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Anthropogenic aerosols exert a cooling influence that offsets part of the greenhouse gas warming. Due to their short tropospheric lifetime of only up to several days, the aerosol forcing responds quickly to emissions. Here we present and discuss the evolution of the aerosol forcing since 2000. There are multiple lines of evidence that allow to robustly conclude that the anthropogenic aerosol effective radiative forcing - both aerosol-radiation and aerosol-cloud interactions - has become globally less negative, i.e. that the trend in aerosol effective radiative forcing changed sign from negative to positive. Bottom-up inventories show that anthropogenic primary aerosol and aerosol precursor emissions declined in most regions of the world; observations related to aerosol burden show declining trends, in particular of the fine-mode particles that make up most of the anthropogenic aerosols; satellite retrievals of cloud droplet numbers show trends consistent in sign, as do observations of top-of-atmosphere radiation. Climate model results, including a revised set that is constrained by observations of the ocean heat content evolution show a consistent sign and magnitude for a positive forcing relative to 2000 due to reduced aerosol effects. This reduction leads to an acceleration of the forcing of climate change, i.e. an increase in forcing by 0.1 to 0.3Wm-2, up to 12% of the total climate forcing in 2019 compared to 1750 according to IPCC. [ABSTRACT FROM AUTHOR]

Soil uptake by H2 -oxidizing bacteria is the main sink of the global hydrogen cycle, accounting for nearly 80% of the atmospheric H2 consumption. Although the H2 uptake is strongly influenced by soil moisture, little attention has been paid to coherently couple the water and hydrogen dynamics in soils. Toward this goal, we improve the mechanistic representation of the H2 uptake as a function of soil moisture and highlight the role of the moisture temporal fluctuations on the biotic consumption of H2 . The results show that, due to the strongly nonlinear relationship between soil moisture and H2 uptake, addressing the dry-wet sequences is necessary to characterize the H2 uptake in semi-arid regions correctly. From novel analytical relationships validated with field data, we also infer the biotic and abiotic limitations in the global soil H2 uptake. It is shown that, diffusion generally limits the uptake in humid temperate and tropical regions, while biotic limitations tend to occur in very arid or cold soils. Finally, we discuss the implications that climate change may have on the H2 soil sink. [ABSTRACT FROM AUTHOR]

The imprint of anthropogenic activities on the marine nitrogen (N) cycle remains challenging to represent in global models, in part because of uncertainties regarding the source of marine N to the atmosphere. While N inputs of terrestrial origin present a truly external perturbation, a significant fraction of N deposition over the ocean arises from oceanic ammonia (NH3) outgassing that is subsequently deposited in other ocean regions. Here, we describe advances in the Geophysical Fluid Dynamics Laboratory's (GFDL) Earth System Model 4 (ESM4.1) aimed at improving the representation of the exchange of N between atmosphere and ocean and its response to changes in ocean acidity and N deposition. We find that the simulated present‐day NH3 outgassing (3.1 TgN yr−1) is 7% lower than under preindustrial conditions, which reflects the compensating effects of increasing CO2 (−16%) and N enrichment of ocean waters (+9%). This change is spatially heterogeneous, with decreases in the open ocean (−13%) and increases in coastal regions (+15%) dominated by coastal N enrichment. The ocean outgassing of ammonia is shown to increase the transport of N from N‐rich to N‐poor ocean regions, where carbon export at 100 m increases by 0.5%. The implications of the strong response of NH3 ocean outgassing to CO2 for the budget of NH3 in the remote marine atmosphere and its imprint in ice cores are discussed. Key Points: Development of an interactive representation of N exchange between ocean and atmosphere in the GFDL ESM4.1, including ammonia outgassingLittle change in the ocean NH3 outgassing over the historical period due to compensation between acidification and increasing N depositionOcean NH3 outgassing increases carbon export at 100 m in N‐limited regions by 0.5% [ABSTRACT FROM AUTHOR]

This contribution describes the ocean biogeochemical component of the Geophysical Fluid Dynamics Laboratory's Earth System Model 4.1 (GFDL‐ESM4.1), assesses GFDL‐ESM4.1's capacity to capture observed ocean biogeochemical patterns, and documents its response to increasing atmospheric CO2. Notable differences relative to the previous generation of GFDL ESM's include enhanced resolution of plankton food web dynamics, refined particle remineralization, and a larger number of exchanges of nutrients across Earth system components. During model spin‐up, the carbon drift rapidly fell below the 10 Pg C per century equilibration criterion established by the Coupled Climate‐Carbon Cycle Model Intercomparison Project (C4MIP). Simulations robustly captured large‐scale observed nutrient distributions, plankton dynamics, and characteristics of the biological pump. The model overexpressed phosphate limitation and open ocean hypoxia in some areas but still yielded realistic surface and deep carbon system properties, including cumulative carbon uptake since preindustrial times and over the last decades that is consistent with observation‐based estimates. The model's response to the direct and radiative effects of a 200% atmospheric CO2 increase from preindustrial conditions (i.e., years 101–120 of a 1% CO2 yr−1 simulation) included (a) a weakened, shoaling organic carbon pump leading to a 38% reduction in the sinking flux at 2,000 m; (b) a two‐thirds reduction in the calcium carbonate pump that nonetheless generated only weak calcite compensation on century time‐scales; and, in contrast to previous GFDL ESMs, (c) a moderate reduction in global net primary production that was amplified at higher trophic levels. We conclude with a discussion of model limitations and priority developments. Plain Language Summary: This paper describes and evaluates the ocean biogeochemical component of the Geophysical Fluid Dynamics Laboratory's Earth System Model 4.1 (GFDL‐ESM4.1). GFDL‐ESM4.1 was developed to study the past, present, and future evolution of the Earth system under scenarios for natural and anthropogenic drivers of Earth system change, including greenhouse gases and aerosols. The response of the ocean's vast carbon and heat reservoirs to accumulating greenhouse gases greatly reduces their atmospheric and terrestrial impacts, but also puts ocean environments and the marine resources they support at risk. Relative to previous models, GFDL‐ESM4.1 improves the representation of (a) ocean food webs connecting plankton and fish; (b) biological processes influencing the sequestration of carbon in the deep ocean; and (c) land‐atmosphere‐ocean nutrient exchanges. While simulations have biases, they capture many critical aspects of the global ocean carbon cycle and ocean ecosystem, including the observed uptake of anthropogenic carbon over the last ~150 yr. Projections suggest that continued CO2 increases could significantly decrease ocean productivity and the ocean's capacity to sequester atmospheric carbon. Key Points: Enhanced plankton food webs, remineralization, and Earth system linkages yield skillful global carbon cycle and ecosystem simulationsOcean productivity estimates improved, but phosphate limitation and hypoxia overestimated in some areasHigh CO2 and associated warming substantially reduce the biological pump and ocean productivity across trophic levels [ABSTRACT FROM AUTHOR]

Abstract. Feedbacks play a fundamental role in determining the magnitude of the response of the climate system to external forcing, such as from anthropogenic emissions. The latest generation of Earth system models include aerosol and chemistry components that interact with each other and with the biosphere. These interactions introduce a complex web of feedbacks which it is important to understand and quantify. This paper addresses the multiple pathways for aerosol and chemical feedbacks in Earth system models. This is achieved by extending previous formalisms which include CO2 concentrations as a state variable to a formalism which in principle includes the concentrations of all climate-active atmospheric constituents. This framework is demonstrated by applying it to the Earth system models participating in CMIP6 with a focus on the non-CO2 reactive gases and aerosols (methane, ozone, sulphate aerosol, organic aerosol and dust). We find that the overall climate feedback through chemistry and aerosols is negative in the CMIP6 Earth system models due to increased negative forcing from aerosols with warmer temperatures. Through diagnosing changes in methane emissions and lifetime we find that if Earth system models were to allow methane to vary interactively, methane positive feedbacks (principally wetland methane emissions and biogenic VOC emissions) would offset much of the aerosol feedbacks. [ABSTRACT FROM AUTHOR]

The top-of-atmosphere (TOA) outgoing longwave flux over the 9.6-μm ozone band is a fundamental quantity for understanding chemistry-climate coupling. However, observed TOA fluxes are hard to estimate as they exhibit considerable variability in space and time that depend on the distributions of clouds, ozone (O3), water vapor (H2O), air temperature (Ta), and surface temperature (Ts). Benchmarking present day fluxes and quantifying the relative influence of their drivers is the first step for estimating climate feedbacks from ozone radiative forcing and predicting its evolution. To that end, we construct observational instantaneous radiative kernels (IRKs) representing the sensitivities of the TOA flux in the 9.6-μm ozone band to the vertical distribution of geophysical variables, including O3, H2O, Ta, and Ts based upon the Aura Tropospheric Emission Spectrometer (TES) measurements. Applying these kernels to present-day simulations from the Chemistry-Climate Model Initiative (CCMI) project as compared to a 2006 reanalysis assimilating satellite observations, we show that the models have large differences in TOA flux, attributable to different geophysical variables. In particular, model simulations continue to diverge from observations in the tropics, as reported in previous studies of the Atmospheric Chemistry Climate Model Inter-comparison Project (ACCMIP) simulations. The principal culprits are tropical mid and upper tropospheric ozone followed by tropical lower tropospheric H2O. Five models out of the eight studied here have TOA flux biases exceeding 100 mWm−2 attributable to tropospheric ozone bias. Another set of five models flux biases over 50 mWm−2 due to H2O. On the other hand, Ta radiative bias is negligible in all models (no more than 30 mWm−2). We found that AM3 and CMAM have the lowest TOA flux biases globally but are a result of cancellation of difference processes. Overall, the multi-model ensemble mean bias is −132.9 ± 98 mWm−2, indicating that they are too atmospherically opaque thereby reducing sensitivity of TOA flux to ozone and potentially an underestimate of ozone radiative forcing. We find that the inter-model TOA OLR difference is well anti-correlated with their ozone band flux bias. This suggests that there is significant radiative compensation in the calculation of model outgoing longwave radiation. [ABSTRACT FROM AUTHOR]

Northern India (23-31°N, 68-90°E) is one of the most densely populated and polluted regions in world. Accurately modeling pollution in the region is difficult due to the extreme conditions with respect to emissions, meteorology, and topography, but it is paramount in order to understand how future changes in emissions and climate may alter the region's pollution regime. We evaluate the ability of a developmental version of the new-generation NOAA GFDL Atmospheric Model, version 4 (AM4) to simulate observed wintertime fine particulate matter (PM2.5) and its relationship to meteorology over Northern India. We compare two simulations of GFDL-AM4 nudged to observed meteorology for the period 1980-2016 driven by pollutant emissions from two global inventories developed in support of the Coupled Model Intercomparison Project Phases 5 (CMIP5) and 6 (CMIP6), and compare results with ground-based observations from India's Central Pollution Control Board (CPCB) for the period 1 October 2015-31 March 2016. Overall, our results indicate that the simulation with CMIP6 emissions produces improved concentrations of pollutants over the region relative to the CMIP5-driven simulation. While the particulate concentrations simulated by AM4 are biased low overall, the model generally simulates the magnitude and daily variability of observed total PM2.5. Nitrate and organic matter are the primary components of PM2.5 over Northern India in the model. On the basis of correlations of the individual model components with total observed PM2.5 and correlations between the two simulations, meteorology is the primary driver of daily variability. The model correctly reproduces the shape and magnitude of the seasonal cycle of PM2.5, but the simulated diurnal cycle misses the early evening rise and secondary maximum found in the observations. Observed PM2.5 abundances are by far the highest within the densely populated Indo-Gangetic Plain, where they are closely related to boundary layer meteorology, specifically relative humidity, wind speed, boundary layer height, and inversion strength. The GFDL AM4 model reproduces the overall observed pollution gradient over Northern India as well as the strength of the meteorology-PM2.5 relationship in most locations. [ABSTRACT FROM AUTHOR]

Current estimates of agricultural ammonia (NH3) emissions in China differ by more than a factor of 2, hindering our understanding of their environmental consequences. Here we apply both bottom-up statistical and top-down inversion methods to quantify NH3 emissions from agriculture in China for the year 2008. We first assimilate satellite observations of NH3 column concentration from the Tropospheric Emission Spectrometer (TES) using the GEOS-Chem adjoint model to optimize Chinese anthropogenic NH3 emissions at the 1/2° × 2/3° horizontal resolution for March-October 2008. Optimized emissions show a strong summer peak with emissions about 50 % higher in summer than spring and fall, which is underestimated in current bottom-up NH3 emission estimates. To reconcile the latter with the top-down results, we revisit the processes of agricultural NH3 emissions, and develop an improved bottom-up inventory of Chinese NH3 emissions from fertilizer application and livestock waste at the 1/2° × 2/3° resolution. Our bottom-up emission inventory includes more detailed information on crop-specific fertilizer application practices and better accounts for meteorological modulation of NH3 emission factors in China. We find that annual anthropogenic NH3 emissions are 11.7 Tg for 2008 with 5.05 Tg from fertilizer application and 5.31 Tg from livestock waste. The two sources together account for 88 % of total anthropogenic NH3 emissions in China. Our bottom-up emission estimates also show a distinct seasonality peaking in summer, consistent with top-down results from the satellite-based inversion. Further evaluations using surface network measurements show that the model driven by our bottom-up emissions well reproduces the observed spatial and seasonal variations of NH3 gas concentrations and ammonium (NH4+) wet deposition fluxes over China, providing additional credibility to the improvements we have made to our agricultural NH3 emission inventory. [ABSTRACT FROM AUTHOR]