Was Sandy caused by global warming?

In the narrowest sense, the answer is mostly no. A similar event could easily have happened hundreds of years ago, and probably did. But it's the wrong question. There are better ones to ask, and then the answers become more complex.

Better questions are: How rare an event was Sandy? What was its relation to the larger-scale climate, and how will the odds of a similar event (however one might define that) change in the future as the climate warms?

I will first present recent work which addresses one aspect of how rare an event Sandy was. Hurricane Sandy's track crossed the New Jersey coastline at an angle closer to perpendicular than any previous hurricane in the historic record, one of the factors contributing to record setting peak-water levels in parts of New Jersey and New York. In order to estimate the occurrence rate of Sandy-like tracks we use a stochastic model built on historical hurricane data from the entire North Atlantic to generate a large sample of synthetic hurricanes. From this synthetic set we calculate that under long-term average climate conditions a hurricane of Sandy's intensity or greater (category 1+) makes NJ landfall at an angle at least as close to perpendicular as Sandy's at an average annual rate of 0.0014 yr-1 (95% confidence range 0.0007 to 0.0023); i.e., a return period of 714 yr (95% confidence range 435 to 1429).

More broadly, I will discuss some other scientific questions that come up when we try to understand the relationship of a storm like Sandy to climate variability and change. This part of the presentation will present results from from recent simulations done by the US CLIVAR hurricane working group, and will also outline some questions that remain almost entirely open.