Earth may be 4.5 billion years old, but our planet stays stylish with accessories like the tropical rain belt, also known as the Intertropical Convergence Zone. This narrow band of clouds circles the globe along the equator and produces the most intense rainfall in the world.
The rain belt follows the seasonal movements of the Sun, bringing wet and dry seasons to tropical regions, but varies on longer scales as well, which can produce severe droughts. As such, the movement of the rain belt can have potentially disastrous effects on people living in the tropics. Understanding rain belt dynamics will only become more critical as the belt responds to a warming climate.
Here Allen examines the role of atmospheric aerosols in driving rain belt behavior. Atmospheric aerosol particles come from a variety of natural sources, like volcanic eruptions and forest fires. However, Allen focuses on shifts in anthropogenic aerosol production, which has undergone a fairly rapid upsurge with substantial results. For example, increased sulfate emissions in the 1980s cooled the Northern Hemisphere and pushed the tropical rain belt southward.
According to the Intergovernmental Panel on Climate Change Fifth Assessment Report, anthropogenic aerosol emissions will decrease in the 21st century, reaching preindustrial values by 2100. Allen used Coupled Model Intercomparison Project Phase 5 models to simulate rain belt behavior under these conditions.
Allen’s models projected a northward shift in the tropical rain belt during the 21st century due primarily to decreasing atmospheric aerosol levels. Furthermore, in models that accounted for indirect effects of the drop in aerosols, the rain belt moved even farther north than predicted by models that left out indirect effects.
Although the variety of aerosol types and sources means their overall role in climate behavior is still uncertain, Allen’s results provide evidence of their importance and the need for further study. A better understanding of atmospheric aerosols and aerosol-cloud interactions has valuable ramifications for tropical communities, agriculture, and economies. (Journal of Geophysical Research: Atmospheres, doi:10.1002/2015JD023623, 2015)
—Lily Strelich, Freelance Writer
Citation: Strelich, L. (2015), Aerosol cutbacks may bring tropical rains farther north, Eos, 96, doi:10.1029/2015EO037855. Published on 23 October 2015.