 This is a fascinating figure from the IPCC. This is actually from the 2001 report. So this one goes back a little farther. This part is CO2, and actually it's more CO2 as you move towards the left here, and that gives you more warming. So this is how much warming you get for various possible futures. So far we're tracking on the high end, but we're really not sure where we'll end up in the long. term. And each of these over here, as we release more CO2, by late in the century we get more warming, and so you can take the warming that you think we'll find, and you can draw a line across. Maybe you think we're going to get three Celsius because we're on the high end of the emissions, and so you draw the line across there. Then what you see are various columns over here. So example one here is unique and threatened systems such as species extinction. If you are a rare and endangered species living in a little national park, and now you need to migrate, and there's cornfields in the way, even a little bit of warming is pretty bad for you. And so you'll notice this is going from orange up to red or into a more saturated color very, very quickly because even a little bit of warming causes a lot of trouble for unique and threatened systems. If you're worried about when do we get more floods and more droughts, it doesn't take a lot of warming before you start getting to more floods and more droughts, and we'll be well into that before the end of the century. We're already seeing some of that now. Some areas poor people in hot places are already hurting now. Rich people in cold places, it'll take more warming before they really get into trouble. And because so much of the Earth's economy is in the rich people in cold places, you don't really worry about them until you get a good bit of warming. And the sort of, we killed the North Atlantic or dumped the Antarctic ice sheet, it takes a lot of warming until we get there. And so in terms of the question how much warming before we get into trouble that bothers a lot of people, it depends what you're talking about. And for unique and threatened systems for rare and endangered species, we're already pushing them hard as well as for poor people in hot places. The world's economy is not yet suffering hugely, but as the warming increases, it will tend to suffer more. And the basic picture, each degree of warming costs more than the previous degree. And that the first degree didn't hurt a huge amount because it's really only hurting the rare things and it's not hurting the economy. At some point, it starts to hurt the economy too.