We’re only midway through spring, yet searing summer temperatures have already started baking some parts of the world.

Cities like Phoenix and Palm Springs, California, closed in on triple digits in March; Phoenix usually doesn’t reach 100 degrees Fahrenheit until May. This month, hundreds of millions of people across India and Pakistan experienced temperatures as high as 120 degrees Fahrenheit, triggering power outages and protests. The heat has also created conditions for thousands of wildfires in the region.

It’s part of a pattern. Last year, heat waves coursed through Africa and Europe during the spring, setting new temperature records in more than a dozen countries. Mexico experienced a series of heat waves beginning in April. A heatwave in Texas in May sent power demand to a record high for the month.

Heat waves are a distinct weather phenomenon where high temperatures linger for days at a time. As global average temperatures climb higher, the frequency and duration of periods of extreme heat are also growing, which is already hurting people around the world.

But the human impacts of heat waves also vary depending on their timing. Climate change is leading to shorter winters,

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