Page 199 - Earth's Climate Past and Future

CHAPTER 10

Orbital-Scale

Changes in Carbon

Dioxide and Methane

Sequences of ice cores several kilometers in length recovered from ice sheets
give scientists access to another very important archive of climate change over
the last several hundred thousand years. One remarkable discovery from air
bubbles trapped in these cores is that two important greenhouse gases, carbon
dioxide (CO ) and methane (CH ), have varied at the same periods as Earth’s
2 4
orbit. Atmospheric CO concentrations match the ~100,000-year rhythm of
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the ice sheets, with high concentrations during warm interglacial intervals
but values 30% lower during maximum glaciations. The carbon taken from
the atmosphere during glacial times ended up in the deep ocean, but the
processes responsible for this transfer are still being explored. Possibilities
include an ocean cooling that increased CO solubility in seawater, greater
2
transfer (“pumping”) of organic carbon from surface to deep waters, and
changes in the patterns of deep-water circulation. Methane levels have fluctu-
ated mainly at the 23,000-year orbital rhythm of precession, largely because of
fluctuations in north-tropical monsoons at that period. The greenhouse gases
act as a driver of ice sheet changes at the 23,000-year cycle, a positive feedback
to the ice at the 41,000-year cycle, and a combination of both in the oscilla-
tions near 100,000 years.