Hawaii boffins: Aerosols add to Amazon rainfall

More pollutants equal less crazy weather

Boffins based in Hawaii say that over the last 60 years emissions of aerosols - soot and other particulates often emitted by diesel engines, coal powerplants etc - have led to increased rainfall in the rainforests of the Amazon and central Africa, and have also resulted in "fewer extreme [weather] events".

Hiroki Tokinaga and Shang-Ping Xie of the University of Hawaii's International Pacific Research Center examined the records of wind speeds in the Atlantic ocean, which suggest that winds have actually increased over the last six decades. However, the two boffins say that this is simply because these records mostly come from the anemometers carried on ships travelling through the region: and ships have got bigger, so that their anemometers are carried higher up and record higher windspeeds.

Correcting for this bias, Tokinaga and Xie found that in fact the tropical Atlantic winds have decreased, not increased. This has meant much less variability in temperatures, "implying fewer extreme events", according to a UK statement accompanying the research. Furthermore, rainfall over the land - both on the African side and the South American - has increased significantly (despite recent reports of massive drought hitting the Amazon, the long-term trend has seen more rain, according to the scientists.)

Tokinaga and Xie write:

We suggest that the observed changes could be associated with cooling by anthropogenic aerosols, an effect that is stronger in the Northern than in the Southern Hemisphere. If the aerosol emissions decrease in the next decades, the tropical Atlantic may experience yet another shift as the greenhouse gas forcing increases.

Aerosol emissions might decrease as measures are taken to shut down dirty powerplants, clean up diesel emissions and eliminate inefficient cooking fires used in poor parts of the world. Alternatively, developing nations may industrialise rapidly on the Chinese model, and emit more cooling soot - thus carrying on the reduction in extreme weather events seen over the past 60 years, and keeping the rainforests rainy. ®