What a future Hurricane Sandy could look like

by Centre of Excellence Associate Investigator Kevin Walsh

Only two months after Hurricane Sandy’s strike on the most densely populated part of the United States, reports indicate that the recovery has been uneven. The New York subway system experienced the greatest flooding in its history but bounced back remarkably quickly: almost all lines were back in operation within days, and as I write this, only isolated sections of the network still remain out of action.

In contrast, many residents of coastal regions of New Jersey and New York are still rebuilding their lives after their houses were destroyed or damaged by the storm. They are also wondering if they will ever have to go through a storm like Sandy again – or suffer through something worse.

For climate scientists, determining the likelihood that a storm like Sandy will occur again poses a number of questions. A relevant question is how unusual was Sandy? The answer is that it was quite unusual but not unprecedented.

Sandy was a strong storm, with maximum winds along the coast slightly in excess of hurricane force, but this is considerably weaker than the strongest storm ever to strike the coastline in this region. For instance, in 1938, a hurricane with much stronger winds struck Long Island, just to the east of Manhattan, killing hundreds. Sandy was a large storm, though, and so its winds were able to affect a much larger region than would a typical hurricane. In short, a storm like Sandy will certainly occur again in this region, although this is not likely to occur soon.

Still, weather systems are not highly respectful of human beings and their need for safety, so it is even possible that there could be another Sandy next year. It’s just not very likely.

What is likely is that if an identical storm to Sandy were to occur in 2050 (call it “future Sandy”), its effects would be slightly worse. The main culprit is sea level rise, a highly confident prediction of climate change science. In the New York region, sea level rise has two main components: the fall of land due to geologically-driven decreases in land height in this region, and the rise of the oceans caused by a combination of thermally-driven expansion and melting land ice.  Projections for New York indicate a likely sea level rise by 2050 of between 18 and 30 cm, and this would be added on top of any sea flooding associated with future Sandy-like storms, the kind of flooding caused by strong winds blowing towards the coast and pushing water ahead of them.

Other predicted changes in future climate might affect “future Sandy”, but their association with such a storm is less clear.

“Future Sandy” will almost certainly take place in an environment with higher average sea surface temperatures. All other things being equal, higher sea temperatures generally lead to storms with stronger winds. The key disclaimer here is the phrase “all other things being equal”, because of course usually they aren’t: many other factors influence storm intensities apart from sea temperatures. Even so, detailed climate modelling studies, as well as some theoretical work, suggest that “future Sandy” will be slightly more intense.

Another effect of higher sea temperatures is higher evaporation, leading to more moisture in the atmosphere and thus a likely substantial increase in the amount of rainfall occurring in tropical cyclones. So “future Sandy” is likely to have more rain. Rain-induced flooding was minor in the real Sandy, though, with sea flooding giving a much larger impact.

Notice here that I am side-stepping the issue of how much worse “real Sandy” was due to the climate change that has already occurred by now, or whether “real Sandy” was caused by climate change. It’s quite easy to say how much existing global warming has affected the elevated sea levels caused by “real Sandy”: they are about 15-20 cm higher since 1900, as that is what has been measured so far once the geological effects are removed.

Other climate change effects on “real Sandy” are much harder to diagnose, since unlike sea level rise, variables like moisture content and sea surface temperatures directly interact with the physical processes that modify hurricanes, making it difficult to determine cause and effect relationships. Also, it is hard to determine whether other factors that affected Sandy are related to climate change rather than to simple bad luck, like the unusual atmospheric circulation patterns that caused “real Sandy” to strike the coast rather than taking a more usual track that would have moved the storm out to sea. Quite a few scientists are working on untangling these issues right now, though, so we can expect to see numerous papers on Sandy emerging in the near future.

But we can be confident that “future Sandy”, our hypothetical 2050 clone of the real 2012 storm, is likely to be slightly stronger, produce slightly more rainfall, and have slightly higher sea levels than “real Sandy”. The use of the word “slightly” is of course cold comfort to those who have had to deal with the impact of real Sandy this year.

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