Global Dimming by Air Pollution and Global Warming by Greenhouse Gases: Global and Regional Perspectives

The global build up of greenhouse gases (GHGs), is the most vexing global environmental issue facing the planet. GHGs warm the surface and the atmosphere with significant implications for, rainfall, retreat of glaciers and sea ice, sea level, among other factors. What is less recognized, however, is a comparably major global problem dealing with air pollution. Until about 10 years ago, air pollution was thought to be just an urban or a local problem. But new data have revealed that, due to fast long-range transport, air pollution is transported across continents and ocean basins, resulting in transoceanic and transcontinental plumes of atmospheric brown clouds (ABCs) containing submicron-size particles, i.e., aerosols. ABCs intercept sunlight by absorbing as well as reflecting it, both of which lead to a large surface dimming. The dimming effect is enhanced further because aerosols nucleate more cloud drops which makes the clouds reflect more solar radiation. The surface cooling from this dimming effect has masked the warming due to GHGs. ABCs are concentrated in regional and megacity hot spots. Long-range transport from these hot spots gives rise to widespread plumes over the adjacent oceans. Such a pattern of regionally concentrated surface dimming and atmospheric solar heating, accompanied by widespread dimming over the oceans, gives rise to large regional effects. Only during the last decade, we have begun to comprehend the surprisingly large regional impacts. The large north–south gradient in the ABC dimming has altered the north–south gradients in sea surface temperatures, which in turn has been shown by models to decrease rainfall over the continents. In addition to their climate effects, ABCs lead to acidification of rain and also result in over few million fatalities worldwide. The surface cooling effect of ABCs may have masked as much 50% of the global warming. This presents a dilemma since efforts to curb air pollution may unmask the ABC cooling effect and enhance the surface warming. Thus efforts to curb GHGs and air pollution should be done under one framework. The uncertainties in our understanding of the ABC effects are large, but we are discovering new ways in which human activities are changing the climate and the environment.

Keywords Global dimming, global warming