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CHAPTER 19 • Future Climatic Change 349

FIGURE 19-7 Vulnerable ice
shelves Ice from the interior of
Greenland and Antarctica flows in ice
streams to the shelves along the
Ice stream margin. In a warmer world, ice shelves
may be vulnerable to destruction by
Ice stream rising ocean temperatures, which may
in turn accelerate flow in the ice
streams. (Adapted from R. A.
Bindschadler et al., “What Is Happening
Sea ice
to the West Antarctic Ice Sheet?” EOS 79
[1998]: 256–65.)
Ice shelf

Bedrock
ridge

The margins of ice sheets can fluctuate rapidly over Because this kind of disequilibrium has not occurred
intervals of a few centuries (Chapter 14), but the great in Earth’s past, no exact analogs for future climates
mass of continent-sized ice sheets respond only over exist. Still, even these partial analogs provide general
many thousands of years (Chapter 9). The greater indications of future climate.
warmth of the next few hundred years will greatly
increase melting along the ice margins, but the central 19-5 Partial Future Analogs: 2 × and 4 ×
portions will melt more slowly. As a result, the main Preindustrial CO Concentrations
bulk of the ice will survive for much longer. The part 2
that survives will be like a gigantic block of ice on a Future CO concentrations will reach somewhere
2
summer day, out of place in a warmer world. between twice and four times the preindustrial value of
Many climate scientists think that most of the 280 ppm (see Figure 19–3).
Greenland ice sheet will survive the pulse of high CO 2 × CO World CO levels are currently 35% higher
2 2 2
levels during the next few centuries and persist into the than the preindustrial level of 280 ppm, and equivalent
subsequent era of decreasing CO concentrations (see CO levels are 60% higher. Rising rates of emissions in
2 2
Figure 19–3), but others disagree. They point out the the early years of the twenty-first century make it highly
ablation that has been occurring at surprisingly high unlikely that atmospheric greenhouse-gas concentrations
elevations on the ice sheet in recent summers and the will fail to reach the equivalent 2 × CO level by the mid-
2
accelerated melting caused by rapid flow of ice in mar- dle of the current century. If this happens, Earth’s average
ginal ice streams (Figure 19–7). Current evidence is temperatures will register a near-complete equilibrium
insufficient to tell which of these views is correct. response to this doubled CO value by the end of the cen-
2
In any case, the world of the future will be a strange tury. For the fast-responding parts of the climate system,
no-analog combination of the slow-responding ice this 2 × CO world will be analogous to the world that
2
sheets and deep ocean and the fast-responding atmos- existed about 10 million years ago.
phere, land surface, vegetation, and upper ocean. Near One striking difference in the world of 10 million
the slow-responding ice sheets, which strongly influence years ago was the much-reduced extent of ice in the Arc-
regional climates by their high albedo and their effect tic Ocean. With less sea ice, the atmosphere extracted
on atmospheric winds, the lingering cold caused by the heat stored in the ocean each summer, and this transfer
ice will suppress part of the response of the atmos- moderated air temperatures during Arctic winters. One
phere and nearby surface ocean to the higher CO lev- consequence of the warmer winters was that the broad
2
els. Farther from the ice, the fast-responding parts bands of permafrost and tundra that now surround the
across most of the climate system will react strongly to Arctic Ocean in Eurasia and North America (Figure
the new warmth. 19–8) were absent, and conifer trees grew in their place.