Atmospheric Aerosols and Climate Change:
A Component of the NOAA OGP Aerosol-Climate Program
Presented by V. Ramaswamy, GFDL September 14, 1995
Funding from the Office of Global Programs within NOAA (available during FY93 and FY95 only) has been used at the Geophysical Fluid Dynamics Laboratory to improve the quantitative understanding of the effects due to aerosols on the disposition of radiative energy within the surface-atmosphere system and to evaluate their impact on Earth's climate and climate change.
On the Radiative Transfer front, the GFDL `benchmark' line-by-line, doubling-adding algorithm (Ramaswamy and Freidenreich, 1991) has been enhanced to account for details of the molecular absorption as well as scattering by particles of any size and composition, and for carrying out calculations at any solar zenith angle. As an example of the applications, computations carried out in the ultra-violet and visible spectrum reveal the sensitivity to aerosol phase function and their vertical distribution. Further, these calculations have enabled the comparison, testing and development of aerosol radiative paramaterizations for general circulation model work.
On the General Circulation Modeling front, the Laboratory's Climate GCM, which has hitherto been used solely for estimating the climatic effects of greenhouse gases, has been modified in order to model the albedo changes initiated by the presence of atmospheric aerosols. Investigation into the characteristics of the climate responses to the albedo forcing of the aerosols has begun and has also involved a comparison with the corresponding results obtained for greenhouse gas increases.
With a view to familiarizing the reader on the direction and scope of research performed, we describe below briefly the highlights of the most recent research, particularly the developments occurring during the past year.
a) Aerosol-related solar biases and their effects on land-surface processes:
Chen and Ramaswamy (1995a) have investigated the effect of biases in the solar radiation absorbed by the atmosphere and surface upon the land-surface processes. This is a central problem to be addressed amidst the increasing evidence that aerosols and, perhaps, clouds cause a larger-than-expected decrease in the solar radiation reaching the surface. A GCM study, with fixed cloud distributions and climatological sea-surface temperatures, indicates that biases in solar radiation lead to alterations in the vertical velocity and precipitation which, in turn affect the components of the surface energy and moisture budgets (e.g., latent heat release, soil moisture). The results of the study imply that the interactions between the atmosphere and land-surface processes are significantly influenced by the presence of absorbers and scatterers in the atmosphere. Since natural and anthropogenic aerosols have these characteristics, the aerosol radiative effects are thus linked directly to the maintenance of climate.
b) Albedo perturbations and global climate change:
Numerical experiments have been undertaken using the GFDL Climate GCM. These experiments have been designed to investigate the response of the atmosphere-mixed layer ocean model, to global (Chen and Ramaswamy, 1995b and c) and regionally imposed albedo perturbations. The regional albedo perturbations mimic the effects due to the anthropogenic sulfate aerosols in the midlatitude northern hemisphere. These responses are compared with the responses to greenhouse gas increases which is essential to gauging the relative influences due to these two different anthropogenic species during the Industrial age (Ramaswamy et al., 1995). The results show that i) the ratio of global surface temperature response to forcing (i.e. climate sensitivity) is essentially invariant, whether the radiative perturbation imposed is global or regional; further, the climate sensitivity for albedo perturbations is virtually similar to that for CO2 increases. This is a topic of crucial importance in climate change considerations (IPCC 1994) and relate to the question of whether radiative forcing is a meaningful way to evaluate climate impacts.
c) Top-of-the-atmosphere clear-sky shortwave flux:
Using the high-spectral resolution radiative transfer model, the computed clear-sky shortwave reflected fluxes at the top-of-the-atmosphere have been compared with the Earth Radiation Budget satellite observations over the Tropical Western and Central Pacific. The comparisons reveal that, in the presence of only the molecular constituents, the computed and observed fluxes do not agree anywhere over the domain considered. The underestimate in the computed fluxes strongly suggests the presence of aerosols and also indicates their significant role in determining the albedo of clear skies and, thus, the clear-sky solar energy absorbed by the surface-atmosphere system. This result emphasizes that even small aerosol optical depths, as estimated for the present-day column and also as estimated for the anthropogenic perturbation over the past century, are capable of resulting in significant radiative flux changes within the atmosphere-surface system.
d) Tropospheric aerosols and lower stratosphere cooling:
While anthropogenic aerosol effects on the climate of the troposphere is the subject of close scrutiny currently, especially their offset of the warming due to the increases in greenhouse gases, an investigation (Ramaswamy and Bowen, 1994) has found that aerosols also exert, surprisingly, a cooling effect in the lower stratosphere which could be a significant fraction of the cooling induced by the greenhouse gases alone (WMO 1994). It is worth noting that the currently known changes in the concentrations of species (gases or aerosols) suggest that all would act to cool the lower stratosphere. A secular global cooling, which is indicated by observations as well, has implications for the heterogenous chemical processes associated with ozone in the stratosphere (WMO 1994). The aerosol-related effect is estimated to be less than the radiative effects expected from a loss of stratospheric ozone.
Chen, C-T., and V. Ramaswamy, A parameterization of the solar radiative characteristics of low clouds and studies with a general circulation model, J. Geophy. Res., 100, 11611-11621, 1995a.
Chen, C-T., and V. Ramaswamy, Sensitivity of simulated global climate to perturbations in low cloud microphysical properties. Part I: Globally uniform perturbations. Accepted for publication in J. of Climate, 1995b.
Chen, C-T., and V. Ramaswamy, Sensitivity of simulated global climate to perturbations in low cloud microphysical properties. Part II: Spatially localized perturbations. Submitted to J. of Climate, 1995c.
Ramaswamy, V., and M. M. Bowen, Effect of changes in radiatively active species upon the lower stratospheric temperatures, J. Geophys. Res., 99, 18909-18921, 1994.
Ramaswamy, V., and S. M. Freidenreich, Solar radiative line-by-line computations of water vapor absorption and water cloud extinction in inhomogeneous atmospheres, J. Geophys. Res, 96, 9139-9152, 1991.
Ramaswamy, V. et al., What are the observed and anticipated meteorological and climatic responses to aerosol forcing? In: Aerosol Forcings of Climate, R.J. Charlson and J. Heintznberg Eds., pp384-399. Wiley and Sons, Ltd., Chicester U.K., 1995.
Shine, K. P., K. Labitzke, V. Ramaswamy, P. Simon, S. Solomon, and W-C. Wang, Radiative Forcing and Temperature Trends, In WMO/UNEP Scientific Assessment of Stratospheric Ozone 1994, Geneva, Chapter 8, 1995.
Shine, K. P., Y. Fouquart, V. Ramaswamy, S. Solomon, and J. Srinivasan, Radiative Forcing, Intergovernmental Panel on Climate Change 1994, Cambridge University Press, 163-203, 1995.