 This wonderful plot from Noah and the IPCC is about where carbon is in the Earth's system and where it's going. The numbers of how much is in a place are in gigatons of carbon, billions of tons, and how fast it's going is in billions of tons per year of gigatons of carbon per year. The black numbers are before humans started messing with it, and a number like this one up here is how much carbon was in the atmosphere before human influence. The red number up here is how much humans have added to the atmosphere. The numbers with arrows are how much was going naturally in a particular direction before humans were in, and then how much the human change has been. And so you can see fluxes of carbon and you can see reservoirs of carbon. Perhaps the key one, they have estimated that before humans got into the game there were about 3700 gigatons of carbon as fossil fuels, of which we have burned only 244, so there's lots more carbon that can be burned in the system. They show a flux here of 6.4, that's probably about 9 now, but you'll notice that at that rate there's actually something like 400 years of fossil fuels remaining. Now if everyone burned it at the same rate as we do in the United States, that would drop it down to 100 or less, and if population continues growing it would be even less than that. And if we start to get into trouble when half of it is burned, then most of you students actually will live to see the time when we're getting into economic difficulties because fossil fuels are starting to get scarce. Nonetheless, it's pretty clear that there is much more fossil fuel in the earth than we've burned so far, and so we can make a lot bigger change than we've done so far.