Page 270 - Earth's Climate Past and Future

246 PART IV • Deglacial Climate Changes

effect of this vegetation-albedo feedback almost the southwest coast of Norway. The estimated changes
doubled the initial insolation warming of high northern in sea surface temperature reconstructed from these
latitudes (Figure 13–18C). assemblages show a gradual cooling since 6000 years
Sea ice also contributed to these far-northern cli- ago (Figure 13–19D).
mate changes. High summer insolation caused the sea The boundary between tundra to the north and
ice margin in the model to thin and retreat northward, boreal forest to the south is still another climatic indi-
and this change propagated into the rest of the yearly cator in Asia and North America. This boundary in
cycle, with delayed refreezing of seasonal sea ice in northern Canada was well north of its present limit
autumn, thinner and less extensive sea ice in winter, and 6000 years ago but has since advanced southward by up
earlier melting of sea ice in spring. As a result, despite to 300 km (Figure 13–19E). This shift from forest to
the fact that the winter insolation values were consider- tundra vegetation suggests cooler summers and a
ably lower than those today, the reduced sea ice cover in shorter growing season.
winter and larger areas of open water near the coasts Still another indication of summer cooling in the
moderated the winter cooling of the continents. As a last several thousand years (not shown) comes from
result, the simulated annual average change in this mountain glaciers. Like the much larger ice sheets,
region showed a considerable warming. mountain glaciers at high latitudes will melt if summer
Renewed Cooling in the Last Several Thousand insolation increases, but mountain glaciers respond
Years During the last 6000 years, Earth’s tilt has slowly to climate changes within just a few decades. Before
decreased and its precessional motion has moved the 5000 years ago, mountain glaciers were small, but
northern hemisphere summer solstice toward the aphe- since that time their size has increased in most regions,
lion (distant-pass) position. These combined orbital consistent with the evidence of progressive cooling
changes have produced a 5% decrease in summer inso- driven by a long-term decrease in summer insolation.
lation and a 5% increase in winter insolation at high
latitudes since 6000 years ago (see Figure 13–12). As a Current and Future Orbital-Scale
result, summer temperatures have fallen significantly Climatic Change
during the last several thousand years in several regions
at high northern latitudes (Figure 13–19). Astronomy tells us not only the changes in Earth’s orbit
Evidence of cooler summers comes from ice cores that have already occurred but those that will occur in
taken from small ice caps in several parts of the Arctic. the future. This knowledge of the future gives us a good
Ice from the tiny Agassiz ice cap on Ellesmere Island, basis for predicting the course climate would follow if it
in far northern Canada, shows that summer melting responds only to orbital forcing (changes in precession
episodes were far more frequent before 5000 years and tilt). [Note that this analysis refers only to natural
ago than they have been since that time (Figure changes occurring at orbital scales. Other factors that
13–19A). This evidence supports a trend toward cooler will determine our near-term climatic future will be
summers. examined in Part V.]
A second region where cooling is evident over the Today June 21 occurs near the July 4 aphelion
last several thousand years is the high-latitude Atlantic (distant-pass) position in Earth’s eccentric orbit around
Ocean off the coast of Greenland, a region that today the Sun. In another 10,000 years, Earth will have
has a sea-ice cover in winter. Ocean sediment cores returned to the opposite configuration. June 21 will
from this area contain shells of diatoms that once lived occur at perihelion, when Earth is closest to the Sun,
in these waters. The diatom species present before just as it did 10,000 years ago (Figure 13–20 left).
about 5000 years ago indicate that sea ice was absent or Because the 23,000-year cycle of orbital precession con-
scarce in this region (Figure 13–19B). trols changes in the tropical monsoons, this shift in
A third piece of evidence that indicates cool- Earth’s orbital shift will increase the amount of summer
ing in recent millennia is the increase in size of small insolation across the northern tropics and drive a
glaciers on Arctic islands. Glacier margins on Arctic stronger monsoon over North Africa and southern Asia.
islands were located well back from their modern Predicting the effect on higher latitudes is more dif-
positions between 8000 and 3500 years ago (Figure ficult. In the next 10,000 years, the tilt of Earth’s axis
13–19C), and some of the ice caps melted entirely. will have fallen to the next minimum (Figure 13–20
These glaciers have reappeared or grown since 3500 center), and the resulting decrease in summer insolation
years ago, oscillating toward progressively larger sizes, will tend to cool climate. But the insolation decrease
consistent with cooler summer temperatures and less caused by the change in tilt will be opposed by the
melting. insolation increase caused by the change in precession.
A fourth indication of cooling is changes in the The combined signal (Figure 13–20 right) shows a
abundance of temperature-sensitive diatom species off larger effect of tilt than of precession.