Question No. 26

The IPCC Fourth Assessment Report claims that "anthropogenic warming could lead to some impacts that are abrupt or irreversible." However, I have read that CO2 levels were once much higher on earth in the past. Why did the earth's atmosphere not experience "runaway" positive feedbacks during those eras?
Answered 11 November 2009.
Question author: Harold Vance.
Asked 5 November 2009.

The main substance that is responsible for the greenhouse effect on Earth is water, which is present in the atmosphere in the form of gas (water vapor) and liquid (clouds). Atmospheric moisture content rises exponentially with increasing sea surface temperature. This enhances the greenhouse effect, leads to a temperature rise and, by consequence, a further increase of moisture content, and so on. This is the essence of the positive feedback of the runaway greenhouse effect.

Atmospheric concentration of carbon dioxide does not depend on temperature. Therefore, CO2 accumulation in the atmosphere cannot lead to a runaway greenhouse effect. The absorption band of CO2 covers 19% of the thermal radiation spectrum of Earth. Not taking the absorption band broadening into account, even if the CO2 concentration increases practically infinitely (not twice or thrice, but hundreds of time compared to the preindustrial period), the greenhouse effect will not grow by no more than 20%.

The main question is why the climate of Earth with its liquid hydrosphere remains stable despite the positive feedback associated with the temperature dependence of water vapor concentration, i.e. why the hydrosphere does not either evaporate or freeze. This question has not been and is not considered by IPCC and raises little interest in the meteorological community. Without a clear understanding of the nature of climate stability it is not possible to give any responsible account of what is going on with the Earth's climate under anthropogenic impact. Our research shows that a life-compatible global mean surface temperature corresponding to the liquid state of water is stabilized by the functioning of the natural ecosystems via biogenic control of surface evaporation in the ocean and evapotranspiration on land. Power of the global biota of Earth (measured in units of organic carbon turnover) exceeds the power of modern civilization by an order of magnitude (i.e., by about ten times).

Cultivation of natural ecosystems and clearing natural forests caused by global population growth leads to the destabilization of climate.

Further reading:

On the occasion of Russia signing the Kyoto protocol: Climate warming or climate collapse?

Makarieva A.M., Gorshkov V.G. (2001) The greenhouse effect and the stability of the global mean surface temperature. Doklady Earth Sciences, 377, 210-214. No abstract. PDF (0.2 Mb).

Gorshkov V.G., Makarieva A.M. (2002) Greenhouse effect dependence on atmospheric concentrations of greenhouse substances and the nature of climate stability on Earth. Atmospheric Chemistry and Physics Discussions, 2, 289-337. Abstract. PDF (0.3 Mb). Screen version (0.5 Mb).