It’s official: We can now say global warming has made some weather events worse

Flood waters saturate the Tall Timbers subdivision of Haughton , La., on Wednesday. (AP Photo/Mike Silva)

From Hurricane Katrina to Superstorm Sandy, to last year’s devastating heat wave in India, the question never ceases to arise: Did global warming cause this?

That question has long made scientists squirm. They know the atmosphere is a complicated place. They know weather events are the result of a huge array of factors — like, say, the El Niño event that has been making weather really, really weird lately. And they know their notion of causation itself is far more complex and multifaceted than the colloquial understanding of the term “cause.”

But now, says a new report from the National Academy of Sciences exploring the attribution of extreme events to climate change, scientists can be at least a little bit less conservative about this.

In the past, a typical climate scientist’s response to questions about climate change’s role in any given extreme weather event was “we cannot attribute any single event to climate change,” says the report, which was composed by a committee of 10 scientists led by David Titley of Penn State University. “The science has advanced to the point that this is no longer true as an unqualified blanket statement.”

Rather, the new report argues, careful studies can be conducted that suggest that the probability of a given event occurring was increased because of the current state of the planet. This can be done by observing how far a single event, like a heat wave, is outside the norm of prior events, or by using computer models to determine how often an event like it would occur with or without human greenhouse gas emissions (which can be included or excluded from the model).

As with most things in science, it’s best to use several methods, combining the approaches above, to reach a stronger conclusion.

“In many cases, it is now often possible to make and defend quantitative statements about the extent to which human-induced climate change (or another causal factor, such as a specific mode of natural variability) has influenced either the magnitude or the probability of occurrence of specific types of events or event classes,” says the report.

But this is also far easier for some types of events than others. Major heat waves and extreme heat events are the easiest to attribute, the report says. After all, the reasoning isn’t difficult: With the world as a whole hotter, it’s no surprise to see heat records being broken, or more precisely, the breaking of considerably more heat records than cold records.

When it comes to saying climate change influenced a weather event, “our reports say that you can do this for some events now, especially heat/cold (higher confidence) and heavy rainfall/drought (medium confidence),” said Titley, who headed the study.

Granted, such claims still have to be backed up by a serious scientific study. A great example of this kind of attribution occurred last year, when a study of Australian temperature records found both that vastly more hot records than cold records were being set (over 12 to 1 was the ratio from 2000 to 2014), but also that in a climate model only including greenhouse gases produced a similar pattern of record-breaking.

Then, there are events that are related to rainfall or precipitation. That includes both downpours and droughts. Here, too, the study says attribution can sometimes be done, because the physics is often rather simple, particularly for heavy rain events.

“Heavy rainfall is influenced by a moister atmosphere, which is a relatively direct consequence of human-induced warming, though not as direct as the increase in temperature itself,” the report finds.

But there are also “greater levels of uncertainty for events that are not directly temperature related,” the research finds. It says that the uncertainty is highest when it comes to attributing wildfires (which, after all, can be started by human carelessness), extra-tropical cyclones (winter storms or blizzards), and severe-convective storms (thunderstorms that can produce tornadoes) to climate change.

There is “little or no confidence in the attribution” of the last two kinds of events, the study finds.

And there remain some other caveats, said J. Marshall Shepherd, an atmospheric scientist at the University of Georgia who also served on the committee. “We caution against extrapolating from one study to make big sweeping statements about all aspects of climate change,” Shepherd said. “We also caution that there is some selection bias in what events are studied.”

In the end, then, the report says you still can’t say climate change “caused” an event, but it may well have made that type of event more likely, or worse. But you still have to do your homework to say even that.