African Floods: Caused by Global Warming?

As Africa suffers from outbreaks of disease in the wake of deadly, devastating floods, a new study by NASA raises a startling question: Might these floods have been caused, in part, by global warming? Tropical rainfall has increased in the past 25 years, as warmer oceans leads to more evaporation in the tropics, NASA scientists recently reported. Here's a snapshot of the devastation that increased rains can bring, based on facts on the ground in Africa this weekend, as reported by the Associated Press:

Torrential downpours and flash floods across Africa have submerged whole towns and washed away bridges, farms and schools. This summer's rains have killed at least 150 people, displaced hundreds of thousands and prompted the U.N. to warn Saturday of a rising risk of disease outbreaks. In eastern Uganda, nine people have been reported killed and 150,000 have been made homeless since early August. Another 400,000 -- mainly subsistence farmers -- have lost their livelihoods after their fields were flooded or roads washed away and the rains are forecast to worsen in the next month.

On the other side of the continent, Ghana in west Africa has also been heavily hit. Three regions in the north, the country's traditional breadbasket, have been declared an official disaster zone after whole towns and villages were submerged. Torrential rains between July and August killed at least 18 persons and displaced a quarter of a million.... More than a million people across at least 17 countries have been affected.

How is that linked to global warming? Well, it is not. At least, not scientifically. No one weather event can be said to have been caused by global warming. But it is consistent with a trend that NASA's Goddard Space Flight Center scientists have linked to models that predicted an increase in tropical rain resulting from global warming. An analysis of 27 years of data shows that the planet's wettest areas -- the tropics -- are getting wetter.

They got 5% more rain in 2005 than in 1979, and 2006 appears to have tied 2005 as a record wet year. "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%," lead author Guojun Gu, a research scientist at Goddard Space Flight Center in Greenbelt, Md., was quoted by NASA as saying. From NASA's Earth Observatory Web site:

This map above shows patterns of rainfall change between 1979 and 2005. Tropical oceans experienced the greatest increases. In large areas of the Pacific and Indian Oceans, rainfall increased by more than half a millimeter per day each decade. Northern South America and Southeast Asia also experienced wetter weather. Most land areas, however, experienced decreasing rainfall amounts. This unevenness in rainfall changes is not unexpected.

Many models predict that rising temperatures will make wet areas wetter and dry areas drier. Global average rainfall may increase, but the increase may come in fewer, but heavier storms. Climate scientists predict that a warming trend in Earth's atmosphere and surface temperatures would produce an accelerated recycling of water between land, sea and air. Warmer temperatures increase the evaporation of water from the ocean and land and allow air to hold more moisture. Eventually, clouds form that produce rain and snow.

"A warming climate is the most plausible cause of this observed trend in tropical rainfall," co-author Robert F. Adler, senior scientist at Goddard's Laboratory for Atmospheres, was quoted as saying. Another hitch in linking the rains in Africa to global warming: Most of the excess rainfall detected by NASA scientists occurred over the open ocean -- not over land.