Heavy rainstorms like those that caused last week’s devasting flooding across a 12-county region of West Virginia are almost certainly made more frequent and more intense by global warming that is fueled by the buildup of greenhouse gas emissions, some of the nation’s top climate science experts say.
A half-dozen climate scientists from around the country — asked what West Virginians should understand about the flooding and climate change — all said that residents should know that the overwhelming scientific evidence shows that the warming of the planet’s atmosphere is increasing the occurrence of and the seriousness of heavy rains.
“The frequency is increasing and the intensity is increasing,” said Jake Crouch, a Beckley native who is a climate scientist with the climate monitoring branch of the National Oceanic and Atmospheric Administration’s National Centers for Environmental Information.
Explained simply, warmer air holds more water, leading to stronger and more frequent heavy precipitation events. Confidence in the link between heavy rain and climate change is strong partly because there is a relatively direct connection between a more moist atmosphere and human-induced warming. This relationship is not as direct as for an increase in temperature, but more direct than for something like a tropical cyclone.
“One of the robust predictions of climate science is that the heaviest rain events will become more frequent as the climate warms,” said Andrew Dessler, a professor of atmospheric sciences at Texas A&M University. “We have good statistical evidence that this is indeed occurring.”
Dessler pointed, for example, to the findings of a special report on extreme weather that was produced by the U.N.’s Intergovernmental Panel on Climate Change in 2012. That report concluded that it is likely that the frequency of heavy precipitation or the proportion of total rainfall from heavy rainfalls will increase this century over many areas of the globe. In some regions, it is expected that, even while total precipitation could decrease, increases in heavy precipitation events will also occur.
“This is one of our most confident expectations for the outcome of continued global warming,” said Ken Kunkel, a climate scientist at North Carolina State University and NOAA’s National Climatic Data Center. “Global warming will, with almost complete certainty, lead to higher amounts of water vapor in the atmosphere.”
Across the world, most scientists say that drastic reductions in carbon dioxide emissions, like those from coal-fired power plants, are urgently needed to avoid the most serious impacts of climate change. Just this week, 31 leading science organizations sent a letter to Congress to caution lawmakers that climate change is already occurring, citing among other things concerns about extreme weather events.
“The severity of climate change impacts is increasing and is expected to increase substantially in the coming decades,” the letter said.
In West Virginia, public officials ignore such warnings. Many of them question or ridicule the science, and officials at the federal and state level continue to vigorously oppose efforts by the Obama administration to force cuts in greenhouse gas emissions from coal plants.
Generally, there is little focus in the public debate about such matters on the potential for serious local effects from changes in the climate system, though one state group, Friends of Blackwater’s Allegheny Highlands Climate Change Impacts Initiative, has been trying to bring more attention to the issue.
And, in a 2013 report on mitigation of various potential disaster threats, the state Division of Homeland Security and Emergency Management noted that, “Climate change is both a present threat and a slow-onset disaster. It acts as an amplifier of existing natural hazards.”
“Extreme weather events have become more frequent over the past 40 to 50 years, and this trend is projected to continue,” the report said. “Climate change is expected to have a significant effect on communities, including those in West Virginia. For instance, more frequent intense precipitation events may translate into more frequent flash flooding episodes.”
The report added, “As climate science evolves and improves, future updates to this plan might consider including climate change as a parameter in the ranking or scoring of natural hazards.”
Most of the scientists contacted by the Gazette-Mail about the flood pointed to the findings of the National Climate Assessment, a lengthy and detailed report from a collection of federal agencies combining various scientific findings about climate change in the U.S.
“The heaviest rainfalls have become heavier and more frequent, and the amount of rain falling on the heaviest rain days has also increased,” that report states. “There has also been an increase in flooding events in the Midwest and Northeast, where the largest increases in heavy rain amounts have occurred.”
Among other things, the report says that West Virginia’s region has experienced a greater increase in extreme precipitation than any other region in the country. Between 1958 and 2010, the region saw more than a 70 percent increase in the amount of preciptiation falling in heavy events, which are defined as the heaviest 1 percent of daily events.
“This increase, combined with coastal and riverine flooding due to sea level rise and storm surge, creates increased risks,” the report said. “This will increase the vulnerability of the region’s residents, especially its most disadvantaged populations.”
One study that is noted in the assessment concluded that a two-day rainfall that might have occurred only once every five years in the early 20th century has become almost 40 percent more frequent since then.
Marshall Shepherd, director for the atmospheric science program at the University of Georgia, said that the West Virginia storms appear to have led to such heavy rainfall because of a particular meteorological setup that included a process called “training,” in which storms line up over the same spot like the cars of a train.
“The topography was an amplifier,” Shepherd said. “When such rains fall in mountain-valley systems, the drainage into the ‘vulnerable’ valley regions where people live and work is evident.”
Shepherd served recently on a National Academy of Science Panel that examined the latest efforts at climate change attribution, or the science of linking climate changes to specific weather events.
In its report, released in March, that expert panel concluded that the ability of scientists to accurately perform these attribution studies has increased greatly.
“In the past, a typical climate scientists response to questions about climate change’s role in any given extreme weather event was, ‘We cannot attribute any single event to climate change,’” the report said. “The science has advanced to the point that this is no longer true as an unqualified blanket statement. In many cases, it is now often possible to make and defend quantitative statements about the extent to which human-induced climate change (or another causal factor, such as a specific mode of natural variability), has influenced the magnitude or the probability of occurrence of specific types of events or event classes.”
Recent studies, for example, have looked at climate change attribution following the 2010 Russian heat wave, the 2011 Texas drought and the ongoing California drought. The American Meteorological Society publishes an annual journal special issue that examines event attribution.
Climate scientists noted that attribution studies take months or years of analysis before they can be published in the peer-reviewed literature. At the same time, the authors of the National Academy report on the subject said that the public and the media often ask the wrong questions about climate change following major extreme weather events.
“The question often asked by the public and the media, ‘Was this extreme event caused by anthropogenic climate change, yes or no?’ is not well posed, because ‘cause’ can have several different meanings,” the academy report said. “In this case, a more informative rephrasing of the question could be ‘are events of this severity becoming more or less likely because of climate change?’”
The report also emphasized that, while “many conditions must align to set up a particular [weather] event” and “extreme events are generally influenced by a specific weather situation,” all “events occur in a climate system that has been changed by human influences.”
Chris Field, a Stanford University climate scientist who chaired the panel that wrote the IPCC’s extreme weather report, said, “The most important three things people need to know are that climate change is real, it is happening now, and all of us are vulnerable.”
“The horrifying recent floods in West Virginia starkly demonstrate the vulnerability,” Field said. “For an extreme to become a disaster, there need to be three things, 1) a hazard or triggering event, 2) valuable assets at risk, and 3) a lack of sufficient preparation.
“It is always tragic to see the havoc caused by a disaster like the floods in West Virginia,” Field said. “We can minimize the risk of future losses through a combination of steps to reduce vulnerability and actions to limit the amount of climate change that occurs.”
Reach Ken Ward Jr. at firstname.lastname@example.org, 304-348-1702 or follow @kenwardjr on Twitter.