Asia's air pollution may be keeping tropical storm activity down

From the Law of Unintended Consequences and The Clean Air Act, comes this bit of news. Since the 1970’s The Clean Air Act has benefited breathing in many American cities with tangible results (just look at Los Angeles), but it may have had a role in increasing tropical storm activity.

This new paper suggests that due to the reduction of aerosols and particulates in the atmosphere might have been the main cause of a recent increase in tropical storm frequency in the North Atlantic.

Aerosol levels have increased since the start of industrial revolution, but as we know there have been periods when aerosol emissions declined; the Great Depression, World War II and after clean air legislation was enacted in Europe and the United States in the 1970s and 1980s.

The paper suggests that these periods of reduced emissions eventually increased tropical storm frequency.

Anthropogenic aerosol forcing of Atlantic tropical storms

N. J. Dunstone, et al. Nature Geoscience (2013) doi:10.1038/ngeo1854

Received 04 December 2012 Accepted 15 May 2013 Published online 23 June 2013

The frequency of tropical storms in the North Atlantic region varies markedly on decadal timescales^{1, 2, 3, 4}, with profound socio-economic impacts^{5, 6}. Climate models largely reproduce the observed variability when forced by observed sea surface temperatures^{1, 8, 10}. However, the relative importance of natural variability and external influences such as greenhouse gases, dust, sulphate and volcanic aerosols on sea surface temperatures, and hence tropical storms, is highly uncertain^{11, 12, 13, 14, 15, 16}. Here, we assess the effect of individual climate drivers on the frequency of North Atlantic tropical storms between 1860 and 2050, using simulations from a collection of climate models¹⁷. We show that anthropogenic aerosols lowered the frequency of tropical storms over the twentieth century. However, sharp declines in anthropogenic aerosol levels over the North Atlantic at the end of the twentieth century allowed the frequency of tropical storms to increase. In simulations with a model that comprehensively incorporates aerosol effects (HadGEM2-ES; ref. 18), decadal variability in tropical storm frequency is well reproduced through aerosol-induced north–south shifts in the Hadley circulation. However, this mechanism changes in future projections. Our results raise the possibility that external factors, particularly anthropogenic aerosols, could be the dominant cause of historical tropical storm variability, and highlight the potential importance of future changes in aerosol emissions.

h/t to Marc Hendrickx