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16.2. Global temperatures are likely to increase by several degrees Centigrade as CO2 and other greenhouse gases are added to the atmosphere.

It depends on which part of the climate system you have in mind. The degree of reaction of each part of the climate system to the future warming will be determined by its response time (see table 15.1). Because the main part of the peak in greenhouse-gas concentrations and the resulting peak in warmth will last for about 200 years, the fast-responding parts of the climate system will be drastically altered, but the slow-responding parts will be much less affected.

At the slow-responding extreme, ice sheets take thousands of years to react to changes in climate. As a result, the lower and warmer margins of the Greenland ice sheet will begin to melt in this warmer climate of the future, but the bulk of the ice sheet may have been relatively little affected by the time the greenhouse-gas pulse begins to fade (although recent analyses point to greater vulnerability). The deep-frozen ice sheet on Antarctica will probably be little affected. Air masses there will become somewhat less frigid and thus able to hold and carry more moisture toward the pole. Rates of snow accumulation will increase on the high central portion of the ice sheet. On the other hand, along the lower ice-sheet

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16.3. Several kinds of satellite-based measurements show that Arctic sea ice has retreated during the last several decades.

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16.3. Several kinds of satellite-based measurements show that Arctic sea ice has retreated during the last several decades.

margins, a slightly warmer ocean may increase the removal of ice. Whether the Antarctic ice sheet as a whole will grow or shrink depends on which process wins this battle. Most likely, it will not change in size very much.

The situation is clearer for mountain glaciers. Because these small bodies of ice respond to climate change within decades, they will be heavily impacted by future warming, and most will disappear completely. At present, virtually every mountain glacier on Earth is already melting in response to the industrial-era warming. Within the last century, some 70 percent of the glaciers in Glacier National Park have disappeared (mostly the smaller ones). At current rates, the larger ones will disappear by 2030 from "Glacierless National Park."

A similar fate may be in store for sea ice, which also responds relatively quickly to changes in climate. Summer sea ice may completely disappear from the central Arctic Ocean, and winter sea ice may no longer reach the coasts of North America and northern Eurasia. Here again, a trend in this direction is currently under way. Several kinds of satellite measurements over the last three decades show a 6 percent retreat in the area ofArctic sea ice (fig. 16.3). More ominously, radar measurements from submarines transiting the Arctic Ocean show a 40 percent decrease in sea-ice thickness from a regional average of 3.2 meters (10 feet) in the 1960s to just 1.8 meters (6 feet) in the mid-1990s. If this thinning continues in future decades, rates of areal retreat will accelerate. Seasonal snow limits in the Northern Hemisphere are another of the fast-responding parts of the Arctic climate system, and satellite measurements show that the extent of Northern Hemisphere snow has decreased in the last three decades. These sea-ice and snow-cover trends might yet prove to be part of a natural cycle of oscillations, or they may represent the first steps in a longer-term Arctic thaw that will intensify in the next two centuries. Time will tell us which is true.

The ring of permafrost that surrounds the Arctic responds somewhat more slowly to climate. Most of the frozen ground lies too deep to be easily reached by the seasonal heat from the summer Sun. The future greenhouse warming will warm and melt the surface layer and turn it into a quagmire for a much longer interval of the year than at present, but it will not melt much of the deeper layer of frozen ground. Permafrost (and tundra) will also shrink or disappear from highmountain regions farther south.

In temperate latitudes, the length of the growing season will increase, and with it the fraction of the year when forests and grasslands are green. On average, the warm season will expand by about a month at either end. Future Aprils will be like modern Mays, and future Novembers like modern Octobers. Again, such a shift is now under way: the growing season measured both by satellites and by ground observations has expanded by about a week in the spring and by half a week in the autumn over the last two decades. Part of this change probably reflects the greater availability of atmospheric CO2 in giving plants a fertilizer boost, but part of it is also due to the warming in recent decades. Another significant change at temperate latitudes will be a reduced number of bitterly cold outbreaks of polar air masses in winter.

At tropical and subtropical latitudes, where future temperature increases will be smaller, the main concern will be drought and floods. As global temperature rises, evaporation will increase, because rates of evaporation are determined mainly by temperature. With more water vapor drawn into the atmosphere each year, more rain must fall. If the distribution of rain were perfectly even, the increase in precipitation in each area might balance the increase in evaporation, with little or no net effect. But rainfall is notoriously patchy, especially in the warmth of summer when thunderstorms let loose the most torrential rains of the year. As a result, the most likely outcome will be both more extensive droughts in some areas and more severe storms in others, with relatively little predictability as to location.

If the future greenhouse-gas concentrations and temperature trends follow the "business-as-usual" scenario and reach values four times the preindustrial concentration (figs. 16.1, 16.2), all of the trends just described will be intensified. A 4 X CO2 level of greenhouse gases is equivalent to the levels that existed some 50 million years or more ago when no permanent ice existed anywhere on Earth, even in Antarctica. For this large a future warming, the present-day Antarctic ice sheet would be out of equilibrium and would join the Greenland ice sheet in shrinking. But once again, the peak centuries of the greenhouse interval would pass too quickly for a large fraction of the south polar ice to melt. Future owners of coastal property will thank the sluggish response of these ice sheets. If all the water locked up in the two ice sheets were released, sea level would rise by 66 meters (205 feet).

In summary, whether we eventually reach the 2 X CO2 level, the 4 X CO2 level, or (more likely) something in between, the future greenhouse warming will be large. Having inadvertently stopped a small glaciation from developing in the last several thousand years, we will now melt much of the world's sea ice and mountain glaciers in the next century or two and push back the seasonal limits of snow cover, but we will leave the two great ice sheets largely intact.

Our impacts on the climate system will change an important part of Earth's basic appearance from even the distant perspective of space: most of the north polar region that is now white will be repainted in hues of dark blue (where sea ice melts back) and dark green (where snow-covered tundra gives way to boreal conifer forest). As satellite photos accumulated over many decades begin to show that we are repainting Earth's northern pole, the scale of our impact on climate will become obvious to all.

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