CO2 and temperature increase in close synchrony in the geological records, illustrating that once the temperature begins to rise (which it can do for a number of reasons) CO2 becomes the dominant force to continue increasing the temperature. By contrast, CO2 is often slower to decline in atmospheric concentration than the temperature declines. What does this mean?
. A bit of preliminary information is needed as context. The primary source of energy in the Earth’s climate system is incoming solar radiation, termed total solar irradiance, or TSI. At equilibrium, because of the requirement of energy conservation, the Earth’s radiative budget must balance such that TSI is equal to outgoing longwave radiation at the top of the atmosphere (a state known as radiative equilibrium). Since the radiation emitted by a body is a function of surface temperature, the Earth’s ‘effective temperature is the temperature at which radiative equilibrium is achieved assuming the Earth acts like a blackbody. The majority (75%) of the greenhouse effect is due to the warming effects of water vapour and clouds, with the non-condensing greenhouse gasses (predominantly CO2 and CH4) accounting for the remaining 25%. However, at the temperatures and pressures typical of the Earth’s surface, water vapour and clouds act as feedbacks rather than drivers of the greenhouse effect, with CO2 and CH4, and the other non-condensing GHGs (for example, N2O) determining the overall strength of the greenhouse effect. (Lacis, A. A., Schmidt, G. A., Rind, D. & Ruedy, R. A. Atmospheric CO2: principal control knob governing Earth’s temperature. Science 330, 356–359 (2010).)
If we examine the full 800,000 year record we can see that CO2 change tracks closely with the temperature change. In addition, there are other factors that affect the amount of heat retained; surface reflectance or albedo of the Earth surface for example. Ice reflects about 95% of the incoming solar irradiance, while water absorbs over 99%. So melting ice is also an important factor. All of these interact as the Earth’s orbit and angle to the sun vary over time modifying the incoming solar irradiance.
The graph shows it takes longer to cool down than to warm up. It also shows the CO2 sometimes lags behind the temperature decrease. This means the CO2 is less influential as the temperature declines. Instead other factors such as the increasing albedo as more snow and ice form are important. Plants, especially trees declining slowly in abundance (because they have a built in structure) so the change is slow. To create an ice age, the ocean must evaporate huge amounts of water and deposit it as snow on the land where it gradually compresses to ice. This takes longer than to melt the ice when the temperature was increasing. Finally dust is an important factor in changing the landscape and ocean. As plants die, they gradually expose the land. We know that increasing dust is associated with the declining temperature. As dust enters the ocean, productivity increases. As the plankton builds new bodies, carbon is capture and removed from the atmosphere, causing cooling.
The second graph examines the a past interglacial period in a bit more detail. You can see the onset of CO2 change at about 88,600 years ago, while the onset of temperature increase is a little later at 88,000 years, 600 years before temperature rises. At 84,100 years, CO2 begins to drop about 800 years before temperature falls.
The third graph illustrates the relationship between temperature and dust in the Antarctic. The build up of dust coincides with the decline in temperatures in the ice core record. As the dust reaches the maximum extent and the temperature is at its lowest, there is an abrupt warming.
And there is still another very important factor in triggering warm periods or cold period: the Milankovitch cycles. Although the graph is quite complex, if you examine it closely you will see that the temperature changes are tied to the highs and lows of the strength of the solar energy striking the Earth at about 65N. The relationship is not exact, but is primarily acting as a trigger rather than tracking exactly with the changes in temperature.
Our planet Earth is a complicated system with many interacting parts, so the explanations are equally complex and sometimes hard to understand.