Multiple large heatwaves the size of Mongolia occurred at the same time nearly every day during the warm seasons of the 2010s across the Northern Hemisphere, according to a new study.  Using ERA5 climate data from 1979 to 2019, the researchers found that the number of heatwaves occurring simultaneously in the mid- to high-latitudes of the Northern Hemisphere was seven times greater in the 2010s than in the 1980s. On average, there were concurrent heatwaves on 143 days each year of the 2010s -- almost every day of the 153 days of the warm months of May through September. The concurrent heat events also grew hotter and larger: their intensity rose by 17% and their geographic extent increased 46%.