Global Warming May Have Fueled March Heat Wave Odds

By Andrew Freedman

According to several top scientists, the March heat wave that has shattered records across a wide swath of the U.S. bears some of the hallmarks of global warming.

In email conversations on Wednesday and Thursday, those same scientific researchers who specialize in studying the role climate change plays in influencing individual extreme events — a burgeoning field known as  “extreme event attribution” — said global warming may have made March's soaring temperatures more likely to occur, although they add that natural variability has played a key role as well.

Land surface temperature departures from average from March 8-15. Click on the image for a larger version. Credit: NASA.

Since March 12, more than 7,000 warm temperature records (warm daily highs and warm overnight lows) have been set or tied, including numerous all-time monthly high temperature records.

Although studies have not yet been conducted on the main factors that triggered this heat wave and whether global warming may have tilted the odds in favor of the event, scientific studies of previous heat events clearly show that global warming increases the odds of heat extremes, in much the same way as using steroids boosts the chances that a baseball player will hit more home runs in a given year.

Gabi Hegerl, Chair of Climate System Science at the University of Edinburgh, said there is evidence that extreme heat events have become more common and more severe, including at the regional level in parts of the U.S. ”This is consistent with observing more and stronger heat waves,” she said.

Hegerl said that in order to draw conclusions about global warming’s role in this particular heat wave, one would need to conduct modeling studies where you compare the odds of this event occurring with and without added greenhouse gases in the atmosphere, such as carbon dioxide, “to see how much the warming has changed the odds.”

The reasoning behind such an approach is because both global-warming trends and shifts in atmospheric circulation may contribute to a certain event, so scientists need to observe the whole climate system at play to investigate how often an event with these extreme characteristics takes place under the two alternative scenarios.

Surface temperature departures from average from march 13-19, showing the heat in the U.S. and Canada (in orange and red). Click on image for a larger version. Credit: NOAA/ESRL.

Studies that have followed this probability-based approach, such as one that examined the deadly European heat wave of 2003, and another that investigated the brutal Russian heat wave of 2010, have found that changes in greenhouse gases can significantly increase the odds that such severe events would occur.

A study published in the Proceedings of the National Academy of Sciences in October 2011 found that the local warming trend in the Moscow area had increased the expected number of records per decade there by five times what it otherwise would have been. The study also found that there is an 80 percent chance that the 2010 July monthly heat record would not have occurred without global warming.

This study would suggest that global warming is an enabler of extreme heat events, making them more likely to occur, but not necessarily causing each specific one, or accounting for each characteristic of a particular event.

In some ways global warming acts as an accomplice in a crime, not necessarily pulling the trigger, but still playing a significant role.

One view that some researchers expressed is that since temperatures are exceeding average values and breaking records by such wide margins, it’s likely that the comparatively modest global warming to date — U.S. temperatures have warmed by an average of about 1 degree Fahrenheit during the past century — doesn’t account for the full magnitude of what’s been going on in so much of the country this month.

As to whether global warming might be contributing to the recent hot wave, Randall M. Dole, a deputy director of research at NOAA’s Earth System Research Laboratory (ESRL) in Boulder, Colo., said, “Absolutely. The planet as a whole is warming, the continents on average are warming faster than the oceans, so there is a great body of scientific evidence that would support such an interpretation. The question is how much.”

“ . . . It's hard to attribute more than a modest fraction of the event magnitude to [manmade climate change] itself. It could well have made a truly extreme event even warmer,” Dole said.

So, according to him, global warming stacked the deck in favor of an unusual heat event, but does not fully account for how intense this event has been. He pointed to the setup of weather systems and a lack of extensive snow cover as being partly responsible for exceeding records by such wide margins.

Satellite picture taken March 21, 2012, showing the blocking high and heat dome in place over the Great Lakes and East, and a slow-moving storm over the South Central states. Click on the image for a larger version. Credit: NOAA.

Unlike other heat waves studied recently, this one occurred during March, which is normally a season of transition when the presence or lack of snow cover can make a huge difference for daily high and low temperatures. Snow reflects incoming sunlight, and tends to keep the surrounding air cooler than it would otherwise be. The lack of an extensive snowpack this year helped allow temperatures to soar to record levels during this heat wave, Dole said.

It’s also worth noting, a few of the researchers said, that this event featured a massive dome of high pressure that blocked the progression of weather systems. Such “blocking patterns” are often associated with temperature and precipitation extremes, and were present during the 2010 Russian heat wave and the 2003 European heat wave, as well. However, there is a lot of uncertainty about what causes blocking events or how global warming influences them.

Martin Hoerling, a research meteorologist at ESRL, agreed with Dole, saying that although global warming is likely playing a role in this event, it probably did not play a major one. “Meteorology, not climate change, is the main ingredient in the current March 2011 U.S. extreme warmth,” he wrote. Of climate change, he said, “. . . its contribution to the magnitude of current conditions (+30°F departures [from average]) is quite small (but not zero) indeed.”

Kevin Trenberth, a climate scientist at the National Center for Atmospheric Research, said he views the situation somewhat differently. “Indeed [greenhouse gas-driven] warming is not dominant, but I suspect when all the evidence is in we will find that the event likely would not have occurred without global warming, the odds will be so low,” he said.

It will take many months for scientists to conduct the rigorous analyses necessary to determine how global warming may have set the stage for this heat wave. For now, weather forecasters are left scratching their heads over the staggering records that have been set. In some places, such as Rochester, Minn., the overnight low temperature on March 18 was so warm that it broke the daily high temperature record.

In Chicago, where 80°F temperatures are rare even in April, the temperature exceeded that mark for a record eight days this month — including an 87°F high temperature on March 21. Chicago’s average temperature for the first 21 days of March would tie for the seventh-warmest April on record there, the National Weather Service reported.