The Causes Of Dimming And Brightening

Dimming and brightening are related to aerosol loading of the atmosphere and the influences of aerosols on atmospheric transmittance. The influence of natural aerosols from volcanic eruptions can be seen in the sharp declines in Eg# for the year or two following the eruptions of El Chichon in 1983 and Pina-tubo in 1991 [54]. Stanhill and Cohen [23] reviewed the possible causes for dimming in the context of a simplified expression:

where Eg# is estimated from the extraterrestrial irradiance at the top of the atmosphere, Eo, modified by a chain of five transmissivities t which quantify the solar scattering and absorbing properties of the different components of the atmosphere. These include tr, representing Rayleigh scattering; tg, permanent gas absorption; tw, absorption by water vapour; and ta and tc, the absorption and scattering by the aerosols and cloud components, respectively. The only factor whose known changes and influence on global radiation are large enough to cause changes of the magnitude observed is aerosol loading. Aerosol influences on radiation include direct effects, that is, absorption, reflection and scattering of radiation by aerosols, and indirect effects, referring to aerosol mediated changes in cloud albedo (the Twomey effect), rain suppression (the Albrecht effect), and cloud lifetime. The large changes in Eg# can be pinned to some extent on anthropogenic pollution, as suggested by the large dimming in urban mega-cities and the industrialised zone of the Northern Hemisphere. The connection between dimming and aerosols has been clearly demonstrated (e.g. [55]), and known changes in aerosol loading of the atmosphere are well correlated with the transition from dimming to brightening in the 1980s [56].

Prior to the twenty-first century scientists studying aerosols had suspected that aerosol influences on climate were far larger than was being acknowledged and Satheesh and Ramanathan [57] demonstrated the large radiative forcing that can be caused by aerosols. As the evidence for worldwide dimming of a magnitude of several percent has mounted scientists who were studying aerosol influences have begun to implement the full extent of aerosol influences in models of earth's climate (e.g. [58,59]).

0 0

Post a comment