Anthropogenic Aerosols are Uniquely Nonuniform Agents of Climate Change
A high resolution climate model provides a snapshot of the global distribution of aerosols in the atmosphere. Credit: NOAA GFDL.
Because aerosols only last in the atmosphere for days to weeks, they remain concentrated in regions of intense human activity, creating horizontal gradients in climate (and other) effects between polluted and unpolluted regions. Certain aerosol species absorb sunlight within the atmosphere, but deplete it at the surface, creating vertically opposing radiative effects between the surface and atmosphere.
My current research centers on two crucial questions:
How does the spatial pattern of anthropogenic aerosols' forcing (and, more fundamentally, where aerosols are emitted) influence their regional and global climate impacts?
What unique mitigation challenges, societal impacts, and policy opportunities does the heterogeneous, short-lived nature of aerosol forcing create?
Aug 2017 Competing Atmosphere and Surface- Driven Impacts of Absorbing Aerosols on the East Asian Summertime Climate
Apr 2012 Tropical Troposphere-Only Responses to Absorbing Aerosols
July 2010 Two Opposing Effects of Absorbing Aerosols on Global-Mean Precipitation
Aug. 2018 Divergent Global-Scale Temperature Effects from Identical Aerosols Emitted in Different Regions
Aug. 2018 Global and Arctic Climate Sensitivity Enhanced by Changes in North Pacific Heat Flux
Nov. 2017 Navigating the Flood of Information: Evaluating and Integrating Climate Science into Groundwater Planning in California
Competing Atmospheric- and Surface-Driven Impacts of Absorbing Aerosols
on East Asian Summertime Climate
Persad, G. G., D. J., Paynter, Y. Ming, and V. Ramaswamy. J. Climate https://doi.org/10.1175/JCLI-D-16-0860.1
Figure 1. Absorbing aerosols (like soot) in East Asia affect climate by both absorbing sunlight in the atmosphere and depleting it at the surface---two mechanisms that we decompose for the first time using a new suite of climate model simulations.
Winner of an American Meteorological Society Graduate Student Presentation Award
East Asia has some of the world’s highest atmospheric concentrations of shortwave-absorbing black carbon aerosol, peaking during the summer monsoon months. The impact of this “absorbing aerosol” on regional climate manifests through its ability both to increase shortwave radiation absorbed in the atmosphere and to decrease shortwave radiation reaching the surface. However, no study to date has decomposed the separate, combined, and competing impacts of absorbing aerosol’s atmospheric and surface effects on East Asian summertime climate – an issue of particular importance for predicting how regional climate will respond to projected changes in aerosol emissions. The goal of this work is to fill this important gap by (1) conducting a new set of idealized forcing simulations that cleanly decompose absorbing aerosol’s regional atmospheric and surface effects and (2) developing physical theory for how these effects influence East Asian summertime climate both in isolation and in tandem.
Surface shortwave reductions caused by absorbing aerosols weaken the East Asian Summer Monsoon (EASM) by reducing land-sea temperature contrast, while the atmospheric heating they induce strengthens the EASM by invigorating convection.
Absorbing aerosols’ surface energy effects dominate their atmospheric effects; even purely absorbing aerosol weakens the EASM.
Although both absorbing and scattering aerosols weaken the EASM, absorbing aerosols do so by less than purely scattering ones due to the compensating effects of absorbing aerosols’ atmospheric heating.
The East Asian summer monsoon and the broader Asian monsoon complex provide half the world’s population with the majority of their water resources. Understanding anthropogenic factors influencing their strength is, thus, vital for water security. This work also provides new, detailed physical theory for how regional climate responds to absorbing aerosols – pollutants whose emissions rate and global distribution is expected to shift substantially in the coming decades.
Spatially Similar Surface Energy Flux Perturbations due to Greenhouse Gases and Aerosols
Persad, G. G., Y. Ming, and V. Ramaswamy. Submitted.
Figure 1. Spatial patterns of the surface heat flux change due to present-day greenhouse gases (c) and aerosols (d) are more well correlated (R=-0.6) than the spatial patterns of their respective top-of-atmosphere radiative forcings (a, b; R=-0.4), suggesting fast atmosphere and land processes alone can efficiently homogenize the patterns of response to differing initial forcing patterns.
Awarded an American Geophysical Union 2014 Outstanding Student Paper Award (Atmospheric Sciences)
This research addresses a significant question posed by the work of Shang-Ping Xie and collaborators in their 2013 Nature Geoscience paper: to what extent do similarities in the spatial distribution of the climate response to aerosols and greenhouse gases manifests on fast, policy-relevant timescales? The spatial structure in the climate response to greenhouse gases and aerosols is one of the key “fingerprints” used in detection and attribution of human signals in climate phenomena. Our understanding of regional climate change is similarly impacted by constraining the spatial pattern of response to different anthropogenic climate forcers, like greenhouse gases and aerosols.
Our work identifies that even when only incorporating atmosphere and land mechanisms that operate on month-long timescales, the spatial structure of the climate response to greenhouse gases and aerosols (as manifested in the perturbation to the surface energy flux) is substantially similar, though of opposite sign (R=-0.57). The landmasses rapidly heat in response to greenhouse gases and cool in response to aerosols. This acts as a geographically fixed perturbation to the extratropical circulation, which produces patterns of surface energy change that are relatively insensitive to the structure of the initial forcing.
The Role of Aerosol Absorption in Driving Clear-Sky Solar Dimming over East Asia
Persad, G. G., Y. Ming, and V. Ramaswamy, 2014: The Role of Aerosol Absorption in Driving Solar Dimming over East Asia. J. Geophys. Res. Atmos., 119, 410–20. doi: 10.1002/2014JD021577.
Figure 1. The clear-sky surface solar radiation (SSR) and atmospheric absorption anomalies over East Asia are shown for AM3 and AM2.1 with their linear trend values. Absorption contributes strongly to the overall reduction in surface solar radiation in both models.
Awarded an American Geophysical Union 2013 Outstanding Student Paper Award (Atmospheric Sciences)
The amount of solar radiation reaching the surface is a major driver of the surface energy balance and influences regional circulation and precipitation. Over East Asia, observations show a significant reduction in clear-sky surface solar radiation since the 1960s, colloquially referred to as “dimming,” primarily caused by large regional increases in anthropogenic aerosol emissions. The impact that this dimming has on regional climate can be expected to depend on its relative contribution from aerosol absorption versus aerosol scattering. However, little work has been done to quantify the role of aerosol absorption in East Asian dimming or to analyze the aerosol mechanisms responsible.
This work constitutes a novel mechanistic investigation of the drivers of East Asian clear-sky dimming that sheds light both on the nature of the observed dimming trends and on the behavior of the aerosol formulations in GFDL’s models. We analyze the relative contribution to East Asia’s clear-sky dimming trend from aerosol absorption versus aerosol scattering using two generations of GFDL’s atmospheric general circulation model, AM2.1 and AM3, which have disparate aerosol formulations. We also use a standalone version of the models’ radiative transfer calculation to quantify the impact of various aerosol characteristics on the surface solar radiation and atmospheric shortwave absorption.
Our key findings are:
GFDL’s AM2.1 and AM3 models simulate large reductions in clear-sky surface solar radiation over East Asia since the 1960s that are attributable to increased anthropogenic aerosol emissions and are comparable to observations.
Atmospheric absorption of solar radiation by black carbon aerosol is responsible for as much as half of the reduction in clear-sky surface solar radiation (colloquially known as “dimming”) over East Asia since the 1960s.
The difference in the mixing state and the column burden of sulfate and black carbon between AM3 and AM2.1 have compensating effects on the aerosol absorption over East Asia, resulting in similar absorption and dimming despite significantly different aerosol formulations.
Our results demonstrate that absorption can drive a significant portion of dimming over East Asia and that it is thus vital to understand the mechanisms controlling absorption in different models. These findings strongly support NOAA’s mission to understand and predict changes in climate and illuminate a previously under-analyzed aspect of the regional anthropogenic climate perturbation in East Asia.
Tropical Troposphere-Only Responses to Absorbing Aerosols
Persad, G. G., Y. Ming, and V. Ramaswamy, 2012: Tropical Troposphere-Only Responses to Absorbing Aerosols. J. Climate, 25, 2471-2480. doi: 10.1175/JCLI-D-11-00122.1
Figure 1. Radiative Flux Perturbation (RFP) values for a globally uniform layer of black carbon in the mid-troposphere are shown. Positive values correspond to a warming to the system, while negative values correspond to a cooling to the system. The South Pacific Subsidence Region (dashed box) is characterized by positive values associated with low cloud loss that is primarily due to black carbon “cloud burning.” The Western Pacific Warm Pool convective region (dotted box) is characterized by negative values associated with an increase in middle cloud that is primarily dependent on the model’s cumulus cloud parameterization.
Citation from Impact of Anthropogenic Absorbing Aerosols on Clouds and Precipitation: A Review of Recent Progresses (2012, Atmospheric Research):
"This study along with a few others advances current understanding of the effect of absorbing aerosol on low clouds, from considering the effect of aerosol heating on thermodynamical profile alone to including the dynamical adjustment on the height of PBL along with thermodynamical effects associated with large-scale vertical motion."
An ongoing challenge in quantifying aerosols’ impact on the climate is determining an optimal way of calculating aerosols’ radiative forcing. For absorbing aerosols, in particular, studies have shown that a forcing calculation that does not include the tropospheric response to absorbing aerosol (instantaneous forcing) is a poor proxy for the change in global mean surface temperature caused by the aerosol. Radiative flux perturbation, a forcing calculation that does include the tropospheric response, is a much better temperature proxy, but runs the risk of incorporating nonphysical processes. In this paper, we investigate the physical mechanisms behind the tropospheric response to absorbing aerosols in GFDL’s AM2.1 Atmospheric General Circulation Model in order to shed light on whether it may be robustly included in the calculation of absorbing aerosols’ radiative forcing.
We conduct a series of highly idealized experiments in which AM2.1 is run with a globally uniform layer of black carbon, and compare the top-of-atmosphere fluxes with those of a pre-industrial control run after the troposphere has been allowed to respond to the aerosol. By analyzing where differences exist between these fluxes and those that occur before the troposphere responds, we can better understand what tropospheric processes would be incorporated into a radiative flux perturbation calculation and whether they are physically robust.
The tropospheric response manifests primarily in cloud changes and differs based on the large-scale environment. Over regions where the large-scale air is subsiding (sinking), absorbing aerosol heating near the boundary layer top causes a substantial reduction in low cloud amount, both thermodynamically (by decreasing relative humidity) and dynamically (by lowering cloud top). Over convective regions, on the other hand, heating above the boundary layer hinders the vertical development of deep cumulus clouds, causing horizontal detrainment of cloudy air and stronger cloud reflection. However, much of the convective cloud change appears to be dependent on the model’s cumulus parameterization. This mechanistic understanding of the tropospheric response allows us to better characterize the physical robustness of forcing values derived from the radiative flux perturbation over different regions.
Regions that are generally characterized by subsiding air on the large-scale (e.g. the Eastern Subtropical Pacific) experience a reduction of low cloud in the presence of absorbing aerosol that is primarily attributable to “cloud burning” dictated by a negative correlation between atmospheric temperature and relative humidity.
Regions of deep convection (e.g. the Western Pacific Warm Pool) experience an increase in mid-level cloud amount, which may be due to the horizontal detrainment of cloudy air forced by absorbing aerosol heating in the mid-troposphere, but also appears to be dependent on the model’s cumulus parameterization.
The tropospheric response over subsiding regions appears to be largely the result of first-order physics that is expected to be transferable across different models, while the response over convective regions appears to result from a mixture of physical and nonphysical mechanisms. Care should, therefore, be taken when including the tropospheric response to absorbing aerosols in calculations of its radiative forcing over convective regions.
For the majority of signals, however, radiative flux perturbation can be expected to be a physically robust measure of absorbing aerosol radiative forcing for AM2.1.
Two Opposing Effects of Absorbing Aerosols on Global-Mean Precipitation
Ming, Y., V. Ramaswamy, and G. Persad, 2010: Two Opposing Effects of Absorbing Aerosols on Global-mean Precipitation. Geophys. Res. Lett., 37, L13701. doi: 10.1029/2010GL042895.
Figure 1. Scatter plot of the relative change in precipitation (%) due to black carbon at different levels in the atmosphere. The x-axis shows precipitation changes directly simulated by a global climate model. The y-axis shows the same change as predicted by a new thermodynamic theory.
Absorbing aerosols affect global‐mean precipitation primarily in two ways. They give rise to stronger shortwave atmospheric heating, which acts to suppress precipitation. Depending on the top‐of‐the‐atmosphere radiative flux change, they can also warm up the surface with a tendency to increase precipitation.
In the work, we present a theoretical framework that takes into account both effects, and apply it to analyze the hydrological responses to increased black carbon burden simulated with a general circulation model. We find that the damping effect of atmospheric heating can outweigh the enhancing effect of surface warming, resulting in a net decrease in precipitation.