Attribution of Recent Climate Change - Earlier Climate Changes

Earlier Climate Changes

Factors other than increased CO₂ concentrations can initiate warming or cooling episodes (see, e.g., orbital forcing). The ice core record shows that on some occasions temperature starts rising hundreds of years before CO₂ increases. Such results confirm that the relationship between CO₂ and climate can go in both directions: changes in CO₂ concentrations affect climate, while changes in climate can affect CO₂ concentrations. One proposed mechanism for this effect is increased release of sequestered CO₂ from oceans as circulation patterns shift, perhaps abruptly, in response to climate change.

A more speculative and polemical inference sometimes drawn is that the causal relationship between temperature rises and global CO₂ concentrations is only one-way, so that historical increases in CO₂ have been nothing more than the product of independently rising temperatures. However, a strictly "one-way" view of the relationship between CO₂ and temperature contradicts basic results in physics, specifically the fact that the absorption and emission of infrared radiation by CO₂ increases as its atmospheric concentration increases.

First principles as well as empirical observation suggest that positive feedbacks from CO₂ concentrations amplify warming initially caused by other factors:

Close analysis of the relationship between the two curves shows that, within the uncertainties of matching their timescales, the temperature led by a few centuries. This is expected, since it was changes in the Earth’s orbital parameters (including the shape of its orbit around the Sun, and the tilt of Earth’s axis) that caused the small initial temperature rise. This then raised atmospheric CO₂ levels, in part by outgassing from the oceans, causing the temperature to rise further. By amplifying each other’s response, this "positive feedback" can turn a small initial perturbation into a large climate change. There is therefore no surprise that the temperature and CO₂ rose in parallel, with the temperature initially in advance. In the current case, the situation is different, because human actions are raising the CO₂ level, and we are starting to observe the temperature response.

Present CO₂ levels greatly exceed the range found in the ice core data. Isotopic analysis of atmospheric CO₂ confirms that fossil fuel burning is the source of most of the CO₂ increase, unlike during prior interglacial periods. As noted above, models that include increased CO₂ levels when simulating recent climate match the observed data far better than those that do not.