Thanks to unbridled greenhouse gas emissions, our planet is stitching together a climate version of Dr. Frankenstein’s monster. We still have ice from the warmer parts of the Pleistocene even as our temperature approaches the warmer Pliocene levels of 3 million years ago. Meanwhile, our CO2 level is between the Pliocene and the Miocene of 10 million years ago, and we risk an Eocene hothouse not seen in 40 million years.
At some point, this unnatural fusion of incongruous climate parts must resolve into a new equilibrium—but at what point? And what does that equilibrium look like? Much of that is up to us, based on how fast we reach net-zero greenhouse gas emissions. But it’s also up to our planet—how “sensitive” it is to greenhouse gases and how quickly it reacts to changes.
Discovering our planet’s sensitivity to greenhouse gases has been a “holy grail” for scientists since the 1970s, but it has stubbornly resisted attempts to constrain it. The best we can do is a wide range: 1.5° to 4.5° C of warming if CO2 levels double. That’s a huge temperature range, and we’re likely to double preindustrial CO2 levels this century even as we want to avoid warming above 2° C. Narrowing this range will be key to understanding what our Frankenstein-like climate will look like when it settles into a new equilibrium.