The last time the atmosphere had 400 ppm CO2 was millions of years ago. That was before humans were even . . . however humans ended up here.
The plot above shows a recent reconstruction of global temperature since the end of the last ice age over 10,000 years ago. The beginning of civilization is on that blue line.
Ice ages normally end fast and restart slowly. The restart toward a new ice age is the cooling that started around 6000 years before present. It was caused by slow orbital changes that drive the ice ages.
We reversed it.
Earth should still be cooling. It stopped because we started dumping burned coal into the air, raising atmospheric CO2 from around 280 ppm to about 394 ppm now. We're increasing CO2 at about 2 ppm per year, which is orders of magnitude faster than it happens naturally.
If we do nothing more the climate will stabilize at a level never before experienced by any human. But we can't seem to stop.
If we don't stop, two things will happen.
First, natural feedbacks will take off, out of control. Forests will burn, frozen carbon in the Arctic will thaw and be released, and lots of things like those. Second, our emissions will add even more CO2.
The feared end state is a CO2 level of around 1000 ppm. The last time that happened was tens of millions of years ago, when the planet was around 30 F hotter than now.
Earth has been there before, so no worries, right?
Well, making that change so fast is a little like driving around a corner. Everything is fine at normal speed. But at 120 mph, you have a problem.
That's what we're doing: going to a place mankind has never been, at a speed of approach that is insanely fast.
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