by Tim Storer
The ability to accurately predict and prepare for weather changes in the 21st century will be an invaluable asset to nations and policymakers across the globe, and because extreme hurricanes have traditionally been among the most destructive weather patterns, prediction of their patterns/intensities is useful. Because it is so difficult to directly predict storm activity, researchers have sought to take a roundabout approach: first investigate connections between local/global temperatures and past hurricanes, and then use future temperature predictions to predict future storms. Because the high winds associated with hurricanes are so closely accompanied by increased sea levels, called surges, the measured levels are a good measure of hurricanes. In addition, the researchers note that surge levels have shown to be better than wind speeds at indicating the damage potential of hurricanes. In the United States, the greatest hurricane threats are usually tropical storms along the eastern seaboard, and these storms are the primary focus. It has been widely predicted that global temperatures are expected to rise in the upcoming century, and researchers have now found that global temperatures are a surprisingly effective indicator of storm surge levels (Grinsted et al. 2013). Additionally, it was estimated that there would be many more Katrina-scale events in the upcoming decades, by at least a factor of two.
Aslak Grinsted and others at Massachusetts Institute of Technology sought to use a thorough and unbiased record of cyclone activity, and see how those patterns vary with warming patterns. By analyzing the probability distribution over a long span of time (from 1923 until near present day), the researchers expect to be able to predict the distribution of even the most rare and extreme events in future decades. Data on storm surges were collected from six different tide-measuring stations in the southeastern U.S., and then seasonal changes, tide effects, and sea level rise were factored out so just changes due to hurricanes could be isolated. These surge levels were matched with global temperatures, both sum totals and temperatures from specific locations. Somewhat ambiguously, hurricane Katrina was chosen as a benchmark for a destructive hurricane, and the results often refer to the number of “Katrinas” expected in the future, which includes all events that are of equal or higher destructive potential.
It was seen that, with a few exceptions, average global surface temperature was a better indicator of Atlantic cyclone activity than almost all specific areas. Some of the notably well-predicting areas include deserts, such as those in Africa and Australia. It was postulated that this possibly counterintuitive connection is because these climates are caused similar global conditions as cause hurricanes. Each of these notable areas is not a random occurrence, and there are physical theories that back each of the statistical findings.
The most significant finding comes from combining all the research of temperature correlations and climate models. Researchers found that a global average increase of 1°C would cause the frequency of “Katrinas” to increase by a factor of 2–7, depending on which climate model is trusted. Even more daunting, this prediction does not include the additional risk associated with general rising sea levels. Interestingly, a much more minor increase in normal/smaller storm events was predicted, which is in contrast with other studies, that actually predict a decrease. This is yet another example of the polarization/intensification of climate that is predicted to occur within our lifetimes.
Lastly, it was predicted that 0.4 °C warming increases the frequency of Katrinas by a factor of two. Because this amount of warming is less than the increase in average temperature over the 20th century, researchers conclude that a threshold has been crossed: any given Katrina-caliber surge is more likely caused by global warming than not. This is a remarkable finding, and these daunting predictions should give warning to urban planners, and give motivation to policymakers to fight anthropogenic climate change.
Grinsted, A., Moore, J.C., Jevrejeva, S., 2013. Projected Atlantic hurricane surge threat from rising temperatures, K. Emanuel, 5369–5373, doi: 10.1073/pnas.1209980110