How Scientists Link Climate Change to Worse Heat Waves, Droughts, Wildfires and Floods

For several decades, scientists have gathered evidence that the global warming caused by greenhouse gas emissions is making some types of extreme weather more likely or more intense. It’s a bigger challenge to connect climate change to a particular weather event at a specific time and place. But the science that enables researchers to do just that has advanced over the past 20 years to the point where individual heat spells, storms, floods, droughts and wildfires are now routinely tied to global warming.

Researchers first classify a weather event as extreme by putting it in the context of observations of the same type of event in the same area, ideally over a long period of history. Weather can be capricious, so just because an event is extreme doesn’t mean humanity’s greenhouse gas pollution played a role. A possible connection is assessed by using computer models to create two virtual worlds. One of them, the so-called counterfactual world, is built by keeping carbon concentrations constant at a level in the past before people began burning fossil fuels. In the other, actual concentrations are plugged in. Then researchers compare the weather event in the two scenarios. This methodology doesn’t determine whether global warming caused the event — but rather whether it made it more likely, more severe or both. One of these three was the determination in 71% of the more than 500 extreme weather events or trends reviewed by researchers since 2011, according to a count up to August maintained by CarbonBrief.org, a UK-based nonprofit that covers developments in climate science.