The warmth of the earth is dependent on the balance between incoming solar radiation and outgoing radiation from the earth. Warmth from the earth’s core and from mankind’s direct activity are relatively unimportant. Since the sun’s radiation is essentially constant, what is important is the reflectivity of the earth, oceans and atmosphere known as the albedo and the heat trapping of gases. The gases are CO2, water vapor, and methane, with an honorable mention to nitrous oxide (N2O), a byproduct of agriculture. What terrifies climate scientists is the existence of positive feedback loops, for example, the warmer it gets, the more water vapor is in the atmosphere which traps radiation thereby accelerating warming and adding still more water vapor to the atmosphere. At some point the earth is warm enough that the radiation out into space equals the warmth being trapped, unfortunately for Venus, that level is 464°C.
CO2
CO2 in the atmosphere traps the long wavelength infra-red reradiated from the earth’s surface. The sun, which we see as a yellow ball, puts out shorter wavelength radiation which penetrates the atmosphere quite well, if it is not reflected from the tops of clouds or by sulfur particles from coal burning, ship bunker fuel, other emissions, and yes, volcanoes. When earth surfaces, including pavement and trees absorb that radiation, they re-radiate some of it as heat, which is longer wavelength infra-red, and CO2 does a great job of capturing the energy in that radiation and trapping it in the atmosphere. This is a good thing as without it the earth would freeze and be around -17°C. But too much of a good thing….
Unfortunately, CO2 in the atmosphere has a natural half-life in the atmosphere of around 1,000 years. Once CO2 enters the atmosphere, it stays there and continues to warm the earth. As a result, even if we stopped adding CO2 to the atmosphere by 2050, while we might hit some target for 2100, the earth doesn’t care about calendars and will continue to warm for another hundred plus years. Hence, we really do need to pull CO2 out of the atmosphere at some point.
Prior to man’s use of fossil fuels around 250 years ago, the CO2 concentration in the atmosphere was around 280 PPM (parts per million) and it is now approaching 418 PPM, the highest it has been for 3 million years, and has gotten there in an incredibly short time. This overwhelms the earth’s natural absorption and buffer mechanisms, and life’s ability to adjust.
CO2 is also absorbed in the oceans, indeed more than half of the CO2 emitted is in the oceans where it is acidifying the upper layers of the oceans and interfering with shell, coral, and diatom formation on which life in the ocean depends. If we reduce the amount of CO2 in the atmosphere, the partial pressure, this CO2 in the oceans will return to the atmosphere. To avoid doing twice the work in the sky, we need to reduce the CO2 in the oceans via reducing the ocean’s acidity, helping with the ocean biosphere problem.
The amount of CO2 we have dumped into the biosphere is around 1,500 gigatons. As we discussed in What’s a Gigaton, a gigaton is a lot and 1,500 gigatons is a staggering amount.
But this CO2 it is the product of the entire society we have built since 1750 including population growth from 650mm to 7,800 million people, and a life that is incomparably better for almost everyone in the world than 1750, all powered by burning fossil fuels. Even if we heroically reduce carbon emissions to zero over the next 30 years from our current equivalent of around 50 Gt/year, this is still an average of 25GT/yr x 30 = 750 more Gigatons. For round numbers, to get us back to where we were for hundreds of thousands of years, we need to ultimately pull 2,000 Gigatons out of the biosphere.
We will discuss water vapor, methane, and nitrous oxide in a later essay. But as a preview water vapor is a very potent gas at trapping heat---it’s why Jacob in the Bible complained about the cold of the desert, where there is very little moisture and why it is always warmer on a cloudy night. But many types of clouds have white tops that are good at reflecting the sun’s radiation. Water vapor is the key problem in building climate models.