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Unformatted text preview: Making It Comfortable 1 Chapter Thirteen Making It Comfortable: Running Water and Temperature Control The temperature of a planet’s surface depends on the luminosity of the star about which it orbits and on the planet’s distance from this star. It also depends on the reﬂ ectivity of its surface and on the “greenhouse power” of its atmosphere. Earth and Venus exemplify the importance of the latter of these inﬂ uences. Although nearly equal in size and bulk composition, the Venusian ground surface is 400 degrees centigrade warmer than Earth’s. The reason for this difference is that most of the Venusian carbon is in the atmosphere as CO 2 creating a powerful thermal blanket. By contrast, Earth’s carbon is nearly all stored in sediments as carbonate minerals and organic residues. The CO 2 content of the Earth’s atmosphere has probably changed with time in response to changes in the convective motions in the Earth’s mantle. These motions cause sections of the Earth’s sedimentary cover to be dragged into the mantle and heated to the point where sediment- bound carbon is converted to CO 2 gas, which is carried back to the surface with volcanic magmas. Changes in the rate and pattern of the mantle’s ﬂ ow and on the composition of the subducted sedi- ment undoubtedly have led to changes in the rate at which CO 2 is added to the atmosphere. Hence, the Earth’s climate is linked to its tectonics. The CO 2 content is also inﬂ uenced by biogeochemical cycles taking place on our planet’s surface, especially by the rate of release of the element calcium from soils. Although CO 2 given off from the Earth’s interior provides an insurance policy against permanent freeze up, recent studies of ancient glacial deposits capped by thick deposits of calcium carbonate strongly suggest that at least twice in its history the Earth became a “snowball”. The climate on Earth is also sensitive to details of planetary architecture. Because of its moon, its equatorial bulge and its tilted rotation axis, the Earth undergoes a top-like precession once every 26,000 years. Because of the gravitational tug of the major planets, the Earth’s orbital tilt and orbital shape change cyclically on the time scales of 40,000 and 100,000 years. Scientists now believe that the changes in the seasonal and latitudinal distribution of the sunlight reaching the Earth generated by these orbital cycles paced waxing and waning of the Earth’s polar ice caps. But, contrary to expectation, these ice-cap cycles are not simple sinusoids. Instead, they are modu- lated by jumps in climate from one of its quasi-stable states of operation to another. These jumps appear to be triggered by reorganizations of the ocean’s thermohaline circulation....
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This note was uploaded on 02/08/2011 for the course EAS 1601 taught by Professor Lynch during the Spring '08 term at Georgia Institute of Technology.
- Spring '08