The rain band near the equator that determines the supply of freshwater to nearly a billion people throughout the tropics and subtropics has been creeping north for more than 300 years, probably because of a warmer world, according to research published in the July issue of Nature Geoscience.
If the band continues to migrate at just less than a mile (1.4 kilometers) a year, which is the average for all the years it has been moving north, then some Pacific islands near the equator - even those that currently enjoy abundant rainfall - may be drier within decades and starved of freshwater by midcentury or sooner. The prospect of additional warming because of greenhouse gases means that situation could happen even sooner.
Lead author Julian Sachs, associate professor of oceanography at the University of Washington, said:
We're talking about the most prominent rainfall feature on the planet, one that many people depend on as the source of their freshwater because there is no groundwater to speak of where they live. In addition many other people who live in the tropics but farther afield from the Pacific could be affected because this band of rain shapes atmospheric circulation patterns throughout the world.
The researchers found evidence of salt-tolerant microbes buried in lake sediments, corresponding to a period 400 - 1,000 years ago, suggesting a period of reduced rainfall. Additional evidence was provided by ratios of hydrogen isotopes of material in the sediments that can only be explained by large changes in precipitation. The findings are based on sediment cores from lakes and lagoons on Palau, Washington, Christmas and Galapagos islands.