| Summary: | Extreme heat occurred over Alaska in June–July 2019, posing risks to infrastructure,
ecosystem, and human health. It is vital to improve our understanding of the causes of such events
and the extent to which anthropogenic forcing may alter their likelihood and magnitude. Here, we use
multiple large ensembles of climate models, comprising thousands of simulated years, to investigate these
issues. Our results suggest that the presence of anthropogenic radiative forcing increased the likelihood of
the 2019 extreme heat event by as much as 6%. Further we show the rate of occurrence of such an extreme
heat event is likely to substantially increase in the future with increasing levels of atmospheric greenhouse
gases. While uncertainty in projected climate risk from model choice leads to a broad range of future
extreme heat event probabilities, some models project that with rapidly increasing levels of greenhouse
gases the likelihood of such events would exceed 75% by 2090.
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