In the midst of a stifling Washington, D.C. summer two years ago, former President Obama appeared in the White House to paint a grim picture of the challenge global warming posed to the country.
The August 2015 press conference, at which Obama announced the Clean Power Plan, was supposed to be held on the White House's South Lawn, but it was moved indoors at the last moment, after officials decided it was too hot to do the event outside. Obama might have been tempted to reference D.C.'s heat wave in his remarks that day on global warming, but the President chose his words judiciously — careful not to overstep the scientific understanding of climate change and extreme weather. "While we can't say any single weather event is entirely caused by climate change, we've seen stronger storms, deeper droughts, longer wildfire seasons," he said.
For years, careful climate scientists — and the politicians like Obama who listened to them — have avoided saying that any particular event was directly caused by climate change, even as they called for urgent action to address the issue. But researchers now say they can use a variety of approaches to show that climate change is all but certainly causing and worsening extreme weather events.
A comprehensive new report from scientists at 13 federal agencies, published this week by the New York Times as it awaits review by the Trump administration, highlights the change in thinking. The scientists behind the report, leaders in their respective fields, say researchers can use statistical analysis, modeling and other methods to determine how much climate change increased the likelihood of a given event.
Researchers behind the report point to the sweltering heat wave that swept Europe in the summer of 2003 and Australian temperature extremes in 2013 as cases with "relatively strong evidence for human contribution." Other events, like the a 2011 Texas heat wave and drought, are thought to have other primary causes, like the La Niña weather pattern. Generally, extreme temperature events are the easiest to attribute to climate change, followed by drought and heavy rain. The science around the size and severity of extreme storms remains mixed, according to the report.
The report notes that every extreme weather event analyzed to date would be theoretically possible without climate change, even if many would be incredibly unlikely. That's not the case with the record high global temperature averages the world has experienced in recent years, which would have been "essentially impossible" without human influence, according to the report.
Still, in many cases, scientists can now say with confidence that climate change worsened the effects of some events, even if the phenomenon did not cause the entire event. The report points to Hurricane Sandy as a moment when climate change exacerbated the damage, because rising sea levels increased the threat of storm surge, which contributed to flooding in coastal areas in the Northeast. Answering the bigger question of whether climate change increased the intensity or likelihood of storm would be much more difficult, according to the report.
The notion that scientists can assess how much climate change plays into a particular event is not entirely new, but its inclusion now in the National Climate Assessment, the draft report released by the Times this week, gives it a stamp of legitimacy. The National Climate Assessment, produced every four years by Congressional mandate, represents research that has been firmly established in the scientific literature. This view of climate change's relationship to extreme weather also received an important endorsement last year from the National Academy of Science in a report devoted to the topic.
"The days of saying no single weather event can be linked to climate change are over," said Heidi Cullen, chief scientist at Climate Central, a nonprofit that reports on global warming, last year. "This report makes a really important contribution in linking global warming to extreme weather."
Scientists behind the draft National Climate Assessment told the New York Times that they fear the Trump administration may try to scuttle the document because it does not fit with the White House's agenda. President Trump has called climate change a "hoax" and sought to diminish its role in federal policymaking. Press Secretary Sarah Sanders criticized the Times in a statement for not verifying the report with the White House and noted that parts of it have previously been published in draft form. Regardless, now that the report has been published in full, its conclusions are beyond the President's power to kill.