The World in 2050: Four Forces Shaping Civilization's Northern Future

276 By averaging model simulations over a twenty-year period (2046-2064), this map smooths out most of the short-term variability described earlier, thus revealing the strength of the underlying greenhouse effect. Yet even after this smoothing process, we still find a geographically uneven pattern of warming. For map source see next endnote.

277 IPCC AR4, Figure 10.8, Chapter 10, p. 766 (Full citation: G. A. Meehl et al., Chapter 10, “Global Climate Projections,” in S. Solomon, D. Qin, M. Manning, Z. Chen, M. Marquis, K. B. Averyt, M. Tignor, H. L. Miller, eds., Climate Change 2007: The Physical Science Basis. Contribution of Working Group I to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change (Cambridge, UK, and New York: Cambridge University Press, 2007). See Chapter 1 for more on the IPCC Assessment Reports.

278 These outcomes are called SRES scenarios, of which three are shown here (i.e., each row is a different SRES scenario). There are many economic, social, and political choices contained within different SRES scenarios, but the differences are not important for our purposes here. SRES refers to the IPCC Special Report on Emissions Scenarios. They are grouped into four families (A1, A2, B1, and B2) exploring alternative development pathways, covering a wide range of demographic, economic, and technological driving forces and resultant greenhouse gas emissions. B1 describes a convergent, globalized world with a rapid transition toward a service and information economy. The A1 family assumes rapid economic growth, a global population that peaks around 2050, and rapidly advancing energy technology, with A1B assuming a balance between fossil and nonfossil energy. A2 describes a nonglobalized world with high population growth, slow economic development, and slow technological change. For more, see N. Nakicenovic, R. Swart, eds., Special Report on Emissions Scenarios: A Special Report of Working Group III of the Intergovernmental Panel on Climate Change (Cambridge, UK: Cambridge University Press, 2000), 570 pp.

279 The three SRES scenarios shown, which I have renamed for clarity, are B1, A1B, and A2, respectively. There are a number of other scenarios but these three illustrate a representative cross-section from the IPCC AR4 Assessment.

280 P.217, R. Henson, The Rough Guide to Climate Change (London: Penguin Books Ltd., 2008).

281 These are discussed further in Chapter 9.

282 Projected temperature increases average about 50% higher over land than over oceans. The stubborn bull’s-eye marks where warm, north-flowing waters of the Meridional Overturning Current (MOC)—also known as the North Atlantic Deep Water Formation (NADW)—cool and sink. Weakened MOC overturning is expected to counter the climate warming effect locally in this area. There are other physical reasons why the warming effect is amplified in the high northern latitudes, including low evaporation rate, a thinner atmosphere, and reduced albedo (reflectivity) over land. But the most important reason by far is the disappearance of sea ice over the Arctic Ocean, changing it from a high-albedo surface that reflects incoming sunlight back out to space to an open ocean that absorbs it.

283 E.g., Figure 10.12, IPCC AR4, Chapter 10, p. 769. The models also concur pretty well in the Mediterranean region, southern South America, and the western United States, where precipitation is projected to decrease. They concur well around the equator, over the southern oceans around Antarctica, and throughout the northern high latitudes, where it is projected to increase. Except for Canada’s western prairies, precipitation is projected to rise significantly across the northern territories and oceans of all eight NORC countries.

284 Among other things the Clausius-Clapeyron relation, i.e., a warmer atmosphere holds more water vapor.

285 The 2050 projections are from P. C. D. Milly et al., “Global Pattern of Trends in Streamflow and Water Availability in a Changing Climate,” Nature 438 (2005): 347-350. That the projected northern runoff increases surpass all bounds of natural climate variability is shown by Hulme et al., “Relative Impacts of Human-Induced Climate Change and Natural Climate Variability,” Nature 397, no. 6721 (1999): 688-691. The twentieth-century river discharge increases appeared first and most strongly in Russia, B. J. Peterson et al., “Increasing River Discharge to the Arctic Ocean,” Science 298, no. 5601 (2003): 2171-2173; J. W. McClelland et al., “A Pan-Arctic Evaluation of Changes in River Discharge during the Latter Half of the Twentieth Century,” Geophysical Research Letters 33, no. 6 (2006): L06715. In Canada, runoff experienced late-century declines in total runoff to Hudson’s Bay but increases in the Northwest Territories. S. J. Dery, “Characteristics and Trends of River Discharge into Hudson, James, and Ungava Bays, 1964-2000,” Journal of Climate 18, no. 14 (2005): 2540-2557; J. M. St. Jacques, D. J. Sauchyn, “Increasing Winter Base-flow and Mean Annual Streamflow from Possible Permafrost Thawing in the Northwest Territories, Canada,” Geophysical Research Letters 36 (2009): L01401. An excellent recent synopsis is A. K. Rennermalm, E. F. Wood, T. J. Troy, “Observed Changes of Pan-Arctic Cold-Season Minimum Monthly River Discharge,” Climate Dynamics, DOI: 10.888/1748- 9326 /4/2/024011.

286 L. C. Smith et al., “Rising Minimum Daily Flows in Northern Eurasian Rivers: A Growing Influence of Groundwater in the High-Latitude Hydrologic Cycle,” Journal of Geophysical Research 112, G4, (2007): G04S47.

287 Ice caps are large glacier masses on land. Unlike Antarctica, a continent buried beneath mile-thick glaciers and surrounded by oceans, the Arctic is an ocean surrounded by continents. It is thinly covered with just one to two meters of seasonally frozen ocean water called “sea ice.”

288 The Fall Meeting of the American Geophysical Union, which convenes each December in San Francisco, California.

289 The Arctic Ocean freezes over completely in winter but partially opens in summer. The annual sea-ice minimum occurs in September.

290 By September 2009 sea-ice cover was nearing recovery to its old trajectory of linear decline. However, the extreme reductions of 2007-2009 were a major excursion from the long-term trend and clearly demonstrate the surprising rapidity with which the Arctic’s summer sea-ice cover can disappear.

291 Unlike land-based glaciers, the formation or melting of sea ice does not significantly raise sea level because the volume of buoyant ice is compensated by the volume of water displaced (Archimedes’ Principle). A slight exception (about 4%) to this does arise because sea ice is fresher than the ocean water it is displacing (thus taking up slightly more volume than the equivalent mass of sea water).

292 This albedo feedback works in the opposite direction, too, by amplifying global cooling trends. If global climate cools, then Arctic sea ice expands, reflecting more sunlight, thus causing more local cooling and more sea- ice formation, and so on.

293 Sea ice does form around the edge of the Antarctic continent, but its areal extent is much less than in the Arctic Ocean and it does not survive the summer. Other reasons for the warming contrast between the Arctic and Antarctica include the strong circumpolar vortex around the southern oceans, which divorce Antarctica somewhat from the global atmospheric circulation, and the cold high elevations of interior Antarctica, where air temperatures will never reach the melting point, unlike the Arctic Ocean, which is at sea level.

294 The sea-ice albedo feedback is the most important factor causing the global climate warming signal to be amplified in the northern high latitudes, but there are also others. Reduced albedo over land (from less snow), a thinner atmosphere, and low evaporation in cold Arctic air are some of the other positive warming feedbacks operating in the region. The transition to a new summertime ice-free state is likely to happen rapidly once the ice pack thins to a vulnerable state. M. C. Serreze, M. M. Holland, J. Stroeve, “Perspectives on the Arctic’s Shrinking Sea-Ice Cover,” Science 315, no. 5815 (2007): 1533-1536. Not all northern albedo feedbacks are positive—for example, more forest fires, an expected consequence of rising temperatures, actually raise albedo over the long term. E. A. Lyons, Y. Jin, J. T. Randerson, “Changes in Surface Albedo after Fire in Boreal Forest Ecosystems of Interior Alaska Assessed Using MODIS Satellite Observations,” Journal of Geophysical Research 113: (2008) G02012.

295 Based on projections of the NCAR CCSM3 climate model. You can view these results in D. M. Lawrence, A. G. Slater, R. A. Tomas, M. M. Holland, and C. Deser, “Accelerated Arctic Land Warming and Permafrost Degradation during Rapid Sea Ice Loss,” Geophysical Research Letters 35, no. 11, (2008): L11506, DOI:10.1029/2008GL033985.