Research Briefs

Climate variation, change, or both?

A man with an umbrella walks through floodwaters in Ambala, India, July 2010

Monsoon floods washed through Ambala, India, on July 6, 2010. Flooding killed thousands and affected tens of millions of people across Pakistan and India in the summer of 2010. (Wikimedia Commons, photo by Harsh Mangal.)

Burning fossil fuels has led to a warmer, moister atmosphere and a shifting background for extreme weather and climate events, according to a new study by NCAR scientist Kevin Trenberth. Published in the journal Climatic Change, the analysis puts a wide range of noteworthy weather events from the last two years in the context of a warming and moistening global climate.

Following an extreme event, such as a flood or heat wave, people often ask if global warming was the cause. Although Trenberth notes that natural variability plays an important role in such events, he adds that they are also affected to some extent by recent changes in the global atmosphere.

Temperatures have increased globally by around 0.9°F (0.5°C) since the 1970s. Trenberth estimates that, as a result, water vapor above the oceans has increased by about 4% in that time due to increased evaporation. Large storm systems gather and concentrate moisture from vast areas, and the extra water vapor releases additional heat and buoyancy as it condenses, says Trenberth. A storm’s precipitation amounts may be boosted by anywhere from 5 to 10%, depending on wind shear and other local factors.

Some of the warmest ocean temperatures and greatest amounts of water vapor occur during and after an El Niño event, as light winds and sunny skies allow heat to build across the tropical Indian and Atlantic oceans. Trenberth notes that in late 2010 and early 2011, during and after the most recent El Niño, the storms that produced extreme flooding in parts of China, India, Pakistan, Colombia, and Australia all drew on very moist air masses associated with sea-surface temperatures at record or near-record highs.

A number of groups now examine extreme events using computer models of atmospheric behavior to determine how much the risk of such an event has increased due to greenhouse gases. Trenberth cautions that such findings are not yet definitive, as models typically have great difficulty in simulating extremes well, especially for tropical precipitation. Even apart from such studies, he says, there is evidence for a modest but growing human influence on extreme events.

Kevin E. Trenberth, "Framing the way to relate climate extremes to climate change,” Climatic Change, 2012; DOI: 10.1007/s10584-012-0441-5