THIS Heat Wave IS Climate Change! (But Not Necessarily THAT One)

Sweltering heat and extreme drought has been wreaking havoc on the U.S. over the past few months, resulting in wilted crops and record wildfires. January to June was the warmest first half of the year on record. June saw over 170 all-time high temperature records broken or tied across the nation, with Colorado experiencing its warmest June on record and seven other states—including Arizona, New Mexico, and Utah—had June temperatures in their top ten warmest. May was the second warmest on record and 26 states—including Colorado and Texas—had temperatures ranking among their ten warmest.

Meanwhile, drought expanded to the point where 81 percent of the contiguous U.S. is classified as at least abnormally dry, and 63 percent is experiencing moderate to exceptional drought conditions (see figure). The main culprit, according to Climate Central: La Niña. However, other factors likely enhanced drought conditions. The winter of 2011-2012, for example, brought below-average snowfall to the Rockies, and, combined with above-average temperatures, led to earlier snowmelt and drier soils.

Scientists believe another key factor that played a role in enhancing extreme drought conditions is warming temperatures associated with climate change. The extreme temperatures made drought conditions warmer than they would be otherwise, heightening the drought and priming the Southwest for intense wildfires. But is climate change also making these extremes—exceptional drought, intense wildfires, record warm temperatures—more common? Can it be blamed for the increased numbers of climate extremes that have been occurring over the past two years?

A few months ago, I asked whether we can attribute individual extreme climate events to climate change. The scientific consensus then was that it’s still too early to attribute specific events to climate change, but that climate change definitely plays a role in some extremes. More recently, new reports shed more light on the topic and suggest attribution is possible in some cases.

Climatecommunication.org released a report in June specifically focused on identifying the link, if any, between climate change and extreme temperatures. The authors summarized the current understanding on the relationship between heat waves—a period of several days to weeks of abnormally hot weather—and climate change based on the research published through May 2012. They confirmed that climate change has increased the frequency and severity of heat waves across the globe.

According to the authors, “extremely warm nights that used to come once in 20 years now occur every 10 years,” and 10 percent of the globe now experiences extremely hot summers, up from 0.1 to 0.2 percent for the period 1951-1980. Models can only reproduce these observations when they include human influences, thus demonstrating that these extreme temperatures would not have occurred without a human influence on climate. Looking at a specific event, the Russian heat wave of July 2010, the authors cite studies showing that there was an 80 percent chance that the event would not have occurred without global warming.

Keeping the focus on individual specific events, NOAA and the Bulletin of the American Meteorological Society (BAMS) released a real-time attribution paper along with the 2011 State of the Climate report. Meant to be the first edition of an annual report, the paper assesses the causes and changes in extremes around the globe in 2011, and then specifically examines six extreme events and explains them from a climate perspective.

The extremes they examined include the Thailand floods, drought in eastern Africa, drought in Texas, extreme warm temperatures in western Europe, warm autumn in central England, and extreme cold temperatures in the UK. As expected, being able to attribute one event to climate change depends on the particular event itself and the natural climate conditions occurring at the time. What’s more, such attribution requires consideration of non-climate-related factors such as land use and water management. This was illustrated in the case of the Thailand floods, in which the authors found that the rainfall associated with the floods was not that unusual but that changes in nonclimatic factors probably played a bigger role in creating the devastating effects of the rains.

The authors of the BAMS paper did find that climate change, in particular long-term warming and associated heat waves, definitely played a role in creating the extreme warm temperatures in Europe last year, as well as the drought in Texas. In fact, the authors say that the “conditions leading to droughts such as the one that occurred in Texas in 2011 are, at least in the case of temperature, distinctly more probable than they were 40-50 years ago.”

Associating specific extreme events to climate change is challenging because there is always going to be natural variability and because there has always been extreme weather. So how do we determine if one event is attributable to climate change or if it was going to occur anyway? Scientists typically have said it’s not possible to make that distinction. But the new State of the Climate report, which will hopefully become an annual publication, demonstrates the new thinking is that attribution statements are possible, as long as natural variability and nonclimatic factors are evaluated in each particular event to determine the probability of the event occurring naturally.

Addendum 8/6/12:

It feels like every few weeks there’s a new study on climate extremes and their association with climate change, and every one seems to move us closer to making attribution statements about more and more extremes. A new paper published today in PNAS by NASA scientists concluded that the “climate dice” are now loaded to a degree in which “we can say with a high degree of confidence that events such as the extreme summer heat in the Moscow region in 2010 and Texas in 2011 were a consequence of global warming.”

This idea of climate dice they refer to is an interesting way of thinking about how global warming is predicted to (and currently is) changing the odds of more extremes occurring. The idea is that in a climate with only natural variability, there would be a 33% chance for the occurrence of colder than average temperatures, a 33% chance for average temperatures, and a 33% chance for warmer than average temperatures (a 6-sided die would have two sides of each, hence “climate dice”).

The authors found that in the summer, colder than average temperatures now occur about 10% of the time while warmer than average temperatures now occur about 75% of the time. But probably their most important finding is that extremely hot summer temperatures, which have been almost absent in the past and only covered a few tenths of one percent of the global land area, are now occurring over about 10% of global land area.

My guess is that as more and more extremes continue to occur (such as the extreme heat and drought currently covering a large portion of the U.S.), that we’ll see more and more of these studies being published, making more and more bold attribution statements.