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This post is written by our guest blogger Bill McGuire. Bill McGuire is Professor Emeritus of Geophysical & Climate Hazards at UCL and author of Waking the Giant: How a Changing Climate Triggers Earthquakes, Tsunamis and Volcanic Eruptions. He was a contributor to the IPCC 2012 report on Climate Change & Extreme Events and Disasters.
Just a couple of weeks ago, it looked as if Florida would be facing the most powerful hurricane to strike the US mainland since Andrew near obliterated part of the Miami metropolitan area in 1992. Now, we know that the state had a miraculous escape. Having trashed the northern Bahamas, Hurricane Dorian – arguably the most powerful Atlantic storm ever to make landfall – veered hard right before reaching the state and followed a track that curved northwards, paralleling the east coast, but staying just offshore. Coastal communities from Florida to Canada experienced powerful winds, biblical deluges, and flooding, but were spared the 220 mph winds and 23ft storm surge that destroyed or severely damaged at least 13,000 homes in the Bahamas.
For the US east coast, however, this is no more than a respite. As global heating makes tropical cyclones in all the ocean basins more powerful, it can only be a matter of time before a category five storm – the most powerful there is – scores a direct hit on Miami or some other east coast city. When this happens, the level of destruction and the cost of rebuilding will be prodigious. And this won’t be a once in a lifetime event either. As the most powerful hurricanes become more and more frequent, so the major urban centres of the eastern seaboard, from the sunshine state all the way up to Canada, can expect to be repeatedly pummelled. Combined with ever more rapidly climbing sea levels, this is likely to leave – later this century – much of the urbanised east barely habitable.
Hurricane Dorian itself was significantly more powerful and therefore more destructive, than it would have been had human activities not been pumping the atmosphere full of carbon for the past couple of centuries. As carbon levels rise and the world gets hotter, the oceans heat up too, and become more effective at spawning bigger and nastier hurricanes. For every one degree of warming, the maximum sustained wind speeds of a hurricane increase by around seven percent. Bad enough in its own right if you are on the receiving end. But even worse, this translates into a 23 percent increase in the level of potential destruction. Something the 70,000 inhabitants of the Bahamas, who still desperately need relief, would attest to.
Because warmer air can hold more moisture, hurricanes are also getting much wetter. Rainfall is heavier and more sustained, leading to greater levels of flooding that adds to the damage bill and makes relief and reconstruction more problematical and costlier. And it’s not just the rain that’s the problem. The more intense a hurricane (in other words how low is the pressure in the eye), the higher the sea-surface swells beneath it, and the bigger the accompanying storm surge. In the case of Dorian, a very low-pressure core of just 910 millibars, resulted in a storm surge in excess of 20ft that travelled right across the low lying islands of the northern Bahamas. The destructive power of such surges is comparable to a tsunami and even bigger ones can be expected as global heating super-charges hurricanes even more.
Dorian’s behaviour also demonstrated a couple of other features that don’t bode well for the future. Firstly, the hurricane intensified incredibly quickly, from a category two to a category five storm. This so-called explosive intensification seems to be happening more commonly on our hotter planet. Secondly, hurricanes seem to be moving more slowly, meaning that they hang out longer over a particular point. Dorian effectively stopped for time over the northern Bahamas, greatly increasing levels of damage due to wind and flooding. The bad news is that recent research has revealed that tropical cyclones globally have slowed by 10 percent, while hurricanes impacting upon the eastern US have slowed by a full 17 percent.
The bottom line is that, bad as it was, Hurricane Dorian cannot be regarded an exceptional event. Far from it, the storm provides a glimpse of what may well become the new normal. On the day of the Global Climate Strike – the greatest ever environmental demonstration – America needs to be aware of what global heating and climate breakdown will mean for its coastal communities. It should also remember that this year’s hurricane season still has almost two months still to run.