Asian brown cloud - fact and fantasy

摘要

The wide publicity given to the release of a United Nations Environment Programme (UNEP) report on the so-called Asian Brown Cloud and its multifarious impacts on health, agriculture and climate on both regional and global scales, has led to considerable concern. We find that the UNEP news release (and hence the media reports based on it) is a blend of observations and scientifically sound deductions on the one hand and sensational statements with little scientific basis on the other. The UNEP report is based on the findings of an international programme called the Indian Ocean Experiment (INDOEX). The term Asian Brown Cloud was coined by leaders of INDOEX to describe the brown haze occurring during the period January to March, over the South Asian region and the tropical Indian Ocean, Arabian Sea and Bay of Bengal. It is important to note that, the haze is not a permanent feature of the atmosphere over the Asian region and the surrounding seas. It occurs only during January-March, in the season foUowing the southwest monsoon and northeast monsoon seasons. t is suggested in the UNEP report that the impact of the haze assessed with the help of an atmospheric general circulation model is a decrease in rainfall in northwest Asia (including Saudi Arabia, Pakistan, Afghanistan). However, we find that the model simulation of the rainfall patterns over this region is particularly poor and hence the reliability of this projection is suspect. Also, the expected magnitude of the impact on crop yields is small and there is no basis for the statement in the UNEP news release that the 'vast blanket of pollution across South Asia is damaging agriculture'.