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.
- Spring '08