Numerous large heatwaves the size of Mongolia happened at the same time approximately every day during the warm seasons of the 2010s throughout the Northern Hemisphere, according to a study guided by scientists from the Washington State University (WSU).
Applying climate data from 1979 to 2019, the scientists learned that the number of heatwaves taking place concurrently in the mid- to high-latitudes of the Northern Hemisphere was seven times more in the 2010s than in the 1980s. On average, there were simultaneous heatwaves on 143 days each year of the 2010s — nearly every day of the 153 days of the warm months of May to September.
The simultaneous heat events also became hotter and larger: their intensity grew by 17% and their geographic extent grew to 46%.
More than one heatwave occurring at the same time often has worse societal impacts than a single event. If certain regions are dependent on one another, for instance for agriculture or trade, and they’re both undergoing stresses at the same time, they may not be able to respond to both events.
Cassandra Rogers, Study Lead Author and Post-Doctoral Researcher, WSU
Details of the study have been published in the Journal of Climate.
Heatwaves could result in disasters from crop failures to wildfires. Simultaneous heatwaves can increase those threats, the authors highlighted, draining the ability of nations to offer mutual aid in crises as was witnessed during the numerous wildfires in the United States, Australia and Canada related to the 2019 and 2020 heatwaves.
An earlier study also discovered that concurrent heatwaves caused approximately a 4% decrease in crop production worldwide.
The study described large heatwaves as high-temperature events spanning three days or more and covering no less than 1.6 million km2 (around 620,000 square miles), which is approximately equivalent to the size of Iran or Mongolia.
The scientists examined ERA5 data generated by the European Center for Medium-Range Weather Forecasts, which combines massive amounts of observational data from weather stations on land, aircraft and water buoys as well as data from satellites with weather forecasting capacities.
ERA5 offers complete global estimates of hourly data for different climate variables from 1979, when satellite data became available, which is why the research concentrated on this time period.
With this observational data, the scientists learned that the main driver of the heatwaves was the overall increase in global mean temperature because of climate change. The world has warmed 1 °C (about 1.8 °F) over the last 100 years with the vast majority of the increase, two-thirds, happening since 1975.
The team also discovered that increasing incidence of two hemisphere-wide circulation patterns rendered certain areas more susceptible to simultaneous heatwaves, including eastern North America, East Asia, eastern and northern Europe and eastern Siberia.
The research adds more proof for the need to control greenhouse gas emissions and alleviate climate change, the scientists said, and the unrelenting increase in temperature means the world should get ready for more simultaneous heatwaves.
As a society, we are not currently adapted to the types of climate events we’re experiencing right now. It’s important to understand how we can reduce our vulnerability and adapt our systems to be more resilient to these kind of heat events that have cascading societal impacts.
Deepti Singh, Study Co-Author and Associate Professor, School of the Environment, WSU
Besides Rogers and Singh, authors involved in this study include Kai Kornhuber of Columbia University, Sarah Perkins-Kirkpatrick of the University of New South Wales in Australia, and Paul Loikith of Portland State University. This study received support from the National Science Foundation and the Australian Research Council.
Rogers, C.D.W., et al. (2021) Six-fold increase in historical Northern Hemisphere concurrent large heatwaves driven by warming and changing atmospheric circulations. Journal of Climate. doi.org/10.1175/JCLI-D-21-0200.1.