Attributing Weather Events to Climate Change Is the Easy Part

There's a phrase that comes up a lot in news reports that do discuss the relationship between climate change and extreme weather that get the connection completely backward. Here it is in the New York Times story (11/12/13) Peter quoted:

Yet scientists remain cautious about drawing links between extreme storms like this typhoon and climate change. There is not enough data, they say, to draw conclusions about any single storm.

And on NBC Nightly News (11/11/13): "While scientists can't say whether climate change contributed to this particular typhoon, they believe global warming is making storms stronger."

Here's Bryan Walsh (11/11/13) on Time's website: "The reality is that it remains extremely difficult to attribute specific weather events to climate change." And Brad Plumer in the Washington Post (11/12/13):

Detecting a clear trend here is difficult, as the Intergovernmental Panel on Climate Change (IPCC) concluded. And it's even harder to say whether the strength of a single storm like Haiyan can be attributed to man-made climate change.

The reason all these statements are backwards is that attributing particular weather events to climate change is ridiculously easy: Every weather event in the modern world is attributable to climate change. This is because weather is a chaotic system, which is to say it varies wildly based on initial conditions. Once we raised global temperature by a degree Celsius-which is an enormous intervention in the physical world-we irrevocably changed all weather, producing an entirely different set of events than the ones that would have otherwise occurred.

So climate change caused Typhoon Haiyan-in the sense that Haiyan would not have happened in the absence of climate change. Note that this is the most basic and obvious meaning of the word "cause."

Now, when journalists say that it's difficult to attribute a particular weather event to climate change, they're presumably saying something more complicated-perhaps something like "it's difficult to say whether a similar event could not have happened in the absence of climate change." Even with Haiyan-a storm more ferocious than any seen before in recorded history (Scientific American, 11/11/13)-you can always speculate that perhaps that record would have one day been broken in a world where people hadn't altered the atmosphere.

But to talk about "cause" on this level is misleading to news consumers-who I think, when they hear that, really are imagining some storms happening and others not on the basis of climate change. And it's unhelpful in terms of setting public policy, because it's defining "cause" in a way that makes it impossible to connect weather disasters to human activity.

It is possible, and important, to compare current weather events with the historical record, and point out how the current pattern differs. But it's not always easy to say what that pattern is, particularly since the climate change is an ongoing process, with a different average temperature this decade than there was last decade and than there will be next decade. The current weather is not the "new normal"; living humans will never see anything that can be referred to as "normal" again.

Journalists can help clear up this complicated situation by stressing the one thing that we know for sure: We changed the climate, and this is the weather we got as a result.