Page 141 - Earth's Climate Past and Future

PART III

Orbital-Scale Climate

Change

records containing orbital changes over this time span (Chapter 10). Because climate scientists now have in
come from ocean sediments, which are dated by hand accurate records of both the orbital forcing and
radiometric methods and by orbital “tuning.” A wide the internal climatic responses, the mechanisms of

array of records covering the last 650,000 years also orbital-scale changes in ice sheets are gradually
comes from ice cores, which are dated by counting becoming clearer.
annual layers and by orbital tuning. These techniques In Part III we address the following major
make it possible to resolve time to within a few questions:
thousand years in both kinds of records. Because ocean
sediments and ice cores are multichannel recorders
that carry several kinds of climate signals side by side,
• How do orbital variations drive the strength of tropical
scientists can also determine the relative timing of
monsoons?
climatic responses in the oceans and on land, including
• How do changes in Earth’s orbit affect the size of
the ice sheets.
northern hemisphere ice sheets?
The major orbital cycles have been detected in
• What controls orbital-scale fluctuations of
records of several important climatic responses on
atmospheric greenhouse gases?
Earth: the strength of the low-latitude African and
• What is the origin of the 41,000-year ice-age cycle
Asian monsoons (Chapter 8), the size of north polar ice
between 2.75 and 0.9 Myr and the variations at
sheets (Chapter 9), and the concentrations of important 100,000 years during the last 0.9 Myr?
greenhouse gases (CO and CH ) through time
2 4