte et al. 2013] Paleo
    ipcc

18. [Bindoff et al. 2013] Detección y atribución
    ipcc

19. [Jiménez et al. 2013] This work uses a WRF numerical simulation from 1960 to 2005 performed at a high horizontal resolution (2 km) to analyze the surface wind variability over a complex terrain region located in northern Iberia. A shorter slice of this simulation has been used in a previous study to demonstrate the ability of the WRF model in reproducing the observed wind variability during the period 1992–2005. Learning from that validation exercise, the extended simu- lation is herein used to inspect the wind behavior where and when observations are not available and to determine the main synoptic mechanisms responsible for the surface wind variability. A principal component analysis was applied to the daily mean wind. Two principal modes of variation accumulate a large percentage of the wind variability (83.7%). The first mode reflects the channeling of the flow between the large mountain systems in northern Iberia modulated by the smaller topographic features of the region. The second mode further contributes to stress the differentiated wind behavior over the mountains and val- leys. Both modes show significant contributions at the higher frequencies during the whole analyzed period, with different contributions at lower frequencies during the dif- ferent decades. A strong relationship was found between these two modes and the zonal and meridional large scale pressure gradients over the area. This relationship is described in the context of the influence of standard circu- lation modes relevant in the European region like the North Atlantic Oscillation, the East Atlantic pattern, East Atlantic/ Western Russia pattern, and the Scandinavian pattern.
    Dynamical downscaling wrf for wind

20. [Jiménez-Guerrero et al. 2013]
    ABSTRACT: The ESCENA (2008 to 2012) project is a Spanish initiative, which applies the dynam- ical downscaling technique to generate climate change scenarios based on an ensemble of Regional Climate Models (RCMs) consisting of PROMES, WRF, MM5 or REMO over Peninsular Spain and the Balearic and Canary Islands using a high resolution of 25 km. We describe the mean fields and interannual variability for temperature and precipitation in an ensemble of simulations forced by the high resolution ERA-Interim reanalysis (1990 to 2007) and compare them to the Spain02 observed data set. Maximum surface air temperature shows seasonal cold biases up to –2.5K in all models and it is clearly underestimated during the coldest seasons, but less so dur- ing summertime (JJA). Generally, there is a better agreement between observed and simulated minimum surface air temperature, which is slightly overestimated (up to +2K) especially during wintertime (DJF). Regarding precipitation, all models except PROMES tend to show low dry biases during all seasons, especially for autumn on the Mediterranean coast of the Iberian Penin- sula. With respect to the interannual variability, the PROMES simulations overestimate the stan- dard deviation of maximum surface air temperature, while the remaining models tend to slightly underestimate it, and most models tend to underestimate the standard deviation of precipitation. The results highlight the ability of these RCMs to reproduce the mean fields and the interannual variability in a very complex terrain such as the Iberian Peninsula, showing a great diversity of cli- matic behavior. The evaluation of the ensemble results indicates a great improvement in the tem- poral correlation and the representation of the spatial patterns of temperature and precipitation for all seasons with respect to the individual models.
    RCM Iberia, climate change

21. [Jungclaus et al. 2013] Abstract: MPI-ESM is a new version of the global Earth system model developed at the Max Planck Institute for Meteorology. This paper describes the ocean state and circulation as well as basic aspects of variability in simulations contributing to the fifth phase of the Coupled Model Intercomparison Project (CMIP5). The performance of the ocean/sea-ice model MPIOM, coupled to a new version of the atmosphere model ECHAM6 and modules for land surface and ocean biogeochemistry, is assessed for two model versions with different grid resolution in the ocean. The low-resolution configuration has a nominal resolution of 1.5°, whereas the higher resolution version features a quasiuniform, eddy-permitting global resolution of 0.4°. The paper focuses on important oceanic features, such as surface temperature and salinity, water mass distribution, large-scale circulation, and heat and freshwater transports. In general, these integral quantities are simulated well in comparison with observational estimates, and improvements in comparison with the predecessor system are documented; for example, for tropical variability and sea ice representation. Introducing an eddy-permitting grid configuration in the ocean leads to improvements, in particular, in the representation of interior water mass properties in the Atlantic and in the representation of important ocean currents, such as the Agulhas and Equatorial current systems. In general, however, there are more similarities than differences between the two grid configurations, and several shortcomings, known from earlier versions of the coupled model, prevail.
    MPIOM

22. [Landrum et al. 2012]
    An overview of the Last Millennium (LM) simulation of the Community Climate System Model version 4 (CCSM4) at a nominal latitude-longitude resolution is presented. The CCSM4 LM simulation reproduces many large-scale climate patterns suggested by historical and proxy-data records including Northern Hemisphere (NH) and Southern Hemisphere (SH) surface temperatures cooling from the Medieval Climate Anomaly (MCA) to the Little Ice Age (LIA) and warming in the 20th century. High latitudes of both hemispheres show polar amplification of the cooling from MCA to LIA associated with sea ice increases. The LM simulation does not reproduce La Nina-like cooling in the eastern Pacific during the MCA relative to the LIA, as has been suggested by proxy reconstructions. Dry Medieval conditions over the southwestern and central US are simulated in agreement with a variety of proxy indicators for these regions. CCSM4 captures some reconstructed patterns of temperature changes over Europe and North America, but not those of precipitation in the Asian monsoon region, to large volcanic eruptions. Only the Atlantic Multidecadal Oscillation (AMO) has higher variance and increased power at centennial periods in the LM simulation compared with the 1850 CE non-transient run, suggesting long-term oceanic response to natural forcings. The North Atlantic Oscillation (NAO), Pacific Decadal Oscillation (PDO), and El Nino-Southern Oscillation (ENSO) variability modes show little or no change. CCSM4 does not simulate a persistent positive NAO or a prolonged period of negative PDO during the MCA, as suggested by the proxy reconstructions.
    Climate simulation, last millennium.

23. [Liu et al. 2013] As a result of global warming, precipitation is likely to increase in high latitudes and the tropics and to decrease in already dry subtropical regions1. The absolute magnitude and regional details of such changes, however, remain intensely debated2,3. As is well known from El Nin o studies, sea-surface-temperature gradients across the tropical Pacific Ocean can strongly influence global rainfall4,5. Palaeoproxy evidence indicates that the difference between the warm west Pacific and the colder east Pacific increased in past periods when the Earth warmed as a result of increased solar radiation6 9. In contrast, in most model projections of future greenhouse warming this gradient weakens2,10,11. It has not been clear how to reconcile these two findings. Here we show in climate model simulations that the tropical Pacific sea-surface-temperature gradient increaseswhen the warming is due to increased solar radiation and decreases when it is due to increased greenhouse-gas forcing. For the same global surface temperature increase the latter pattern produces less rainfall, notably over tropical land, which explains why in the model the late twentieth century is warmer than in the Medieval Warm Period (around AD 1000 1250) but precipitation is less. This difference is consistent with the global tropospheric energy budget12, which requires a balance between the latent heat released in precipitation and radiative cooling. The tropospheric cooling is less for increased greenhouse gases, which add radiative absorbers to the troposphere, than for increased solar heating, which is concentrated at the Earth s surface. Thus warming due to increased greenhouse gases produces a climate signature different from that of warming due to solar radiation changes.
    Global temperature, precip, past1000, thermostat model.

24. [Moberg 2013] Comparison of simulated and reconstructed past climate variability within the last millennium provides an opportunity to aid the understanding and interpretation of palaeoclimate proxy data and to test hypotheses regarding external forcings, feedback mechanisms and internal climate variability under conditions close to those of the present day. Most such comparisons have been made at the Northern Hemispheric scale, of which a selection of recent results is briefly discussed here. Uncertainties in climate and forcing reconstructions, along with the simplified representations of the true climate system represented by climate models, limit our possibility to draw certain conclusions regarding the nature of forced and unforced climate variability. Additionally, hemispheric-scale temperature variations have been comparatively small, wherefore the last millennium is apparently not a particularly useful period for estimating climate sensitivity. Nevertheless, several investigators have concluded that Northern Hemispheric-scale decadal-mean temperatures in the last millennium show a significant influence from natural external forcing, where volcanic forcing is significantly detectable while solar forcing is less robustly detected. The amplitude of centennial-scale variations in solar forcing has been a subject for much debate, but current understanding of solar physics implies that these variations have been small similar in magnitude to those within recent sunspot cycles and thus they have not been a main driver of climate in the last millennium. This interpretation is supported by various comparisons between forced climate model simulations and temperature proxy data. Anthropogenic greenhouse gas and aerosol forcing has been detected by the end of Northern Hemispheric temperature reconstructions.
    Last millennium, model-data comparison

25. [Oleson et al. 2013] Tech rep CLM 4.5
    LSM, CLM

26. [Ortega et al. 2013] Abstract. Studies addressing climate variability during the last millennium generally focus on variables with a direct influence on climate variability, like the fast thermal response to varying radiative forcing, or the large-scale changes in atmospheric dynamics (e.g. North Atlantic Oscillation). The ocean responds to these variations by slowly integrating in depth the upper heat flux changes, thus producing a delayed influence on ocean heat content (OHC) that can later impact low frequency SST (sea surface temperature) variability through reemergence processes. In this study, both the externally and internally driven variations of the OHC during the last millennium are investigated using a set of fully coupled simulations with the ECHO-G (coupled climate model ECHAMA4 and ocean model HOPE-G) atmosphere–ocean general circulation model (AOGCM). When compared to observations for the last 55 yr, the model tends to overestimate the global trends and underestimate the decadal OHC variability. Extending the analysis back to the last one thousand years, the main impact of the radiative forcing is an OHC increase at high latitudes, explained to some extent by a reduction in cloud cover and the subsequent increase of short-wave radiation at the surface. This OHC response is dominated by the effect of volcanism in the preindustrial era, and by the fast increase of GHGs during the last 150 yr. Likewise, salient impacts from internal climate variability are observed at regional scales. For instance, upper temperature in the equatorial Pacific is controlled by ENSO (El Niño Southern Oscillation) variability from interannual to multidecadal timescales. Also, both the Pacific Decadal Oscillation (PDO) and the Atlantic Multidecadal Oscillation (AMO) modulate intermittently the interdecadal OHC variability in the North Pacific and Mid Atlantic, respectively. The NAO, through its influence on North Atlantic surface heat fluxes and convection, also plays an important role on the OHC at multiple timescales, leading first to a cooling in the Labrador and Irminger seas, and later on to a North Atlantic warming, associated with a delayed impact on the AMO.
    ECHO-g

27. [2k 2013] Past global climate changes had strong regional expression. To elucidate their spatio-temporal pattern, we reconstructed past temperatures for seven continental-scale regions during the past one to two millennia. The most coherent feature in nearly all of the regional temperature reconstructions is a long-term cooling trend, which ended late in the nineteenth century. At multi- decadal to centennial scales, temperature variability shows distinctly di erent regional patterns, with more similarity within each hemisphere than between them. There were no globally synchronous multi-decadal warm or cold intervals that de ne a worldwide Medieval Warm Period or Little Ice Age, but all reconstructions show generally cold conditions between ad 1580 and 1880, punctuated in some regions by warm decades during the eighteenth century. The transition to these colder conditions occurred earlier in the Arctic, Europe and Asia than in North America or the Southern Hemisphere regions. Recent warming reversed the long-term cooling; during the period ad 1971–2000, the area-weighted average reconstructed temperature was higher than any other time in nearly 1,400 years.
    Last millennium, model-data comparison

28. [Phipps et al. 2013] The past 1500 years provides a valuable opportunity to study the response of the climate system to external forcings. However, the integration of paleoclimate proxies with climate modeling is critical to improving our understanding of climate dynamics. In this paper, we therefore use a climate system model and proxy records to study the role of natural and anthropogenic forcings in driving the global climate. The inverse and forward approaches to paleoclimate data-model comparison are applied, and sources of uncertainty are identiffed and discussed. In the 1rst of two case studies, the climate model simulations are compared with multi2 proxy temperature reconstructions. Robust solar and volcanic signals are detected in South2 ern Hemisphere temperatures, with a possible volcanic signal detected in the Northern Hemi2 sphere. The anthropogenic signal dominates during the industrial period. We also and that seasonal and geographical biases may cause multi-proxy reconstructions to overestimate the magnitude of the long-term pre-industrial cooling trend. In the second case study, the model simulations are compared with a coral d18O record from the central Pacific Ocean. We find that greenhouse gases, solar irradiance and volcanic eruptions all influence the mean state of the central Pacific, but there is no evidence that natural or anthropogenic forcings have any systematic impact on El Nin o-Southern Oscillation. The proxy-climate relationship is found to change over time, challenging the assumption of stationarity that underlies the interpretation of paleoclimate proxies. These case studies demonstrate the value of paleocli mate data-model comparison, but also highlight the limitations of current techniques and demonstrate the need to develop alternative approaches.
    Last millennium, model-data comparison

29. [Reick et al. 2013] Abstract: The purpose of this paper is to give a rather comprehensive description of the models for natural and anthropogenically driven changes in biogeography as implemented in the land component JSBACH of the Max Planck Institute Earth system model (MPI-ESM). The model for natural land cover change (DYNVEG) features two types of competition: between the classes of grasses and woody types (trees, shrubs) controlled by disturbances (fire, windthrow) and within those vegetation classes between different plant functional types based on relative net primary productivity advantages. As part of this model, the distribution of land unhospitable to vegetation (hot and cold deserts) is determined dynamically from plant productivity under the prevailing climate conditions. The model for anthropogenic land cover change implements the land use transition approach by Hurtt et al. (2006). Our implementation is based on the assumption that historically pastures have been preferentially established on former grasslands (“pasture rule”). We demonstrate that due to the pasture rule, deforestation reduces global forest area between 1850 and 2005 by 15% less than without. Because of the pasture rule the land cover distribution depends on the full history of land use transitions. This has implications for the dynamics of natural land cover change because assumptions must be made on how agriculturalists react to a changing natural vegetation in their environment. A separate model representing this process has been developed so that natural and anthropogenic land cover change can be simulated consistently. Certain aspects of our model implementation are illustrated by selected results from the recent CMIP5 simulations.
    MPIESM lulc

30. [Rohde et al. 2013] A new mathematical framework is presented for producing maps and large-scale averages of temperature changes from weather station thermometer data for the purposes of climate analysis. The method allows inclusion of short and discontinuous temperature records, so nearly all digitally archived thermometer data can be used. The framework uses the statistical method known as Kriging to interpolate data from stations to arbitrary locations on the Earth. Iterative weighting is used to reduce the influence of statistical outliers. Statistical uncertainties are calculated by subdividing the data and comparing the results from statistically independent subsamples using the Jackknife method. Spatial uncertainties from periods with sparse geographical sampling are estimated by calculating the error made when we analyze post-1960 data using similarly sparse spatial sampling. Rather than “homogenize” the raw data, an automated procedure identifies discontinuities in the data; the data are then broken into two parts at those times, and the parts treated as separate records. We apply this new framework to the Global Historical Climatology Network (GHCN) monthly land temperature dataset, and obtain a new global land temperature reconstruction from 1800 to the present. In so doing, we find results in close agreement with prior estimates made by the groups at NOAA, NASA, and at the Hadley Center/Climate Research Unit in the UK. We find that the global land mean temperature increased by 0.89 ± 0.06°C in the difference of the Jan 2000-Dec 2009 average from the Jan 1950-Dec 1959 average (95 confidence for statistical and spatial uncertainties)
    Global temperature avgs

31. [Romero-Viana et al. 2013] Isabel Lake is a hypersaline crater-lake on Isabel Island, Mexico, situated in the eastern tropical Pacific, an area highly sensitive to hydrological changes. Today, annual rainfall mostly occurs during the wet season, from June to October, when the northern edge of the Intertropical Convergence Zone (ITCZ) extends over the island. In order to evaluate the potential of sedimentary lipid biomarker signatures as a proxy of past hydroclimatic variability we have performed a calibration analysis comparing changes in biomarker distribution in the upper 16 cm of the sediment core with a regional instrumental data set. Annual laminations present in the sediment sequence allow for precise chronological control (1942 2006). More than 80 different lipid compounds were identified in the sediment and could be assigned to three major groups of source organisms: (1) algal populations; (2) a mixed community of ciliates, bacteria and cyanobacteria; and (3) photosynthetic sulfur bacteria. We found that the observed changes in the relative contribution of the different lipid biomarkers to the sediment record were determined by the regional rainfall variability over the last 65 years. The planktonic community of Isabel Lake was highly sensitive to salinity fluctuations related to rainfall variability; seasonal precipitation results in freshwater input into the lake, driving an annual algal bloom and a relative decrease in the abundance of the more halotolerant populations of (cyano) bacteria and ciliates. Consequently, the concentration ratio between the two most abundant biomarkers in the Isabel Lake sediments, n-alkyl diols and tetrahymanol (which we define as the DiTe index), representing algal and ciliate planktonic populations, respectively, was significantly correlated with the seasonal rainfall anomaly (r=0.68, pb0.01). We propose that the DiTe index is a proxy of changes in the aquatic ecosystem of Isabel Lake and, by extension, regional hydrological changes in a sensitive climatic area of the eastern tropical Pacific.
    Last millennium, precipitation

32. [Schurer et al. 2013] Reconstructions of past climate show notable temperature variability over the past millennium, with relatively warm conditions during the Medieval Climate Anomaly  (MCA) and a relatively cold Little Ice Age  (LIA). We use multi-model simulations of the past millennium together with a wide range of reconstructions of Northern Hemispheric mean annual temperature to separate climate variability from 850 to 1950CE into components attributable to external forcing and internal climate variability. We find that external forcing contributed significantly to long-term temperature variations irrespective of the proxy reconstruction, particularly from 1400 onwards. Over the MCA alone, however, the effect of forcing is only detectable in about half of the reconstructions considered, and the response to forcing in the models cannot explain the warm conditions around 1000CE seen in some reconstructions. We use the residual from the detection analysis to estimate internal variability independent from climate modelling and find that the recent observed 50-year and 100-year hemispheric temperature trends are substantially larger than any of the internally-generated trends even using the large residuals over the MCA. We find variations in solar output and explosive volcanism to be the main drivers of climate change from 1400-1900, but for the first time we are also able to detect a significant contribution from greenhouse gas variations to the cold conditions during 1600-1800. The proxy reconstructions tend to show a smaller forced response than is simulated by the models. We show that this discrepancy is likely to be, at least partly, associated with the difference in the response to large volcanic eruptions between reconstructions and model simulations.
    Last millennium, temperature simulations

33. [Shi et al. 2013] ABSTRACT: Previous studies have either exclusively used annual tree-ring data or have combined tree-ring series with other, lower temporal resolution proxy series. Both approaches can lead to significant uncertainties, as tree-rings may underestimate the amplitude of past temperature variations, and the validity of non-annual records cannot be clearly assessed. In this study, we assembled 45 published Northern Hemisphere (NH) temperature proxy records covering the past millennium, each of which satisfied 3 essential criteria: the series must be of annual resolution, span at least a thousand years, and represent an explicit temperature signal. Suitable climate archives included ice cores, varved lake sediments, tree-rings and speleothems. We reconstructed the average annual land temperature series for the NH over the last millennium by applying 3 different reconstruction techniques: (1) principal components (PC) plus second-order autoregressive model (AR2), (2) composite plus scale (CPS) and (3) regularized errors-in-variables approach (EIV). Our reconstruction is in excellent agreement with 6 climate model simulations (including the first 5 models derived from the fifth phase of the Coupled Model Intercomparison Project (CMIP5) and an earth system model of intermediate complexity (LOVECLIM), showing similar temperatures at multidecadal timescales; however, all simulations appear to underestimate the temperature during the Medieval Warm Period (MWP). A comparison with other NH reconstructions shows that our results are consistent with earlier studies. These results indicate that well-validated annual proxy series should be used to minimize proxy-based artifacts, and that these proxy series contain sufficient information to reconstruct the low-frequency climate variability over the past millennium.
    Last millennium, temperature reconstruction

34. [Brovkin et al. 2013] ABSTRACT: In recent generation Earth system models (ESMs), land-surface grid cells are represented as tiles covered by different plant functional types such as trees or grasses. Here, we present an evaluation of the vegetation-cover module of the ESM developed at the Max Planck Institute for Meteorology in Hamburg, Germany (MPI-ESM) for present-day conditions. The vegetation continuous fields (VCF) product that is based on satellite observations in 2001 is used to evaluate the fractional distributions of woody vegetation cover and bare ground. The model performance is quantified using two metrics: a square of the Pearson correlation coefficient, r2, and the root-mean- square error (RMSE). On a global scale, r2 and RMSE of modeled tree cover are equal to 0.61 and 0.19, respectively, which we consider as satisfactory values. The model simulates tree cover and bare ground with r2 higher for the Northern Hemisphere (0.66) than for the Southern Hemisphere (0.48–0.50). We complement this analysis with an evaluation of the simulated land-surface albedo using the difference in net surface radiation. On a global scale, the correlation between modeled and observed albedos is high during all seasons, whereas the main disagreement occurs in spring in the high northern latitudes. This discrepancy can be attributed to a high sensitivity of the land-surface albedo to the simulated snow cover and snow- masking effect of trees. By contrast, the tropics are characterized by very high correlation and relatively low RMSE (5.4–6.5 W/m2) during all seasons. The presented approach could be applied for an evaluation of vegetation cover and land-surface albedo simulated by different ESMs.
    Land cover and land surface albedo

35. [Stevens et al. 2013] Abstract: ECHAM6, the sixth generation of the atmospheric general circulation model ECHAM, is described. Major changes with respect to its predecessor affect the representation of shortwave radiative transfer, the height of the model top. Minor changes have been made to model tuning and convective triggering. Several model configurations, differing in horizontal and vertical resolution, are compared. As horizontal resolution is increased beyond T63, the simulated climate improves but changes are incremental; major biases appear to be limited by the parameterization of small-scale physical processes, such as clouds and convection. Higher vertical resolution in the middle atmosphere leads to a systematic reduction in temperature biases in the upper troposphere, and a better representation of the middle atmosphere and its modes of variability. ECHAM6 represents the present climate as well as, or better than, its predecessor. The most marked improvements are evident in the circulation of the extratropics. ECHAM6 continues to have a good representation of tropical variability. A number of biases, however, remain. These include a poor representation of low-level clouds, systematic shifts in major precipitation features, biases in the partitioning of precipitation between land and sea (particularly in the tropics), and midlatitude jets that appear to be insufficiently poleward. The response of ECHAM6 to increasing concentrations of greenhouse gases is similar to that of ECHAM5. The equilibrium climate sensitivity of the mixed-resolution (T63L95) configuration is between 2.9 and 3.4 K and is somewhat larger for the 47 level model. Cloud feedbacks and adjustments contribute positively to warming from increasing greenhouse gases.,
    MPIESM

36. [Wolff et al. 2013] In this chapter, we describe and explain some of the patterns observed in the behaviour of Earth s climate system. We explain some of the causes of the climate s natural variability, setting contemporary climate change in its longer-term context. We describe the various lines of evidence about climate forcing and the feedbacks that determine the responses to perturbations, and the way in which reconstructions of past climates can be used in combination with models and contemporary observations of change.
    Natural perturbations