Hansen et al 2006 Global Temperature Changes

Sea level rise lagged tropical temperature by a few

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Unformatted text preview: today in some of those periods (10). In contrast, sea level was 25–35 m higher the last time that the Earth was 2–3°C warmer than today, i.e., during the Middle Pliocene about three million years ago (32). Ice sheet response time can be investigated from paleoclimate evidence, but inferences are limited by imprecise dating of climate and sea level changes and by the slow pace of weak paleoclimate forcings compared with stronger rapidly increasing human-made forcings. Sea level rise lagged tropical temperature by a few thousand years in some cases (28), but in others, such as Meltwater Pulse 1A 14,000 years ago (33), sea level rise and tropical temperature increase were nearly synchronous. Intergovernmental Panel on Climate Change (10) assumes negligible contribution to 2100 sea level change from loss of Greenland and Antarctic ice, but that conclusion is implausible (17, 34). BAU warming of 2–3°C would bathe most of Greenland and West Antarctic in melt-water during lengthened melt seasons. Multiple positive feedbacks, including reduced surface albedo, loss of buttressing ice shelves, dynamical response of ice streams to increased melt-water, and lowered ice surface altitude would assure a large fraction of the equilibrium ice sheet response within a few centuries, at most (34). Sea level rise could be substantial even in the AS, 1 m per century, and cause problems for humanity due to high population in coastal areas (10). However, AS problems would be dwarfed by the disastrous BAU, which could yield sea level rise of several meters per century with eventual rise of tens of meters, enough to transform global coastlines. Extinction of animal and plant species presents a picture analogous to that for sea level. Extinctions are already occurring as a result of various stresses, mostly human-made, including climate change (35). Plant and animal distributions are a reflection of the regional climates to which they are adapted. Thus, plants and animals attempt to migrate in response to climate change, but their 14292 www.pnas.org cgi doi 10.1073 pnas.0606291103 Fig. 6. Poleward migration rate of isotherms in surface observations (A and B) and in climate model simulations (17) for 2000 –2100 for Intergovernmental Panel on Climate Change scenario A2 (10) and an alternative scenario of forcings that keeps global warming after 2000 less than 1°C (17) (C and D). Numbers in upper right are global means excluding the tropical band. paths may be blocked by human-constructed obstacles or natural barriers such as coastlines. A study of 1,700 biological species (36) found poleward migration of 6 km per decade and vertical migration in alpine regions of 6 m per decade in the second half of the 20th century, within a factor of two of the average poleward migration rate of surface isotherms (Fig. 6A) during 1950–1995. More rapid warming in 1975–2005 yields an average isotherm migration rate of 40 km per decade in the Northern Hemisphere (Fig. 6B), exceeding known paleoclimate rates of ch...
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This document was uploaded on 03/15/2014 for the course MEA 570 at N.C. State.

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