Page 212 - Earth's Climate Past and Future

188 PART III • Orbital-Scale Climate Change

Greenland ice cores of 11,000 to 10,500 years ago, Orbital-Scale Climatic Roles: CO and CH
coincident with the time of the most recent July insola- 2 4
tion maximum. Because we know the basic timing of CO and CH vari-
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The methane signal in Figure 10–16 closely resem- ations during the last several hundred thousand years,
bles the monsoon response signal (see Chapter 8). Peak we can attempt to assess the role these gases played in
methane values match predicted peaks in monsoon orbital-scale climatic variations. Milankovitch proposed
intensity not just in timing but also in amplitude, with that summer insolation drives ice sheets at the 41,000-
the largest methane peaks lining up with the strongest year and 23,000-year cycles with a lag of approximately
insolation maxima. This match suggests that a connection 5000 years (Figure 10–17A), but how do the greenhouse
exists between past CH concentrations and changes in gases fit into this framework?
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monsoon strength. The answer to this question turns out to be dif-
Monsoon fluctuations determine the amount of ferent for the orbital cycles of precession and tilt. At the
precipitation that falls in southern Asia. Heavy rainfall 23,000-year precession cycle (Figure 10–17B), both
saturates the ground, reduces its ability to absorb water, gases respond on or near the “early” tempo of the Sun
and increases the amount of standing water in swampy (northern hemisphere July insolation). Because the
areas. Vegetation that grows and decays each summer processes that generate CO and CH have very short
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uses up the oxygen dissolved in the water and creates response times, the gases respond quickly to the forcing
the reducing conditions needed to generate methane. from the Sun. The 23,000-year methane response arises
The extent of these boggy areas expanded during wet from year-by-year midsummer heating of northern
monsoon maxima and shrank during monsoon minima continents, especially Asia. The 23,000-year CO signal
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at the 23,000-year precession cycle, as did the methane lags behind that of methane by ~1000 years, but it still
emissions. has a phase much closer to that of the Sun than to that
Wetlands in circum-Arctic regions probably also of the ice. The 23,000-year CO response could result
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contributed to the 23,000-year variations in methane. from a range of processes, some acting very quickly
In far-northern bogs, summer warmth, rather than the (yearly changes in carbon pumping) and others some-
precipitation control that prevails in tropical monsoon what more slowly (the hundreds of years required for
regions, is the main control on methane releases. For changes in deep-ocean circulation). Because the green-
most of the year, temperatures in the far north are too house gases have early phases like that of the Sun, they
cold for plants to grow and produce methane, but the are a part of the forcing of the ice sheet response at
warmth of the short summer season allows methane to 23,000 years, an addition to the initial forcing provided
be generated and released. The primary tempo of by summer insolation.
orbital changes in summer insolation across Asia occurs In contrast, the response of both gases at the 41,000-
at the 23,000-year cycle, and heating of the continent year tilt cycle falls on or close to that of the ice sheets
varies mainly at this cycle, as do methane releases from (Figure 10–17C). This timing rules out any possibility
northern wetlands. (Changes linked to ice sheets and that the gases are forcing the ice sheets at the tilt cycle. If
ice-driven responses also affect northern methane they were forcing the ice, the slow-responding ice sheets
releases but to a lesser extent.) would lag behind by thousands of years, but no such lag
Although peaks in the 23,000-year CH signal are exists. The observed phasing at the 41,000-year cycle can
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generally well matched in amplitude to the strength of be explained only if the ice sheets are controlling CO 2
summer insolation maxima, the methane minima are and CH with little or no lag in the gas response.
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not. Many of the minima reach similar values despite the The ice-driven gas signals, particularly the signal of
fact that the insolation minima vary widely in amplitude. CO , provide immediate positive feedback to the ice
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The lower side of the methane trend has a truncated or sheets at the 41,000-year cycle. As ice sheets grow, they
“clipped” look, apparently because some tropical wet- drive the CO concentration in the atmosphere down,
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lands survive even the strongest insolation forcing and the lower CO concentration cools the climate and
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toward extreme drying. Likely candidates are wetlands helps the ice grow even faster. When the ice melts, CO 2
very near the equator, which remain in the tropical wet concentrations rise, climate warms, and ice melting
zone for any climate. accelerates.
The greenhouse-gas role in the prominent oscillations
at a period near 100,000 years is not as clear. The very
IN SUMMARY, prominent variations in methane at the small amount of insolation forcing at the 100,000-year
23,000-year cycle are linked primarily to changes in eccentricity period has nearly the same phase as the
strength of the summer monsoon in tropical and changes in orbital eccentricity, but it has a negligible effect
subtropical regions and secondarily to variable on the ice sheets (Chapter 9). In the absence of measurable
heating of Asia and its northern wetlands.
insolation forcing, the only timing comparison we can