[MUSIC] How much CO2 is too much? Well, the first question is why are concerned in particular about CO2 rather than say methane, which is another greenhouse gas? Well, the answer to that is that CO2, as you know, kind of accumulates in the biosphere, whereas methane is a transient greenhouse gas that breaks down in about a decade. So the long-term climate change that we're worried about is mostly that driven by CO2. So I've seen a sort of evolution of people thinking on this question over the decades, actually, so in the 80s and 90s, people were talking in terms of stabilizing atmospheric CO2 concentration. So trying to come up with how much CO2 we can release that would make the concentration rise and then plateau at some level like 750 parts per million, or 550 is doubling the pre-anthropogenic value of about 280, or 450 or something like that. So, when you release CO2 to the atmosphere, as you now know, some of it dissolves in the ocean and some of it goes into the land biosphere and only some of it continues to reside in the atmosphere. And so there were efforts to use carbon cycle models to figure out, given some set of emissions, what the concentration would be. And then figuring out how to exactly follow along one of these stabilization scenarios was another sort of inverse problem like the borehole temperatures, where you had to use the model to sort of go backwards from the effect back to the cause. So this was a little dodgy, because slight changes in the trajectory could have very large changes in what they said we could release this year or next year, so it's kind of a confusing situation. [MUSIC]