Page 135 - Earth's Climate Past and Future

CHAPTER 6 • From Greenhouse to Icehouse: The Last 50 Million Years 111

FIGURE 6-17 Tibet and the
monsoon Heating of the
Tibetan Plateau draws in
Rising air
Monsoon precipitation moisture from the Indian Ocean
on the Himalayas Tibetan Plateau and enhances the intensity of
the warm, moist summer
monsoon on its southern
(Himalayan) margin

Moisture from Himalayas
Indian Ocean

hypothesis that physical weathering is stronger today
IN SUMMARY, the cause of global cooling during the
on a global basis than in earlier times, but this conclu-
last 50 Myr remains uncertain. The most likely
sion remains tentative because of the sediment lost to
culprit is a decrease in atmospheric CO , but it is
subduction. 2
unclear whether this change was driven by
Prediction 3: Unusual Chemical Weathering The
decreased input tied to seafloor spreading or
final test of the uplift weathering hypothesis is whether or
increased removal tied to chemical weathering.
not the global average rate of chemical weathering is
higher today than in the past. Unfortunately, chemical
weathering rates are difficult to determine even on
regional scales, much less for the entire Earth. Future Climate Change at Tectonic
Climate scientists quantify modern rates of chemi- Time Scales
cal weathering on a regional basis by measuring the
total amount of ions dissolved and transported in Despite the wealth of evidence that climate has cooled
rivers. This measure reflects the amount of chemical over the last 50 Myr, it is impossible to predict the
weathering within the watershed drained by each river, changes in climate that will be caused by tectonic
but modern disturbances of natural weathering pro- processes. One reason is that scientists disagree about
cesses by humans complicate such studies. In addition, whether past cooling was driven by uplift, slower
it is difficult to distinguish between the ions provided seafloor spreading, or other factors. In any case, future
by slow weathering of silicate rocks (hydrolysis) and changes in plate tectonic processes are inherently
those resulting from rapid dissolution of carbonate unpredictable because the driving forces are not fully
rocks. Only hydrolysis affects the CO balance in the understood. If we cannot accurately predict the opera-
2
atmosphere (Chapter 3). Another limitation is strategic: tion of tectonic processes in the future, we cannot pre-
it is impossible to study enough rivers to reach an dict their climatic effects. The safest prediction—that
accurate estimate of the global weathering rate because long-term (tectonic-scale) climate will continue to
too many rivers contribute significantly to the global cool—is really not a prediction at all, just a forward pro-
total. jection of past trends.
It is even more difficult to reconstruct rates of The tectonic-scale climatic trends of the distant
chemical weathering during earlier intervals (Box 6–2). future will also be influenced by positive and negative
As a result, the case for unusual chemical weathering feedback processes. We have already seen that negative
today rests only on a plausibility argument based on feedback from chemical weathering is an integral part
several observations: the unusual height and extent of of the BLAG hypothesis (Chapter 4), but a rough cal-
the Tibetan-Himalayan complex, the unusual strength culation shows that it could also have acted to offset
of the monsoon rains, and the unusual volume of sedi- much of the increase in chemical weathering driven by
ments deposited in the nearby ocean. By inference, this uplift (Figure 6-18). In this calculation, we assume that
combination of favorable factors should promote uplift affected an area equivalent to 1% of the total
unusually rapid chemical weathering and cause CO area of continental crust, approximately the size of
2
removal from the atmosphere. But inference is not the Himalayas and the southern parts of the Tibetan
proof. Plateau. We also assume that uplift increased the rate of