Greg Faluvegi

Robust estimates of historical changes in aerosols are key for accurate constraints on climate sensitivity. Dry deposition is a primary sink of aerosols from the atmosphere. However, most global climate models do not accurately represent observed strong dependencies of dry deposition following turbulent transport on aerosol size. It is unclear whether there is a substantial impact of mischaracterized aerosol deposition velocities on historical aerosol changes. Here we describe improved mechanistic representation of aerosol dry deposition in the NASA Goddard Institute for Space Studies (GISS) global climate model, ModelE, and illustrate the impact on 1850–2000 changes in global aerosol burdens as well as aerosol direct and cloud albedo effects using a set of 1850 and 2000 time slice simulations. We employ two aerosol configurations of ModelE (a “bulk” mass‐based configuration and a configuration that more explicitly represents aerosol size distributions, internal mixing, and microphysics) to explore how model structural differences in aerosol representation alter the response to representation of dry deposition. Both configurations show larger historical increases in the global burdens of non‐dust aerosols with the new dry deposition scheme, by 11% in the simpler mass‐based configuration and 23% in the more complex microphysical configuration. Historical radiative forcing responses, which vary in magnitude from 5% to 12% as well as sign, depend on the aerosol configuration.

For the population of a given US city, the risk of premature death associated with heat exposure increases as temperatures rise, but risks in hotter cities are generally lower than in cooler cities at equivalent temperatures due to factors such as acclimatization. Those living in especially hot neighborhoods within cities might therefore suffer much more than average if such adaptation is only at the city‐wide level, whereas they might not experience greatly increased risk if adjustment is at the neighborhood level. To compare these possibilities, we use high spatial resolution temperature data to evaluated heat‐related deaths assuming either adjustment at the city‐wide or at the neighborhood scale in 10 large US cities. On average, we find that if inhabitants are adjusted to their local conditions, a neighborhood that was 10°C hotter than a cooler one would experience only about 1.0–1.5 excess heat deaths per year per 100,000 persons. By contrast, if inhabitants are acclimatized to city‐wide temperatures, the hotter neighborhood would experience about 15 excess deaths per year per 100,000 persons. Using idealized analyses, we demonstrate that current city‐wide epidemiological data do not differentiate between these differing adjustments. Given the very large effects of assumptions about neighborhood‐level acclimatization found here, as well as the fact that current literature is conflicting on the spatial scale of acclimatization, more neighborhood‐level epidemiological data are urgently needed to determine the health impacts of variations in heat exposure within urban areas, better constrain projected changes, and inform mitigation efforts.

Observed trends in tropospheric ozone, an important air pollutant and short‐lived climate forcer (SLCF), are estimated using available surface and ozonesonde profile data for 1993–2019, using a coherent methodology, and compared to modeled trends (1995–2015) from the Arctic Monitoring Assessment Program SLCF 2021 assessment. Increases in observed surface ozone at Arctic coastal sites, notably during winter, and concurrent decreasing trends in surface carbon monoxide, are generally captured by multi‐model median trends. Wintertime increases are also estimated in the free troposphere at most Arctic sites, with decreases during spring months. Winter trends tend to be overestimated by the multi‐model medians. Springtime surface ozone increases in northern coastal Alaska are not simulated while negative springtime trends in northern Scandinavia are not always reproduced. Possible reasons for observed changes and model performance are discussed including decreasing precursor emissions, changing ozone dry deposition, and variability in large‐scale meteorology.

Exposure to fine particulate matter (PM_2.5) air pollution is associated with large‐scale health consequences, but the ranges in estimates of global air pollution exposure and PM_2.5‐related global premature mortality remain understudied. Using four model/observation‐based PM_2.5 data sets and six Coupled Model Intercomparison Project Phase 6 (CMIP6) climate models, we compare ranges in current PM_2.5‐related mortality estimates based on model/observation combinations to the range of projected emissions reductions and the resulting impacts on global health from CMIP6 models. Although estimates of current mortality are sensitive to the PM_2.5 data set (6.54–9.23 million/year using the central estimate from the Global Exposure Mortality Model), the projected near‐term benefits of emissions reductions for reduced mortality are much more certain. Specifically, until the middle of the century, uncertainty ranges in projected avoided deaths are less than the magnitude of ranges in present‐day mortality estimates, even when projected future changing socio‐demographic factors are considered. Under a low‐emissions scenario, avoided cumulative early deaths could reach 275 million (range: 243–351 million) by 2100, assuming no future socio‐demographic changes. Avoided cumulative deaths could reach 124 million (range: 74–246 million) if changing future socio‐demographic factors are considered. As the global population ages and becomes more sensitive to breathing polluted air, there will be an increasing need for policies that both reduce air pollution and improve public health.

Acetone is an abundant volatile organic compound in the atmosphere with important influence on ozone and oxidation capacity. Direct sources include anthropogenic, terrestrial vegetation, oceanic, and biomass burning emissions. Acetone is also produced chemically from other volatile organic compounds. Sinks include deposition onto the land and ocean surfaces, as well as chemical loss. Acetone’s lifetime is long enough to allow transport and reactions with other compounds remote from its sources. The latest NASA Goddard Institute for Space Studies (GISS) Earth System Model, ModelE2.1, simulates a variety of Earth system interactions. Previously, acetone had a very simplistic representation in the ModelE chemical scheme. This study assesses a more sophisticated acetone scheme, in which acetone is a full 3-dimensional tracer, with explicit sources, sinks and atmospheric transport. We evaluate the new global acetone budget in the context of past literature. Anthropogenic emissions, vegetation emissions, biomass burning, and deposition representations agree well with previous studies. Chemistry and the ocean contribute to both sources and sinks of acetone, with their net values agreeing with the literature, although their individual source and sink terms appear to be overestimated for chemistry and underestimated for ocean fluxes. We find the production of acetone from precursor hydrocarbon oxidation has strong leverage on the overall chemical source, indicating the importance of accurate molar yields for this source. Spatial distributions reveal that ocean uptake of acetone is strongest in northern latitudes, while production is mainly in mid-southern latitudes. The seasonality of acetone-related processes was also studied in conjunction with field measurements around the world. These comparisons show promising agreement, but have shortcomings at urban locations, since the model’s resolution is too coarse to capture behavior in high-emission areas. Overall, our analysis of the acetone budget aids the development of this tracer in the GISS ModelE2.1, a crucial step to understanding the role of acetone in the atmosphere.

Africa is highly vulnerable to climate change but emits a small portion of global greenhouse gases. Additionally, decarbonization might lead to a ‘climate penalty’ whereby reductions in cooling aerosols offset temperature benefits from CO₂reductions for several decades. However, climate change impacts conditions other than temperatures, including precipitation. Using the NASA Goddard Institute for Space Studies climate model, we find that although African emissions cuts have weak impacts on projected African temperatures, they significantly impact rainfall. Whereas business-as-usual increasing emissions lead to substantial drying over tropical Northern Hemisphere Africa during local summer, that drying is essentially eliminated under a sustainable development pathway. The reduction in cooling aerosols is responsible for ~33–90% of the avoided drying in our model, with the remainder largely attributable to reduced absorbing aerosols. African policy choices may therefore greatly reduce regional African summer drying, giving parts of Africa substantial leverage over their own climate and air quality future.

COVID‐19 pandemic responses affected atmospheric composition and climate. These effects depend on the background emissions, climate, and season in which they occur. Although using multiple scenarios is common in explorations of long‐term climate change, they are rarely used to explore atmospheric composition or climate changes in response to transient emission perturbations on the scale of COVID‐19 lockdowns. We used the ModelE Earth system model to evaluate how atmospheric and climate impacts depend on the decade and season in which lockdowns occurred. Global COVID‐19‐related anomalies in aerosols and trace gases differed by up to an order of magnitude or more when comparing lockdowns in 1980, 2008, 2020, and 2051. Regional atmospheric composition anomalies tended to be largest when emissions were near a historical peak: 1980 in Europe and temperate North America, 2008 or 2020 in eastern Asia, and 2051 in south Asia. Regional aerosol direct effect anomalies were almost always less than 0.1 W m⁻² during the first pandemic year, but over 0.1 W m⁻² in Europe and exceeded 0.2 W m⁻² in Europe and temperate North America in 1980, generally changing in tandem with regional emissions. In contrast, direct effect anomalies in Asia were positive in 1980 and negative in 2008, suggesting they may be primarily determined by exogenous emission anomalies. Shifting COVID‐19 onset in 2020 by 3, 6, or 9 months also altered atmospheric composition on the order of 2%–25% globally. In all scenarios, changes in surface temperature or precipitation appeared unrelated to local atmospheric compositional changes.

Trends in tropospheric ozone, an important air pollutant and short-lived climate forcer (SLCF), are estimated using available surface and ozonesonde profile data for 1993-2019. Using a coherent methodology, observed trends are compared to modeled trends (1995-2015) from the Arctic Monitoring Assessment Programme SLCF 2021 assessment. Statistically significant increases in observed surface ozone at Arctic coastal sites, notably during winter, and concurrent decreasing trends in surface carbon monoxide, are generally captured by multi-model median (MMM) trends. Wintertime increases are also estimated in the free troposphere at most Arctic sites, but tend to be overestimated by the MMMs. Springtime surface ozone increases in northern coastal Alaska are not simulated while negative springtime trends in northern Scandinavia are not always reproduced. Possible reasons for observed changes and model behavior are discussed, including decreasing precursor emissions, changing ozone sinks, and variability in large-scale meteorology.

Precipitation has increased across the arid Central Asia region over recent decades. However, the underlying mechanisms of this trend are poorly understood. Here, we analyze multi-model simulations from the Precipitation Driver and Response Model Intercomparison Project (PDRMIP) to investigate potential drivers of the observed precipitation trend. We find that anthropogenic sulfate aerosols over remote polluted regions in South and East Asia lead to increased summer precipitation, especially convective and extreme precipitation, in arid Central Asia. Elevated concentrations of sulfate aerosols over remote polluted Asia cause an equatorward shift of the Asian Westerly Jet Stream through a fast response to cooling of the local atmosphere at mid-latitudes. This shift favours moisture supply from low-latitudes and moisture flux convergence over arid Central Asia, which is confirmed by a moisture budget analysis. High levels of absorbing black carbon lead to opposing changes in the Asian Westerly Jet Stream and reduced local precipitation, which can mask the impact of sulfate aerosols. This teleconnection between arid Central Asia precipitation and anthropogenic aerosols in remote Asian polluted regions highlights long-range impacts of anthropogenic aerosols on atmospheric circulations and the hydrological cycle.

Over the CMIP6 historical period (1850–2014), aerosols provided the largest negative forcing compared to all other climate forcings via their ability to absorb or scatter solar radiation and alter clouds. Aerosols played an important role in counterbalancing warming by greenhouse gases (GHGs). Here we study aerosol forcing trends in the CMIP6 simulations of the NASA Goddard Institute for Space Studies (GISS) ocean‐atmosphere ModelE version 2.1 (GISS‐E2.1‐G) using a fully coupled atmospheric composition configuration, including interactive gas‐phase chemistry, and either an aerosol microphysical (MATRIX) or a mass‐based aerosol (OMA) module. Simulations of the CMIP6 historical period are analyzed as well as four Shared Socioeconomic Pathway (SSP) future scenarios for 2015–2100: SSP1‐2.6, SSP2‐4.5, SSP3‐7.0, and SSP5‐8.5. The main conclusion of this study is that aerosol forcing in the GISS model has reached its turning point, switching from globally increasing to a decreasing trend in the first decade of the 21st century. This result is robust, independent of which aerosol module or SSP scenario is used. Non‐linear aerosol‐cloud interactions dominate as a forcing agent over aerosol‐radiation interactions. Aerosols' ability to counterbalance GHG forcing on the global scale is today at a level comparable to that at the beginning of the last century. In the 1980s, the decade of largest global aerosol loads, aerosols balanced up to 80% of GHG forcing. As a consequence, global warming of the last decades, which is primarily driven by greenhouse gases, has been augmented by the effect of decreasing aerosol cooling in our model. By the end of this century, following the SSP scenarios, aerosols will only counterbalance 0%–20% of GHG forcing, depending on model and on scenario.