We Don't Know All About Hurricanes--But We Know Enough to Act
Sandy instantly brought a new kind of national media attention to the influence of global warming on weather disasters. During the week before the election, the storm even induced Mayor Bloomberg to force the issue into the presidential campaign with his endorsement of President Obama. It should have been there long before that, but never mind. After several years of near-silence on climate from our political leaders and the mainstream media, the renewed attention is profoundly welcome to climate scientists and all who take us seriously.
Did global warming cause Sandy? Of course it’s not that simple. But the real issue — to what extent we can expect weather disasters of various kinds to increase in frequency and severity — is as serious as a crippled northeastern United States, and for several years up to now a creative and well-funded denialist movement has kept it largely out of the mainstream political discussion.
Let’s first repeat the basics. 1. The climate is warming due to human emissions of greenhouse gases, mainly from the burning of coal, oil, and natural gas. Anyone who doesn’t accept that by now is either uninformed, dishonest, misled by the dishonest, or blinded by the psychic difficulty of accepting the threat to our fossil-fueled society that warming implies. 2. The weather fluctuates naturally, and a wide range of weather disasters would occur even in an unchanging climate.
Most of us understand that no individual event can be directly and entirely attributed to global warming. Rather, warming changes the odds that extreme events will occur. But different types of extreme events – heat waves, floods, droughts, hurricanes, tornadoes – are influenced in different ways, and to different degrees, by climate. We understand some of those influences much better than others. Our degree of understanding determines the certainty of the statements we can make about the future.
It’s simple to understand why heat waves – like the devastating, crop-killing one in the US this past summer – become much more likely as the climate warms. Yes, there is natural variability in temperature. Assume that variability doesn’t change, but add a few degrees of global warming. The frequency with which any given temperature threshold (whatever we want to consider a heat wave) is exceeded will increase. The real climate is a little more complicated than that, but not a lot more. So we know that more and worse heat waves are pretty much guaranteed.
Hurricanes are more subtle. They require sufficiently warm sea surface temperatures, so it’s natural to think that they will form more often as the ocean warms. But the sea surface temperature threshold for hurricane formation is not a fixed number. It’s a property of the climate, and can change as the climate does. Hurricanes actually depend not on the absolute sea surface temperature, but on the difference between that and the atmospheric temperature. Both the oceans and atmosphere are warming. The sea surface temperature required for hurricane formation will increase about as fast as the actual sea surface temperature does, and we don’t have any simple rules to tell us what hurricanes will do.
Our best tools here are computer models designed specifically for this problem (close relatives of those which predicted Sandy’s landfall so far ahead of time). The latest and best models – still not perfect, but much better than any we had a few years ago – are in agreement that the number of tropical cyclones worldwide will more likely decrease than increase, but that their intensities will increase. We don’t fully understand this result yet, but when all the best models agree we have to take them seriously.
If “fewer, but more intense” sounds confusing (is that bad or good?), the relation of a storm like Sandy to climate is even more ambiguous. What made Sandy so unusual was not the original hurricane itself, or even the hybridization process by which it merged with a winter storm. The freak part was the left turn it took so that it hit the Jersey coast at the worst possible angle for storm surge. We can say very little, at this point, about how this left turn might have been related to climate. There are ideas, but they are still speculative.
The relation of droughts to warming is closer to that of heat waves; more droughts in many areas are almost certain. Same for floods (counterintuitive as that may seem). Tornadoes, on the other hand, are even more poorly understood than hurricanes. We just don’t know yet what warming will do to them.
Perhaps most relevant to the present disaster, we know that sea level is rising as the climate warms. It has increased by nearly one foot on the Atlantic seaboard since 1900. The question now is how fast it will rise in the future; that depends on how soon big chunks of ice in Greenland and Antarctica break off and fall into the sea. So storm surge, the real killer in this event, will get worse even if hurricanes don’t change at all, because any given surge just piles more water on top of what is already there. That by itself should be enough to spur us to action.
More fundamentally, the precautionary principle tells us that if there is even a decent chance that human-induced global warming will make weather disasters more common – and the chance is better than decent, even if we can’t say for every type of disaster exactly how much more common or exactly how or where – that should be enough to motivate us to act. The argument that the cost of serious action is large can at least be a contribution to a productive debate; denial of the reality of warming cannot. We climate scientists can’t yet answer every question with certainty, but for a long time now our answers to the most basic questions have been solid enough that they should be front and center in every federal election, and the basis for U.S. action.
Adam Sobel is professor of Applied Physics and Applied Mathematics and of Earth and Environmental Sciences at the Fu Foundation School of Engineering and Applied Sciences, and at Lamont-Doherty Earth Observatory at Columbia University.