Jim Hansen's Answer to the Climate Question:
Warmer by +3°C, if we double CO2
No Computer Models. Climate models cannot be understood by the non-expert. Fortunately James Hansen, one of the world's leading climate scientists takes a paleoclimate approach that does not rely on computer models. This approach, explained in his recent book Storms, will be described here.
James Hansen's uses paleoclimate science to explains how we know that climate change is strong and real. Paleoclimate science studies the Earth's climate before human records. Of course not much is known, but temperature at various latitudes have been estimated and the extent of ice (from snowfall) has been estimated from pre-historic sea levels.
More accurate. In spite of these limits, Hansen claims that "climate sensitivity" has be estimated more accurately with paleoclimate science than with large computer models.
The key climate question is: "How sensitive is the climate to greenhouse gases? This is called "climate sensitivity" and it's usually described by how much the Earth will warm if we double the CO2 from what it was in 1750. Hansen's answer is that the Earth would warm 3°C, or 5.4°F.
So far we've increased CO2 from 280 to 387 ppm, or 38%, so we should have already warmed 1.1°C, or 2°F. That's a little more than we've seen so far, but that could be because of natural fluctuations or man-made counter-effects, such as small sulfur particles from coal-fired power plants in China—they cool the Earth as volcanoes do.
The estimation procedure looks at what happened as the Earth warmed up from the last ice age.
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.