In less than a lifetime, about six and a half decades, heat waves have evolved from sporadic, isolated episodes to massive, cascading weather events that can blanket vast swathes of the planet for months or even years at a time.

The largest heat wave of the 1960s reached about 6 per cent of the planet as it drifted for 46 days over the Atlantic Ocean, parts of Latin America and West Africa.

The largest one this decade swept across almost 85 per cent of the globe during its 367-day lifetime, stretching into the polar latitudes and touching virtually every human being on Earth.

To continue calling such sprawling phenomena “heat waves” would be like calling a monsoon a rain shower. To understand the new reality of these immense expanses of heat, Reuters consulted with two dozen meteorologists and climate researchers – and analysed temperatures for every hour of every day since 1960 for more than one million grid squares covering the entire globe. Out of those results we developed a new concept: the “heat storm”.

Simply put, heat storms are supersized heat waves. Reuters uses a conservative benchmark to define an ordinary heat wave: five or more consecutive days of abnormally high temperatures. Then we define the largest heat waves – those that at their peak cover at least 1 per cent of the globe, roughly half the size of Canada – as heat storms.

Applying this definition, Reuters identified a startling transformation. Heat storms were rare in the 1960s, when they averaged about five active storms per year.

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