Man-made climate change a factor in 2012 extreme weather, study

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Darren McCollester

By Renee Lewis / Al Jazeera America / Sept. 7, 2013

Man-made climate change may have contributed to half of the extreme weather events of 2012, scientists said in a new report in the Bulletin of the American Meteorological Society (AMS).

The report (PDF), published Friday by scientists with the National Oceanic and Atmospheric Administration (NOAA), is based on studies of about a dozen extreme weather events from around the world in 2012 — from northern Europe’s unusually wet summer to Hurricane Sandy.

The scientists were aiming to answer the question from which their field of study, known as “climate attribution science,” was born: How many of these events were caused by man-made climate change as opposed to natural variability?

Their conclusion, based on factors such as sea-surface temperatures, changes in incoming solar radiation and many others, is that human influence varies from one event to another. But in about half of the extreme events studied, they found that man-made climate change was a contributing factor.

Attempting to decipher whether or not climate change is the dominant factor in these events is a complex process.

Using an analogy, the AMS report explained: “Adding just a little bit of speed to your highway commute each month can substantially raise the odds that you’ll get hurt some day. But if an accident does occur, the primary cause may not be your speed itself: it could be a wet road or a texting driver.”

Likewise, while climate models indicate that compounded human effects increase the frequency of extreme-weather events — “much like speeding increases the chances of having an accident” — natural variability may still be the primary factor in any individual event.

Hurricane Sandy’s intensity, for example, was reportedly compounded by both human effects and natural variability.

Global warming caused by human atmospheric pollution makes storms more frequent and intense, the report said. Hurricane Sandy happened to hit the East Coast at peak storm-tide levels and close to local high tide — what researchers behind the report call natural variability. Further aggravating the intensity of the storm was Sea Level Rise (SLR), which has been shown to make flooding events worse.

‘Century scale events’

In 2012 the United States experienced its most severe drought in decades, with more than half of all counties in the country listed as natural-disaster areas by the Department of Agriculture.

“For much of the central U.S., such conditions of combined scarcity of precipitation and elevated temperature had not been experienced since the Dust Bowl years of 1934 and 1936,” the report said.

The drought resulted in low or even zero crop yields, reduced livestock inventory, increases in food prices and at least 123 direct human deaths, according to the report, which called it a “century scale event.”

The report attributed the drought to a combination of factors, including increased surface heating due to human pollution — more specifically, greenhouse-gas emissions.

A recently convened Intergovernmental Panel on Climate Change (IPCC) examining extreme weather events said it was confident that the central United States would experience an increase in duration and intensity of droughts in the coming years.

The report seemingly supports IPCC’s claim, finding that such large-scale droughts are four times as likely amid current temperatures than at pre-industrial levels. A model scenario developed by the NOAA scientists found that increases in temperature resulted in decreases of overall precipitation.

Higher temperatures have also been blamed for extensive ice loss in the Arctic, but the report found that it is not that simple. Melting ice, the report says, cannot be wholly attributed to human effects, but to a natural, gradual warming of Earth.

Yet global warming from man-made atmospheric pollution did contribute to a large storm which transited over the Arctic, breaking up the ice and sending a significant amount to warmer waters.

Some climate models project that the Arctic will be ice-free in summer within a decade — spelling disaster for low-lying countries such as Bangladesh and Pacific Island nations already feeling the effects of SLR.

Future of climate change

NOAA scientists behind the report say they hope to help provide “information that governments, organizations, and individuals can use to manage climate risks and opportunities.”

According to the Environmental Protection Agency (EPA), global average temperatures have already increased more than 1.4 degrees Fahrenheit over the last 100 years. Scientists project the Earth’s average temperatures to rise between 2 and 12 degrees Fahrenheit by 2100.

Increasing the average global temperature by a degree or two can have serious global consequences.

For every 2 degrees Fahrenheit of warming, the EPA says it expects a 5 to 15 percent reduction in crop yields; a 3 to 10 percent increase in rain during heavy precipitation events which can lead to flooding; a 5 to 10 percent decrease in stream flow and some river basins; and a 200 to 400 percent increase in the area burned by wildfire in parts of the western United States.

The magnitude and frequency of future climate change depends on the rate at which levels of greenhouse gas emissions increase in the atmosphere, and how strongly temperature, precipitation and sea levels respond to expected increases in emissions, the report said.

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