A look at weather extremes and the big-picture climate connections.

“Strong” Links of Manmade Heat, Rainfall Extremes: Study

Following on the heels of the March heat wave, which was one of the most remarkable extreme weather events on record in the U.S., are two new scientific papers that discuss the relationship between extreme weather events and global climate change.

Both studies help frame the discussion about how climate change is stacking the deck in favor of extreme events, including heat waves.

In particular, one of the papers, published Sunday in the journal Nature Climate Change, contains this provocative statement regarding what is already known about the relationship between manmade climate change and extreme events:

“In 1988, Jim Hansen famously stated in a congressional hearing that “it is time to stop waffling so much and say that the evidence is pretty strong that the greenhouse effect is here.” We conclude that now, more than 20 years later, the evidence is strong that anthropogenic, unprecedented heat and rainfall extremes are here – and are causing intense human suffering.”

Sea surface temperatures, along with long-term climate change, can influence temperature and precipitation extremes. Credit: Kevin Trenberth/Climatic Change.

The authors, Dim Coumou and Stefan Rahmstorf of the Potsdam Institute for Climate Impacts Research, raise the possibility that complex interactions may influence the magnitude of extremes, causing temperatures during a heat wave, for example, to far outpace the amount of background warming that has taken place during the past century.

Such interactions could include changes in soil moisture, which can enhance warm temperatures during periods of dry weather, as more of the sun’s energy is able to go towards directly heating the air. Studies have implicated soil moisture deficits in contributing to both the European heat wave of 2003 and the Russian heat wave of 2010.

Another complicating factor, the study says, are unusual atmospheric circulation patterns.

The March heat wave in the U.S. and Canada, for example, occurred during a period when a so-called “blocking pattern” was in place in the atmosphere, and weather systems over North America were progressing eastward extremely slowly, locking a heat dome in place across the Midwest and East for more than a week.

Coumou and Rahmstorf suggest that “nonlinear interactions” like these may help explain why some extreme events cross over into unprecedented territory.

A "blocking pattern" in the upper atmosphere helped keep the March heat wave going in the Midwest and East, as seen in this annotated satellite image from March 21. Click on image for a larger version. Credit: NOAA.

During the March heat wave, more than 7,000 warm temperature records were set or tied (warm daily highs and warm overnight low temperatures), and numerous records were set for the warmest temperature on record for the month of March. Many of the records that were set blew well past the old benchmarks. In some cases, the overnight low temperature was so warm that it also constituted a record daytime high, which is an extremely rare occurrence.

The bottom line, the researchers say, is that global warming makes such extreme events more likely to take place, and raises the likelihood of more intense events as well. They call for improved modeling of extreme events, noting that current models are not skilled at simulating extreme precipitation events and atmospheric blocking situations.

Coumou and Rahmstorf also wrote a blog post for realclimate.org, discussing heat wave-related questions such as: “How much hotter did global warming make this heat wave?”

The other paper, by Kevin Trenberth of the National Center for Atmospheric Research in Boulder, Colo., also argues that small shifts in the average climate can lead to large changes in extremes. Trenberth details recent extreme events around the world and how they may be linked to a combination of natural variability and manmade climate change.

Trenberth argues that a key connection between extreme weather and climate change is found via an increase in sea surface temperatures, which adds moisture to the atmosphere, and can give storms added potency.

He also pointedly rejects the most common question journalists ask about extreme events and global warming.

“Scientists are frequently asked about an event “Is it caused by climate change?” The answer is that no events are “caused by climate change” or global warming, but all events have a contribution,” he states. “In reality the wrong question is being asked: the question is poorly posed and has no satisfactory answer. The answer is 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.”

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