Search for: All records

Abstract Understanding and predicting heatwave risk is a societal imperative in the face of climate change. Anthropogenic aerosol emissions impact heat extremes more strongly per unit of mean warming than do greenhouse gases, but the influence of aerosols’ evolving spatial pattern on time-varying heatwave hazard and resulting population exposure has been largely ignored. Aerosols’ spatially heterogeneous forcing is often co-located with population centers due to aerosols’ industrial sources and short atmospheric lifetime, potentially resulting in amplified exposure to aerosol-driven climate effects. Here, we quantify the influence of historical and projected future changes in aerosol emissions through 2100 on global patterns of heatwave hazard (i.e. the frequency of heatwave days) and exposure (i.e. population-weighted hazard) using the NCAR Community Earth System Model v1 single forcing large ensemble (LE). Our results show that increased aerosol emissions since 1920 have suppressed heatwave frequency (HWF) over populated regions by roughly half through present-day—a trend that is now reversing with shifting emission patterns and net global declining emissions. This may already be leading to an aerosol-driven acceleration in HWF, a signal that is amplified in populated regions. Aerosols’ influence on heatwaves is strongly co-located with population, creating out-sized exposure, which evolves through time with aerosols’ evolving emissions pattern within this LE. Our results suggest that near-term changes in aerosol emissions will be a disproportionate driver of trends in heatwave exposure, meriting dedicated future study, and that aerosols’ evolving spatial pattern should be considered in attempts to attribute recent heatwave trends to human activity. 

Abstract The regional climate impacts of anthropogenic aerosol emissions and irrigation growth in South Asia have conventionally been studied separately. These forcings have overlapping influences on surface temperature and atmospheric stability, but detection and attribution simulations typically quantify the impact of individual time‐evolving climate forcings, which does not account for nonlinear interactions between forcings or their impacts, when forcings evolve in tandem. Using transient simulations in GISS ModelE 2.1‐G, we assess the summertime surface energy balance in five different sub‐regions of South Asia by comparing the linear addition of anthropogenic aerosol and land use single‐forcing historical simulations with novel dual‐forcing simulations. We find that the combination of aerosol emissions and irrigation changes between preindustrial and present‐day increases aerosol hydration and cloud cover more strongly than does the linear addition of the individual forcings. This results in a strong nonlinear decrease in downwelling shortwave radiation, which drives subsequent nonlinearities in the surface energy balance through a relative suppression of energy availability at the surface. While aerosols and irrigation are each credited with suppressing monsoon winds and delaying onset, combined simulations of both forcings suggest that surface pressure is nonlinearly reduced over the northern Indian Subcontinent. This results in a net increase in 850 mb winds from the Bay of Bengal toward northwest India and Pakistan in combined simulations, suppressing the weakening of summer monsoon winds from single forcing results. The nonlinearities identified in our study suggest that the current framework for detection and attribution may not adequately account for potential interactions between forcings. 

Abstract The aerosol indirect effect (AIE) dominates uncertainty in total anthropogenic aerosol forcing in phase 6 of the Coupled Model Intercomparison Project (CMIP6) models. AIE strength depends on meteorological conditions that have been shown to change between preindustrial (PI) and present-day (PD) climates, such as cloud cover and atmospheric moisture. Hence, AIE strength may depend on background climate state, impacting the dependence of model-based AIE estimates on experiment design or the evolution of AIE strength with intensifying climate change, which has not previously been explicitly evaluated. Using atmosphere-only simulations with prescribed observed sea surface temperatures (SSTs) and sea ice in the National Center for Atmospheric Research (NCAR) Community Earth System Model 2, version 2.1.3 (CESM2), Community Atmosphere Model, version 6.0 (CAM6), model, we impose a PD (2000) aerosol perturbation onto a PI (1850), PD, and PD with a uniform 4 K increase in the SST (PD + 4 K) background climate to assess the dependence of the total aerosol effective radiative forcing (ERF) and AIE on background climate. We find statistically insignificant increases in aerosol ERF when estimated in the different background climates, almost entirely from increases in direct ERF but with some regionally significant compensating signals in PD + 4 K. The absence of an AIE dependence on background climate in our PD simulation may be tied to documented differences in cloud responses to the observed SSTs used in our simulations versus SSTs produced by the fully coupled models from which most cloud feedback studies are derived, known as the “pattern effect.” Our findings indicate that AIE and aerosol forcing overall may not have a strong dependence on the background climate state in the near future but could regionally under extreme climate change. Significance StatementDiverse model representations of aerosol–cloud interactions strongly contribute to uncertainty in historical anthropogenic aerosol forcing and are associated with uncertainty in climate sensitivity. This study aims to highlight the dependence of aerosol indirect effects on the background climate state in Community Earth System Model 2, version 2.1.3 (CESM2), Community Atmosphere Model, version 6.0 (CAM6), by identifying microphysical and meteorological changes between aerosol-driven atmospheric responses in present-day and preindustrial climate states to understand anthropogenic aerosol-driven forcing more thoroughly. 

Abstract Anthropogenic aerosols (AER) and greenhouse gases (GHG)—the leading drivers of the forced historical change—produce different large‐scale climate response patterns, with correlations trending from negative to positive over the past century. To understand what caused the time‐evolving comparison between GHG and AER response patterns, we apply a low‐frequency component analysis to historical surface ocean changes from CESM1 single‐forcing large‐ensemble simulations. While GHG response is characterized by its first leading mode, AER response consists of two distinct modes. The first one, featuring long‐term global AER increase and global cooling, opposes GHG response patterns up to the mid‐twentieth century. The second one, featuring multidecadal variations in AER distributions and interhemispheric asymmetric surface ocean changes, appears to reinforce the GHG warming effect over recent decades. AER thus can have both competing and synergistic effects with GHG as their emissions change temporally and spatially. 

Abstract. In 2020, the International Maritime Organization (IMO) implemented strict new regulations on the emissions of sulfate aerosol from the world's shipping fleet. This can be expected to lead to a reduction in aerosol-driven cooling, unmasking a portion of greenhouse gas warming. The magnitude of the effect is uncertain, however, due to the large remaining uncertainties in the climate response to aerosols. Here, we investigate this question using an 18-member ensemble of fully coupled climate simulations evenly sampling key modes of climate variability with the NCAR model, the Community Earth System Model version 2 (CESM2). We show that, while there is a clear physical response of the climate system to the IMO regulations, including a surface temperature increase, we do not find global mean temperature influence that is significantly different from zero. The 20-year average global mean warming for 2020–2040 is +0.03 °C, with a 5 %–95 % confidence range of [-0.09,0.19], reflecting the weakness of the perturbation relative to internal variability. We do, however, find a robust, non-zero regional temperature response in part of the North Atlantic. We also find that the maximum annual mean and ensemble mean warming occurs around 1 decade after the perturbation in 2029, which means that the IMO regulations have likely had very limited influence on observed global warming to date. We further discuss our results in light of other, recent publications that have reached different conclusions. Overall, while the IMO regulations may contribute up to 0.16 °C [-0.17,0.52] to the global mean surface temperature in individual years during this decade, consistent with some early studies, such a response is unlikely to have been discernible above internal variability by the end of 2023 and is in fact consistent with zero throughout the 2020–2040 period.