 This is a plot from the IPCC, from 2007, their fourth assessment. Down here, this is basically going to more CO2 as we go into the future, and so these different things are scenarios for how much CO2 we face. This is the history of temperature going from 1900 up through 2000, and then the future of temperature as we look into the future. The warming to date is sort of one degree. That's not terribly big. The uncertainty is somewhere between one degree Fahrenheit and one degree Celsius, and so the warming is sort of one degree. If in the year 2000, we had stabilized the composition of the atmosphere, so no more changes happened in the air, we actually would have expected a little more warming, as shown on this orange curve down here, because the ocean has to catch up right now. A lot of heat is going from the air into the ocean, and as the ocean warms, the air will catch up, but not a lot more warming. These others show various paths in the future depending on how much CO2 we emit. So far, we're tracking very near or just above the uppermost of these, but we haven't gone very far yet. The things to notice, first of all, is that in all futures in which we don't do a lot to reduce CO2, the warming that is coming is very big compared to the warming which has happened. Now, the world does not end in 2100, and you'll notice that all of these curves are still heading up at least slowly and possibly rapidly as we go into the future. Some students are going to live off of this graph. You'll also notice over on the side that the uncertainties, as so often happens in this, are mostly on the bad side. So there's a most likely value, and it could be somewhat less, or there's more room on the high side. If it's more, it could be even more. And so what you see is that if we don't do a lot to head off CO2 emissions, the warming so far is very small compared to the warming that comes from the uncertainties mostly on the bad side.