Warmer by +3°C if we double CO2
James Hansen’s new book, Storms explains—better than any other non-technical source—how we know that climate change is strong and real. But it’s still tricky. So zFacts will simplify it for you here on this page and some sub-pages (for footnotes).
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.
How does Hansen find the answer?
He looks at what happened as the Earth warmed up from the last ice age.
1. It warmed 5°C from 20,000 years ago (ice age) until 1750.
2. Two things caused that, (1) less ice, and (2) more greenhouse gases (GHGs).
3. Less ice caused 3.5 Watts of “climate forcing” and GHGs caused 3 Watts.
4. So 6.5 Watts causes a 5°C temperature increase, or 3/4° per Watt.
5. Doubling CO2 will cause 4 Watts of forcing, so that’s 3°C of warming.
Three Scientific (non-partisan) Books. Fresh, Ultra-Competent and Surprising
Storms of My Grandchildren Hansen’s struggle to get the word out on the new climate evidence. He criticizes Republicans for suppressing science and Democrats for blocking the solution. But most important, he explains the new evidence on climate.
Carbonomics Written a year ago, Stoft explains the reasons Copenhagen is failing and prescribes a simple fix. He also explains Hansen’s fully refunded carbon tax.
Sustainable Energy—No Hot Air MacKay, a brilliant and humorous physicist, Chief UK Scientific Advisor for Climate Change. As forthright as the other two.
All Three Support Effective Action But Rethink What’s Really Needed
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.