Inverse Calculation of Aerosol Forcing
Change in global mean surface temperature (GMST) is driven by total forcing, the major components of which are thought to be increased concentrations of greenhouse gases and aerosols. While greenhouse gas forcing is relatively well known, aerosol forcing remains quite uncertain, with central value (1750-2005) of -1.2 W m-2 [range -0.6 to -2.4 W m-2, 5-95%; IPCC AR4, 2007; negative forcing denotes cooling influence]. Forward calculations of aerosol forcing are based on modeled aerosol loading and properties and/or from observed loadings of aerosols and correlations of aerosol loadings and cloud albedo. Alternatively aerosol forcing can be inferred by inverse calculations based on assumed climate sensitivity, measured change in GMST, and forcings other than by aerosols. Here I rely on recent empirical determination of Earth's equilibrium climate sensitivity (Schwartz, 2007) as the quotient of climate system time constant τ and effective heat capacity C. τ is determined from autocorrelation of GMST over 1880-2004 as 5 ± 1 yr (uncertainties are 1-sigma estimates). Effective heat capacity is determined from change in global ocean heat content with increasing GMST over 1955- 2000, 14 ± 6 W yr m-2 K-1, equivalent to 110 m of ocean water; other sinks raise the effective planetary heat capacity C to 17 ± 7 W yr m-2 K-1. The resultant equilibrium climate sensitivity, 0.30 ± 0.14 K/(W m-2), corresponds to an equilibrium temperature increase for doubled CO2 of 1.1 ± 0.5 K, well lower than current IPCC estimates. The short time constant implies that GMST is in near equilibrium with applied forcings and hence that net climate forcing over the twentieth century can be obtained from the observed temperature increase over this period, 0.57 ± 0.08 K, as 1.9 ± 0.9 W m-2. For radiative forcing by incremental greenhouse gases, 2.2 ± 0.3 W m-2, other forcings over the twentieth century, mainly by incremental tropospheric aerosols, are inferred to be slight, -0.3 ± 1.0 W m-2. While the central value of this estimate is quite small, the range encompasses both fairly substantial negative forcing as well as possible positive forcing. These considerations emphasize the need for improved forward calculations of aerosol forcing to constrain estimates of climate sensitivity and to permit evaluation of climate models from their skill in reproducing temperature change over the twentieth century. --- Schwartz S. E. Heat capacity, time constant, and sensitivity of Earth's climate system. J. Geophys. Res. in press, 2007; http://www.ecd.bnl.gov/steve/pubs/HeatCapacity.pdf
Global and Regional Climate Changes due to Soot Aerosols
Soot from burning of fossil fuels, biomass and biofuels, is emerging as a major factor in global and regional climate changes. Globally, its positive forcing may be second only to carbon dioxide. Regionally, however, particularly over Asia, Africa and the Arctic, its effects may be just as important as that due to CO2. It is estimated to contribute to multi- decadal or longer long-term trends in surface and atmospheric temperatures, monsoon circulation and rainfall patterns, and early melting of snow packs and sea ice. Our understanding of the magnitude of the soot emission, its atmospheric concentrations, the radiative forcing both directly and through its interactions with clouds, sea ice and snow surfaces are all at a very preliminary stage. One of the most challenging aspects of the soot-climate issue is that, because of the large spatial and temporal variations of soot concentrations in the atmosphere, it can alter the sea surface temperature gradient (between northern and southern hemisphere) which is one of the important regulators of the inter-tropical-convergence-zone and the monsoon circulation and thus the land rainfall. I will summarize our current understanding of the soot-climate interactions and highlight major outstanding issues relating to global and regional climate, including the sensitivity of the Asian monsoon system, the tropical and extra tropical glaciers, and the regional precipitation patterns to large soot loading in the tropical regions. Finally, I will discuss reduction of soot emissions as a potential option for mitigating large climate changes.
Aerosol Optical Depth Tendencies Over the United States Over the Last Decade
A recent decadal study of aerosol optical depth (AOD) over the U.S. reveals a general nationwide decrease, but also geographic differences. These new data from NOAA's SURFRAD surface radiation budget network show high AOD in summer and low AOD in winter. The western stations have the lowest AOD and the eastern stations have the highest. Western stations also show a secondary springtime maximum caused by Asian dust. The abundance of aerosols in the east and their relatively small size is attributed to organic emissions, high humidity, and the predisposition for stagnant air in summer. These characteristics agree with previously published results, however, national and station decadal AOD tendencies show interesting features that may have climate change implications. Nationally, 500nm AOD decreased from 1997 through 2006 by about 0.02, which is similar to recent results reported for the oceans. However, all U.S. stations do not share this tendency. Results show that aerosols are decreasing in the eastern U.S., however, in the west a decrease is only observed at Desert Rock, Nev. Fort Peck, Mont. and Boulder, Colo. show a very slight increase over the past decade that is likely caused by an upsurge in wildfires from 2000 through 2006. When the four years with the most abundant wildfires are removed, decadal tendencies at Boulder and Fort Peck are steady. If climate change is causing drier conditions in the western U.S., then higher than normal numbers of forest fires are likely to occur, and the observed AOD increase should be considered part of the background. In that case, the additional blockage of sunlight by direct and indirect effects of the fire-induced aerosols would act to slightly offset the warming attributed to increasing greenhouse gases. If the increasing trend in western wildfires does not continue, then the recent AOD increase in the intermountain western U.S. can be considered temporary, and western AOD should be expected to remain steady, as suggested by the observed tendencies of the low-fire years. The only way to answer these and other questions related to aerosols and climate is to continue to build AOD times series into future decades so that tendencies can be determined with more certainty.
Aerosol Optical Depth, Climate Sensitivity and Global Warming
Recent satellite and ground-based observations suggest that the global average of the tropospheric aerosol optical depth has been decreasing during at least the last decade. Consequently, the observed global warming is the result of an increasing concentration of carbon dioxide and a decreasing concentration of tropospheric aerosols. The climate sensitivity parameter, which relates the top-of-the-atmosphere radiative forcing to the change of the global surface air temperature, is derived from analyses of satellite observations of the aerosol optical depth, changes in the carbon dioxide concentration, and the increase in the global temperature. Considering the last decade, when both the decreasing aerosol optical depth and the increasing carbon dioxide concentration have been causing warming, we deduce the climate sensitivity to be 0.4 (with uncertainty of 0.1)K/Wm-2. This value corresponds to a warming of about 1.6 (with uncertainty of 0.4) deg C due to doubling the amount of carbon dioxide from its pre-industrial level. The deduced value is close to the lower end of the IPCC 4AR assessment of 2 to 4.5 deg C (with 66% probability). It is also in agreement with the recent results (0.44 and 0.41 K/Wm-2) of experiments with cloud resolving models embedded within GCMs.
Asian Aerosols: A Geophysical Fluid Dynamics Laboratory general circulation model sensitivity study of model response to aerosol optical depth and aerosol absorption
Atmospheric absorption by black carbon (BC) aerosol heats the atmosphere while simultaneously cooling the surface and reducing latent and sensible heat fluxes from the land. Recent studies have shown that absorbing BC aerosol can have a large impact on regional climates, including modification of the hydrological cycle. However, significant uncertainties remain with regards to (a) the total amount of all aerosol species and (b) the amount of aerosol absorption. Here we present a GCM sensitivity study focusing on the influences due to total aerosol amount and aerosol absorption in the south and east Asian regions. Six experiments are conducted to test the equilibrium response of the GFDL AM2 GCM (under conditions of prescribed, observed sea surface temperatures) to (i) changes in aerosol absorption caused by changes in BC aerosol amount, and (ii) aerosol extinction optical depth increases corresponding to the year 1990 relative to a control case of 1950. In order to systematically explore the uncertainties in aerosol loading and absorption, the sensitivity experiments are classified into four regimes: low extinction optical depth, low absorption; low extinction optical depth, high absorption; high extinction optical depth, low absorption; and high extinction optical depth, high absorption. Changes in surface temperature and changes in the hydrological cycle are generally insignificant when lower aerosol extinction optical depths are considered. For higher extinction optical depths, the change in the modeled regional circulation relative to the control circulation over south and east Asia is affected by the amount of aerosol absorption and contrasts sharply to the regional circulation change associated with increasing only scattering aerosols. When increasing absorbing aerosols over the region, low-level convergence and increases in vertical velocity overcome the stabilizing effects of the absorbing aerosol and enhance the monsoonal circulation and precipitation rate in northwestern India relative to the control run. In contrast, the presence of a purely scattering aerosol weakens the monsoonal circulation relative to the control run and inhibits precipitation in this same region. This study has potential implications for aerosol reduction strategies that seek to mitigate air pollution concerns. At higher optical depths, if absorbing aerosol is present, reduction of scattering aerosol alone may have a lesser effect on precipitation changes, implying that reductions in black carbon aerosol should be undertaken at the same time as reductions in sulfate aerosol.
Coupled Climate Model Simulations to Bracket the Impacts of Increasing Asian Aerosols Emissions and Aggressive Future Clean Air Policies
We report ensemble simulations of the climatic impacts of changing anthropogenic aerosols (sulfate, organic and black carbon), which bracket two policy scenarios: increased emissions over China and India by a factor of three over current levels and a global reduction of aerosols by a factor of ten, using the NCAR-CCSM3 and NASA- GISS coupled ocean atmosphere models. Tripling the anthropogenic aerosols over China and India has a small cooling effect (about -0.12°C) on the global mean surface air temperature with a slight reduction in global mean precipitation by ~ -0.8%. On the other hand, global reduction of anthropogenic aerosols by a factor of ten would warm the global surface temperatures by 0.4 °C - 0.8 °C in less than 10 years after the reduction takes place as well as an increase in global precipitation by 3.0% - 3.3%. Comparisons of NCAR and NASA model simulations also suggest that the indirect effects of aerosols are about 1-2 times the direct effects of aerosols. Tripling Asian anthropogenic aerosols results in regional cooling and a reduction in precipitation primarily in Asia, with cooling (warming) also noted over the high latitudes of Northern (Southern) Hemisphere. Warming and increase in precipitation in the case of global reduction of aerosols are concentrated mainly over polluted land areas in both hemispheres. Tropical regions experience large changes in precipitation in both scenarios. We provide new insights into the climate model sensitivities of global mean temperatures and rainfall to aerosol forcing. Our results underscore the urgency of reducing greenhouse gas accumulation rates as the world reduces air pollution to improve human health and that potential increased Asian pollution, offsets only a small fraction of the warming by greenhouse gases. http://aerosols.lanl.gov
Anthropocene changes in desert area: Sensitivity to climate model predictions
Changes in desert area due to humans have important implications from a local, regional to global level. Here we focus on the latter in order to better understand estimated changes in desert dust aerosols and the associated iron deposition into oceans. Using 17 model simulations from the World Climate Research Programme's Coupled Model Intercomparison Project phase 3 multi-model dataset and the BIOME4 equilibrium vegetation model we estimate changes in desert dust source areas due to climate change and carbon dioxide fertilization. If we assume no carbon dioxide fertilization, the mean of the model predictions is that desert areas expand from the 1880s to the 2080s, due to increased aridity. If we allow for carbon dioxide fertilization, the desert areas become smaller. Thus better understanding carbon dioxide fertilization is important for predicting desert response to climate. There is substantial spread in the model simulation predictions for regional and global averages.