Orbitallyinduced Milankovitch climate change

Climate is extremely complex to explain in physical terms. The study of the main climate processes responsible for climate variation requires the construction of mathematical models to describe quantitatively the interacting physical elements. Reconstructions of icesheet variations over the last million years indicate that climate, directly or through feedback mechanisms, is sensitive to variations in the orbital parameters. Climate models have been used to reconstruct the relationship between orbital variations and climate in the past. Few studies have, however, attempted to predict future climate based on variations in the orbital parameters (e.g. Berger and Loutre, 1994). Climate reconstructions for the past 120,000 years and the next tens to hundred thousand years show common features, despite some minor discrepancies. The model reconstructions show, after removing potential anthropogenic effects, that the world's climate will soon begin to deteriorate towards glacial/stadial conditions (Fig. 7.1).

Oscillatory cooling with progressively colder periods is expected at approximately 5000, 23,000 and 60,000 years from now. The glacial episode around 60,000 years is expected to reach a similar magnitude to the last glacial maximum at 18-20,000 kyr bp. The model simulations all indicate that climates as warm as the present Holocene have been and will be rare. A climate such as that of the Holocene will not occur again before 120,000 years from the present. In the absence of anthropogenic forcing, model result also indicate that the long-term cooling trend which started about 6000 yr bp will continue for the next 5000 years. This first minimum will be followed by stabilization phases approximately 15,000 years and 25,000 years from now. In the northern hemisphere, the next glacial maximum will occur approximately 55,000 yr after present, leading to an average cooling of 0.01 °C per century. This is negligible on a human time-scale, and also within the expected temperature rise for the twenty-first century as a result of increased greenhouse gas concentrations. The build-up of northern hemisphere ice sheets, with an

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Figure 7.1 Predicted ice volume in the northern hemisphere resulting from astronomical and CO2 forcing. (Modified from Berger and Loutre, 1994)

Figure 7.1 Predicted ice volume in the northern hemisphere resulting from astronomical and CO2 forcing. (Modified from Berger and Loutre, 1994)

estimated volume of 27 x 106 km3 within 55,000 years after present, will correspond to a glacio-eustatic sea-level drop of about 70 m (corresponding to 10 cm per century).

In a model for the next 5000 years, Loutre (1993) assumed that the pre-industrial C02 concentration will rise from 280 to 710ppmv within the next 500 years and then decrease to 450ppmv and 350ppmv within 1000 and 1500 years from now, respectively. According to the model, the surface air temperature in the northern hemisphere will increase by 3°C, from 15°C to 18°C, over the next 500 years. The temperature changes are expected to be particularly large north of 65°N during spring and autumn, as a response to a reduction in the extent of ice and snow which is a positive feedback mechanism. Between 500 and 1500 years from now, the surface air temperature will decrease and reach a minimum between 16 and 17°C. According to the model, temperature is expected to increase slightly up to 5000 years from now due to melting of the Greenland ice sheet, which is expected to disappear at the end of the period. Great uncertainties remain, however, between the greenhouse and orbital forcing and future climate change. Three possible scenarios describing the relationship between enhanced greenhouse warming and orbital forcing have been suggested (Goodess et al., 1992). The first scenario is that a relatively short (ca. 1000 years) period of greenhouse gas-induced warming will be followed by 'natural' glacial-interglacial cycles. The second option is that the next glacial period will be delayed and less severe. The final scenario is that enhanced greenhouse warming will reduce the positive feedback mechanisms to such an extent that future glaciations will be prevented (the irreversible greenhouse effect). In conclusion, modelling results indicate that the pattern and amplitude of climatic conditions in the past million years will be experienced over the next hundreds of thousands of years. However, enhanced greenhouse-gas emissions may cause some deviations from the pattern of orbitally driven climate changes seen in the past.

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