IPCC Assessment of Anthropogenically Driven Climate Change

A major goal of the Third Assessment of the Intergovernmental Panel on Climate Change (IPCC) has been to identify the possible role of humans in influencing the recent variations in global climate. In order to address this question, two central terms have to be clarified first (Intergovernmental Panel on Climate Change, 2001). Detection is the process of demonstrating that an observed change is significantly different (in a statistical sense) than can be explained by natural variability, and attribution implies the establishment of cause and effect of an observed climate variation with a defined degree of confidence, including the assessment of competing hypotheses.

The objective lies in detecting a shift in climate and attributing it to an anthropogenic cause. Since climate change (i.e., an anthropogenically caused change in climate parameters; see above) always occurs against the background of climate variability, the detection and attribution of anthropogenic factors amount to a statistical "signal to noise" problem. The first task lies in establishing that the recently observed changes deviate from the internal variability of the climate system. To that end, numerical global climate models are utilized to determine the magnitude of global mean temperature anomalies and compare these with the observed trends for the last 140 years (see above). This comparison clearly reveals that the recent changes cannot be accounted for by internal variability alone (Intergovernmental Panel on Climate Change, 2001). Moreover, reconstructions of past temperatures over the last 1000 years indicate that the more recent temperature changes are unlikely to be entirely of natural origin, notwithstanding the uncertainties of paleotem-perature reconstructions (Intergovernmental Panel on Climate Change, 2001).

The next step consists of determining whether or not the recent temperature changes can be attributed to a combination of natural and anthropogenic forcing. Here again, numerical fully coupled ocean-atmosphere climate models have been employed in the IPCC Third Assessment and have been run for the last 140 years with different forcing scenarios (Figure 4). In the first case, only natural forcings, that is, solar and volcanic variability, have been employed and the resultant temperatures have been compared with observed global mean temperatures. The agreement between the two data sets is reasonable during the first half of the 20th century, less satisfactory for the late 19th century, and poor for the later decades of the 20th century (Intergovernmental Panel on Climate Change, 2001). A second set of models was run with anthropogenic forcings, that is, well-mixed greenhouse gases of

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Figure 4: Measured surface temperatures for the period 1860-2000 and combined for land and ocean surfaces; seasonal deviations relative to the average temperatures for 1961-1990 as well as two-standard error uncertainties (Intergovernmental Panel on Climate Change, 2001) are shown.

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Figure 4: Measured surface temperatures for the period 1860-2000 and combined for land and ocean surfaces; seasonal deviations relative to the average temperatures for 1961-1990 as well as two-standard error uncertainties (Intergovernmental Panel on Climate Change, 2001) are shown.

increasing concentration, changes in stratospheric and tropospheric ozone, and the direct and indirect effects of sulfate aerosols. The comparison between model results and observed temperature is better than in the first set of models, but reveals significant differences for the second half of the 20th century (Intergovernmental Panel on Climate Change, 2001). The third model run was performed by employing both natural and anthropogenic forcings. While showing minor discrepancies in detail, the overall agreement between model results and data is by far the best of all three simulations (Intergovernmental Panel on Climate Change, 2001). More specifically, the IPCC assessment reveals that, over the last 50 years, the estimated range and magnitude of global warming due to increasing greenhouse gas concentrations alone are comparable with or larger than the observed increases in global mean temperatures (Intergovernmental Panel on Climate Change, 2001). Thus, despite remaining uncertainties with respect to detection and attribution, the IPCC concludes that the human-induced increase in atmospheric greenhouse gas concentrations is a primary cause of the observed climate change of the last century.

Manfred A. Lange

See also Albedo; Climate Oscillations; Glacier Mass Balance; Greenhouse Gas Emissions; Ice Sheets; Intergovernmental Panel on Climate Change (IPCC); Permafrost Retreat; Quaternary Period; Thermohaline Circulation

Further Reading

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Alley, R.B. & M.L. Bender, "Greenland ice cores: frozen in time." Scientific American, 278 (1998): 66-71 AMAP, AMAP Assessment Report: Arctic Pollution Issues, Oslo: Arctic Monitoring and Assessment Programme (AMAP), 1998

Baldwin, M.P., L.J. Gray, T.J. Dunkerton, K. Hamilton, P.H. Haynes, W.J. Randel, J.R. Holton, M.J. Alexander, I. Hirota, T. Horinouchi, D.B.A. Jones, J.S. Kinnersley, C. Marquardt, K. Sato & M. Takahashi, "The quasi-biennial oscillation." Reviews of Geophysics, 39 (2001): 179-229 Barnola, J.M., D. Raynaud, Y.S. Korotkevich & C.J. Lorius, "Vostok ice core provides 160,000-year record of atmospheric CO2." Nature, 329 (1987): 408-414 Bond, C.R. & R. Lotti, "Iceberg discharge into the North Atlantic on millennial time scales during the last glaciation." Science, 267(5200) (1995): 1005-1010 Clark, P.U., R.B. Alley & D. Pollard, "Northern hemisphere ice-sheet influences on global climate change." Science, 286 (1999): 1104-1111 Gregory, J.M. & J. Oerlemans, "Simulated future sea-level rise due to glacier melt based on regionally and seasonally resolved temperature changes." Nature, 391 (1998): 474-476

Haeberli, W., "Glacier fluctuations and climate change detection—operational elements of a worldwide monitoring strategy" WMO Bulletin, 44(1) (1995): 23-31 Haeberli, W., P. Müller, P. Alean & H. Bösch, "Glacier Changes Following the Little Ice Age—A Survey of the International Data Basis and its Perspectives." In Glacier Fluctuations and Climatic Change, edited by J. Oerlemans, Dordrecht: Kluwer, 1989, pp. 77-101 Hansen, J.E., A. Lacis, D. Rind, G. Russel, P. Stone, I. Fung, R. Rudey & J. Lerner, "Climate Sensitivity: Aanalysis of Feedback Mechanisms." In Climate Processes and Climate Sensitivity, edited by J.E. Hansen & T. Takahashi, Washington, District of Columbia: American Geophysical Union, 1984, pp. 130-163 Harvey, L.D.D., "Climate impact of ice-age aerosols." Nature,

334 (1988): 333-335 Holloway, G. & T. Sou, "Is arctic sea ice rapidly thinning?." Ice and Climate News, 1 (2001): 2-5 Imbrie, J., E.A. Boyle, S.C. Clemens, A. Duffy, W.R. Howard, G. Kukla, J. Kutzbach, D.G. Martinson, A. McIntyre, A.C. Mix, B. Molfino, J.J. Morley, L.C. Peterson, N.G. Pisias, W.L. Prell, M.E. Paymo, N.J. Shackleton & J.R. Toggweiler, "On the structure and origin of major glaciation cycles: 1. Linear responses to Milankovitch forcing." Paleoceanography, 7 (1992): 701-738

Imbrie, J., A. Berger, E.A. Boyle, S.C. Clemens, A. Duffy, W.R. Howard, G. Kukla, J. Kutzbach, D.G. Martinson, A. McIntyre, A.C. Mix, B. Molfino, J.J. Morley, L.C. Peterson, N.G. Pisias, W.L. Prell, M.E. Paymo, N.J. Shackleton & J.R. Toggweiler, "On the structure and origin of major glaciation cycles: 2. The 100,000-year cycle." Paleoceanography, 8 (1993): 699-735 Intergovernmental Panel on Climate Change, Summary for Policymakers. A Report of Working Group I of the Intergovernmental Panel on Climate Change, Geneva: Intergovernmental Panel on Climate Change, 2001 (available at http://www.ipcc.ch/pub/spm22-01.pdf) Johannessen, O.M., E.V. Shalina & M.W. Miles, "Satellite evidence for an Arctic sea ice cover in transformation." Science, 286 (1999): 1937-1939 Lachenbruch, A.H. & B.V. Marshall, "Changing climate: geot-hermal evidence from permafrost in the Alaskan Arctic." Science, 234 (1980): 689-696 Legendre, L., S.F. Ackley, G.S. Diekmann, B. Gulliksen, R. Horner, T. Hoshiai, I.A. Melnikov & W.S. Reeburg, "Ecology of sea ice biota." Polar Biology, 12 (1992): 429-444 Manabe, S. & A.J. Broccoli, "The influence of continental ice sheets on the climate of an ice age." Journal of Geophysical Research, 90 (1985): 2167-2190 National Academy of Sciences, Climate Change Science— An Analysis of Some Key Questions, Washington, District of Columbia: National Academy Press, 2001 Oerlemans, J., "Quantifying global warming from the retreat of glaciers." Science, 264 (1994): 243-245 Osterkamp, T., "Evidence for warming and thawing of discontinuous permafrost in Alaska." EOS, 75(44) (1994): 85 Osterkamp, T.E. & V.E. Romanovsky, "Characteristics of changing permafrost temperatures in the Alaskan Arctic, USA." Arctic and Alpine Research, 28(3) (1996): 267-273 Raynaud, D., J. Jouzel, J.M. Barnola, J. Chappelaz, R.J. Delmas & C.J. Lorius, "The ice core record of greenhouse gases." Science, 259 (1993): 926-934 Rothrock, D.A., Y. Yu & G.A. Maykut, "Thinning of the Arctic sea ice cover." Geophysical Research Letters, 26(23) (1999): 3469-3472

Storch, H.v. & K. Hasselmann, "Climate Change and Ocean Forecasting." In The Ocean and the Poles: Grand Challenges for European Cooperation, edited by G. Hempel, Jena, Stuttgart, New York: G. Vischer Verlag, 1996, pp. 33-58

Thomas, D. & G. Dieckmann, "Life in a frozen lattice." New

Scientist, 142 (1994): 33-37 Vinnikov, K.Y., A. Robock, R.J. Stouffer, J.E. Walsh, C.L. Parkinson, D.J. Cavalieri, J.F.B. Mitchell, D. Garrett & V.F. Zakharov, "Global warming and northern hemisphere sea ice extent." Science, 286 (1999): 1934-1937 Weller, G. & M.A. Lange (editors), Impacts of Global Climate Change in the Arctic Regions—Report from a Workshop on the Impacts of Global Change, Oslo, Norway: International Arctic Science Committee, 1999 Winsor, P., "Arctic sea ice thicknesses remained constant during the 1990s." Geophysical Research Letters, 28(6) (2001): 1039-1041

CLIMATE: ENVIRONMENTAL INITIATIVES

Over the past two decades, a number of national and international research programs and environmental initiatives have arisen to address the global challenges posed by climate change. The Arctic is both an important part of the Earth's climate system due to polar feedback processes and in turn predicted to warm more than the global mean during global warming. A number of climate initiatives addressed specifically at the Arctic or that relate to Arctic environments are described below.

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