CMIP5 Earth System Models Research Articles

Sources of Uncertainty in Future Projections of the Carbon Cycle

Abstract The inclusion of carbon cycle processes within CMIP5 Earth system models provides the opportunity to explore the relative importance of differences in scenario and climate model representation to future land and ocean carbon fluxes. A two-way analysis of variance (ANOVA) approach was used to quantify the variability owing to differences between scenarios and between climate models at different lead times. For global ocean carbon fluxes, the variance attributed to differences between representative concentration pathway scenarios exceeds the variance attributed to differences between climate models by around 2025, completely dominating by 2100. This contrasts with global land carbon fluxes, where the variance attributed to differences between climate models continues to dominate beyond 2100. This suggests that modeled processes that determine ocean fluxes are currently better constrained than those of land fluxes; thus, one can be more confident in linking different future socioeconomic pathways to consequences of ocean carbon uptake than for land carbon uptake. The contribution of internal variance is negligible for ocean fluxes and small for land fluxes, indicating that there is little dependence on the initial conditions. The apparent agreement in atmosphere–ocean carbon fluxes, globally, masks strong climate model differences at a regional level. The North Atlantic and Southern Ocean are key regions, where differences in modeled processes represent an important source of variability in projected regional fluxes.

Ocean warming expands habitat of a rich natural resource and benefits a national economy.

Geographic redistribution of living natural resources changes access and thereby harvesting opportunities between countries. Internationally shared fish resources can be sensitive to shifts in the marine environment and this may have great impact on the economies of countries and regions that rely most heavily on fisheries to provide employment and food supply. Here we present a climate change-related biotic expansion of a rich natural resource with substantial economic consequences, namely the appearance of northeast Atlantic mackerel (Scomber scombrus) in Greenlandic waters. In recent years, the summer temperature has reached record highs in the Irminger Current, and this development has expanded the available and realized mackerel habitat in time and space. Observations in the Irminger Current in east Greenland in 2011 of this temperature-sensitive epipelagic fish were the first records so far northwest in the Atlantic. This change in migration pattern was followed by a rapid development of a large-scale fishery of substantial importance for the national economy of Greenland (23% of Greenland's export value of all goods in 2014). A pelagic trawl survey was conducted in mid-summer 2014 and the results showed that the bulk of ~1million Mg (=t) of mackerel in the Irminger Current in southeast Greenland were located in the relatively warm (>8.5°C) surface layer. Mackerel was also observed in southwest Greenland. Finally, 15 CMIP5 Earth System Model projections of future marine climate were used to evaluate the epipelagic environment in Greenland. These projections for moderate and high CO2 emission scenarios (representative concentration pathways [RCP] 4.5 and 8.5) suggest how the available mackerel habitat may expand further in space and time. Overall, our results indicate that, if the stock remains large, productive, and continues its current migration pattern, then climate change has provided Greenland with a new unique opportunity for commercial exploitation. However, positive cases like this should not be cherry-picked and misused as arguments against timely and effective mitigation of climate change.

Influence of urban resilience measures in the magnitude and behaviour of energy fluxes in the city of Porto (Portugal) under a climate change scenario

Different urban resilience measures, such as the increase of urban green areas and the application of white roofs, were evaluated with the WRF-SUEWS modelling system. The case study consists of five heat waves occurring in Porto (Portugal) urban area in a future climate scenario. Meteorological forcing and boundary data were downscaled for Porto urban area from the CMIP5 earth system model MPI-ESM, for the Representative Concentration Pathway RCP8.5 scenario. The influence of different resilience measures on the energy balance components was quantified and compared between each other. Results show that the inclusion of green urban areas increases the evaporation and the availability of surface moisture, redirecting the energy to the form of latent heat flux (maximum increase of +200Wm−2) rather than to sensible heat. The application of white roofs increases the solar radiation reflection, due to the higher albedo of such surfaces, reducing both sensible and storage heat flux (maximum reductions of −62.8 and −35Wm−2, respectively). The conjugations of the individual benefits related to each resilience measure shows that this measure is the most effective one in terms of improving the thermal comfort of the urban population, particularly due to the reduction of both sensible and storage heat flux. The obtained results contribute to the knowledge of the surface-atmosphere exchanges and can be of great importance for stakeholders and decision-makers.

Inconsistent strategies to spin up models in CMIP5: implications for ocean biogeochemical model performance assessment

Abstract. During the fifth phase of the Coupled Model Intercomparison Project (CMIP5) substantial efforts were made to systematically assess the skill of Earth system models. One goal was to check how realistically representative marine biogeochemical tracer distributions could be reproduced by models. In routine assessments model historical hindcasts were compared with available modern biogeochemical observations. However, these assessments considered neither how close modeled biogeochemical reservoirs were to equilibrium nor the sensitivity of model performance to initial conditions or to the spin-up protocols. Here, we explore how the large diversity in spin-up protocols used for marine biogeochemistry in CMIP5 Earth system models (ESMs) contributes to model-to-model differences in the simulated fields. We take advantage of a 500-year spin-up simulation of IPSL-CM5A-LR to quantify the influence of the spin-up protocol on model ability to reproduce relevant data fields. Amplification of biases in selected biogeochemical fields (O2, NO3, Alk-DIC) is assessed as a function of spin-up duration. We demonstrate that a relationship between spin-up duration and assessment metrics emerges from our model results and holds when confronted with a larger ensemble of CMIP5 models. This shows that drift has implications for performance assessment in addition to possibly aliasing estimates of climate change impact. Our study suggests that differences in spin-up protocols could explain a substantial part of model disparities, constituting a source of model-to-model uncertainty. This requires more attention in future model intercomparison exercises in order to provide quantitatively more correct ESM results on marine biogeochemistry and carbon cycle feedbacks.

Changes in interannual climate sensitivities of terrestrial carbon fluxes during the 21st century predicted by CMIP5 Earth System Models

AbstractTerrestrial carbon fluxes are sensitive to climate change, but the interannual climate sensitivity of the land carbon cycle can also change with time. We analyzed the changes in responses of net biome production (NBP), net primary production (NPP), and heterotrophic respiration (Rh) to interannual climate variations over the 21st century in the Earth System Models (ESMs) from the Coupled Model Intercomparison Project 5. Under Representative Concentration Pathway (RCP) 4.5, interannual temperature sensitivities of NBP (γTempNBP), NPP (γTempNPP), and Rh (γTempRh) remain relatively stable at global scale, yet with large differences among ESMs and spatial heterogeneity. Modeled γTempNPP and γTempRh appear to increase in parallel in boreal regions, resulting in unchanged γTempNBP. Tropical γTempNBP decreases in most models, due to decreasing γTempNPP and relatively stable γTempRh. Across models, the changes in γTempNBP can be mainly explained by changes in γTempNPP rather than changes in γTempRh, at both global and regional scales. Interannual precipitation sensitivities of global NBP (γPrecNBP), NPP (γPrecNPP), and Rh (γPrecRh) are predicted not to change significantly, with large differences among ESMs. Across models, the changes in γPrecNBP can be mainly explained by changes in γPrecNPP rather than changes in γPrecRh in temperate regions, but not in other regions. Changes in the interannual climate sensitivities of carbon fluxes are consistent across RCPs 4.5, 6.0, and 8.5 but larger in more intensive scenarios. More effort should be considered to improve terrestrial carbon flux responses to interannual climate variability, e.g., incorporating biogeochemical processes of nutrient limitation, permafrost dynamics, and microbial decomposition.

A latitudinally banded phytoplankton response to 21st century climate change in the Southern Ocean across the CMIP5 model suite

Abstract. Changes in Southern Ocean (SO) phytoplankton distributions with future warming have the potential to significantly alter nutrient and carbon cycles as well as higher trophic level productivity both locally and throughout the global ocean. Here we investigate the response of SO phytoplankton productivity and biomass to 21st century climate change across the CMIP5 Earth System Model suite. The models predict a zonally banded pattern of phytoplankton abundance and production changes within four regions: the subtropical (~ 30 to 40° S), transitional (~ 40 to 50° S), subpolar (~ 50 to 65° S) and Antarctic (south of ~ 65° S) bands. We find that shifts in bottom-up variables (nitrate, iron and light availability) drive changes in phytoplankton abundance and production on not only interannual, but also decadal and 100-year timescales – the timescales most relevant to climate change. Spatial patterns in the modelled mechanisms driving these biomass trends qualitatively agree with recent observations, though longer-term records are needed to separate the effects of climate change from those of interannual variability. Because much past observational work has focused on understanding the effects of the Southern Annular Mode (SAM) on biology, future work should attempt to quantify the precise influence of an increasingly positive SAM on SO biology within the CMIP5 models. Continued long-term in situ and satellite measurements of SO biology are clearly needed to confirm model findings.

Extending the relationship between global warming and cumulative carbon emissions to multi-millennial timescales

The transient climate response to cumulative carbon emissions (TCRE) is a highly policy-relevant quantity in climate science. The TCRE suggests that peak warming is linearly proportional to cumulative carbon emissions and nearly independent of the emissions scenario. Here, we use simulations of the Earth System Model (ESM) from the Geophysical Fluid Dynamics Laboratory (GFDL) to show that global mean surface temperature may increase by 0.5 °C after carbon emissions are stopped at 2 °C global warming, implying an increase in the coefficient relating global warming to cumulative carbon emissions on multi-centennial timescales. The simulations also suggest a 20% lower quota on cumulative carbon emissions allowed to achieve a policy-driven limit on global warming. ESM estimates from the Coupled Model Intercomparison Project Phase 5 (CMIP5–ESMs) qualitatively agree on this result, whereas Earth System Models of Intermediate Complexity (EMICs) simulations, used in the IPCC 5th assessment report to assess the robustness of TCRE on multi-centennial timescales, suggest a post-emissions decrease in temperature. The reason for this discrepancy lies in the smaller simulated realized warming fraction in CMIP5–ESMs, including GFDL ESM2M, than in EMICs when carbon emissions increase. The temperature response to cumulative carbon emissions can be characterized by three different phases and the linear TCRE framework is only valid during the first phase when carbon emissions increase. For longer timescales, when emissions tape off, two new metrics are introduced that better characterize the time-dependent temperature response to cumulative carbon emissions: the equilibrium climate response to cumulative carbon emissions and the multi-millennial climate response to cumulative carbon emissions.

Observed allocations of productivity and biomass, and turnover times in tropical forests are not accurately represented in CMIP5 Earth system models

A significant fraction of anthropogenic CO2 emissions is assimilated by tropical forests and stored as biomass, slowing the accumulation of CO2 in the atmosphere. Because different plant tissues have different functional roles and turnover times, predictions of carbon balance of tropical forests depend on how earth system models (ESMs) represent the dynamic allocation of productivity to different tree compartments. This study shows that observed allocation of productivity, biomass, and turnover times of main tree compartments (leaves, wood, and roots) are not accurately represented in Coupled Model Intercomparison Project Phase 5 ESMs. In particular, observations indicate that biomass saturates with increasing productivity. In contrast, most models predict continuous increases in biomass with increases in productivity. This bias may lead to an over-prediction of carbon uptake in response to CO2 or climate-driven changes in productivity. Compartment-specific productivity and biomass are useful benchmarks to assess terrestrial ecosystem model performance. Improvements in the predicted allocation patterns and turnover times by ESMs will reduce uncertainties in climate predictions.

Continued increase in atmospheric CO<sub>2</sub> seasonal amplitude in the 21st century projected by the CMIP5 Earth system models

Abstract. In the Northern Hemisphere, atmospheric CO2 concentration declines in spring and summer, and rises in fall and winter. Ground-based and aircraft-based observation records indicate that the amplitude of this seasonal cycle has increased in the past. Will this trend continue in the future? In this paper, we analyzed simulations for historical (1850–2005) and future (RCP8.5, 2006–2100) periods produced by 10 Earth system models participating in the fifth phase of the Coupled Model Intercomparison Project (CMIP5). Our results present a model consensus that the increase of CO2 seasonal amplitude continues throughout the 21st century. Multi-model ensemble relative amplitude of detrended global mean CO2 seasonal cycle increases by 62 ± 19% in 2081–2090, compared to 1961–1970. This amplitude increase corresponds to a 68 ± 25% increase in net biosphere production (NBP). The results show that the increase of NBP amplitude mainly comes from enhanced ecosystem uptake during Northern Hemisphere growing season under future CO2 and temperature conditions. Separate analyses on net primary production (NPP) and respiration reveal that enhanced ecosystem carbon uptake contributes about 75% of the amplitude increase. Stimulated by higher CO2 concentration and high-latitude warming, enhanced NPP likely outcompetes increased respiration at higher temperature, resulting in a higher net uptake during the northern growing season. The zonal distribution and spatial pattern of NBP change suggest that regions north of 45° N dominate the amplitude increase. Models that simulate a stronger carbon uptake also tend to show a larger increase of NBP seasonal amplitude, and the cross-model correlation is significant (R=0.73, p< 0.05).

CO2-Induced Sahel Greening in Three CMIP5 Earth System Models

Abstract The existence and productivity of vegetation is the basis for food and energy supply in the Sahel. Past changes in climate and vegetation abundance have raised the question whether the region could become greener in the future as a result of higher CO2 levels. By analyzing three Earth system models (ESMs) from phase 5 of the Coupled Model Intercomparison Project (CMIP5) with dynamic vegetation, the authors demonstrate why an answer to this question remains elusive in contrast to more robust projections of vegetation cover in the extratropics. First, it depends on the location and the time scale whether vegetation expands or retreats. Until the end of the twenty-first century, the three models agree on a substantial greening in the central and eastern Sahel due to increased CO2 levels. This trend is reversed thereafter, and vegetation retreats in particular in the western Sahel because the beneficial effect of CO2 fertilization is short lived compared to climate change. Second, the vegetation cover changes are driven by different processes in different models (most importantly, precipitation change and CO2 fertilization). As these processes tend to oppose each other, the greening and browning trends are not a reliable result despite the apparent model agreement. The authors also find that the effect of vegetation dynamics on the surface energy balance crucially depends on the location. In contrast to the results of many previous studies, the Sahel appears as a hotspot where the physiological effects of CO2 can exert a cooling because vegetation structure and distribution overcompensate for the decreased stomatal conductance.

A height dependent evaluation of wind and temperature over Europe in the CMIP5 Earth System Models

To date, evaluation studies of global circulation models (GCMs) generally focus on large-scale circulation variables. However, since the resolution of GCMs is increasing and their interaction with the surface is improving, GCM representations of near-surface variables are becoming increasingly realistic. For downscaling practices, this implies that near-surface vari- ables might become suitable as predictors in statistical models, and might increase the added value of dynamical models. This study focuses on the representation of wind and temperature in the lowest 1.5 km of the atmosphere over Europe as simulated by 6 of the earth system models (ESMs) from the Coupled Model Intercomparison Project Phase 5. The evaluation is based on the representation of the variables' probability density functions (PDFs) using ERA-Interim reanalysis data as the reference. The PDF biases are analyzed according to their scale and origin. Above coastal bays and capes, small-scale biases in the ESMs result in unskillful wind speed PDFs up to 400 m. High orography affects wind speeds throughout the lowest 1.5 km of the atmosphere, espe- cially during summer and night. Apart from these small-scale biases, the surface wind speed PDFs north of 45° N are well represented by all the ESMs. Therefore, these PDFs can be considered skillful inputs for statistical downscaling practices. South of 45° N, winds are affected by a large- scale bias originating from errors in the representation of the large-scale circulation, especially during winter. For temperature, near-surface levels as well as upper-atmospheric levels are affected by small-scale and large-scale biases. Large-scale biases are adopted by the downscaling models, underlining the importance of model evaluation before downscaling.

To what extent can interannual CO2 variability constrain carbon cycle sensitivity to climate change in CMIP5 Earth System Models?

AbstractWe analyze the carbon‐climate feedback in eight Earth System Models from phase 5 of the Coupled Model Intercomparison Project (CMIP5). We focus on tropical land carbon change and find decreases (−31.02 to −169.32 GtC K−1) indicating tropical ecosystems will release carbon as temperature warms, thus contributing to a positive feedback identified in earlier studies. We further investigate the relationship between tropical land carbon change and sensitivity of historical atmospheric CO2 growth rate to tropical temperature variability and find a weak linear relationship. This sensitivity for most models is stronger than observed. We further use this “emergent constraint” to constrain uncertainties in model‐projected future carbon‐climate changes and find little effect in narrowing the model spread, but the mean sensitivity is slightly smaller. This contrasts with earlier Coupled Carbon Cycle Climate Model Intercomparison Project results, highlighting the challenge in constraining future projections by modern observations and the necessity for evaluating such relationships continuously.

Emergent constraints on climate‐carbon cycle feedbacks in the CMIP5 Earth system models

AbstractAn emergent linear relationship between the long‐term sensitivity of tropical land carbon storage to climate warming (γLT) and the short‐term sensitivity of atmospheric carbon dioxide (CO2) to interannual temperature variability (γIAV) has previously been identified by Cox et al. (2013) across an ensemble of Earth system models (ESMs) participating in the Coupled Climate‐Carbon Cycle Model Intercomparison Project (C4MIP). Here we examine whether such a constraint also holds for a new set of eight ESMs participating in Phase 5 of the Coupled Model Intercomparison Project. A wide spread in tropical land carbon storage is found for the quadrupling of atmospheric CO2, which is of the order of 252 ± 112 GtC when carbon‐climate feedbacks are enabled. Correspondingly, the spread in γLT is wide (−49 ± 40 GtC/K) and thus remains one of the key uncertainties in climate projections. A tight correlation is found between the long‐term sensitivity of tropical land carbon and the short‐term sensitivity of atmospheric CO2 (γLT versus γIAV), which enables the projections to be constrained with observations. The observed short‐term sensitivity of CO2 (−4.4 ± 0.9 GtC/yr/K) sharpens the range of γLT to −44 ± 14 GtC/K, which overlaps with the probability density function derived from the C4MIP models (−53 ± 17 GtC/K) by Cox et al. (2013), even though the lines relating γLT and γIAV differ in the two cases. Emergent constraints of this type provide a means to focus ESM evaluation against observations on the metrics most relevant to projections of future climate change.

Transient climate changes in a perturbed parameter ensemble of emissions-driven earth system model simulations

We describe results from a 57-member ensemble of transient climate change simulations, featuring simultaneous perturbations to 54 parameters in the atmosphere, ocean, sulphur cycle and terrestrial ecosystem components of an earth system model (ESM). These emissions-driven simulations are compared against the CMIP3 multi-model ensemble of physical climate system models, used extensively to inform previous assessments of regional climate change, and also against emissions-driven simulations from ESMs contributed to the CMIP5 archive. Members of our earth system perturbed parameter ensemble (ESPPE) are competitive with CMIP3 and CMIP5 models in their simulations of historical climate. In particular, they perform reasonably well in comparison with HadGEM2-ES, a more sophisticated and expensive earth system model contributed to CMIP5. The ESPPE therefore provides a computationally cost-effective tool to explore interactions between earth system processes. In response to a non-intervention emissions scenario, the ESPPE simulates distributions of future regional temperature change characterised by wide ranges, and warm shifts, compared to those of CMIP3 models. These differences partly reflect the uncertain influence of global carbon cycle feedbacks in the ESPPE. In addition, the regional effects of interactions between different earth system feedbacks, particularly involving physical and ecosystem processes, shift and widen the ESPPE spread in normalised patterns of surface temperature and precipitation change in many regions. Significant differences from CMIP3 also arise from the use of parametric perturbations (rather than a multimodel ensemble) to represent model uncertainties, and this is also the case when ESPPE results are compared against parallel emissions-driven simulations from CMIP5 ESMs. When driven by an aggressive mitigation scenario, the ESPPE and HadGEM2-ES reveal significant but uncertain impacts in limiting temperature increases during the second half of the twenty-first century. Emissions-driven simulations create scope for development of errors in properties that were previously prescribed in coupled ocean–atmosphere models, such as historical CO2 concentrations and vegetation distributions. In this context, historical intra-ensemble variations in the airborne fraction of CO2 emissions, and in summer soil moisture in northern hemisphere continental regions, are shown to be potentially useful constraints, subject to uncertainties in the relevant observations. Our results suggest that future climate-related risks can be assessed more comprehensively by updating projection methodologies to support formal combination of emissions-driven perturbed parameter and multi-model earth system model simulations with suitable observational constraints. This would provide scenarios underpinned by a more complete representation of the chain of uncertainties from anthropogenic emissions to future climate outcomes.

Future ocean acidification in the Canada Basin and surrounding Arctic Ocean from CMIP5 earth system models

Six Earth system models that include an interactive carbon cycle and have contributed results to the 5th Coupled Model Intercomparison Project (CMIP5) are evaluated with respect to Arctic Ocean acidification. Projections under Representative Concentration Pathways (RCPs) 8.5 and 4.5 consistently show reductions in the bidecadal mean surface pH from about 8.1 in 1986–2005 to 7.7/7.9 by 2066–2085 in the Canada Basin, closely linked to reductions in the calcium carbonate saturation state ΩA,C from about 1.4 (2.0) to 0.7 (1.0) for aragonite (calcite) for RCP8.5. The large but opposite effects of dilution and biological drawdown of DIC and dilution of alkalinity lead to a small seasonal amplitude change in Ω, as well as intermodel differences in the timing and sign of the summer minimum. The Canada Basin shows a characteristic layering in Ω: affected by ice melt and inflowing Pacific water, shallow undersaturated layers form at the surface and subsurface, creating a shallow saturation horizon which expands from the surface downward. This is in addition to the globally observed deep saturation horizon which is continuously expanding upward with increasing CO2 uptake. The Eurasian Basin becomes undersaturated much later than the rest of the Arctic. These CMIP5 model results strengthen earlier findings, although large intermodel differences remain: Below 200 m ΩA varies by up to 1.0 in the Canada Basin and the deep saturation horizon varies from 2000 to 4000 m among the models. Differences of projected acidification changes are primarily related to sea ice retreat and responses of wind mixing and stratification.

Carbon–Concentration and Carbon–Climate Feedbacks in CMIP5 Earth System Models

AbstractThe magnitude and evolution of parameters that characterize feedbacks in the coupled carbon–climate system are compared across nine Earth system models (ESMs). The analysis is based on results from biogeochemically, radiatively, and fully coupled simulations in which CO2 increases at a rate of 1% yr−1. These simulations are part of phase 5 of the Coupled Model Intercomparison Project (CMIP5). The CO2 fluxes between the atmosphere and underlying land and ocean respond to changes in atmospheric CO2 concentration and to changes in temperature and other climate variables. The carbon–concentration and carbon–climate feedback parameters characterize the response of the CO2 flux between the atmosphere and the underlying surface to these changes. Feedback parameters are calculated using two different approaches. The two approaches are equivalent and either may be used to calculate the contribution of the feedback terms to diagnosed cumulative emissions. The contribution of carbon–concentration feedback to diagnosed cumulative emissions that are consistent with the 1% increasing CO2 concentration scenario is about 4.5 times larger than the carbon–climate feedback. Differences in the modeled responses of the carbon budget to changes in CO2 and temperature are seen to be 3–4 times larger for the land components compared to the ocean components of participating models. The feedback parameters depend on the state of the system as well the forcing scenario but nevertheless provide insight into the behavior of the coupled carbon–climate system and a useful common framework for comparing models.

Twenty-First-Century Compatible CO2 Emissions and Airborne Fraction Simulated by CMIP5 Earth System Models under Four Representative Concentration Pathways

Abstract The carbon cycle is a crucial Earth system component affecting climate and atmospheric composition. The response of natural carbon uptake to CO2 and climate change will determine anthropogenic emissions compatible with a target CO2 pathway. For phase 5 of the Coupled Model Intercomparison Project (CMIP5), four future representative concentration pathways (RCPs) have been generated by integrated assessment models (IAMs) and used as scenarios by state-of-the-art climate models, enabling quantification of compatible carbon emissions for the four scenarios by complex, process-based models. Here, the authors present results from 15 such Earth system GCMs for future changes in land and ocean carbon storage and the implications for anthropogenic emissions. The results are consistent with the underlying scenarios but show substantial model spread. Uncertainty in land carbon uptake due to differences among models is comparable with the spread across scenarios. Model estimates of historical fossil-fuel emissions agree well with reconstructions, and future projections for representative concentration pathway 2.6 (RCP2.6) and RCP4.5 are consistent with the IAMs. For high-end scenarios (RCP6.0 and RCP8.5), GCMs simulate smaller compatible emissions than the IAMs, indicating a larger climate–carbon cycle feedback in the GCMs in these scenarios. For the RCP2.6 mitigation scenario, an average reduction of 50% in emissions by 2050 from 1990 levels is required but with very large model spread (14%–96%). The models also disagree on both the requirement for sustained negative emissions to achieve the RCP2.6 CO2 concentration and the success of this scenario to restrict global warming below 2°C. All models agree that the future airborne fraction depends strongly on the emissions profile with higher airborne fraction for higher emissions scenarios.

How well do CMIP5 Earth System Models simulate present climate conditions in Europe and Africa?

The present study assesses the ability of seven Earth System Models (ESMs) from the Coupled Model Intercomparison Project Phase 5 to reproduce present climate conditions in Europe and Africa. This is done from a downscaling perspective, taking into account the requirements of both statistical and dynamical approaches. ECMWF’s ERA-Interim reanalysis is used as reference for an evaluation of circulation, temperature and humidity variables on daily timescale, which is based on distributional similarity scores. To additionally obtain an estimate of reanalysis uncertainty, ERA-Interim’s deviation from the Japanese Meteorological Agency JRA-25 reanalysis is calculated. Areas with considerable differences between both reanalyses do not allow for a proper assessment, since ESM performance is sensitive to the choice of reanalysis. For use in statistical downscaling studies, ESM performance is computed on the grid-box scale and mapped over a large spatial domain covering Europe and Africa, additionally highlighting those regions where significant distributional differences remain even for the centered/zero-mean time series. For use in dynamical downscaling studies, performance is specifically assessed along the lateral boundaries of the three CORDEX domains defined for Europe, the Mediterranean Basin and Africa.

Causes of variation in soil carbon simulations from CMIP5 Earth system models and comparison with observations

Abstract. Stocks of soil organic carbon represent a large component of the carbon cycle that may participate in climate change feedbacks, particularly on decadal and centennial timescales. For Earth system models (ESMs), the ability to accurately represent the global distribution of existing soil carbon stocks is a prerequisite for accurately predicting future carbon–climate feedbacks. We compared soil carbon simulations from 11 model centers to empirical data from the Harmonized World Soil Database (HWSD) and the Northern Circumpolar Soil Carbon Database (NCSCD). Model estimates of global soil carbon stocks ranged from 510 to 3040 Pg C, compared to an estimate of 1260 Pg C (with a 95% confidence interval of 890–1660 Pg C) from the HWSD. Model simulations for the high northern latitudes fell between 60 and 820 Pg C, compared to 500 Pg C (with a 95% confidence interval of 380–620 Pg C) for the NCSCD and 290 Pg C for the HWSD. Global soil carbon varied 5.9 fold across models in response to a 2.6-fold variation in global net primary productivity (NPP) and a 3.6-fold variation in global soil carbon turnover times. Model–data agreement was moderate at the biome level (R2 values ranged from 0.38 to 0.97 with a mean of 0.75); however, the spatial distribution of soil carbon simulated by the ESMs at the 1° scale was not well correlated with the HWSD (Pearson correlation coefficients less than 0.4 and root mean square errors from 9.4 to 20.8 kg C m−2). In northern latitudes where the two data sets overlapped, agreement between the HWSD and the NCSCD was poor (Pearson correlation coefficient 0.33), indicating uncertainty in empirical estimates of soil carbon. We found that a reduced complexity model dependent on NPP and soil temperature explained much of the 1° spatial variation in soil carbon within most ESMs (R2 values between 0.62 and 0.93 for 9 of 11 model centers). However, the same reduced complexity model only explained 10% of the spatial variation in HWSD soil carbon when driven by observations of NPP and temperature, implying that other drivers or processes may be more important in explaining observed soil carbon distributions. The reduced complexity model also showed that differences in simulated soil carbon across ESMs were driven by differences in simulated NPP and the parameterization of soil heterotrophic respiration (inter-model R2 = 0.93), not by structural differences between the models. Overall, our results suggest that despite fair global-scale agreement with observational data and moderate agreement at the biome scale, most ESMs cannot reproduce grid-scale variation in soil carbon and may be missing key processes. Future work should focus on improving the simulation of driving variables for soil carbon stocks and modifying model structures to include additional processes.

Arctic sea ice decline and ice export in the CMIP5 historical simulations

Arctic sea ice properties and Fram Strait ice export from six CMIP5 Global Climate and Earth System Models are evaluated and investigated for the period 1957–2005. Over the last decades most ensemble members simulate a decreasing September sea ice area and a slow, general thinning of the sea ice cover. While the different ensemble members both under- and overestimate the decline in observed September sea ice area, none of the members reproduce the observed thinning.This study is a first attempt to evaluate the Fram Strait ice area export in the CMIP5 models, and the role it has played for Arctic sea ice area and thickness. Five of the six models evaluated reproduce the seasonal cycle and the inter-annual variance of the ice area export in the Fram Strait reasonably well. The simulated southward export of sea ice in the Fram Strait constitutes a major fraction of the Arctic sea ice in these five models; 10–18% of the sea ice covered Arctic Basin is annually exported. For the same models the year-to-year variability in Fram Strait ice volume export carries 35% of the year-to-year variability in the Arctic Basin sea ice volume.We have found low but significant correlations on inter-annual timescales between the Fram Strait ice export, both in terms of area and volume, and the Arctic Basin sea ice thickness. All six models show that an increase in ice area export leads a decrease in the sea ice thickness. This inverse relationship also holds when considering the long-term trends; the larger the increase in Fram Strait ice area export, the larger the thinning of the Arctic Basin sea ice cover and the larger the loss in the September sea ice area. The different ensemble members show both negative and positive ice export trends. Focusing on the model with the largest number of ensemble members (10), we have been able to quantify the effect of the ice area export on the Arctic Basin sea ice for this particular model. For this model an increase of the ice area export similar to the estimated trend (from NCEP) can explain almost 20% of the total simulated decline in sea ice area and thickness.