Wednesday, June 08, 2011

Why the cooling?

(This series of geology stories, explaining some of what the JOIDES Resolution contributes to human knowledge, begins on June 3. If you are a new visitor, scroll down and read from the beginning.)

So why has the Earth's average climate been cooling for the past 3 million years?

Three things determine the Earth's average temperature:

1) How much sunlight falls on the planet. This is determined by those subtle variations in the Earth's orbit, and they are generally well understood. They mostly account for the wiggles in the light blue line yesterday, but have no net long-term effects over millions of years. It could be that the Sun's output varies over the time scale of our graph, but there is no evidence that this is so.

2) The Earth's average albedo -- the degree to which the Earth's surface reflects sunlight back into space. This depends on cloud cover, sea ice, land ice, deserts, etc. For example, as the Earth cools, more ice forms, reflecting more sunlight back into space, cooling the Earth further -- positive feedback. This doesn't help much solving our riddle.

3) "Greenhouse" gases. Gases in the atmosphere such as carbon dioxide and methane trap the sun's heat, like glass in a greenhouse. Carbon dioxide (CO2) is the most important.

Which brings us to the graph below, which should be compared to yesterday's. Click to enlarge.

The graph represents the amount of CO2 in the atmosphere over the same 5 million year period, determined, or estimated, by various methods. For the past 800,000 years this is known directly from bubbles of past atmosphere trapped deep in the ice caps of Greenland and Antarctica. The other colors on the graph are based on less direct observations, such as the ratios of carbon isotopes in organic matter recovered from sea cores. The width of the color bands indicates the degree of uncertainty in the data.

What is clear is that the general trend of atmospheric CO2 parallels the temperature curve of yesterday. Here then is a strong correlation between greenhouse CO2 and climate, but we have to be careful about which is the cause and which the effect.

Now look at the two horizontal black lines, representing atmospheric CO2 in 1830 and today. This is the warning bell that has climatologists concerned about human-induced global warming. Yesterday I said "all else being equal." All else is not equal. We are pouring CO2 into the atmosphere, more CO2 today than at almost any time in the past 5 million years!

In yesterday's graph, instead of the expected drop in temperature in the next thousands of years -- if CO2 is the controlling factor -- the curve will climb toward the greater warmth of 3 to 5 million years ago. And relatively quickly.

There is still the question of why CO2 started dropping 3 million years ago. Can the JR resolve that question?

Still more tomorrow.