It is "extremely likely" that human activities are the "dominant cause" of global warming, according to the most comprehensive study ever of climate science by U.S. government researchers.
The climate report, obtained by NPR, notes that the past 115 years are "the warmest in the history of modern civilization." The global average temperature has increased by about 1.8 degrees Fahrenheit over that period. Greenhouse gases from industry and agriculture are by far the biggest contributor to warming.
The findings contradict statements by President Trump and many of his Cabinet members, who have openly questioned the role humans play in changing the climate.
"I believe that measuring with precision human activity on the climate is something very challenging to do," EPA Administrator Scott Pruitt said in an interview earlier this year. "There's tremendous disagreement about the degree of impact."
That is not consistent with the conclusions of the 600-plus-page Climate Science Special Report, which is part of an even larger scientific review known as the fourth National Climate Assessment. The NCA4, as it's known, is the nation's most authoritative assessment of climate science. The report's authors include experts from leading scientific agencies, including the National Oceanic and Atmospheric Administration, NASA and the Department of Energy, as well as academic scientists.
The report states that the global climate will continue to warm. How much, it says, "will depend primarily on the amount of greenhouse gases (especially carbon dioxide) emitted globally." Without major reductions in emissions, it says, the increase in annual average global temperature could reach 9 degrees Fahrenheit relative to pre-industrial times. Efforts to reduce emissions, it says, would slow the rate of warming.
"This is good, solid climate science," says Richard Alley, a geoscientist at Penn State University, who says he made minor contributions to the report's conclusions on sea level rise. "This has been reviewed so many times in so many ways, and it's taking what we know from ... a couple of centuries of climate science and applying it to the U.S."
The assessments are required by an act of Congress; the last one was published in 2014. Alley says this year's goes further in attributing changes in weather to the warming climate, especially weather extremes. "More heat waves and fewer cold snaps, this is very clear," he says. The report also notes that warmer temperatures have contributed to the rise in forest fires in the West and that the incidence of those fires is expected to keep rising.
Some of the clearest effects involve sea level rise. "Coastal flooding, you raise the mean level of the ocean, everything else equal you get more coastal flooding," Alley says. The report notes that sea level has risen 7 to 8 inches since 1900, and 3 inches of that occurred since 1993. The report says that rate is faster than during any century over the past 2,800 years.
The report also points out that heavy rainfall is increasing in intensity and frequency across the U.S., especially in the Northeast, and that is expected to keep increasing.
Other connections are harder to nail down, Alley says, such as whether a particular hurricane can be attributed to climate change.
"The Climate Science Special Report is like going to a doctor and being given a report on your vital signs," says environmental scientist Rachel Licker of the Union of Concerned Scientists. She notes that the authors assessed more than 1,500 scientific studies and reports in making their conclusions.
Alley adds that the new report "does a better job of seeing the human fingerprint in what's happening." He says that while he hasn't read all of it yet, he sees no evidence that it has been soft-pedaled or understates the certainty of the science.
Alley notes that "there's a little rumbling" among climate scientists who are concerned that the Trump administration will ignore this effort. "I think the authors really are interested in seeing [the report] used wisely by policymakers to help the economy as well as the environment."
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