That’s some weather we’re having. Is it climate change?

In the public’s mind, it’s impossible to separate the climate from the weather. Each significant weather event seems to be accompanied by discussions of its implications for climate change; is it an example of what to expect, or clear indications that climate change isn’t happening?

Often lost in the public discussion is that determining the role of climate change in a specific weather event is a challenging but interesting scientific problem. It’s also one with immense practical implications. As regions rebuild after a damaging event, it’s important that these efforts be informed by what we should expect in the future.

This month’s edition of the Bulletin of the American Meteorological Society tackles this problem, termed “attribution,” in a big way: 22 different studies of weather events rolled into a single report entitled “Explaining Extreme Events of 2013.”

There’s no one single agreed upon method of doing attribution studies, and several studies often look at the same event from different perspectives. The link between climate change and the events varies from strong to non-existent, as well. Both of these factors ensure that the issue provides an excellent perspective on the challenges of attribution, as well as the sort of information it produces.

Dry in California

A handful of studies focus on California’s 2013 drought, which was the most severe dry period in the historic record. From a weather perspective, the dryness had been linked to a persistent ridge of low pressure air over the northwest Pacific—it even picked up the nickname “the Ridiculously Resilient Ridge” in the popular press. The ridge blocked the normal storm track out of Alaska that brings winter rains to the US West Coast, and it appears to have been the immediate cause of the drought.

One of the papers that looks at the drought shows that the event truly was rare. It estimates that, were the climate stable, events like it would only come along every 420 years, and we’d be unlikely to see another for at least 285 years. Another paper fed climate models the sea surface temperatures that prevailed at the time, showing that they reproduce the ridge and the lack of rainfall. Since the sea surface has been warming along with the climate, you might think that would point a finger at climate change. And, to an extent, it does.

But things aren’t quite that certain. It turns out that the warming oceans also pump humidity into the atmosphere, and some of that ultimately ends up falling as rain in California. When both factors are considered, “The above two effects appear to counteract each other,” one of the papers concludes. In other words, if you look at an average year, the extremes might be larger, but there’s no obvious trend toward drought due to the warming Pacific in this region.

Hot across the Pacific

Australia suffered through a difficult heat wave last year, but it wasn’t the only country to do so; Japan and Korea did as well. Here, the link with climate change is obvious. When the average temperature goes up, it shifts the entire bell curve of expected temperatures with it. As a result, extremely rare hot events become more likely. A number of papers use different means of showing that this simple, neat explanation actually does work for these heat waves. “CMIP5 simulations suggest that the extremely warm year observed over Australia and the far western Pacific during 2013 was largely attributable to human forcing of the climate system,” reads one example. “Anthropogenic climate change has caused a very large increase in the likelihood of extreme events such as the record Australia-wide average temperatures,” reads another.

The same thing is true of a drought that occurred across the Tasman Sea in New Zealand. Extreme heat of the sort that Korea experienced last year is now 10 times more likely due to climate change. Heat waves in Europe and Japan are approached in a similar way.

But it’s important to note that almost all of these studies simply said that we should expect more extreme events like this due to climate change, not that this particular weather event can be traced to a specific change in the climate. So, this is less a specific attribution and more a recognition of increased probabilities. The one exception seems to be a study that looked at what happened in China, which assigned about 60 percent of the increased risk to human influences.