Is it now possible to blame extreme weather on global warming?

That’s the question that Leo Hickman, writing for The Guardian’s Environment Blog, posed to a number of climate scientists on July 3. Here are some of the replies:

  • Kerry Emanuel, professor of atmospheric science at the Massachusetts Institute of Technology: “In my view, the only responsible statement scientists can make about this regards the probabilities of such events with and without climate change. We should be able to say something like “the annual probability of a heat wave of magnitude A and duration B before the advent of climate change was x but as a result of climate change has increased to y and is expected to further increase to between z1 and z2”. It would take some work to actually fill in the numbers x,y, z1, and z2, but there are studies along these lines for events such as the 2003 European heat wave. In my view, any statement that goes appreciably beyond statements like this one probably involves spin of one kind or another. In addition, once could talk about the particular routes by which climate change affects particular events. For example, the fires in the Rockies have apparently been affected by the ill health of many trees, owing to a population explosion among pine beetles, which is in turn partly owing to climate change.”
  • Professor Michael Mann, director of the Earth System Science Center at the Penn State Department of Meteorology: “I like to use the analogy of loaded dice. Here in the US, we’ve seen a doubling in the frequency of record-breaking heat, relative to what we would expect from chance alone. So far this year, we’re seeing those records broken at nearly 10 times the rate we would expect without global warming. So there is no question in my mind that the “signal” of climate change has now emerged in our day-to-day weather. We are seeing the loading of the random weather dice toward more “sixes”. We are seeing and feeling climate change in the more extreme heat we are witnessing this summer, the outbreak of massive forest fires like the one engulfing Colorado over the past week, and more extreme weather events like the Derecho that knocked out power for millions in the eastern US during a record-breaking heat spell.”
  • Michael Oppenheimer, professor of geosciences and international affairs at Princeton University’s Woodrow Wilson School and Department of Geosciences: “The link between extreme events which have occurred recently and the build-up of the greenhouse gases is best represented by the “loading the dice” analogy – as the world warms, the likelihood of occurrence (frequency), intensity, and/or geographic extent of many types of extreme events is increasing. The events are individual data points in a broader pattern, akin to pixels on a computer screen. You can’t say much from any one pixel, but a picture emerges when you step back and look at the pattern. That said, for a few types of extreme events, particularly heat waves, it is sometimes possible to connect the pixel to the bigger picture more directly. The best case is the European heat wave of 2003. According to computer simulations of climate, the likelihood that such an event would occur was about doubled by the buildup of the greenhouse gases. A few other events have been examined using similar techniques, including the 2010 heat wave in Russia. As for the willingness of scientists to make such statements: as the climate signal due to the ever-increasing greenhouse effect strengthens and emerges more and more from the noise in the system, and as statistical techniques for doing such “fingerprinting” studies as I mention above improves, scientists have become more confident in making such claims, which is to be expected.”
  • Harold Brooks, head of the mesoscale applications group at NOAA’s National Severe Storms Laboratory: “Attribution of extremes is challenging. We’re faced with separate, but related, questions. First, how much did the warming of the planet contribute directly to the extreme event? Second, how much more likely was the event because of a warmer planet? For things that are closely related to temperature (e.g., heat waves. fires), the first question can be addressed in a relatively straightforward manner and, typically, the answers are conservative. Even with a degree or two of global warming, the direct contribution to extreme heat, such as in the southern Plains of the US in 2011 and in much of the US in 2012, is small. The second question is more challenging to address. There are two issues that need to be considered. First is a statistical problem about how the likelihood of low probability events changes as the average condition changes. For example, if you flip a fair coin 100 times, on average you get 50 heads and 95% of the time, you’ll get between 40 and 60 heads and 2 or 3 times, you’ll get 65 heads. If you get a weighted coin that is 55% likely to be heads, it will 10 times as likely that you’ll get 65 heads. The small change in the average chance means the chance of an extreme becomes much more likely. The same thing happens for temperature extremes, but there’s another issue. Did the change in the average temperature make it more likely that the flow in the atmosphere was even more likely to occur than just by chance? For instance, when it doesn’t rain much over a large area, the ground dries out and heats up. The atmosphere responds to this by flowing around the area of hot air in a way that makes rain even less likely in the hot area, leading to more heating of the ground, reinforcing the flow around that area. For things that aren’t temperature, we have to work to understand the relationship between the global temperature and the phenomenon in question. For instance, we understand that warming the planet will likely lead to a more intense water cycle, with heavier rain when it rains and longer periods without rain in between. On the other hand, our understanding of how global scale atmospheric changes affect things like tornadoes and severe thunderstorms is that global warming will make some of the ingredients for them more likely and others less likely. As a result, it appears that long-term trends in tornado occurrence or intensity are unlikely to be large. Even without the planet warming, we would expect to see some years with many tornadoes and others with few tornadoes.”
  • Michael F. Wehner, staff scientist at the Lawrence Berkeley National Laboratory: “Whenever an extreme weather event occurs, it is natural for the public to ask, “Is this event a result of global warming?” This is not the quite the correct question to ask, as to date, all individual weather events observed could have happened prior to the human intervention in the climate system, however unlikely that may have been. The more relevant question to ask is ‘How has the risk of this event changed because of climate change?’ This risk of extreme weather, particularly very severe heat waves, has already changed significantly due to human induced global warming. For instance, the chances of the 2003 European summer heat wave, responsible for as many as 70000 additional deaths, at least doubled and likely increased by a factor of 4 to 10. The chances of the 2010 Russian and 2011 Texas events also undoubtedly increased. While these events could have occurred without the human changes to the climate, it is important to know that the amount of climate change that we have experienced so far is very small to what is projected to occur by the middle and end of this century. By 2100, today’s most extreme weather events will seem relatively normal.”

Editor’s note: Other responses in the complete article include those of Kevin Trenberth, a climate scientist at the US National Center for Atmospheric Research, Dr. Peter Stott, head of climate monitoring and attribution at the Met Office Hadley Centre, Dr. Clare Goodess, senior researcher at the University of East Anglia’s Climatic Research Unit, and Dr. Doug Smith who leads decadal climate prediction research and development at the Met Office Hadley Centre.

Image 1 for article titled "Is it now possible to blame extreme weather on global warming?"
The High Park Wildfire burns on the Arapaho and Roosevelt National Forests and Pawnee National Grassland on Thursday, June 10, 2012. The 2012 Colorado wildfires are an ongoing unusually devastating series of Colorado wildfires and include several separate fires in June and July 2012. At least 34,500 residents were evacuated in June. In 2011-2012, Colorado had an extremely dry winter, with only 13% of the average precipitation. The summer also saw temperatures near and in excess of 100ºF across the state, and most of the state had relative humidity in the teens and single digits. Dry thunderstorms and arsonists were also possible contributing factors, though the sources of some of the fires have not yet been determined (Wikipedia: 2012 Colorado wildfires)

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Land surface temperatures of March 8–15, 2012. Land surface temperatures are distinct from the air temperatures that meteorological stations typically measure. In March 2012, one of the greatest heat waves was observed in many regions of North America. First very warm air pushed northward west of the Great Lakes region, and subsequently spread eastward. This air mass movement was propelled by an unusually intense low level southerly jet that stretched from Louisiana to western Wisconsin. Once this warm surge inundated the area, a remarkably prolonged period of record setting temperatures ensued. NOAA’s National Climate Data Center reported that over 7000 daily record high temperatures were broken over the U.S. from 1 March thru 27 March. In some places the temperature exceeded 30 °C (86 °F) with anomalies up to +22°C. Temperature records across much of southern Canada were also demolished. In March, 2012, the United Kingdom experienced a heat wave with temperature anomalies of +10 °C in many places. In late June 2012, much of North America began experiencing a heat wave, as heat spread east from the Rocky Mountains. During the heat wave, the June 2012 North American derecho caused violent storms that downed trees and power lines, leaving 3 million people in the eastern U. S. without power on June 30 (Wikipedia: Heat wave)

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Tropical Storm Beryl was the strongest off-season Atlantic tropical cyclone to make landfall in the United States on record. The second tropical cyclone of the 2012 Atlantic hurricane season, Beryl developed on May 26, 2012 from a low-pressure system near the East Coast of the United States. Initially subtropical, the storm slowly acquired tropical characteristics as it tracked across warmer sea surface temperatures and within an environment of decreasing vertical wind shear. Late on May 27, Beryl transitioned into a tropical cyclone while less than 120 miles (190 km) from North Florida. Early on May 28, the storm moved ashore near Jacksonville Beach, Florida, with peak winds of 70 mph (110 km/h). It quickly weakened into a tropical depression, and dropped heavy rainfall while moving slowly across the southeastern United States. A cold front turned the storm to the northeast, and Beryl became extratropical on May 30 (Wikipedia: Tropical Storm Beryl [2012])

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A composite radar image of the June 2012 Eastern United States derecho. A derecho is a widespread, long-lived, straight-line windstorm that is associated with a fast-moving band of severe thunderstorms. The June 2012 Mid-Atlantic and Midwest derecho was one of the most destructive and deadly fast-moving severe thunderstorms in North American history. The progressive derecho tracked across a large section of the Midwestern United States and across the central Appalachians into the Mid-Atlantic States on the afternoon and evening of June 29, 2012, and into the early morning of June 30, 2012. It resulted in at least 22 deaths, widespread damage and millions of power outages across the entire affected region. The storm prompted the issuance of four separate severe thunderstorm watches by the Storm Prediction Center. A second storm in the late afternoon caused another watch to be issued (Wikipedia: June 2012 North American derecho)