Page 137 - Earth's Climate Past and Future

CHAPTER 6 • From Greenhouse to Icehouse: The Last 50 Million Years 113

FIGURE 6-18 Negative feedback
50-fold increase in Lower CO , from chemical weathering? If
2
weathering of global cooling
uplifted regions increased chemical weathering in
1% of Earth's localized regions of uplift causes
surface uplifted Reduced global climate cooling, the resulting
cooling
reduction of chemical weathering in
other regions may act as a negative
feedback that moderates much of the
Lower temperature, cooling driven by uplift.
precipitation, vegetation
Less CO
2 in regions not uplifted
removal

50%
decrease
in chemical
weathering
Weathering budget:
(0.01) (50) ≈ (0.99) (0.5)
Fraction Fractional Fraction Fractional
of land increase of land decrease
uplifted in rate of not in rate of
weathering uplifted weathering

ing effects of the chemical weathering thermostat may erode the preexisting cover of already weathered soils,
have balanced (or nearly balanced) the effects of uplift- expose fresh underlying bedrock, and grind the rock
driven weathering during the last 50 Myr. down to finer particles. This debris is then deposited in
But the possibility also exists that the process of global extensive moraine ridges at the ice margins.
cooling could produce positive feedbacks that keep If the ice sheets simply remain at or near their max-
driving climate toward even colder conditions. These imum extents for millions of years (as the Antarctic ice
feedbacks result from the increased amounts of freshly sheet seems to have done), they would probably slow
fragmented rock generated by ice (Figure 6-19A). If the the overall rate of chemical weathering. Weathering
mountain glaciers grind large volumes of bedrock as may increase along the ice margins where piles of frag-
climate cools, the rate of exposure of freshly fragmented mented debris are exposed, but these localized increases
rock should increase, thereby speeding up rates of chemi- would probably be overwhelmed by the larger decrease
cal weathering, pulling more CO out of the atmosphere, in weathering caused by ice cover across much of the
2
and causing additional cooling (Figure 6-19B). This kind continent.
of feedback can happen both in regions of active uplift If, however, the ice sheets fluctuate in size (as they
and in high terrain that is no longer being uplifted (see have done in the northern hemisphere), these variations
Box 6-1). should cause a net increase in chemical weathering.
An increase in weathering is particularly likely if the When shorter-term climatic variations cause the ice
lower limits of the mountain glaciers move up and sheets to melt back from their maximum extents or
down the sides of mountains, alternately grinding fresh even disappear (as has happened in North America and
rock and then exposing it to the atmosphere for chemi- Europe since 20,000 years ago), vast areas of finely
cal weathering (Figure 6-19B). These kinds of fluctua- ground debris are exposed (Figure 6-19C). This fresh
tions result from the orbital-scale cycles of warming and debris can then be rapidly weathered during the warmer
cooling we will examine in Part III. climates during intervals between glaciations. The net
A similar positive feedback may occur in connection result is an increase in chemical weathering.
with large continental-scale ice sheets (Figure 6-19C). Further research is obviously needed to assess the
As global climate cools and vast ice sheets appear, they role of tectonic forcing and internal feedbacks, both