The Indian Ocean Experiment and the Asian brown cloud

Abstract

The Indian Ocean Experiment (INDOEX) was sponsored by research agencies within Europe, India and USA, and was mainly concerned with the haze over south Asia and the adjacent Indian Ocean. It excluded other equally or even more polluted areas in Asia. The Asian Brown Cloud is a followon international research project that includes all of Asia. The brown haze is a worldwide phenomenon and should not be assumed to be just an Indian or an Asian problem. UNEP had commissioned a panel in 2001 to provide an early assessment of the societal implications of INDOEX findings. The panel published its report in August 2002 which was accompanied by a press release prepared by UNEP. This article clarifies the scientific basis of the brown haze in response to a recent article by Srinivasan and Gadgil (hereafter referred to as SG). The south Asian brown haze covers most of the Arabian Sea, Bay of Bengal and the south Asian region. It occurs every year, and extends from about November to April and possibly longer. The black carbon and other species in the haze reduce the average radiative heating of the ocean by as much as 10% and enhance the atmospheric solar radiative heating by 50 to 100%. These findings are at variance with SG's perceptions that the haze occurs only during January to March, and that the aerosol forcing used by UNEP was unrealistically large because it used 1999 values and ignored IR effects of aerosols. INDOEX and UNEP did not rely just on 1999 values, but used data for 1996 to 1999, and also accounted for the compensating IR effects. The long duration of the haze, its black carbon content, the large perturbation to the radiative energy budget of the region and its simulated impact on the rainfall distribution, if proved correct, have significant implications to the regional water budget, agriculture and health. The link between anthropogenic aerosols and reduction of monsoonal rainfall in south Asia also has been made by over fifteen model studies preceding the UNEP report. We do not find any reason to modify the findings, the recommendations and the caveats in the UNEP report. The press release, while its direct quotes of the report are accurate, should have given more emphasis to the caveats in the report.