Can We Attribute One Specific Extreme Weather Event to Climate Change?

Last year the U.S. experienced a record 14 separate billion-dollar weather and climate disasters (the previous record of nine was set in 2008) totaling $53 billion dollars in damage (NOAA). Amazingly, 2011 ranks only fifth in damage amount (in dollars) from billion-dollar disasters; the top four are 2005, 1988, 2008, and 1980 (red dashed line in figure 1). And these were only the disasters in the U.S.; Europe and China also experienced record drought and temperatures in 2011, while Thailand and Australia dealt with record floods.

If you were to ask a scientist if these extremes can be attributed to climate change, the response would mostly likely be “we can’t attribute any one event to climate change.” He or she would then usually follow with “What we can say is that with warmer temperatures, we would expect to see more high-temperature records being broken relative to low-temperature records, and we expect to see more precipitation extremes, both wet and dry.” These are both accurate statements; it is hard to tease out natural variability from one specific event. However, with more and more records being broken, I wonder if there is a point when we can begin attributing one specific event to climate change. Does this point exist, and if so, have we already reached it? Or are there specific types of extremes that we can attribute to climate change?

Many studies on climate extremes have come out recently, including a report by the Intergovernmental Panel on Climate Change (IPCC) that describes the current knowledge on observed and projected changes in extreme weather and climate events, as well as risk management and adaptation options. The report states that in general, “single extreme events cannot be simply and directly attributed to anthropogenic climate change” because there is always the possibility that the event may have occurred by natural variability alone. The authors go on to say, however, that for “certain classes of regional, long-duration extremes (of heat and rainfall),” it is possible to argue that anthropogenic climate change has increased the probability of such an extreme occurring.

They also describe specific types of extremes and how likely—or unlikely—it is that the occurrences of these extremes have changed due to human influences. They describe it as likely that human influences have led to an increase in record daily minimum and maximum temperatures globally and an increase in extreme coastal high water due to rising sea level. There is only medium confidence, however, that human influences have led to an increase in heavy precipitation events on a global scale. As for tropical cyclones, there is low confidence that any changes can be attributed to human influences. Lastly, the authors state there is only low to medium confidence that there have been any changes in the number of droughts, tornadoes, or floods, let alone whether humans have influenced them or not. However, the IPCC tends to be conservative when describing the effects of climate change, so other studies might prove more useful in our search for single event attribution.

Sure enough, a study just recently published in Nature Climate Change reviews data on certain extremes—specifically heatwaves, rainfall extremes and tropical cyclones—over the past decade to evaluate whether these extremes are related to climatic warming. The authors, Coumou and Rahmstorf of the Potsdam Institute for Climate Impact Research in Germany, describe that about twice as many daily record highs as record lows have been observed in the U.S. over the past decade—this increase has actually risen by a factor of 2.7 since June 2010 and by a factor of over 8 during the summer of 2011 (according to another recent publication in Climatic Change). What’s more, Coumou and Rahmstorf explain that monthly heat records around the globe are now three times more likely to occur than in a stationary climate—one without human influence—and in Moscow this number is fivefold. In a stationary climate this ratio should be 1 to 1: equal numbers of record highs and record lows. This, along with statistical analyses and climate modelling led the authors to argue there is strong evidence linking at least some of these events—or an increase in their numbers—to human influence on climate.

This claim also applies to extreme rainfall events, which, say Coumou and Rahmstorf, have increased by about 33 percent in the U.S. over the past 100 years and nearly eightfold in European winters over the past 150 years. This is because a warmer atmosphere can hold more water vapor, thus the average moisture content of the atmosphere has increased. According to the authors, models show that “over approximately two-thirds of the Northern Hemisphere land area, greenhouse gases have contributed to the observed intensification of annual maxima of daily and five-daily precipitation amounts during the second half of the twentieth century.”

Tropical storms, on the other hand, prove more difficult to analyze mostly because of short data sets and an incomplete understanding of the driving forces. Coumou and Rahmstorf show that the intensity of tropical storms has significantly increased since satellite records began in the 1970s; however, they explain that scientists are still unsure if this increase is outside the realm of natural variability.

Scientists agree that with a warmer climate, we should expect to see a higher number of heat waves, an increase in daily maximum and minimum temperatures, an increase in the frequency of heavy precipitation, an increase in the intensity of tropical cyclones with a potential decrease in the number of cyclones, an increase in the intensity of droughts in some areas, and an increase in coastal high water levels (see previous blog on what we can expect to see in the Southwest). There is evidence that the globe is already experiencing these extremes, but it appears that it is still difficult to attribute one specific event to climate change. We can say, however, that most of the recent extremes we have been experiencing in the past decade—especially in regards to heat waves, maximum and minimum temperatures, and heavy precipitation events—would probably not have occurred without human influences on the climate.

So what do you say the next time someone asks you if a specific extreme weather event is caused by climate change? I think Kevin Trenberth provides the best answer in his Climatic Change publication mentioned briefly above: “The answer is that no events are ‘caused by climate change’ or global warming, but […] that all weather events are affected by climate change because the environment in which they occur is warmer and moister than it used to be.”