 This is one cute eastern cottontail rabbit. Young ones often have a little white mark on their forehead. If you have one cute rabbit, you have one cute rabbit. But if you have two rabbits, before you know it, you may have four rabbits. And then you may have eight rabbits. And then you may have sixteen rabbits. And then thirty-two rabbits. And then sixty-four rabbits. And more rabbits. This is an example of what is often called exponential growth. The more you have, the faster the growth. And it clearly can't go on forever. Or the whole world would be rabbits. Now this is a picture of something that looks like exponential growth. But this happens to be the growth of the size of the world economy per person over time from the year 1500 on your left up to the year 2000 on your right. And so this is something like dollars per person per year or euros per person per year. And you can see it cranking up very rapidly. The economy has grown. And that's what economists normally assume is going to happen. But exponential growth is exponential growth. And it can't go on to infinity. You can't actually just keep running up forever and ever and ever. No. Because ultimately there are limits. You run out of things infinitely. You can't be reached. The interesting question then is what does the future hold? Will the growth roll over to some sort of stable economy? Will the growth spike up and then crash down before stabilizing or before crashing completely? And those are very interesting questions that drive a lot of people to ask very big things. But the sort of will grow forever is built into some models or at least that we aren't close enough to rolling over that we have to worry about it. If we are close to rolling over and that starts to show up, then our future generations won't be as wealthy as we think they are and they won't have as easy a time dealing with climate change as we think it is. And it would be wiser for us now to do more to head off climate change.