Being subjected to different natural and anthropogenic forcing, the Earth's climate changes at a variety of time-scales. In the course of the history of the Earth, a general cooling trend can be observed. The turn from the generally warm Paleozoic and, especially, Mesozoic epochs to the Quaternary is a marked example of this general cooling. This latter turn has been followed by the periodic variations of the Earth's climate in the Quaternary with its alternate glacials and interglacials. The current interglacial, the Holocene, is lasted unprecedently long, about 11 ky. In the last millennium, substantial variations related to natural and anthropogenic forcing were observed. The last century and a half are unprecedented in terms of the rate of the climate change.
To apply the knowledge about past climates for future climate changes, a method of paleontology's analogs has been suggested. The backbone of this method is an assumption that for future warmer periods it is possible to find a past warm epoch, which may approximate the climate state for this future epoch. This method has been proved to be useful as a diagnostic tool in inferring about possible climate changes. However, quantitative results from this method are to be treated with caution. The causes involved in an expected greenhouse warming and in the formation of the past warm epochs may be quite different. In particular, the Eemian interglacial and the Holocene optimum are cited frequently as analogs of future greenhouse warming. However, for both these paleoepochs, only summer warming has been demonstrated while the values for annual mean temperature change remain uncertain (see above). In addition, reconstructions for these warm paleoepochs represent mean conditions for rather long intervals (centuries or millennia) while the current warming progresses at much shorter timescales (decades) as well.
As the past epochs represent climate states quite different from the present-day one, they may be useful for validating climate models. For instance, if we compare model simulations and available reconstructions of past epochs with high atmospheric concentration of carbon dioxide, we get that the most probable range of the climate model sensitivity to the doubling of the carbon dioxide in the atmosphere is 3.0 ± 1.5 °C.
See also-. Temperature Patterns.
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