“Climate change is natural and normal—it’s happened at other points in history.”

It’s true that there have been periods of global warming and cooling—also related to spikes and lulls in greenhouse gases—during the Earth’s long history. But those historic increases in CO2 should be a warning to us: They led to serious environmental disruptions, including mass extinctions. Today, humans are emitting greenhouse gases at a far higher rate than any previous increase in history.