We normally think of ice as inflexible and brittle. You cannot, for example,
bend an icicle noticeably without breaking it. On a small scale, ice resembles
glass, but on a large scale, and over a long period of time, it takes on a much dif-
ferent character. For a moment, ignore the scaling laws of physics, and suppose
that you are hundreds of meters tall, and that you perceive time to pass in such
a way that a year seems like a minute. With time and space thus altered in scale,
glaciers do not appear hard and stiff, but instead they are semiliquid, flowing
down mountain slopes and spreading out over huge regions of land. On this
scale, ice behaves more like thick molasses than like glass. From this expanded
time-space perspective, you would have no trouble envisioning how, given
enough snowfall in the polar regions and enough time, glaciers can spread out
until they cover much of the land surface of the earth.
If snow accumulates enough to form a glacier and then continues to build up
on a mountain as the glacier expands downslope, the land at lower altitudes is
gradually covered up by the glacier. Eventually the front of the glacier encoun-
ters a climate warm enough to halt its progress, and from that time on, the rate
at which the ice melts at the edge of the glacier will exactly balance the forward
expansion. If the rate of snowfall increases along with a cooling trend in the cli-
mate, the glacier will move farther, and the equilibrium point will move to a
lower elevation. If there is still more snow and a still cooler climate, the glacier
may move out of the mountain entirely and begin to invade the plains below.
This has evidently taken place numerous times in geologic history. Suppose that
the most recent glacial invasion of this type was not the last, and that, left untam-
pered with, nature will produce more ice ages in the future?
IT TAKES TIME
Imagine, for a moment, that we are headed for another ice age. Imagine that
20,000 years from now, much of North America, Europe, and northern Asia will
once again be covered by ice. What sorts of weather changes will our descendants
observe in the coming centuries? The following is a hypothetical description of
the climate in North America as an ice age approaches and establishes itself.
There will not be much of a difference in the general weather until about
3000 A.D. Then, early in the 4th Millennium, winters begin to get more severe,
especially in the mountains. The last snows of the season in the northern United
States occur in late May or early June. The Great Lakes freeze up every year,
as they did in the worst winters of the 19th, 20th, and 21st centuries. In the
southern United States, snow regularly falls as far south as central Texas and the
CHAPTER 9 The Past and Future Climate
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