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An Analysis on Extreme Heat Waves and Long-Term Climate Change

The Pacific Northwest heat wave in 2021 killed hundreds, crumbled roads, melted power lines, and sparked a deadly wildfire when temperatures reached a top of 121 ℉. The extreme temperatures astonished climate scientists.

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“The 2021 Pacific Northwest heat wave appears to be the result of climate change and extraordinarily bad luck with natural variability,” says UCLA’s Karen McKinnon. Image Credit: Ketut Subiyanto/Pexels

Karen McKinnon, a Climate Scientist and Statistician, has conducted new research that confirms the scientific community’s initial shock. According to the UCLA analysis, the heat wave that hit the Pacific Northwest in 2021 was roughly a once-in-10,000-years occurrence.

It was outrageous how extreme and severe that heat wave was. Climate models struggle to capture events this extreme, and most early research puts the chances of it occurring at zero.

Karen McKinnon, Assistant Professor, Atmospheric and Oceanic Sciences, University of California, Los Angeles

McKinnon also works at the UCLA Institute for the Environment and Sustainability.

The study is published in Geophysical Research Letters’ issue dated September 28th, 2022. McKinnon, who also teaches statistics as an Assistant Professor at UCLA College, set out to ascertain two things:

  • Whether the likelihood of such a remarkable heat wave might be determined using climate models;
  • Whether the unusually high temperatures indicated that the likelihood of extreme heat waves is rising more quickly than anticipated.

The researchers studied climate model simulations and historical trends at Washington, Oregon, and British Columbia weather stations to identify the solutions.

The study discovered that climate models might create heat waves akin to the event in 2021 with a probability of happening around once per 10,000 years by grouping foreign regions that are climatologically similar to the Pacific Northwest.

The probability decreased once per 100,000 years in cities subjected to the heat wave’s highest temperatures.

They discovered that, so far, the pace of climate change is increasing both the frequency of heat waves and summer’s average temperature.

McKinnon added, “We don’t see historical evidence of hot temperatures increasing faster than average temperatures during the early summertime when the heatwave occurred. The 2021 Pacific Northwest heat wave appears to be the result of climate change and extraordinarily bad luck with natural variability.

To expand the scope of their data set, the researchers utilized similar regions, such as coastal Alaska, all of British Columbia, Canada, and the Nordic nations. The areas are often on the western shores of continents and have the same northern latitude.

Similar local climatic profiles of positive “skewness”—a lopsided temperature distribution curve with usually mild weather but a history of infrequent but hotter heat waves—form heat waves in response to stationary high-pressure systems.

Using a climate model known as Community Earth System Model 2, or CESM2, maintained by the National Center for Atmospheric Research, the researchers examined 50 climate model simulations from 1850 to 2100.

The simulations employ SSP3-7.0, a feasible emissions scenario created by the United Nations climate committee, which assumes greenhouse gas emissions will double from current levels by 2100.

The largest occurrence in 10,000 years of data transpired in the simulations, with events similar to the heat wave in the Pacific Northwest.

The good news is that we don’t find evidence that events this extreme should start happening regularly. The bad news is the summer of 2022 brought record-breaking heat waves everywhere from the United Kingdom to China to California. We need to continue evaluating whether these very extreme events are telling us something new about how the climate is changing, and whether they confirm or refute our latest findings”, added McKinnon.

However, she added that “if 10,000-year events keep happening, that suggests there may be something missing in the climate model we used” even if she doesn't expect to find that severe occurrences are warming more quickly than average temperatures.

Furthermore, McKinnon warned that even if the likelihood of catastrophic events perfectly tracks average climate change, that is still bad news.

She further stated, “If everything is moving with mean climate change, that can sound like it is not so bad, but look at the severe impacts of the climate change we are already experiencing.

To envision the future, McKinnon is motivated in part by this to keep researching large-scale climate variability and climate extremes.

The Packard Foundation and the National Science Foundation provided funding for the study.

Journal Reference

McKinnon, K. A., et al. (2022) How Unexpected Was the 2021 Pacific Northwest Heatwave? Geophysical Research Letters. doi:10.1029/2022GL100380.


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