Page 376 - Earth's Climate Past and Future

352 PART V • Historical and Future Climate Change

The enormous ice sheet on eastern Antarctica
Arctic Ocean
(Figure 19–11) was very large 10 million years ago.
Today, this frigid ice sheet is starved for snow, with only
a few centimeters per year falling across most of its
high-elevation surface. In a warmer 2 × CO world, the
2
supply of snow should increase and allow faster and
thicker annual accumulations of ice in the interior. This
change toward positive mass balance in the ice sheet
Greenland interior will probably be opposed by faster flow in mar-
Baffin Bay Sea ginal ice streams. At present, it is unclear which of these
two processes would dominate the overall ice mass
G R E E N L A N D
balance in East Antarctica.
The smaller West Antarctic ice sheet was less exten-
sive 10 million years ago than it is now, if it existed at
all. The extensive ice shelves that now fringe this ice
sheet are vulnerable to destabilization because they are
Cross section in contact with an ocean that could become slightly
A B
warmer. Destabilization of the ice shelves can accelerate
flow in ice streams moving to the ocean and draw ice
Arctic
Davis Elevation (m) out of the interior of the continent (see Figure 19–7).
Strait Circle
3000 This evidence suggests that the western Antarctic ice
Denmark 2500 sheet will be vulnerable to greater melting in a 2 × CO
Strait 2
2000 world.
1500
1000 Melting of Greenland and Antarctic ice will cause a
0 future rise in sea level, as will expansion of ocean water
Atlantic Ocean Ice-free as it slowly warms. The 2007 IPCC report projects that
the rate of sea level rise is likely to double from the
rate of 17 cm during the last century to about 30 cm
A B (approximately 1 ft) during the current century. Sea
Depth (km) 2 Ice cap level rises could be much higher than this estimate if the
3
Greenland ice sheet proves to be as vulnerable as some
1
0
scientists predict.
-1
4 × CO World By one projection (see Figure 19–3),
2
0 200 400 600 800 1000 atmospheric CO concentrations could reach values
2
Distance (km) more than four times the preindustrial level between
2200 and 2300. Projections this far into the future are
FIGURE 19-10 Greenland ice sheet Lower margins of the inherently speculative, especially because of the likeli-
Greenland ice sheet will melt rapidly in the warmth of a 2 × CO
2 hood that technological innovations will avert this large
world, but the highest central surface of the ice sheet may be a change. But if we do reach a 4 × CO world, all the
less affected. (From F. Press and R. Siever, Understanding Earth, 2
warming trends described for the 2 × CO world will
2
2d ed., © 1998 by W. H. Freeman and Company, and from R. F. be amplified. These changes will move Earth’s climate
Flint, Glacial and Quaternary Geology, © 1971 by Wiley.)
toward the world of 50 to 100 million years ago, the last
time CO levels were so high.
2
the Greenland ice sheet already in existence (Figure The climate of 50 million years ago was much warmer
19–10) and we need to predict how it will respond in a than today. The Arctic margins were surrounded by a
warmer world. Simulations with ice sheet models indicate mixed forest of hardwoods and evergreens adapted to
that the 2.5°C global warming in a 2 × CO world will relatively mild winters. No sea ice existed in the Arctic.
2
produce widespread melting of the surface of the Green- Temperate beech (Nothofagus) forests existed on an ice-
land ice sheet. free Antarctic continent. Mountain glaciers probably did
As noted earlier, the extent of future melting of not exist anywhere on Earth. A 5°C warming would cause
Greenland ice is highly uncertain. By some projections, glacial ice to retreat 660 m (more than 2000 ft) up the
only the margins will melt. Other scientists think that sides of mountains, enough to eliminate today’s mountain
accelerated flow in marginal ice streams could lead to glaciers.
much greater losses of ice in future decades and possibly In a 4 × CO world, north polar regions will warm
2
near-total loss in future centuries. enough to eliminate shallow permafrost, tundra, summer