2015

  1. [Bony et al. 2015] Fundamental puzzles of climate science remain unsolved because of our limited understanding of how clouds, circulation and climate interact. One example is our inability to provide robust assessments of future global and regional climate changes. However, ongoing advances in our capacity to observe, simulate and conceptualize the climate system now make it possible to fill gaps in our knowledge. We argue that progress can be accelerated by focusing research on a handful of important scientific questions that have become tractable as a result of recent advances. We propose four such questions below; they involve understanding the role of cloud feedbacks and convective organization in climate, and the factors that control the position, the strength and the variability of the tropical rain belts and the extratropical storm tracks.
    Important questions for climate sensitivy and climate science, clouds.

  2. [Cook et al. 2015] Climate model projections suggest widespread drying in the Mediterranean Basin and wetting in Fennoscandia in the coming decades largely as a consequence of greenhouse gas forcing of climate. To place these and other “Old World” climate projections into historical perspective based on more complete estimates of natural hydroclimatic variability, we have developed the “Old World Drought Atlas” (OWDA), a set of year-to-year maps of tree-ring reconstructed summer wetness and dryness over Europe and the Mediterranean Basin during the Common Era. The OWDA matches historical accounts of severe drought and wetness with a spatial completeness not previously available. In addition, megadroughts reconstructed over north-central Europe in the 11th and mid-15th centuries reinforce other evidence from North America and Asia that droughts were more severe, extensive, and prolonged over Northern Hemisphere land areas before the 20th century, with an inadequate understanding of their causes. The OWDA provides new data to determine the causes of Old World drought and wetness and attribute past climate variability to forced and/or internal variability.
    Drought reconstructions, owda

  3. [Durán et al. 2015] Precipitación en Guadarrama, watter vapour.
    Guadarrama, precipitación

  4. [Durán 2015] Precipitación en Guadarrama. Tesis
    Guadarrama, precipitación

  5. [Garc&́#237;a-D&́#237;ez et al. 2015] Abstract Regional Climate Models (RCMs) are widely used tools to add detail to the coarse resolution of global simulations. However, these are known to be affected by biases. Usually, published model evaluations use a reduced number of variables, frequently precipitation and temperature. Due to the complexity of the mod- els, this may not be enough to assess their physical realism (e.g. to enable a fair comparison when weight- ing ensemble members). Furthermore, looking at only a few variables makes difficult to trace model errors. Thus, in many previous studies, these biases are de- scribed but their underlying causes and mechanisms are often left unknown. In this work the ability of a multi-physics ensemble in reproducing the observed cli- matologies of many variables over Europe is analysed. These are temperature, precipitation, cloud cover, ra- diative fluxes and total soil moisture content. It is found that, during winter, the model suffers a significant cold bias over snow covered regions. This is shown to be re- lated with a poor representation of the snow-atmosphere interaction, and is amplified by an albedo feedback. It is shown how two members of the ensemble are able to alleviate this bias, but by generating a too large cloud cover. During summer, a large sensitivity to the cumu- lus parameterization is found, related to large differ- ences in the cloud cover and short wave radiation flux. Results also show that small errors in one variable are sometimes a result of error compensation, so the high dimensionality of the model evaluation problem cannot be disregarded.
    Cordex

  6. [Gonzalez-Hidalgo et al. 2015] ABSTRACT We have developed a new monthly temperature database for mainland Spain by using the complete holding of maximum and minimum monthly mean values stored at the Spanish National Meteorological Agency (AEMet). After an exhaustive quality control exercise, the data set includes 1358 complete series, and a high-resolution grid (0.1 × 0.1) was calculated to analyse monthly trends. There has been an uneven increase in the mean values of maximum and minimum temperatures in the Spanish mainland from 1951 to 2010. The maximum monthly mean temperature values have risen mostly in late winter/early spring and the summer, while the minimum monthly mean temperature values have increased in summer, spring and autumn in southern areas. The spatial pattern of the diurnal temperature range shows a clear north–south gradient in summer, with positive trends in the north and negative trends in the south; furthermore, a negative pattern has been detected in the south in spring and autumn. These areas, particularly the Mediterranean coastland, have been subject to dramatic urbanization and land use changes during the past 30 years that may have affected nocturnal temperatures, in particular. Thus, warming processes in the Iberian Peninsula appear to be influenced both by global factors and more local ones, and their effects should be differentiated spatially.
    Carbon model mpiesm

  7. [Goll et al. 2015] Abstract: The quantification of sources and sinks of carbon from land use and land cover changes (LULCC) is uncertain. We investigated how the parametrization of LULCC and of organic matter decomposition, as well as initial land cover, affects the historical and future carbon fluxes in an Earth System Model (ESM). Using the land component of the Max Planck Institute ESM, we found that the historical (1750–2010) LULCC flux varied up to 25% depending on the fraction of biomass which enters the atmosphere directly due to burning or is used in short-lived products. The uncertainty in the decadal LULCC fluxes of the recent past due to the parametrization of decomposition and direct emissions was 0.6 Pg C yr−1, which is 3 times larger than the uncertainty previously attributed to model and method in general. Preindustrial natural land cover had a larger effect on decadal LULCC fluxes than the aforementioned parameter sensitivity (1.0 Pg C yr−1). Regional differences between reconstructed and dynamically computed land covers, in particular, at low latitudes, led to differences in historical LULCC emissions of 84–114 Pg C, globally. This effect is larger than the effects of forest regrowth, shifting cultivation, or climate feedbacks and comparable to the effect of differences among studies in the terminology of LULCC. In general, we find that the practice of calibrating the net land carbon balance to provide realistic boundary conditions for the climate component of an ESM hampers the applicability of the land component outside its primary field of application.
    Carbon model mpiesm

  8. [Garc&́#237;a-D&́#237;ez et al. 2015] Abstract Regional Climate Models (RCMs) are widely used tools to add detail to the coarse resolution of global simulations. However, these are known to be affected by biases. Usually, published model evaluations use a reduced number of variables, frequently precipitation and temperature. Due to the complexity of the mod- els, this may not be enough to assess their physical realism (e.g. to enable a fair comparison when weight- ing ensemble members). Furthermore, looking at only a few variables makes difficult to trace model errors. Thus, in many previous studies, these biases are de- scribed but their underlying causes and mechanisms are often left unknown. In this work the ability of a multi-physics ensemble in reproducing the observed cli- matologies of many variables over Europe is analysed. These are temperature, precipitation, cloud cover, ra- diative fluxes and total soil moisture content. It is found that, during winter, the model suffers a significant cold bias over snow covered regions. This is shown to be re- lated with a poor representation of the snow-atmosphere interaction, and is amplified by an albedo feedback. It is shown how two members of the ensemble are able to alleviate this bias, but by generating a too large cloud cover. During summer, a large sensitivity to the cumu- lus parameterization is found, related to large differ- ences in the cloud cover and short wave radiation flux. Results also show that small errors in one variable are sometimes a result of error compensation, so the high dimensionality of the model evaluation problem cannot be disregarded.
    Cordex

  9. [Hersbach et al. 2015] Abstract This article describes an ensemble of ten atmospheric model integrations for the years 1899–2010, performed at the European Centre for Medium-Range Weather Forecasts (ECMWF). Horizontal spectral resolution is T159 (about 125 km), using 91 levels in the vertical from the surface up to 1 Pa, and a time step of 1 h. This ensemble, denoted by ERA-20CM, formed the first step toward a twentieth-century reanalysis within ERA-CLIM, a three-year European funded project involving nine partners. Sea-surface temperature and sea-ice cover are prescribed by an ensemble of realizations (HadISST2), as recently produced by the Met Office Hadley Centre within ERA-CLIM. Variation in these realizations reflect uncertainties in the available observational sources on which this product is based. Forcing terms in the model radiation scheme follow CMIP5 recommendations. Any effect of their uncertainty is neglected. These terms include solar forcing, greenhouse gases, ozone and aerosols. Both the ocean surface and radiative forcing incorporate a proper long-term evolution of climate trends in the twentieth century, and the occurrence of major events, such as the El Nin ̃o–Southern Oscillations and volcanic eruptions. No atmospheric observations were assimilated. For this reason ERA-20CM is not able to reproduce actual synoptic situations. However, the ensemble is able to provide a statistical estimate of the climate. Overall, the temperature rise over land is in fair agreement with the CRUTEM4 observational product. Over the last two decades the warming over land exceeds the warming over sea, which is consistent with models participating in the CMIP5 project, as well as with the ECMWF ERA-Interim reanalysis. Some aspects of warming and of the hydrological cycle are discerned, and the model response to volcanic eruptions is qualitatively correct. The data from ERA-20CM are freely available, embracing monthly-mean fields for many atmospheric and ocean-wave quantities, and synoptic fields for a small, essential subset.
    20C reanalysis

  10. [McGregor et al. 2015] The oceans mediate the response of global climate to natural and anthropogenic forcings. Yet for the past 2,000 years — a key interval for understanding the present and future climate response to these forcings — global sea surface temperature changes and the underlying driving mechanisms are poorly constrained. Here we present a global synthesis of sea surface tempera- tures for the Common Era (ce) derived from 57 individual marine reconstructions that meet strict quality control criteria. We observe a cooling trend from 1 to 1800ce that is robust against explicit tests for potential biases in the reconstructions. Between 801 and 1800 ce, the surface cooling trend is qualitatively consistent with an independent synthesis of terrestrial temperature reconstructions, and with a sea surface temperature composite derived from an ensemble of climate model simulations using best estimates of past external radiative forcings. Climate simulations using single and cumulative forcings suggest that the ocean surface cooling trend from 801 to 1800ce is not primarily a response to orbital forcing but arises from a high frequency of explosive volcanism. Our results show that repeated clusters of volcanic eruptions can induce a net negative radiative forcing that results in a centennial and global scale cooling trend via a decline in mixed-layer oceanic heat content.
    Ocean variability LM

  11. [Ortega et al. 2015] The North Atlantic Oscillation (NAO) is the major source of variability in winter atmospheric circulation in the Northern Hemisphere, with large impacts on temperature, precipitation and storm tracks1, and therefore also on strategic sectors such as insurance2, renewable energy production3, crop yields4 and water management5. Recent developments in dynamical methods offer promise to improve seasonal NAO predictions6, but assessing potential predictability on multi-annual timescales requires docu- mentation of past low-frequency variability in the NAO. A recent bi-proxy NAO reconstruction7 spanning the past millennium sug- gested that long-lasting positive NAO conditions were established during medieval times, explaining the particularly warm conditions in Europe during this period; however, these conclusions are debated. Here, we present a yearly NAO reconstruction for the past millennium, based on an initial selection of 48 annually resolved proxy records distributed around the Atlantic Ocean and built through an ensemble of multivariate regressions. We validate the approach in six past-millennium climate simulations, and show that our reconstruction outperforms the bi-proxy index. The final recon- struction shows no persistent positive NAO during the medieval period, but suggests that positive phases were dominant during the thirteenth and fourteenth centuries. The reconstruction also reveals that a positive NAO emerges two years after strong volcanic eruptions, consistent with results obtained from models and satellite observations for the Mt Pinatubo eruption in the Philippines
    NAO, last millennium

  12. [2k PMIP3 2015] Abstract. Estimated external radiative forcings, model results, and proxy-based climate reconstructions have been used over the past several decades to improve our understanding of the mechanisms underlying observed climate variability and change over the past millennium. Here, the recent set of temperature reconstructions at the continentalscale generated by the PAGES 2k project and a collection of state-of-the-art model simulations driven by realistic external forcings are jointly analysed. The first aim is to estimate the consistency between model results and reconstructions for each continental-scale region over the time and frequency domains. Secondly, the links between regions are investigated to determine whether reconstructed global-scale covariability patterns are similar to those identified in model simulations. The third aim is to assess the role of external forcings in the observed temperature variations. From a large set of analyses, we conclude that models are in relatively good agreement with temperature reconstructions for Northern Hemisphere regions, particularly in the Arctic. This is likely due to the relatively large amplitude of the externally forced response across northern and high-latitude regions, which results in a clearly detectable signature in both reconstructions and simulations. Conversely, models disagree strongly with the reconstructions in the Southern Hemisphere. Furthermore, the simulations are more regionally coherent than the reconstructions, perhaps due to an underestimation of the magnitude of internal variability in models or to an overestimation of the response to the external forcing in the Southern Hemisphere. Part of the disagreement might also reflect large uncertainties in the reconstructions, specifically in some Southern Hemisphere regions, which are based on fewer palaeoclimate records than in the Northern Hemisphere.
    last millennium, pages, pmip

  13. [Sherwood et al. 2015] More intensive analyses of climate simulations reveal a need to revise definitions of forcing and feedback and to recognize the new concept of rapid adjustments.
    Radiative forcing, adjustments

  14. [Swenson and Lawrence 2015] Abstract The estimation of groundwater storage variations is important for quantifying available water resources and managing storage surpluses to alleviate storage deficiencies during droughts. This is particu- larly true in semi-arid regions, where multiyear droughts can be common. To complement the local informa- tion provided by soil moisture and well level measurements, land models such as the Community Land Model (CLM) can be used to simulate regional scale water storage variations. CLM includes a bulk aquifer model to simulate saturated water storage dynamics below the model soil column. Aquifer storage increases when it receives recharge from the overlying soil column, and decreases due to lateral flow (i.e., base flow) and capillary rise. In this study, we examine the response of the CLM aquifer model to transitions between low and high recharge inputs, and show that the model simulates unrealistic long-period behavior relative to total water storage (TWS) observations from the Gravity Recovery and Climate Experiment (GRACE). We attribute the model’s poor response to large wetting events to the lack of a finite lower bound- ary in the bulk aquifer model. We show that by removing the bulk aquifer model and adding a zero-flux boundary condition at the base of the soil column, good agreement with GRACE observations can be achieved. In addition, we examine the sensitivity of simulated total water storage to the depth at which the zero-flux boundary is applied, i.e., the thickness of the soil column. Based on comparisons to GRACE, an opti- mal soil thickness map is constructed. Simulations using the modified CLM with the derived soil thickness map are shown to perform as well or better than standard CLM simulations. The improvements in simu- lated, climatically induced, long-period water storage variability will reduce the uncertainty in GRACE-based estimates of anthropogenic groundwater depletion.
    Groundwater dynamics

  15. [Stahl and Felgenson 2015] Given the overwhelming quantity of information available from the environment, how do young learners know what to learn about and what to ignore? We found that 11-month-old infants (N = 110) used violations of prior expectations as special opportunities for learning. The infants were shown events that violated expectations about object behavior or events that were nearly identical but did not violate expectations. The sight of an object that violated expectations enhanced learning and promoted information-seeking behaviors; specifically, infants learned more effectively about objects that committed violations, explored those objects more, and engaged in hypothesis-testing behaviors that reflected the particular kind of violation seen. Thus, early in life, expectancy violations offer a wedge into the problem of what to learn.
    Infancy, learning

  16. [Steffen et al. 2015] Abstract. The planetary boundary (PB) concept, introduced in 2009, aimed to define the environmental limits within which humanity can safely operate. This approach has proved influential in global sustainability policy development. Steffen et al. provide an updated and extended analysis of the PB framework. Of the original nine proposed boundaries, they identify three (including climate change) that might push the Earth system into a new state if crossed and that also have a pervasive influence on the remaining boundaries. They also develop the PB framework so that it can be applied usefully in a regional context. Science, this issue 10.1126/science.1259855 Developments in the planetary boundaries concept provide a framework to support global sustainability. The planetary boundaries framework defines a safe operating space for humanity based on the intrinsic biophysical processes that regulate the stability of the Earth system. Here, we revise and update the planetary boundary framework, with a focus on the underpinning biophysical science, based on targeted input from expert research communities and on more general scientific advances over the past 5 years. Several of the boundaries now have a two-tier approach, reflecting the importance of cross-scale interactions and the regional-level heterogeneity of the processes that underpin the boundaries. Two core boundaries—climate change and biosphere integrity—have been identified, each of which has the potential on its own to drive the Earth system into a new state should they be substantially and persistently transgressed.
    Planet boundaries