Page 272 - Earth's Climate Past and Future

248 PART IV • Deglacial Climate Changes

Precessional index 65°N summer insolation
2
(εsinω) Tilt (cal/cm /day)
0.04 0 –0.04 23° 23.5° –20 0 20 40
50,000
years
from now

10,000 years
from now

Today

10,000 years
ago

50,000
years
ago

FIGURE 13-20 Future summer insolation trends During the next 10,000 years,
precession-dominated insolation at low latitudes of the northern hemisphere will return to
another maximum (left), while tilt-dominated insolation at very high northern latitudes will
continue to fall toward the next minimum (center). The combined effects of tilt and precession
will increase summer insolation, which will not fall to modern levels for another 50,000 years.
(Adapted from A. Berger and M.-F. Loutre, “Modeling the Climatic Response to Astronomical and
CO Forcings,” C. R. Acad. Sci. Paris 323 [1996]: 1–16.)
2

Climate scientists want to be able to predict when An ice-free interval 50,000 years in length presents
the next glaciation will begin, with ice caps starting to scientists with a major problem. No such ice-free
grow on North America, Eurasia, or both. Over the last interval has occurred in the entire 2.75 million years of
million years, ice sheets have been present on northern northern hemisphere glaciation, so why would one
hemisphere continents for about 90% of the time (see occur now? Has the natural operation of the climate
Figure 9–13), and times without any ice have only lasted system begun to shift in recent millennia toward
for 10,000 years or less. This evidence suggests that the unprecedented warmth and an ice-free state? Such an
current interglaciation must be near its natural end. interpretation would seem to be in complete contradic-
One way to estimate when new ice sheets will grow tion to the evidence for gradual cooling during the last
at high northern latitudes is to examine what has hap- few million years (Chapter 9).
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pened at similar times in the past. Records of δ O Another option will be considered in Chapter 15.
changes covering the last 900,000 years are an approxi- The climatic trend of the last several thousand years
mate index of ice volume, but they have a sizeable tem- may not have been natural, because a new factor has
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perature overprint. The current interval of low δ O arisen and prevented a new glaciation that should be
values (minimal ice volume and warm temperatures) has underway by now. The new factor is agricultural
lasted as long as previous interglaciations, and some humans (Chapter 15).
2-D models that simulate longer-term ice sheets predict
that we are at or near the point when ice sheets
should start growing again because of low summer Key Terms
insolation.
Younger Dryas (p. 236) channeled scablands
Other models predict that glaciation is not immi-
polar front (p. 236) (p. 240)
nent and may not occur for the next 50,000 years,
proglacial lakes no-analog vegetation
because summer insolation is now at a minimum and
(p. 238) (p. 243)
will not be this low again for the next 50,000 years
(Figure 13–20 right). It seems that the ice-age cycles
may have “skipped a beat.” If new ice sheets have not Review Questions
begun to form during the present insolation minimum,
why would they do so at the higher insolation levels of 1. What is the best method of measuring the melting
the next 50,000 years? of ice sheets over the last 17,000 years?