This summer was a bit of a washout for the UK*, but globally, rainfall levels have stayed roughly the same for the last thirty years. But NASA researchers now say they have good evidence that the tropics are getting wetter, with more rain falling over the oceans, than on land.
According to a study, published in the August 1 edition of the American Meteorological Society's Journal of Climate, 2005 was the wettest year between 1979 and the present day, followed by 2004, 2003, 2002 and 1998.
"When we look at the whole planet over almost three decades, the total amount of rain falling has changed very little. But in the tropics, where nearly two-thirds of all rain falls, there has been an increase of 5 per cent," said lead author Guojun Gu, a research scientist based at NASA's Goddard Space Flight Center.
The researchers used data from satellites and ground based observation stations to create their global record. Although the record of land based rainfall goes back many years, it is only in the last 30 years that satellite monitoring has allowed researchers to track the amount of rain falling over the seas.
They tracked the impact of various global events, such as volcanic eruptions, and El Niño, which are known to have an effect on the amount of rain that falls. They were then able to take these factors into account when interpreting the data.
El Niño is a cyclical warming of the ocean waters in the central and eastern tropical Pacific. It tends to emerge once every three to seven years and alters weather patterns worldwide.
They found that during El Niño years, the total rainfall in the tropics did not change much, but that when the ocean currents were disrupted, rain tended to fall into the oceans, rather than the land.
Volcanos also have an effect on rainfall: the ash from an eruption hangs high in the atmosphere, seeding the formation of more clouds. This cools the surface of the planet,slowing evaporations, and therefore reducing rainfall. During the period of the study, there were two volcanic eruptions the researchers rated as significant. For two years after each of these, rain fall was roughly five per cent lower than average.
Even once these two influences were corrected for, the long term trends remained the same: rainfall increased, particularly over the seas.
The observed trend is in line with expectations that a warming planet would accelerate the cycling of water from ocean to atmosphere and back again. The team says climate change is a plausible explanation, but says a longer record is needed before any definitive statements can be made.
*This statement was brought to you by the Department of the Bleedin' Obvious, with additional funding from the Under-Statement subcommittee.