A week after heat waves toppled records across Europe late last month, scientists were ready to declare that climate change almost certainly drove the deadly hot stretch.

On August 2, while the same heat wave was turning Greenland’s ice sheets into slush, World Weather Attribution reported that decades of greenhouse-gas emissions had raised the odds of such extreme weather by a factor of as much as 100 in some regions.

“Over France and the Netherlands, such temperatures would have had extremely little chance to occur without human influence on climate,” concluded the group, a kind of international rapid-response team set up to assess the potential role of climate change in extreme weather events that have recently occurred.

Just a few years ago, climate scientists’ standard response to questions about the cause of specific storms, floods, and droughts was, as the National Academies of Sciences noted in a 2016 paper, basically a shrug: “We cannot attribute any single event to climate change.”

But thanks to better climate models, swelling libraries of earlier analyses, and our improving understanding of these systems, researchers can now often say with virtual certainty that climate change made a particular event more likely or more severe.

These studies are giving researchers, policymakers, and the public a clearer sense of how far we’ve already diverged from the 100-year droughts and 1,000-year floods of the recent past and, in turn, what sorts of steps may be required to combat the rising threats.

“With this kind of science, we’re starting to understand what climate change really means,” says Friederike Otto, acting director of the University of Oxford’s Environmental Change Institute and co-investigator at World Weather Attribution.

Crucially, she says, the work also helps researchers test and refine climate models, in part by underscoring when, as with the European heat waves, climate change is accelerating faster than projections would indicate.

Detecting the climate signal

A growing number of regions are exploring or actively setting up efforts to quickly assess the role of climate change in unfolding events, including the European Union’s Copernicus Climate Change Service and the Australian, French, and German meteorological agencies.

World Weather Attribution is a loosely organized international collaboration of researchers at the University of Oxford’s Environmental Change Institute, the Red Cross Red Crescent Climate Centre, and the Royal Netherlands Meteorological Institute. In the wake of a severe weather event, they begin by attempting to determine its characteristics and the conditions in which it formed. The researchers look at measures such as atmospheric pressure patterns and levels of water vapor by analyzing observational data from satellites, weather stations, and other sources.

But to determine cause and effect, you need to run controlled experiments that isolate the possible variables, says Noah Diffenbaugh, a Stanford climate scientist. Since researchers can’t do that on the climate itself, they often rely on powerful computer models.