Special to American-Statesman
Extreme weather, such as droughts, floods, tornadoes, and hurricanes, has long been a subject of intense scientific research. Recently, though, the study of extreme weather has taken a different turn. People are wondering whether extreme weather is becoming more frequent or intense, and whether specific events might be linked to a long-term global rise in temperatures. Science is responding to this demand for information, and ever-increasing numbers of research studies are addressing this issue.
To confidently answer such questions, you need two things: a clear trend in the data and a sound physical reason. Without the sound explanation, a trend might be random chance. Without the trend, the reason might be wrong because it left out some important factor.
For example, numerous studies have found that intense rainfall is actually becoming more intense. What gives scientists confidence that this trend is no fluke is the fact that warmer temperatures increase evaporation, so it makes sense that there's more water vapor in the air, ready to rain out, when those intense rain events happen.
The trouble with many extreme events is that they are rare and hard to observe, so the historical data record may be spotty and have spurious trends. The main problem with hurricane data, for example, is that we didn't have weather satellites until the mid-1960s, so a hurricane out over the open ocean that would have gone undetected before is now easy to spot. Hurricane experts have also found that, presumably because of better detection abilities, there are a lot more short-lived tropical storms and hurricanes now than in the historical data record.
If we ignore the pre-satellite hurricane data, we find a sharp rise in Atlantic hurricanes over the past several decades, coinciding with a sharp rise in Atlantic sea surface temperatures. Aha, this seems to be a clear trend plus a sound physical reason, since we know that warm water is favorable to hurricanes. Is this a clear connection between hurricanes and global warming?
Actually, no. Most evidence points to the key factor being the difference between tropical Atlantic temperatures and temperatures in the rest of the tropics, not just the Atlantic temperature itself. The idea here is that the warmer the Atlantic relative to its surroundings, the more unstable the air will be over the Atlantic. And since global warming isn't supposed to warm the Atlantic Ocean by itself, this hurricane trend is probably not caused by global warming.
Meanwhile, climate models have gradually gotten better at simulating hurricanelike objects. These models are fairly consistent in predicting that the global number of hurricanes per year should actually go down as the climate warms. They also indicate that the most intense hurricanes ought to be even more intense, so it's a good news, bad news kind of thing.
If the models are right, it should take about 50 years or so before a clear trend should emerge in the long-term hurricane data. That's because hurricanes are naturally so erratic in numbers and strength from year to year. Until that trend emerges, or fails to emerge, all we have is a bit of evidence pointing in one direction.
The situation with tornadoes is even worse. Tornadoes are so small that climate models won't be able to simulate them for a long time.
The historical data show a large increase in the number of tornadoes recently, but that could easily be entirely due to the 15-year-old Doppler radar network, the popularity of storm-chasing and increases in population leading to more eyes watching the skies. Scientists must resort to studying whether the large-scale environment for tornadoes is likely to become more or less favorable, but only a few of the environmental factors have been considered so far.
Not content with looking at changes in the statistics of extreme weather events, some scientists are pushing the envelope and trying to determine to what extent man-made climate change, or climate change in general, has contributed to or caused particular extreme weather events.
In some ways, this is not a very good question. One problem with figuring out how much climate change has affected a particular event is that there's nothing to compare it to. Weather is so chaotic that, as a famous example goes, the tiny stir caused by a butterfly flapping its wings in Brazil can lead to a chain reaction in the atmosphere that might ultimately determine whether a tornado forms in Kansas three weeks later. Everything that gets done to the atmosphere leads to a sequence of weather events that's different from what would have happened otherwise.
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