WEATHER IS NOT CLIMATE. OR IS IT?

• Author: Bailey, Ronald
• Position: SCIENCE

NEWS OUTLETS HAVE long cited extreme weather events as examples of how greenhouse gas emissions affect the climate. In response, experts typically would emphasize the distinction between weather and climate, warning that any given hurricane or heat wave cannot be attributed to longterm changes in average temperatures. But it turns out that climatologists and meteorologists sometimes can establish such causal relationships.

"First of all, it's important to highlight that every climate extreme weather event has multiple causes," Friederike Otto, an Oxford University climate researcher associated with the World Weather Attribution (WWA) collaboration, told MIT Technology Review in 2020. "So the question of the role of climate change will never be a yes or no question. It will always be, 'Did climate change make it more likely or less likely, or did climate change not play a role?'"

For the last decade or so, climate researchers like Otto have been working on statistical techniques aimed at estimating the extent to which a warmer world is making weather events more extreme and/or increasing the frequency of such events. One technique is to run climate models using the current levels of atmospheric greenhouse gases to see if they reproduce the relevant observed weather trends for a region. If the models accurately track the actual record of weather events, the researchers next run them assuming pre-industrial greenhouse gas concentrations. The differences in, say, the maximum temperature during a heat wave, the amount of rainfall dumped by a hurricane, or the timing and extent of wildfires provide an estimate of how much man-made warming may contribute to specific extreme events.

Consider the massive heat wave in June 2021 that produced record-breaking temperatures in the Pacific Northwest, including highs of 116 degrees Fahrenheit in Portland, Oregon; 108 degrees in Seattle, Washington; and 121 degrees in Lytton, British Columbia. Under preindustrial conditions, WWA researchers found, the chance of a heat wave like that was essentially zero. "Western North American extreme heat [was] virtually impossible without human-caused climate change," they concluded.

According to the latest comprehensive report on the physical-science basis of man-made climate change from the U.N. Intergovernmental Panel on Climate Change (IPCC), "it is extremely likely that human influence is the main contributor to the observed increase in the intensity and frequency...