2018

1. [Bonan and Doney 2018] ABSTRACT: Many global change stresses on terrestrial and marine ecosystems affect not only ecosystem services that are essential to humankind, but also the trajectory of future climate by altering energy and mass exchanges with the atmosphere. Earth system models, which simulate terrestrial and marine ecosystems and biogeochemical cycles, offer a common framework for ecological research related to climate processes; analyses of vulnerability, impacts, and adaptation; and climate change mitigation. They provide an opportunity to move beyond physical descriptors of atmospheric and oceanic states to societally relevant quantities such as wildfire risk, habitat loss, water availability, and crop, fishery, and timber yields. To achieve this, the science of climate prediction must be extended to a more multifaceted Earth system prediction that includes the biosphere and its resources.
   ESM, biogeochemistry

2. [Bonan et al. 2018] Abstract. Land surface models used in climate models ne- glect the roughness sublayer and parameterize within-canopy turbulence in an ad hoc manner. We implemented a rough- ness sublayer turbulence parameterization in a multilayer canopy model (CLM-ml v0) to test if this theory pro- vides a tractable parameterization extending from the ground through the canopy and the roughness sublayer. We com- pared the canopy model with the Community Land Model (CLM4.5) at seven forest, two grassland, and three cropland AmeriFlux sites over a range of canopy heights, leaf area indexes, and climates. CLM4.5 has pronounced biases dur- ing summer months at forest sites in midday latent heat flux, sensible heat flux, gross primary production, nighttime fric- tion velocity, and the radiative temperature diurnal range. The new canopy model reduces these biases by introduc- ing new physics. Advances in modeling stomatal conduc- tance and canopy physiology beyond what is in CLM4.5 substantially improve model performance at the forest sites. The signature of the roughness sublayer is most evident in nighttime friction velocity and the diurnal cycle of radiative temperature, but is also seen in sensible heat flux. Within- canopy temperature profiles are markedly different compared with profiles obtained using Monin–Obukhov similarity the- ory, and the roughness sublayer produces cooler daytime and warmer nighttime temperatures. The herbaceous sites also show model improvements, but the improvements are re- lated less systematically to the roughness sublayer parame- terization in these canopies. The multilayer canopy with the roughness sublayer turbulence improves simulations com- pared with CLM4.5 while also advancing the theoretical ba- sis for surface flux parameterizations.
   Canopy turbulence

3. [Bonan et al. 2018] The climate deterioration after the most recent African humid period (AHP) is a notable past example of desertification. Evidence points to a human population expansion in northern Africa prior to this, associated with the introduction of pastoralism. Here we consider the role, if any, of this population on the subsequent ecological collapse. Using a climate- vegetation model, we estimate the natural length of the most recent AHP. The model indicates that the system was most susceptible to collapse between 7 and 6 ka; at least 500 years before the observed collapse. This suggests that the inclusion of increasing elements of pastoralism was an effective adaptation to the regional environmental changes. Pastoralism also appears to have slowed the deterioration caused by orbitally-driven climate change. This supports the view that modern pastoralism is not only sustainable, but beneficial for the management of the world’s dryland environments.
   Pastoralism, Sahara drying

4. [Chávez-Arroyo et al. 2018] ABSTRACT: A new method for the long-term prediction of the wind resource based on the concept of statistical-dynamical downscaling is presented. This new approach uses mean sea level pressure maps from global reanalysis data (National Centers for Environmental Prediction Department of Energy Atmospheric Model Intercomparison Project (NCEP-DOE AMIP-II)) and image processing techniques to identify a synthetic reference periodwhich optimally matches the corresponding long-term maps. Four different image processing techniques, averaged into one image similarity error index, are used to evaluate image similarity. A representative set of days is selected by requiring the error index to be minimal. Validation of representativeness in terms of the wind resource for the Iberian domain is performed against 10 years of measured wind data from Navarra (Spain), as well as mesoscale simulations of the Iberian Peninsula. The new approach is shown to outperform not only the industry-standard method but also other recently proposed methods in its capability to achieve mesoscale level representativeness. A particular advantage of the new method is its capability of simultaneously providing a representative period for all potential wind farm sites located within large regional domains without requiring re-running of the method for different candidate sites.
   long-term wind resource; statistical-dynamical downscaling; stratified sampling; mean sea level maps; reanalysis data; image processing

5. [Cuchiara and Rappenglueck 2018] ABSTRACT: We implemented the Weather Research and Forecast (WRF) model and WRF Large-Eddy Simulation (WRF–LES), focusing on calculations for the planetary boundary layer (PBL), and compared the results against a data set of a well-documented campaign, in the Houston–Galveston area, Texas, in summer 2006. A methodology using WRF in a mesoscale and LES was implemented to assess the performance of the model in simulating the evolution and structure of the PBL over Houston during the Vertical Mixing Experi- ment. Also, the WRF model in a real case mode was examined to explore potential differences between the results of each simulation approach. We analyzed both WRF results for key meteorological parameters like wind speed, wind direction and potential temperature, and compared the model results against the observations. The reasonably good agreement of LES results forced with observed surface fluxes provides confidence that LES describes turbulence quantities such as turbulent kinetic energy correctly and warrants further turbulence structure analysis. The LES results indicate a weak but noticeable nighttime turbulent kinetic energy which was produced by wind shear in Houston’s planetary boundary layer and which may likely be related to intermittent tur- bulence. This is supported by observations made at the University of Houston Moody Tower air quality station when intermittent peaks of carbon monoxide occurred in the evening, although the variability in wind conditions was very little.
   LES, turbulence

6. [de Vrese et al. 2018] Abstract: An adapted Earth system model is used to investigate the limitations that future climate and water availability impose on the potential expansion and productivity of croplands. The model maximizes the cropland area under prevailing climate conditions and accounts for an optimized, sustainable irrigation practice, thus allowing us to consider the two-way feedback between climate and agriculture. For three greenhouse gas concentration scenarios (RCP2.6, RCP4.5, RCP8.5), we show that the total cropland area could be extended substantially throughout the 21st century, especially in South America and sub-Saharan Africa, where the rising water demand resulting from increasing temperatures can largely be met by increasing precipitation and irrigation rates. When accounting for the CO2 fertilization effect, only a few agricultural areas have to be abandoned owing to declines in productivity, while increasing temperatures allow for the expansion of croplands even into high northern latitudes. Without the CO2 fertilization effect there is no increase in the overall cropland fraction during the second half of the century but areal losses in increasingly water-stressed regions can be compensated for by an expansion in regions that were previously too cold. However, global yields are more sensitive and, without the benefits of CO2 fertilization, they may decrease when greenhouse gas concentrations exceed the RCP4.5 scenario. For certain regions the situation is even more concerning and guaranteeing food security in dry areas in Northern Africa, the Middle East and South Asia will become increasingly difficult, even for the idealized scenarios investigated in this study.
   Biogeophysical on crops food

7. [Heidkamp et al. 2018] Land surface–atmosphere interaction is one of the most important characteristic for understanding the terrestrial climate system, as it determines the exchange fluxes of energy and water between the land and the overlying air mass. In several current climate models, it is common practice to use an unphysical approach to close the surface energy balance within the uppermost soil layer with finite thickness and heat capacity. In this study, a different approach is investigated by means of a physically based estimation of the canopy heat storage (SkIn+). Therefore, as a first step, results of an offline simulation of the land component JSBACH of the Max Planck Institute Earth system model (MPI-ESM) – constrained with atmospheric observations – are compared to energy fluxes and water fluxes derived from eddy covariance measurements observed at the CASES-99 field experiment in Kansas, where shallow vegetation prevails. This comparison of energy and evapotranspiration fluxes with observations at the site-level provides an assessment of the model's capacity to correctly reproduce the diurnal cycle. Following this, a global coupled land–atmosphere experiment is performed using an AMIP (Atmospheric Model Intercomparison Project) type simulation over 30 years to evaluate the regional impact of the SkIn+ scheme on a longer timescale, in particular, with respect to the effect of the canopy heat storage. The results of the offline experiment show that SkIn+ leads to a warming during the day and to a cooling at night relative to the old reference scheme, thereby improving the performance in the representation of the modeled surface fluxes on diurnal timescales. In particular: nocturnal heat releases unrealistically destroying the stable boundary layer disappear and phase errors are removed. On the global scale, for regions with no or low vegetation and a pronounced diurnal cycle, the nocturnal cooling prevails due to the fact that stable conditions at night maintain the delayed response in temperature, whereas the daytime turbulent exchange amplifies it. For the tropics and boreal forests as well as high latitudes, the scheme tends to warm the system.
   IPCC SR15

8. [Masson-Delmotte et al. 2018] Synthesis report
   IPCC SR15

9. [Kraehenmann et al. 2018] ABSTRACT: We present a 1-km2 gridded German dataset of hour- ly surface climate variables covering the period 1995 to 2012. The dataset comprises 12 variables including temperature, dew point, cloud cover, wind speed and direction, global and direct shortwave radiation, down- and up-welling longwave radiation, sea level pressure, relative humidity and vapour pressure. This dataset was constructed statistically from station data, satellite observations and model data. It is outstanding in terms of spatial and temporal resolution and in the number of climate variables. For each variable, we employed the most suitable gridding method and combined the best of several information sources, including station records, satellite-derived data and data from a regional climate model. A module to estimate urban heat island intensity was integrated for air and dew point temperature. Owing to the low density of available synop stations, the gridded dataset does not capture all variations that may occur at a reso- lution of 1 km2. This applies to areas of complex terrain (all the variables), and in particular to wind speed and the radiation parameters. To achieve maximum precision, we used all obser- vational information when it was available. This, however, leads to inhomogeneities in station network density and affects the long-term consistency of the dataset. A first climate analysis for Germany was conducted. The Rhine River Valley, for exam- ple, exhibited more than 100 summer days in 2003, whereas in 1996, the number was low everywhere in Germany. The dataset is useful for applications in various climate-related studies, haz- ard management and for solar or wind energy applications and it is available via doi.
   Catalysis, CO2 damping

10. [Krinner et al. 2018] Abstract: This paper describes ESM-SnowMIP, an interna- tional coordinated modelling effort to evaluate current snow schemes, including snow schemes that are included in Earth system models, in a wide variety of settings against local and global observations. The project aims to identify crucial pro- cesses and characteristics that need to be improved in snow models in the context of local- and global-scale modelling. A further objective of ESM-SnowMIP is to better quantify snow-related feedbacks in the Earth system. Although it is not part of the sixth phase of the Coupled Model Intercom- parison Project (CMIP6), ESM-SnowMIP is tightly linked to the CMIP6-endorsed Land Surface, Snow and Soil Moisture Model Intercomparison (LS3MIP).
    Clm tech rep

11. [McGuire et al. 2018] We conducted a model-based assessment of changes in perma- frost area and carbon storage for simulations driven by RCP4.5 and RCP8.5 projections between 2010 and 2299 for the northern permafrost region. All models simulating carbon represented soil with depth, a critical structural feature needed to represent the permafrost carbon–climate feedback, but that is not a universal feature of all climate models. Between 2010 and 2299, simulations indicated losses of permafrost between 3 and 5 million km2 for the RCP4.5 climate and between 6 and 16 million km2 for the RCP8.5 climate. For the RCP4.5 projection, cumulative change in soil car- bon varied between 66-Pg C (1015-g carbon) loss to 70-Pg C gain. For the RCP8.5 projection, losses in soil carbon varied between 74 and 652 Pg C (mean loss, 341 Pg C). For the RCP4.5 projection, gains in vegetation carbon were largely responsible for the overall projected net gains in ecosystem carbon by 2299 (8- to 244-Pg C gains). In contrast, for the RCP8.5 projection, gains in vegetation carbon were not great enough to compensate for the losses of carbon projected by four of the five models; changes in ecosystem carbon ranged from a 641-Pg C loss to a 167-Pg C gain (mean, 208-Pg C loss). The models indicate that substantial net losses of ecosystem carbon would not occur until after 2100. This assessment suggests that effective mitigation efforts during the remainder of this cen- tury could attenuate the negative consequences of the permafrost carbon–climate feedback.
    Clm tech rep

12. [Marsicek et al. 2018] Abstract: Cooling during most of the past two millennia has been widely recognized1,2 and has been inferred to be the dominant global temperature trend of the past 11,700 years (the Holocene epoch)3. However, long-term cooling has been difficult to reconcile with global forcing4, and climate models consistently simulate long-term warming4. The divergence between simulations and reconstructions emerges primarily for northern mid-latitudes, for which pronounced cooling has been inferred from marine and coastal records using multiple approaches3. Here we show that temperatures reconstructed from sub-fossil pollen from 642 sites across North America and Europe closely match simulations, and that long-term warming, not cooling, defined the Holocene until around 2,000 years ago. The reconstructions indicate that evidence of long-term cooling was limited to North Atlantic records. Early Holocene temperatures on the continents were more than two degrees Celsius below those of the past two millennia, consistent with the simulated effects of remnant ice sheets in the climate model Community Climate System Model 3 (CCSM3)5. CCSM3 simulates increases in ‘growing degree days’—a measure of the accumulated warmth above five degrees Celsius per year—of more than 300 kelvin days over the Holocene, consistent with inferences from the pollen data. It also simulates a decrease in mean summer temperatures of more than two degrees Celsius, which correlates with reconstructed marine trends and highlights the potential importance of the different subseasonal sensitivities of the records. Despite the differing trends, pollen- and marine-based reconstructions are correlated at millennial-to-centennial scales, probably in response to ice-sheet and meltwater dynamics, and to stochastic dynamics similar to the temperature variations produced by CCSM3. Although our results depend on a single source of palaeoclimatic data (pollen) and a single climate-model simulation, they reinforce the notion that climate models can adequately simulate climates for periods other than the present-day. They also demonstrate that amplified warming in recent decades increased temperatures above the mean of any century during the past 11,000 years.
    Reconciling reconstructions and simulations over the Holocene

13. [Melo-Aguilar et al. 2018] Abstract: Past climate variations may be uncovered via reconstruction methods that use proxy data as predictors. Among them, borehole reconstruction is a well-established technique to recover the long-term past surface air temperature (SAT) evolution. It is based on the assumption that SAT changes are strongly coupled to ground surface temperature (GST) changes and transferred to the subsurface by thermal conduction. We evaluate the SAT–GST coupling during the last millennium (LM) using simulations from the Community Earth System Model LM Ensemble (CESM-LME). The validity of such a premise is explored by analyzing the structure of the SAT–GST covariance during the LM and also by investigating the evolution of the long-term SAT–GST relationship. The multiple and single-forcing simulations in the CESM-LME are used to analyze the SAT–GST relationship within different regions and spatial scales and to derive the influence of the different forcing factors on producing feedback mechanisms that alter the energy balance at the surface. The results indicate that SAT–GST coupling is strong at global and above multi-decadal timescales in CESM-LME, although a relatively small variation in the long-term SAT–GST relationship is also represented. However, at a global scale such variation does not significantly impact the SAT–GST coupling, at local to regional scales this relationship experiences considerable long-term changes mostly after the end of the 19th century. Land use land cover changes are the main driver for locally and regionally decoupling SAT and GST, as they modify the land surface properties such as albedo, surface roughness and hydrology, which in turn modifies the energy fluxes at the surface. Snow cover feedbacks due to the influence of other external forcing are also important for corrupting the long-term SAT–GST coupling. Our findings suggest that such local and regional SAT–GST decoupling processes may represent a source of bias for SAT reconstructions from borehole measurement, since the thermal signature imprinted in the subsurface over the affected regions is not fully representative of the long-term SAT variations.
    Land atmosphere coupling

14. [Qadir et al. 2018] ABSTRACT: CO2 is selectively hydrogenated to HCO2H or hydrocarbons (HCs) by RuFe nanoparticles (NPs) in ionic liquids (ILs) under mild reaction conditions. The generation of HCO2H occurs in ILs containing basic anions, whereas heavy HCs (up to C21 at 150 °C) are formed in the presence of ILs containing nonbasic anions. Remarkably, high values of TONs (400) and a TOF value of 23.52 h-1 for formic acid with a molar ratio of 2.03 per BMI·OAc IL were obtained. Moreover, these NPs exhibited outstanding abilities in the formation of long-chain HCs with efficient catalytic activity (12percent conversion) in a BMI·NTf2 hydrophobic IL. The IL forms a cage around the NPs that controls the diffusion/residence time of the substrates, intermediates, and products. The distinct CO2 hydrogenation pathways (HCO2H or FT via RWGS) catalyzed by the RuFe alloy are directly related to the basicity and hydrophobicity of the IL ion pair (mainly imposed by the anion) and the composition of the metal alloy. The presence of Fe in the RuFe alloy provides enhanced catalytic performance via a metal dilution effect for the formation of HCO2H and via a synergistic effect for the generation of heavy HCs.
    Catalysis, CO2 damping

    RAMAetal18

15. [Ramaswamy et al. 2019a] ABSTRACT: We describe the historical evolution of the conceptualization, formulation, quantification, application, and utilization of ‘‘radiative forcing’’ (RF) of Earth’s climate. B
    Radiative forcing.

16. [Robinson and Shine 2018] ABSTRACT: It is vital for climate justice to pursue a pathway to zero carbon emissions by 2050 to limit global temperature rise to 1.5 C above pre-industrial levels and to minimize the adverse impacts of climate change on people and their human rights. But can such a pathway be achieved without undermining human rights and restricting the right to development? This Perspective discusses the risks of action and inaction to identify a fair and just transition. It compares the risks posed to human rights from climate impacts with the risks posed by climate action and suggests that rights-informed climate action can maximize benefits for people and the planet.
    Climate change justice, rights.

17. [Steiger et al. 2018] ABSTRACT: Hydroclimate extremes critically affect human and natural systems, but there remain many unanswered questions about their causes and how to interpret their dynamics in the past and in climate change projections. These uncertainties are due, in part, to the lack of long-term, spatially resolved hydroclimate reconstructions and information on the underlying physical drivers for many regions. Here we present the first global reconstructions of hydroclimate and associated climate dynamical variables over the past two thousand years. We use a data assimilation approach tailored to reconstruct hydroclimate that optimally combines 2,978 paleoclimate proxy-data time series with the physical constraints of an atmosphere—ocean climate model. The global reconstructions are annually or seasonally resolved and include two spatiotemporal drought indices, near-surface air temperature, an index of North Atlantic variability, the location of the intertropical convergence zone, and monthly Niño indices. This database, called the Paleo Hydrodynamics Data Assimilation product (PHYDA), will provide a critical new platform for investigating the causes of past climate variability and extremes, while informing interpretations of future hydroclimate projections a.
    Drought reconstructions, data assimilation

18. [Wu et al. 2018] Earth system models (ESMs) from phase 5 of the Coupled Model Intercomparison Project (CMIP5) were diagnosed as having large discrepancies in their land carbon turnover times, which partly explains the dif- ferences in the future projections of terrestrial carbon storage from the models. Carvalhais et al. focused on evaluation of model-based ecosystem carbon turnover times teco in relation with climate factors. In this study, teco from models was analyzed separately for biomass and soil carbon pools, and its spatial dependency upon temperature and precipitation was evaluated using observational datasets. The results showed that 8 of 14 models slightly underestimated global biomass carbon turnover times tveg (modeled median of 8 yr vs ob- served 11 yr), and 11 models grossly underestimated the soil carbon turnover time tsoil (modeled median of 16 yr vs observed 26 yr). The underestimation of global carbon turnover times in ESMs was mainly due to values for tveg and tsoil being too low in the high northern latitudes and arid and semiarid regions. In addition, the models did not capture the observed spatial climate sensitivity of carbon turnover time in these regions. Modeled tveg and tsoil values were generally weakly correlated with climate variables, implying that differ- ences between carbon cycle models primarily originated from structural differences rather than from dif- ferences in atmospheric climate models (i.e., related to temperature and precipitation). This study indicates that most models do not reproduce the underlying processes driving regional tveg and tsoil, highlighting the need for improving the model parameterization and adding key processes such as biotic disturbances and permafrost–carbon climate responses.
    CMIP5 biomass and carbon