Canceled: Climate Dynamics Seminar: Dr. Karen McKinnon, “Another record-breaking heatwave? Understanding the trends and drivers of heat extremes”

Please join us for a special seminar presented by Karen McKinnon, Ph.D. The seminar will help build the research community in Climate Dynamics in the Stanford Doerr School of Sustainability.

Karen McKinnon, Ph.D.
Associate Professor of Statistics and the Environment
University of California, Los Angeles
Tuesday, February 4, 2025
12:00 – 1:00 PM
Y2E2 299

“Another record-breaking heatwave? Understanding the trends and drivers of heat extremes”

Heat extremes result from the convergence of atmospheric and land processes across multiple spatial and temporal scales. While we can be quite confident that heat extremes will become more frequent in a warming world, major uncertainties remain in terms of how and why they will change. A recent notable example was the 2021 Pacific Northwest heatwave, which was so extreme that researchers initially struggled to put a nonzero probability on it. Using both in situ observations and a large climate model (CESM2) ensemble, we find that record-smashing extremes are much more likely to occur in regions like the Pacific Northwest that have climatologically positively skewed temperatures, which reflect the existence of processes that converge to cause very large extremes. As a result, the event was consistent with a human-caused warming of the mean combined with a low-probability sampling of an extreme weather pattern. More broadly, do we find evidence in the observational record that summertime extremes are warming faster than typical summertime temperatures? Despite the many recent high-impact heat waves around the world, we find that observations and climate models agree that the hottest summer temperatures have warmed at the same pace as the median from 1959-2023, although the coldest summer temperatures have warmed more slowly. The trends in the extremes compared to the median can be predicted from an equation derived from the surface energy balance, and are due to changes in surface radiative heating, and the fraction of heat that goes into evaporating water, on extreme versus typical days. Finally, we show that, in contrast to the general expectation of humidity increasing in a warmer climate, humidity has been decreasing on hot days across the semiarid subtropics, with concerning implications for fire risk. In the US Southwest, we trace the decrease to declining soil moisture and evapotranspiration, indicating an important land surface control on humidity trends. Collectively, the results provide a framework for understanding the probability of very unusual events in the relatively short observational record, and highlight the important role of energy partitioning at the surface for both heat and humidity trends.