HR: 11:05h
AN: B32A-04 INVITED     [Abstracts]
TI: Changes in Surface Radiation and Associated Effects on Climate
AU: * Wild, M
EM: martin.wild@env.ethz.ch
AF: Institute for Atmospheric and Climate Science ETH, Swiss Federal Institute of Technology, Universit„tsstr. 16, Zurich, 8092 Switzerland
AB: Variations in solar and thermal radiation incident at the Earth's surface profoundly affect the human environment. Increasing greenhouse gas concentrations in the atmosphere enhance the greenhouse effect at the surface manifest as a gradual increase of thermal radiation from the atmosphere down to the surface. Recent analyses of observational records confirm this expectation. Yet not only the thermal fluxes, but also solar fluxes are currently undergoing major changes. A decline of solar radiation at the land surface has become apparent in many observational records prior to 1990, a phenomenon now popularly known as "global dimming." Dimming of solar radiation was probably caused by increased air pollution, and associated levels of aerosol, as well as increasing cloud amounts. The dimming of solar radiation may also be the cause of reduced evaporation between 1960 and 1990 measured worldwide with evaporation pans, documenting the potential influence of solar dimming on the global hydrological cycle. Newly available surface observations from 1990 to present show, however, that the solar dimming did not persist into the 1990s. Instead, a widespread brightening has been observed since the mid-1980s. The brightening is consistent with reduction in cloudiness and an atmosphere which has recently become more transparent for sunlight. This has been favored by reduced aerosol loadings related to enhanced air pollution control and the economic breakdown in formerly communist countries in the late 1980s. The transition towards more sunlight at the Earth's surface since the late 1980s after decades of decline may have significantly affected surface climate and the global hydrological cycle. While from the 1960s to the 1980s the dimming of sunlight might have been able to counterbalance and mask to some extent the increasing greenhouse effect, leading to only moderate global temperature changes observed in this period, this has no longer been the case after the mid 1980s. The recent absence of solar dimming may have allowed the greenhouse effect to finally develop at full size, as seen in accelerated temperature increases in the 1990s. The rate of temperature change at land surfaces has increased by an order of magnitude in the period 1985-2002 where solar dimming was absent, compared to the period with significant dimming (1958-1985). In the 1990s, more energy may therefore have been available at the surface compared to earlier decades, favoring, for example, an accelerated retreat of worldwide glaciers, or higher evaporation in areas of unlimited water supply and associated reduced soil moisture tendencies, with associated implications for the global hydrological cycle. Reference: Wild, M., Gilgen, H., Roesch, A., Ohmura, A., Long, C., Dutton, E., Forgan, B., Kallis, A., Russak, V., Tsvetkov, A., 2005: From dimming to brightening: Decadal changes in solar radiation at the Earth's surface. Science , 308, 847-850.
DE: 0305 Aerosols and particles (0345, 4801, 4906)
DE: 0360 Radiation: transmission and scattering
DE: 1620 Climate dynamics (0429, 3309)
DE: 1626 Global climate models (3337, 4928)
DE: 1631 Land/atmosphere interactions (1218, 1843, 3322)
SC: Biogeosciences [B]
MN: Fall Meeting 2005