Why is Hansen's method believable? These are all the main steps in the calculation, but each one requires several smaller steps to gain a better understanding. These are explained below and on sub-pages. But first, why is this approach (paleoclimate science) better than the huge climate models?
Because it gets the feedbacks right. What are feedbacks? For example, say GHGs warm the Earth by 1°. Warm air holds more water (and cold air less—which is why your lips chap in the winter) so there is more water in the air when temperature increases. Water is a greenhouse gas, so it warms the Earth more, say 1/2°. So the air holds more water. The Earth warms another 1/4°, etc., etc. and the grand total is 2° even though the CO2 only warmed the Earth 1° directly.
Some (positive) feedbacks (like water vapor) amplify the warming while others (negative feedbacks) do the reverse. Some are very tricky—like cloud formation—so the big models cannot get them all calculated right. But when the Earth warmed up in the last 20,000 years all the feedbacks happened and so Hansen's approach takes every feedback into account in exactly the proportion as they actually occurred.
The data are reliable. The required data are tricky, but they are self checking. Scientists have data on the ups and downs of temperature for the last 400,000 years (4 ice ages) from sampling ice that is a couple of miles deep in Antarctica. It's that old at the bottom. And trapped in the ice is a bit of air from that long ago. So they can see what GHGs were in the air.
Now here is the check. They work out (1) how much ice, (2) GHGs, and (3) the Earth's temperature. Then they use a quite-simple formula to predict temperature from ice and GHGs for 400,000 years, and the predicted temperature fits the ups and downs of the measured temperatures amazingly well. You can see for yourself. It's just too amazing to be coincidence.
Under construction Dec. 12, 2009.