Cleaner skies, stormier skies

To the ever-growing list of ways humanity seems to have altered the earth, add another candidate: Air pollution may have had a major soothing influence on storm cycles in the North Atlantic.

That is the finding of a paper published this week, suggesting that industrial pollution from North America and Europe through much of the 20th century may have altered clouds in ways that cooled the ocean surface. That, in turn, may have suppressed storms, and particularly major hurricanes, below the level that would have existed in a purely natural environment.

If the authors are right, the upturn in storms over the last couple of decades may be no accident. It could, instead, be at least partly a consequence of the clean air acts that have reduced pollution around the North Atlantic basin, thus returning the storm cycles to their more natural state.

The possible impact on storms emerged from sophisticated new computer analyses of the climate that attempt to take account of the indirect effects of particles in the air; it is leading-edge science that may or may not hold up over the long haul.

“Our results show changes in pollution may have had a much larger role than previously thought,” said Nick J Dunstone, a researcher with Britain’s meteorological service and the lead author of the new paper. He acknowledged, however, that “this is all quite new” and the science remains un­certain. It may be recalled, the No­rth Atlantic was quiescent in the 1970s and 1980s, especially for major hurricanes, creating a false sense of security and enc­o­uraging coastal development. But starti­ng in the 1990s, storminess increased sharply, and the new study says that may be because clean air laws had started to take effect.

Previous work had suggested pollution could be playing a significant role in storminess, but the new paper is the most detailed exploration yet of the possible mechanism. It was published online Sunday by the journal Nature Geoscience.

The alternative to this new view is the one most climate experts have long held, th­at the variability in storminess in the North Atlantic is a function of large-scale na­tural oscillations in the ocean circulation.

Five scientists not involved in the new work said they found the findings believable in principle – “entirely plausible,” in the words of Kerry A Emanuel, an atmosp­heric scientist at the Massachusetts Institute of Technology. Several experts added, however, that the effect of pollution on storms, even if real, could turn out to be smaller than the new paper proposes.

The main effect on storm patterns would have come from particles of sulfur dioxide that entered the air from the combustion of sulfur-laden fuels like coal and diesel. Water can condense on these particles, and a surfeit of them in the air can change the properties of clouds, causing them to be made up of finer droplets. That, in turn, can brighten the clouds and cause their tops to reflect more sunlight back to space – meaning less sunlight reaches the surface to heat it.

Getting an accurate representation of all this into computer programmes that model the climate has been a long slog for researchers, which is why the evidence of a possible effect on storms is only just emerging. The authors of the paper are working with one of the most advanced of these models in Britain. The recent rese­a­rch using that model suggests that the huge increase in pollution around the North Atlantic basin in the mid-20th century cooled the surface enough to change the large-scale circulation of the atmosphere.