Jun 23, 2015
The debate over climate change has long focused on determining attribution--whether rising greenhouse gases and global warming caused a particular storm, drought, flood, or blizzard. Now, a new study in Nature Climate Change published Monday seeks to shift the underlying question from "if" to "how."
"The climate is changing," wrote National Center for Atmospheric Research scientists Kevin Trenberth and John Fasullo and University of Reading physicist Theodore Shepherd in their study, Attribution of Climate Extreme Events. "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."
As keen observers have noted for years, attributing extreme weather events to climate is the easy part. Trenberth and his team say it's time to start analyzing events from the assumption that climate change does influence all weather systems--and focus instead on how it influences them.
Trenberth explained to the Washington Post on Monday that "the attribution community has been very conservative, they always start from scratch, from a null hypothesis that there's no influence of humans. Yet we've proved over and over that there is, so why do we do it that way?"
The key is understanding the difference between atmospheric dynamics and thermodynamics when it comes to weather patterns.
"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."
As the Post explained:
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.
The Guardianput the concept this way: "The general rule of thumb is that areas which are currently dry will become drier; areas that are currently wet will become wetter; and rains will occur in heavier downbursts."
We should assume that any given weather event "would have occurred anyway," Trenberth told the Post, "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."
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"Given a particular weather pattern, how were the temperatures, precipitation, and associated impacts influenced by climate change?" the researchers posited in their study, among other questions. "Given a flood, where did the moisture come from?... Given a heat wave, how was that influenced by drought, changes in precipitation, and extra heat from global warming?"
The authors listed a number of recent events that sparked the outdated attribution debate, including the 2010 blizzard in Washington, D.C. colloquially referred to as "Snowmageddon"; Superstorm Sandy, which hit New York in 2012; Supertyphoon Haiyan, which devastated the Philippines in 2013; and the Colorado Floods of the same year.
All of those weather systems were worsened by anomalies like high ocean temperatures and human-caused sea level rise, the researchers found. In Colorado, for example, "the extremely high sea surface temperatures and record water vapor amounts that accompanied the event...probably would not have occurred without climate change," the study states.
Meanwhile, in cases like the 2010 heat wave in Russia or the ongoing drought in California, thermodynamics again proved that those events were worsened by climate change, the authors say--even as previous studies found conflicting results when researching if climate change even caused it.
Why do those studies provide such limited results? The Guardian continues:
Without getting too deep in the weeds, the authors also explain why other teams have failed to make a connection between extreme weather and a warming planet. In some cases, they have asked the wrong questions. In other cases, they have used tools that were too crude. For instance, calculations performed in 2014 by another team relied upon climate models that did not have sufficient resolution.
In other words, the researchers conclude, "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."
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