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Was Katrina's Power a Product of Global Warming?
With a unique confluence of geography, expansive lowlands (particularly in the New Orleans area), wetland loss, deforestation, rapid development, large populations of the poor, and a heavy concentration of industry, the Gulf Coast is extremely vulnerable to hurricanes, with or without global warming. Katrina is not the first category 5 hurricane to hit the Gulf Coast (it actually weakened to category 3 shortly before landfall in Louisiana and Mississippi); in fact, of the three previous similar events, two of them occurred in 1935 and 1969, prior to the period of most of the human-induced global warming that has occurred so far. Clearly, then, global warming is not required for an extremely intense hurricane to strike.
But can science tell us whether Katrina's destructiveness was related to global warming? Not directly: science, as a method, is not good at assigning causation for uncontrolled events, and no single weather event can be linked directly to a long-term driver, such as global warming. This inability to draw a definite conclusion, however, in no sense justifies the conclusion that global warming did not influence Katrina.
What science does offer on this question is a general understanding of the physics of tropical storms that can inform reasonable assessment. Because hurricanes draw strength from heat in ocean surface waters, warming the water should generate more powerful hurricanes, on average. Indeed, sea surface temperature records show that the oceans are more than 1 degree F warmer on average today compared to a century ago. On short time scales (days to months), temperatures fluctuate above and below the long-term average, and the water can be warmer or cooler than the average on any given day. But the higher the average, the more likely the water will be warm enough to produce a strong storm on any given day during the hurricane season. Case in point: while Katrina was strengthening from a tropical storm to a category 5 hurricane, as it passed between the Florida Keys and the Gulf Coast, the surface waters in the Gulf of Mexico were unusually warm - about 2 degrees F warmer than normal for this time of year. From this "first-principles" perspective, then, it is no surprise that Katrina became a very powerful storm. While there is no method to determine whether global warming played a role, it is reasonable to say it increased the probability that the Gulf surface water would be unusually warm on any given day, as it was on August 29 when Katrina's intensity peaked.
Beyond inferences from our understanding of storm physics, is there evidence that storms are actually becoming more intense, as we would expect? A study published recently in Nature  found that since the early 1950s, the average intensity of tropical storms has increased globally, and this trend correlates very well through time with the increase in average sea surface temperatures in the tropics. These data show a real trend that fits expectations from our basic understanding of climate, and a powerful storm like Katrina makes sense in this context.
So, although we cannot be certain global warming intensified Katrina per se, it clearly has created circumstances under which powerful storms are more likely to occur at this point in history (and in the future) than they were in the past. Moreover, it would be scientifically unsound to conclude that Katrina was not intensified by global warming. A reasonable assessment of the science suggests that we will face similar events again and that powerful storms are likely to happen more often than we have been accustomed to in the past.