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Radiative Forcing and Climate Response to Projected 21St Century Aerosol Decreases

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Radiative Forcing and Climate Response to Projected 21St Century Aerosol Decreases Atmos. Chem. Phys., 15, 12681–12703, 2015 www.atmos-chem-phys.net/15/12681/2015/ doi:10.5194/acp-15-12681-2015 © Author(s) 2015. CC Attribution 3.0 License. Radiative forcing and climate response to projected 21st century aerosol decreases D. M. Westervelt1, L. W. Horowitz2, V. Naik3, J.-C. Golaz2, and D. L. Mauzerall1,4 1Program in Science, Technology, and Environmental Policy, Woodrow Wilson School of Public and International Affairs, Princeton University, Princeton, NJ, USA 2Geophysical Fluid Dynamics Laboratory, National Oceanic and Atmospheric Administration, Princeton, NJ, USA 3UCAR/NOAA Geophysical Fluid Dynamics Laboratory, Princeton, NJ, USA 4Department of Civil and Environmental Engineering, Princeton University, Princeton, NJ, USA Correspondence to: D. L. Mauzerall ([email protected]) and D. M. Westervelt ([email protected]) Received: 25 February 2015 – Published in Atmos. Chem. Phys. Discuss.: 27 March 2015 Revised: 4 November 2015 – Accepted: 6 November 2015 – Published: 16 November 2015 Abstract. It is widely expected that global emissions of RCP8.5 is about 0.5 K, with similar magnitude decreases in atmospheric aerosols and their precursors will decrease other climate response parameters as well. Regionally and strongly throughout the remainder of the 21st century, due to locally, climate impacts can be much larger than the global emission reduction policies enacted to protect human health. mean, with a 2.1 K warming projected over China, Japan, For instance, global emissions of aerosols and their precur- and Korea due to the reduced aerosol emissions in RCP8.5, sors are projected to decrease by as much as 80 % by the as well as nearly a 0.2 mm day−1 precipitation increase, a year 2100, according to the four Representative Concen- 7 g m−2 LWP decrease, and a 2 µm increase in cloud droplet tration Pathway (RCP) scenarios. The removal of aerosols effective radius. Future aerosol decreases could be responsi- will cause unintended climate consequences, including an ble for 30–40 % of total climate warming (or 10–20 % with unmasking of global warming from long-lived greenhouse weaker aerosol forcing) by 2100 in East Asia, even under the gases. We use the Geophysical Fluid Dynamics Laboratory high greenhouse gas emissions scenario (RCP8.5). The ex- Coupled Climate Model version 3 (GFDL CM3) to simu- pected unmasking of global warming caused by aerosol re- late future climate over the 21st century with and without ductions will require more aggressive greenhouse gas mitiga- the aerosol emission changes projected by each of the RCPs tion policies than anticipated in order to meet desired climate in order to isolate the radiative forcing and climate response targets. resulting from the aerosol reductions. We find that the pro- jected global radiative forcing and climate response due to aerosol decreases do not vary significantly across the four RCPs by 2100, although there is some mid-century varia- 1 Introduction tion, especially in cloud droplet effective radius, that closely follows the RCP emissions and energy consumption pro- The climate effects of atmospheric aerosols represent one of jections. Up to 1 W m−2 of radiative forcing may be un- the most uncertain aspects of current and future climate forc- masked globally from 2005 to 2100 due to reductions in ing and response estimates (Myhre et al., 2013). Whereas aerosol and precursor emissions, leading to average global the greenhouse gas warming influences on climate are rel- temperature increases up to 1 K and global precipitation rate atively well understood, significant questions remain regard- increases up to 0.09 mm day−1. However, when using a ing the magnitude, and in some cases, even the sign (cooling version of CM3 with reduced present-day aerosol radiative or warming), of aerosol-climate interactions. Aerosol radia- forcing (−1.0 W m−2), the global temperature increase for tive forcing of climate can be split into two categories: the direct effect, in which atmospheric aerosols directly scatter Published by Copernicus Publications on behalf of the European Geosciences Union. 12682 D. M. Westervelt et al.: Radiative forcing and climate response or absorb incoming solar radiation; and the indirect effect, in 2010; Shindell et al., 2012a). Black carbon (BC) is an aerosol which aerosols modify cloud properties which in turn affect species that is a strong absorber of incoming solar radiation the radiation budget. For a fixed amount of cloud water, a in the troposphere and thus a climate-warming agent. It is more polluted air mass will have smaller and more numerous also a major contributor to PM2:5 and has an adverse effect cloud droplets, leading to a larger surface area and a brighter on human health (Bond et al., 2013). Despite a clear human cloud (Twomey, 1974). This is known as the cloud albedo health benefit, questions remain whether its reduction will effect. In addition, more aerosol pollution may also result be an effective strategy for avoiding additional warming, due in a longer cloud lifetime due to the tendency of smaller to its frequent co-emission with two strong cooling species, droplets to remain suspended in the atmosphere longer (Al- OC and sulfate (Chen et al., 2010; Reddington et al., 2013). brecht, 1989), although this cloud lifetime effect is not as Some sources of BC are not large sources of species that ex- well understood. On a global average basis, both the direct ert a negative radiative forcing (e.g. diesel fuel combustion), and indirect effects tend to exert a net negative radiative so it has been suggested that these should be preferentially forcing on present-day climate, opposing the positive forcing targeted for control first (Kopp and Mauzerall, 2010; Shin- from greenhouse gases, with the total aerosol effective radia- dell et al., 2012a). tive forcing estimated to be −0.9 W m−2 (uncertainty range Modeling studies have suggested that aerosols also have −1.9 to −0.1 W m−2) (Myhre et al., 2013). This aerosol forc- strong impacts on precipitation, cloud cover, cloud droplet ing has likely offset a significant portion of present-day CO2 size and number, atmospheric circulation, and other climate and other greenhouse gas-induced climate forcing and sub- parameters (Lohmann and Feichter, 2005; Ming and Ra- sequent global warming. Likewise, any changes in future maswamy, 2009, 2011; Ming et al., 2011; Ramanathan et al., anthropogenic aerosols will have implications for the over- 2001; Rosenfeld et al., 2008; Stevens and Feingold, 2009). all net impact on climate. Here we evaluate the changes in An increase in aerosol emissions tends to decrease local to global and regional aerosol burden, climate forcing, and cli- regional precipitation rates (warm rain), through macrophys- mate response due to future decreases in aerosol and pre- ical and microphysical processes: (1) less incoming solar ra- cursor emissions as projected by all of the Representative diation penetrates the troposphere and reaches the surface, Concentration Pathways (RCPs). We build upon the work of resulting in less evaporation (Ramanathan et al., 2001) and Levy et al. (2013) to contrast the global and regional climate (2) smaller and more abundant aerosols lead to smaller cloud response and quantify the expected unmasking of warming droplets, which are less likely to convert to rain drops via due to future aerosol reductions across all of the RCPs. coalescence on the local and regional scale (Radke et al., Emissions of aerosols and their precursors have increased 1989; Rosenfeld, 2000). An exception to (1) is BC or other dramatically since the preindustrial era, due to increasing in- absorbing aerosols, which may have opposing effects on pre- dustrialization and global population. In 2012, air pollution, cipitation through heating the atmosphere (causing stabiliza- mostly in the form of atmospheric aerosols, was responsible tion and reduction in precipitation) and warming the surface for 7 million deaths (3.7 million from ambient air pollution, via redistributed thermal energy (causing an enhancement of 3.3 million from indoor) worldwide (WHO, 2014). Due to ef- precipitation) (Ming et al., 2010). However, the net effect of forts to reduce this enormous human health impact, aerosol increasing aerosol concentrations tends to be suppression of and precursor emissions are expected to decline worldwide precipitation. Thus, the expected reduction of aerosol con- over the next several decades as governments enact and en- centrations should increase rainfall rates (warm rain) glob- force stricter emission control policies. Emissions of sulfur ally. Since aerosols serve as seeds for virtually all liquid dioxide (SO2, a precursor to sulfate aerosol) have already de- cloud formation in the atmosphere, decreases in aerosols may clined about 50 % in North America and western Europe and also be expected to affect cloud cover, cloud liquid water passed their peak in 2005 in China (Klimont et al., 2013). In path, and effective cloud droplet radius. some developing countries, emissions are still rising but are To estimate future aerosol emissions and burden, radia- expected to begin to decline within the next few decades, as a tive forcing, and climate response, we must utilize future consequence of increasing affluence and more environmental projections or scenarios. The current state-of-the-art emis- regulation (van Vuuren et al., 2011a). sions scenarios for global climate modeling are the Repre- While clearly beneficial for human health, declining sentative Concentration Pathways (RCPs) (Lamarque et al., aerosol emissions will result in the unintended consequence 2011; Masui et al., 2011; Riahi et al., 2011; van Ruijven et of unmasking additional climate warming, due to the reduc- al., 2008; Thomson et al., 2011; van Vuuren et al., 2012, tion of the cooling effects from anthropogenic aerosols such 2011a, b). The RCPs are different from previously devel- as sulfate and organic carbon (OC). Thus, careful policy im- oped scenarios in that they are initialized with radiative forc- plementation is necessary in order to maximize reduction of ing beginning and endpoints (2005–2100).
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