If we had precise control over CO2 in the Earth's atmosphere...what would we dial it to? What is a safe level for humanity? Great question! Here's how I arrived at my answer for this great piece by @themadstone (1/8) https://twitter.com/themadstone/status/1347935109731770373

First off, what is too high? Most climate scientists would agree that the current level, 415 ppm, is too high. I would argue, based on past climate, 350 ppm is also too high. That's the average value during the Pliocene (3 million years ago) (2/8)

During the Pliocene, the Greenland ice sheet was mostly gone, some of the Antarctic ice sheet melted, and sea levels were 20 m (!) higher. We don't want that - NYC would be underwater (eventually...it takes a long time to melt ice sheets) (3/8)

With that much sea level rise, we're looking at building sea walls like in The Expanse (best show on TV right now btw...) (4/8)

OK then, what is too low? IMO, no reason to lower CO2 below preindustrial levels, which were 280 ppm. But at that level, eventually the Earth will cool down and we might see another Ice Age. So do we dial it a bit higher then? To what? (5/8)

Past climate change suggests something around 300 ppm is a good target. Below that level, large ice sheets in the Northern Hemisphere start to grow. Here's a paper that tests this with a climate model (6/8) https://www.sciencedirect.com/science/article/pii/S0277379115001572

Here's the context from the history of CO2 over the past 4 Ma. 415 ppm is pretty high, 350 ppm about average for the Pliocene, 300 ppm is close to Pleistocene interglacials. (7/8)

So, 300 ppm seems OK. Of course this assumes infinite international collaboration, technology, and money...so it's probably not realistic...but still, a fun exercise that emphasizes how #pastclimate informs our future. Thanks to @themadstone for the thought experiment /fin