Every time the world witnesses a weather related disaster — most recently, extreme flooding in Texas and Oklahoma and a deadly heatwave in India — the attribution battle begins. Some scientists and commentators seek to explain how the event could have been worsened by climate change, even as others scoff and dismiss the connection.
Meanwhile, researchers turn to climate modeling studies and other approaches to formally examine whether the odds of a given event had shifted in a warmer climate brought on by human-induced greenhouse gas emissions — which they sometimes answer in the affirmative, and sometimes not. This has often led to big debates over, say, whether or not you can blame the California drought, at least in part, on a changing climate.
A new paper in Nature Climate Change, by Kevin Trenberth of the National Center for Atmospheric Research and two colleagues, wants to change this whole process — by changing its assumptions. “I think the attribution community has been very conservative, they always start from scratch, from a null hypothesis that there’s no influence of humans,” said Trenberth. “Yet we’ve proved over and over that there is, so why do we do it that way?”
By contrast, Trenberth and his colleagues — John Fassulo of the National Center for Atmospheric Research and Theodore Shepherd of the University of Reading in Britain — see a different default assumption for interpreting how climate influences weather.
“The climate is changing: we have a new normal,” they wrote. “The environment in which all weather events occur is not what it used to be. All storms, without exception, are different. Even if most of them look just like the ones we used to have, they are not the same. ”
More particularly, Trenberth and his team are arguing, while changes in atmospheric dynamics are very hard to blame on climate change at the present time, thermodynamic changes involving heat and moisture are relatively easy to attribute — the world’s air and oceans are, after all, warmer. So to understand what they’re arguing in the new paper, you first have to understand this distinction that is fundamental to atmospheric science — between dynamics and thermodynamics.
Dynamics governs the large scale motions of the atmosphere — the way in which the fluid flow of air molecules on a rotating planet leads to major patterns, such as gigantic cyclonic storms or the jet stream. In effect, dynamics governs the arrival of a given weather event in a given place, at a given time.
Thermodynamics, in contrast, involves how temperature and moisture shape atmospheric events. Here, hotter temperatures can lead to more evaporation of water — and are also directly related to the ability for more retention of water vapor, or humidity, in the air. (“The water-holding capacity of the atmosphere goes up exponentially at a rate of about 7% per degree Celsius,” noted Trenberth and his colleagues.) Meanwhile, hotter ocean temperatures can also have a variety of effects, such as strengthening storms like hurricanes.
The researchers therefore argue that for a given severe weather event, you should basically set aside whether climate change altered atmospheric dynamics in such a way as to make that particular event more likely. Just table it, at least until computer models or other analytic approaches get more powerful. Instead, they argue, given that an event did happen, to identify a climate influence you should simply ask if thermodynamic factors were at play in such a way as to worsen it (not cause it), and if climate change altered those thermodynamic factors in some way.
“Assume that that weather system would have occurred anyway,” said Trenberth of his methodology, “and then ask the question how the change in the environment affected the outcome, in particular through higher temperatures, grater rainfalls, more rapid drying in the case of the drought — and these things are answerable. But they’re really starting from a viewpoint that all weather systems are in fact being influenced by climate change, and then trying to estimate the magnitude of the influences that are occurring.”
The authors give a number of examples:
* 2010’s “Snowmageddon,” they said, was worsened by anomalously warm sea surface temperatures off the Atlantic coast which fed the storm moisture. “The unusually high [sea surface temperatures] in the tropical Atlantic Ocean (1.5 °C above normal) led to an exceptional amount of moisture flowing into the storm, which resulted in very large amounts of snow,” the authors wrote. “It is this last part that then relates in part to anomalous external influences on the atmosphere through the effects of climate change on SSTs.”* 2012’s Superstorm Sandy, the authors wrote, was strengthened by warmer oceans beneath the storm — partly attributable to climate change — and higher seas (also partly attributable to climate change) which worsened its landfall impact. “Although perhaps only one-half to one-third of the SST increase can be blamed on global warming from human activities, it is readily apparent that the storm surge and associated damage was considerably influenced by climate change,” wrote Trenberth and colleagues.* 2013’s deadly Supertyphoon Haiyan, which some dubbed a true “Category 6,” devastated the Philippines after running across extremely warm seas — warm to a considerable depth — and making landfall at an intensity unrivaled in historical records. This presence of deep warmth, which powered the storm, was partly attributable to Pacific ocean cycles but “there is also a global component through increased [ocean heat content] from the Earth’s energy imbalance,” wrote Trenberth and colleagues — i.e., climate change.* The 2013 Colorado Floods, caused by a staggering rainfall event, reflected the fate of huge volume of atmospheric moisture that originated over a patch of extremely warm water off the west coast of Mexico, and got pulled northward over Colorado by an “atmospheric river.” “The extremely high SSTs and record water vapour amounts that accompanied the event and were instrumental in its evolution probably would not have occurred without climate change,” wrote Trenberth and his colleagues.
The new paper appears likely to create a large debate, given how strongly it challenges traditional methods of attributing weather events to climate change (or, of failing to) — and given the overall contentiousness of the topic. And as we move into a warmer world — with 2015 currently out ahead of all prior recorded years for its temperatures thus far — the debate over the impact of a changing climate on weather events will only intensify.
“Because global warming is real and present, it is not a question as to whether it is playing a role but what that role is,” wrote Trenberth and his colleagues.
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