When tornadoes rip through the Plains States, a hurricane batters the Gulf Coast, or a drought bakes California, at some point the question will inevitably be asked: Was this event caused by climate change? It's a simple question that rarely if ever has a simple answer. But, according to a new report from the National Academies of Sciences, Engineering and Medicine, it's now possible to estimate the influence of climate change on at least some types of extreme events.
The report – Attribution of Extreme Weather Events in the Context of Climate Change – notes that asking "Did climate change cause X?" fails to recognize the context in which extreme weather events take place.
Such events, of course, have always occurred, and would be doing so now even if human activity wasn't affecting the climate. They also require multiple contributory factors, but they do not occur in a vacuum: All weather, extreme or otherwise, is a function of the prevailing climate. So the picture can, depending on how you approach it, sometimes appear muddied.
For example, in the summer of 2010, Russia baked in a heatwave that saw daytime temperatures in Moscow surge past 100F, higher than any temperature listed in records that date back to 1879. One study reviewing that heat wave argued that it was largely natural because the temperature anomalies were greatly in excess of those explainable by long-term warming trends; another, in contrast, concluded that the anthropogenic influence was significant because long-term climate change, though small, greatly increased the probability of exceeding specified temperature thresholds.
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Retired Rear Adm. David W. Titley of Penn State chaired the committee that wrote the report. He noted in a press briefing today that there are better questions to ask than "Did climate change cause this?" He suggested instead: "Are events of this severity becoming more or less likely because of climate change?" or "To what extent was this event more or less intense because of climate change?" That may seem less satisfying, but it allows the subject to be framed more accurately.
Marshall Shepherd, director of the University of Georgia's Atmospheric Sciences Program, a past president of the American Meteorological Society, and a co-author of the new report, uses a sporting analogy to describe the difference: "Home run frequency and length varied naturally in Major League Baseball, but after the steroid era the influence was seen in the home run statistics. But was a certain baseball player's 300th home run caused by steroids? That's an ill-posed question."
The new report argues that, while the causation question may be unhelpful, climate science has, in the words of co-author Adam Sobel of Columbia University, "reached the point where we can look for the human influence on climate in single weather events, and sometimes find it."
Scientists are developing the ability to do this, says the report, by analyzing two worlds: the real one, in which atmospheric carbon dioxide levels are rising as a result of human activity, and an imaginary one in which the Industrial Revolution never happened and the climate is only influenced by solar and volcanic activity. By running computer simulations for both worlds, they are able to gain a sense of how much more likely a particular drought or flooding event is to occur in one world or the other.
To do so accurately requires what Titley called the "three legs of a stool:" sound physical principles - a clear understanding of how a changing climate might affect a particular type of event - consistent observational evidence and the ability for numerical models to replicate the event. By using those criteria as benchmarks, the report found that science was far better at assessing the links between climate and some types of events than others.
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In general, the report concluded, the case is easiest to make with events that are most directly related to temperature, such as extreme heat, because, Sobel noted in the Washington Post, "the chain of causality from global warming to the event is shortest and simplest." That Moscow heatwave, for example, which might have occurred once in a lifetime in the fictional world, is now what Titley called a "once in a mortgage event," which could be expected every 30 years or so.
Such things as drought and extreme rainfall are more complicated, whether because of a shorter observational record, less reliable models or a less robust understanding of the physics - or some combination of the above. As report co-author Heidi Cullen of Climate Central wrote in the New York Times on Friday, the ongoing California drought is an example of this complexity: "While we now know that higher temperatures resulting from global warming are worsening the drought, current evidence indicates that the lack of precipitation in the state is not primarily a result of climate change."
Confidence is low or non-existent in making the climate connection with tornadoes because, observed Sobel, "our models don't yet have enough resolution to simulate them - like a digital camera with too few pixels to see someone's face from far away - their relation to temperature is indirect, and not enough research has been done for us even to be sure how they should be changing."
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Not everyone is a fan of attempting to assess the influence of a changing climate on extreme weather at all. "Disentangling the contribution of climate change with natural variability for a single event is not fruitful or currently possible in my opinion," said Oklahoma University meteorology professor Jason Furtado.
But others counter that, where climate change can feel like a slowly-unfolding and abstract concept, extreme weather events grab the public and media attention, and being able to speak accurately about the climate context in which they take place can help people understand the legitimacy and urgency of addressing global warming.
Additionally, Shepherd argued at Friday's briefing that such efforts are important because, "The question (of whether climate change was responsible for individual weather events) was being abused by both sides of the ledger. It was being abused by those that subscribe to the consensus science and the peer-reviewed literature; it was being abused by some who would say, 'See, they're blaming every extreme event on climate change so we can't believe anything.' We needed an assessment. It was time."
And what that assessment showed, Cullen told Carbon Brief, is that while the science of extreme event attribution may be in its early stages, "The days of saying no single weather event can be linked to climate change are over. For many extreme weather events, the link is now strong."