The IPCC report, a collaboration among 234 authors, cites more than 14,000 studies and references, covering all the shifts that are occurring in the environment, from the way water circulates to the level of moisture in soil. At the core of all these changes is heat. Global average temperatures have ticked up about 1.1°C since the Industrial Revolution, according to the IPCC, but that seemingly small number obscures the enormous and immediate spikes in temperature in particular places.

Heat waves that bring high temperatures that extend for days have become more frequent, and some areas, particularly vulnerable regions like the Arctic, are warming faster than others. These higher temperatures have a range of trickle-down effects: an altered jet stream, more intense drought and even increased precipitation, to name a few.

Any one of those ripple effects would create serious problems if it struck on its own, but when multiple ones land at the same time, the result is exacerbated. That's what is happening right now in the Western U.S., where residents are experiencing what the IPCC has called a "compound extreme event." Heat has evaporated the water supply for farmers and ranchers—not to mention local communities. States have reported hundreds of excess deaths as bodies collapse without air-conditioning in unmanageable temperatures. And heat has led to drought, which has dried up forests and created tinder for wildfires.

"It's the combination of heat waves, drought conditions and also windy conditions that allow fire propagation," says Paola Andrea Arias Gomez, an IPCC co-author and associate professor at the school of the environment at the University of Antioquia in Colombia. That's led to record numbers. Last year California experienced the worst fire season on record; this year the state has experienced nearly three times the acreage burn compared with the same point in 2020.

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