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-Medium-Low (B

-High (SRES A1

Medium-High (A

-Medium-Low (B

-High (SRES A1

Medium-High (A

1850

1900

1950

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2050

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Met Office Hadley Centre

FlG. 1.12. Projected emissions of carbon dioxide (in units of 1012 tonnes of carbon per year) from fossil-fuel burning for four future emission scenarios, together with estimated emissions from 1850-2000. ©Crown copyright (UK Met Office/Hadley Centre 2005).

surface air temperature is accompanied by more frequent extreme high and less frequent extreme low temperatures. There is a decrease in diurnal temperature range in many areas, with nighttime lows increasing more than daytime highs.

Results for the four SRES emission scenarios give a mean global surface temperature warming between 2000 and 2100 that ranges between 1.5 and 6.0 C, depending on the scenario (Fig. 1.13). The B1 scenario, representing perhaps the most likely world view, at least as seen from 2006, gives about a 2 C warming according to the Hadley Centre HadCM3 climate model. Warming over the next three or four decades is similar for all scenarios, due to the thermal inertia of the climate system and the effective long lifetime of carbon dioxide. Temperatures over land are expected to be higher than over the oceans, with a warming between 3 and 8 °C. Overall, the projected rate of warming is much larger than the observed changes during the twentieth century, and, according to palaeoclimate data, is very likely to be without precedent during at least the last 10000 years.

1.10.2.2 Sea-level rise Sea levels can rise under global warming through the thermal expansion of the oceans (the main contribution, see Chapter 11), the influx of glacier, snow and ice-melt water, and melting of the Greenland and Antarctic ice sheets. The IPCC examined sea-level rise from seven climate models driven by the above emission scenarios and found a range of predicted sea-level rise from 0.1 to about 0.9 m (Fig. 1.14). All models give regional variations

FlG. 1.13. Global temperature rise according to the four SRES emission scenarios, predicted by the Hadley Centre climate model (HadCM3). The range of results from other climate models, according to IPCC, are also given. ©Crown copyright (UK Met Office/Hadley Centre 2005)

of sea-level rise that can be twice the global.

1.10.2.3 Precipitation As the climate warms, the Northern Hemisphere sea-ice extent decreases. The globally averaged mean water vapour, evaporation and precipitation increase. Snowfall is predicted to increase over Antarctica according to the Hadley Centre model; the actual extent of this effect, which represents negative feedback since it tends to cool the global climate by increasing the planetary albedo, varies between models. Most agree that tropical areas and high latitudes have increased mean precipitation, while subtropical areas experience a decrease. The intensity of rainfall events increases everywhere, with more interannual variability of northern summer monsoon precipitation. A general decrease in soil moisture in the midcontinental areas during summer is expected.

1.10.2.4 Thermohaline circulation Most models predict a weakening of the Northern Hemisphere thermohaline circulation, which contributes to a reduction in the surface warming in the northern North Atlantic. However, there is still a warming over Europe due to increased greenhouse gases. In experiments where the atmospheric greenhouse-gas concentration is stabilized at twice its present-day value, the North Atlantic circulation recovers from initial weakening within one to several centuries. However, beyond 2100, a complete and possibly irreversible shutdown of the thermohaline circulation becomes a possibility, with

FlG. 1.14. Projected rise in global sea level for the highest- and lowest-emission scenarios based on IPCC estimates. ©Crown copyright (UK Met Office/Hadley Centre 2005)

consequences that have been little explored but are likely to be severe.

1.11 Bibliography

1.11.1 Notes

Most aspects of the climate system summarized in this chapter, including basic definitions of its components, are covered more fully in the references IPCC 2001 and Hadley Centre 2005 below. See also the Intergovernmental Panel on Climate Change Website, at http://www.ipcc.ch/, and the review by Houghton.

For more detailed coverage on the North Atlantic Oscillation (NAO) see Hurrel and Dickson, and Wang. The El Nino Southern Oscillation ENSO is discussed by Hanley et al.; Philander; and Trenberth and Stepaniak; there are good reviews by Wang and Fiedler; Fedorov et al.; and Dijkstra. Recent results on radiation measurements linked to ENSO are presented by Pavlakis et al.

For a review of direct and indirect radiative forcing due to tropospheric aerosols see Haywood and Boucher.

1.11.2 References and further reading

Bolle, H-J. (2003). Mediterranean climate. Variability and trends. Springer, Berlin.

Croll, J. (1867a). On the change in the obliquity of the ecliptic, its influence on the climate of the polar regions and on the level of the sea. Philos. Mag., 33, 426-445.

Croll, J. (1867b). On the eccentricity of the Earth's orbit, and its physical relations to the glacial epoch. Philos. Mag., 33, 119-131.

Dijkstra, H. A. (2006). The ENSO phenomenon: theory and mechanisms. Adv. Geosc., 6, 3-15.

Fedorov, A. V., Harper, S. L., Philander, S. G., Winter, B. and Wittenberg, A. (2003). How predictable is El Nino?, BAMS, 84, 911-919.

Hadley Centre (2005). Climate change and the greenhouse effect. A briefing from the Hadley Centre. UK Met Office, Exeter.

Hanley, D. E., Bourassa, M. A., O'Brien, J. J., Smith, S. R. and Spade, E. R. (2003). A quantitative evaluation of ENSO indices. J. Climate, 6, 1249-1258.

Haywood, J. M. and Boucher, O. (2000). Estimates of the direct and indirect radiative forcing due to tropospheric aerosols: a review. Rev. Geophys., 38, 513543.

Hurrell, J. W. and Dickson, R. R. (2005). Climate variability over the North Atlantic, in Marine ecosystems and climate variation. The North Atlantic - A comparative perspective, ed. N.C. Stenseth, G. Ottersen, J.W. Hurrell and A. Belgrano. Oxford University Press, Oxford.

Houghton, J. T. (2005). Global warming. Rep. Prog. Phys., 68, 1343-1403.

IPCC, 2001: Climate change 2001: The scientific basis. Contribution of working group I to the third assessment report of the Intergovernmental Panel on Climate Change. Houghton, J. T., Ding, Y., Griggs, D. J., Noguer, M., van der Linden, P. J., Dai, X., Maskell, K. and Johnson, C. A. (ed.), Cambridge University Press, Cambridge, United Kingdom and New York, NY, USA.

Milankovitch, M. M. (1941). Canon of insolation and the ice age problem. Konig-lich Serbische Academie, Belgrade. English translation by the Israel Program for Scientifc Translations, United States Department of Commerce and the National Science Foundation, Washington D.C.

Pavlakis, K. G, Hatzidimitriou, D., Drakakis, E., Matsoukas, C., Fotiadi, A., Hatzianastassiou, N., and Vardavas, I. M. (2006). ENSO surface longwave radi ation forcing over the tropical Pacific. Atm. Chem. Phys. Diss., 6, 1-34.

Philander, S. G. (1990). El Nino, La Nina, and the Southern Oscillation. Academic Press, San Diego.

Rossow, W. B. and Schiffer, R. A. (1999). Advances in understanding clouds from ISCCP, Bull. Am. Meteorol. Soc., 80, 2261-2287.

Trenberth, K. E. and Stepaniak, D. P. (2001). Indices of El Nino evolution. J. Climate, 14, 1697-1701.

Wang, C. and Fiedler P. C. (2006). ENSO Variability and the Eastern Tropical Pacific: A review. Prog. Oceanogr., 69, 239-266.

Wang, C. (2005). ENSO, Atlantic climate variability, and the Walker and Hadley circulations. In The Hadley Circulation: Present, Past and Future. H.F Diaz and R.S. Bradley (ed) Kluwer Academic Publishers, Amsterdam.

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