Reports of extreme weather events — the drought in California, the heat wave in India, historic blizzards in New England — always seem to be accompanied by an important caveat: it’s difficult to trace a single weather event’s cause back to climate change. That’s largely because the influence that climate change has on atmospheric circulation — the large scale movement of air around the globe that determines when certain weather patterns arrive where — is difficult to understand. Right now, computer climate models are not robust enough to measure the small impact that climate change, as opposed to natural variability, might have on atmospheric circulation.
But a new study, published in Nature Climate Change, argues that shortcomings in the understanding of how climate change and atmospheric circulation interact shouldn’t stop us from asking a different question: did climate change play a part in worsening the weather event, even if it would have occurred without climate change?
“Assume that that weather system would have occurred anyway,” Kevin Trenberth, senior scientist with the Climate Analysis Section at the National Center for Atmospheric Research and an author of the study, told the Washington Post, “and then ask the question how the change in the environment affected the outcome, in particular through higher temperatures, greater rainfalls, more rapid drying in the case of the drought — and these things are answerable.”
To look at how an extreme weather event might have been influenced by climate change, Trenberth and his colleagues set aside atmospheric circulation dynamics to look at another part of atmospheric science: thermodynamics, which is how moisture and temperature interact. Thermodynamics says that hot temperatures lead to greater evaporation of moisture, and hot air can also hold more moisture. This is why scientists think that with climate change, extreme precipitation events are going to become more common — the atmosphere, as it warms, will simply be able to hold more moisture that can come down as rain or snow.
In understanding how a specific weather event is related to climate change, Trenberth and his colleagues present a few questions researchers might ask, such as:
- Given a particular weather pattern, how were the temperatures, precipitation, and associated impacts influenced by climate change?
- Given a drought, how was the drying enhanced by climate change and how did that influence the moisture deficits and dryness of the soils, and the wildfire risk? Did it lead to a more intense and perhaps longer-lasting drought, as is likely?
- Given a flood, where did the moisture come from? Was it enhanced by high ocean temperatures that might have had a climate change component?
- Given a heat wave, how was that influenced by drought, changes in precipitation, and extra heat from global warming?
- Given extreme snow, where did the moisture come from? Was it related to higher than normal surface sea temperatures off the coast or father afield?
- Given an extreme storm, how was it influenced by anomalous surface sea temperatures and ocean heat content, anomalous moisture transports into the storm, and associated rainfall and latent heating? Was a storm surge worse because of higher sea levels?
Looked at through this lens, the researchers claim that the 2010 snowstorm that hit Washington, D.C. was worsened by ocean temperatures in the tropical Atlantic that were 1.5° Celsius above normal, causing an unusual amount of moisture to be funneled into the storm. They also note that Superstorm Sandy was preceded by warm ocean temperatures — at least partly attributable to climate change — and that human-caused sea level rise contributed to the storm’s damaging surge.
The 2013 floods in Colorado, when looked at through a thermodynamic lens, also have a strong association to climate change, the researchers argue. The rainfall event that caused the floods originated over a patch of Pacific Ocean off the west coast of Mexico, where surface sea temperatures were 1° Celsius above normal — a warming that, Trenberth writes, “probably would not have occurred without climate change.”
Understanding how climate change impacts extreme weather events, the paper concludes, requires thinking about the interaction between storms and the changing environment in a new way — not just whether or not climate change made the storm more likely to occur, but whether or not climate change made the storm worse.
“The climate is changing: we have a new normal,” Trenberth and colleagues write. “The environment in which all weather events occur is not what it used to be. All storms, without exception, are different.”