New research released yesterday links human-caused climate change to six of 12 extreme weather events from 2012, including summer heat waves in the United States and storm surges from Superstorm Sandy.
Teams of scientists from around the world examined the causes behind extreme weather events on five continents and in the Arctic. Their results were published as a special report in the Bulletin of the American Meteorological Society.
One of the stronger linkages between global warming and severe weather was found in an analysis of last year's high July temperatures in the northeastern and north-central United States.
Noah Diffenbaugh, the Stanford researcher who led that report, found that climate change had made the likelihood of such a heat wave four times more likely than in a world without elevated levels of greenhouse gases. He and fellow researcher Martin Scherer were able to determine this by running models with current levels of greenhouse gases as well as ones that reflected preindustrial levels and examining the relative likelihood of the heat wave.
"It's clear that our greenhouse gas emissions have increased the likelihood of some kinds of extremes," Diffenbaugh said.
Other scientists, led by Thomas Knutson, of the National Oceanic and Atmospheric Administration's Geophysical Fluid Dynamics Laboratory, looked at 2012's hot spring temperatures over the eastern United States and also found that human influences contributed about 35 percent to late spring heat that year.
"[A]nthropogenic forcing leads to a factor of 12 increase in the risk of such an event, according to our calculations," the scientists write in the report.
Links to heat are easier to prove
Another extreme of 2012 was the Great Plains drought. Unlike the studies that linked the heat to climate change, researchers did not find that the lack of precipitation was related to climate change. Another research team did find that a winter drought in Spain and Portugal was partially driven by climate change.
Researchers also examined heavy rains and flooding that occurred in many parts of the world last year. They concluded that the heavy rains in Europe last year were likely due to natural variability in the climate system rather than climate change.
Tom Karl, the head of NOAA's National Climatic Data Center, also cautioned that the science of linking precipitation changes to climate change is complex.
"It is very hard to attribute a specific human contribution to a change in precipitation, although we do have more confidence in the changes in heat," Karl said.
In some other parts of the world, climate change was linked, although in a small way, to extreme precipitation events.
New Zealand experienced an extreme two-day rainfall in December 2011; researchers said 1 to 5 percent more moisture was available for that event due to climate change, which is increasing the amount of water vapor in the atmosphere.
Australia also experienced record rainfall in early 2012, and while La Niña, a natural variation, was behind much of that, researchers found that human-caused climate change increased the chance of the above-average rainfall by 5 to 15 percent.
Increased frequency of 'Sandy-like' disasters
Another clear example of where climate change is having an effect is the impact of storm surges on the eastern coast of the United States. Although storms like Superstorm Sandy are incredibly rare, sea-level rise has made a Sandy-level inundation event 50 percent more likely than it was in 1950 in areas like the Battery and Sandy Hook, said William Sweet, a NOAA oceanographer.
The research Sweet conducted suggests that, from Atlantic City south, the type of storm surge that would have been a once-in-a-century event in 1950 is likely to occur every couple decades by 2100, because of sea-level rise.
"We conclude that coastal communities are facing a looming crisis due to climate change related sea-level rise, one that will manifest itself as increased frequency of Sandy-like inundation disasters in the coming decades along the mid-Atlantic and elsewhere, from storms with less intensity and lower storm surge than Sandy," Sweet said.
The entire report contains 19 different examinations of various weather or climate events in 2012. Although 12 events were analyzed, some were investigated in multiple ways using different methodologies.
According to NOAA's Karl, this approach makes the study more robust.
"One of the things we've learned over the past decades is for us to have confidence in our results, what we need to do is to have different approaches to analyzing the data and the model results," Karl said.
This is the second time the Bulletin of the American Meteorological Society has collected information on the previous year's weather extremes and tried to tease out the role of climate change in those events. The researchers involved in the effort stressed that the science of attribution, or of linking specific events to climate change, is still young and evolving.
"The more we do this in the future ... the easier it's going to get," said Peter Stott, a researcher at the U.K. Met Office Hadley Center and an editor of the report. "This is really quite an exciting research area, and it has a real potential to provide answers to people asking questions in their particular location."