Until now, data to support this idea have been sparse. However, scientists have just announced they have found strong evidence to support it.
At a meeting of the American Geophysical Union in San Francisco today, JPL Senior Research Scientist Hartmut Aumann outlined the results of a study based on five years of data from the Atmospheric Infrared Sounder (AIRS) instrument on NASA's Aqua spacecraft. What they found is that very high clouds, which are associated with severe weather, are more frequent when the ocean is warmer. The correlation they found was very strong; for every degree Centigrade the oceans warm, they saw a 45% increase in the number of these clouds!
They observed the clouds as the seasons changed -- changing the ocean surface temperatures -- to get their data. They typically see 6000 such clouds a day, so it sounds like they got reasonably good statistical samples. The clouds are very high in the atmosphere, about 20 km (12 miles) or more over the Earth's surface. In general, linking clouds, rain, and global warming is incredibly difficult because rain is very difficult to model. This new result bypasses that, using the clouds as a proxy for storms. The study is also consistent with a 2005 NASA study which found a 1.5% increase in the global rain rate over the past 18 years.
So what does this mean? Well, it's still not possible to say that any given season will have more or more powerful hurricanes/monsoons due to global warming, because there are so many inputs that drive the storm season. What it does indicate is that over time, over many years, the number of such storms will increase.
Also, remember, this is a global result; local areas may still get colder -- I say this specifically because I always get comments in global warming posts that some place or another is getting more snow or is colder than usual. That's expected. With global warming comes altered weather patterns, so while the Earth overall warms, some places will get wilder weather.
It will be very interesting to see the fallout from this study. Expect to hear Inhofe call it a hoax, and for the usual suspects to raise a hue and a cry. For my part I'll say that I would very much like to see more observations of this. But in general, as we get more and better data, the case for global effects gets stronger. And come January 20, I'm very much hoping to see our government take this seriously for the first time in 8 years.
Fire image courtesy peasap's Flickr photostream; Earth image from NASA.