I’ve been trapped in post-storm hell (no power, no heat for 10 days. Now power is back, but still no internet at home, which is frustrating, but no big deal), and so I haven’t been able to post this until now.

I’ve been getting a bunch of questions from people in response to an earlier post of mine about global warming, where I said that we can’t blame specific weather events on global warming. The questions come down to: “Can we say that hurricane Sandy and yesterday’s NorEaster were caused by global warming?”

I try to be really careful about things like this. Increasing the amount of energy in the environment definitely has an effect on weather patterns. But for the most part, that effect is statistical. That is, we can’t generally say that a specific extreme weather event wouldn’t have happened without global warming. We can just say that we expect extreme weather events to become much more common.

But what about hurricane Sandy?

Yes, it was caused by global warming.

How can I say that so definitively?

There were a lot of observations made around this particular hurricane. What made it such a severe event is a combination of three primary factors.

• The ocean water over which it developed is warmer that historically normal. Warm water is, simply, fuel for hurricanes. We know this from years of observation. And we know that the water was warmer, by a couple of degrees, than it would normally be in this season. This is a direct cause for the power of the storm, for the fact that as it moved north, it continued to become stronger rather than weakening. Those warm waters are, by definition global warming: they’re one of the things we measure when we’re measuring global temperature trends.
• Hurricane Sandy took a pretty dramatic left turn as it came north, which is what swept it into the east coast of the US. That is a very unusual trajectory. Why did it do that? Because of an unusual weather pattern in the Northeast Atlantic, called a negative North Atlantic oscillation (-NAO). And where did the -NAO come from? Our best models strongly suggest that it resulted, at least in part, from icemelt from Greenland. This is less certain than the first factor, but still likely enough that we can be pretty confident.
• Hurricane Sandy merged with another weather front as it came inland, which intensified it as it came ashore. This one doesn’t have any direct relation to global warming: the front that it merged with is typical autumn weather on the east coast.

  

So of the three factors that caused the severe hurricane, one of them is absolutely, undeniably global warming. The second is very probably linked to global warming. And the third isn’t.

This is important to understand. We shouldn’t make broad statements about causation when we can’t prove them. But we also shouldn’t refrain from making definitely statements about causation when we can.

The NorEaster that we’re now recovering from falls in to that first class. We simply don’t know if it would have happened without the hurricane. The best models that I’ve seen suggest that it probably wouldn’t have happened without the effects of the earlier hurricane, but it’s just not certain enough to draw a definitive conclusion.

But the Hurricane? There is absolutely no way that anyone can honestly look at the data, and conclude that it was not caused by warming. Anyone who says otherwise is, quite simply, a liar.