MIT12_009S11_lec12_16

MIT12_009S11_lec12_16 - 5 5.1 Natural climate change:...

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5 Natural climate change: cycles 5.1 Climatic cycles Earth’s climate has always fluctuated. Climate fluctuations since the 19th century: Image created by Robert A. Rohde / Global Warming Art. Climate fluctuations for the last two millenia: Image created by Robert A. Rohde / Global Warming Art. Climate fluctuations for the last 450 Kyr exhibit the 100-Kyr periodicity of glacial cycles : 69 Glacial
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Image created by Robert A. Rohde / Global Warming Art. Climate and CO 2 fluctuations for the last 420 Kyr: -400 -300 -200 -100 0 -10 -5 0 5 time (kyr relative to present) temperature fluctuation (deg C) 150 200 250 300 CO2 (ppm) This correlation between p CO 2 and climate was highlighted in Al Gore’s Flm An Inconvenient Truth . The covariation of these two signals suggests a strong relation between CO 2 and climate, but its explanation remains one of the great unsolved problems of earth science. Climate fluctuations for the last 5 Myr show that the 100-Kyr cycle began about 1 Ma, and was preceded by the dominance of a 41-Kyr cycle: 70
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Image created by Robert A. Rohde / Global Warming Art. Climate fluctuations for the last 65 Myr: Image created by Robert A. Rohde / Global Warming Art. Climate fluctuations for the last 540 Myr: Image created by Robert A. Rohde / Global Warming Art. 71
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5.2 Milankovitch hypothesis: an introduction Reference: Muller and Macdonald [29]. Milutin Milankovitch (1879–1958) proposed that variations in the precession, obliquity, and eccentricity of Earth’s orbit are responsible for the glacial cy- cles. Similar but less well-developed ideas were proposed in the 19th century by Joseph Ad´ emar and James Croll. Milankovitch’s ideas gained prominence in the 1970s, when evidence of glacial cycles was found in deep sea cores [30]. Let us Frst take a qualitative look at the three principal orbital parameters. 5.2.1 Precession, obliquity, and eccentricity www.meted.ucar.edu 72 This image has been removed due to copyright restrictions.
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Here are movies illustrating precession, obliquity, and eccentricity. Precession is the slow change in the direction of the North Pole. Precession results from torques exerted by the Moon and Sun on Earth’s equatorial bulge. This movement is analogous to that of a tilted top or gyroscope. The period of precession is about 25.8 Kyr. Obliquity is the angle of the tilt of the Earth’s pole towards the Sun. In other words, it is the angle at which the North Pole tilts towards the Sun in summer. Today the obliquity is 23 . 5 . Over the last 800 Kyr it has varied between about 22 and 24 . 5 . Obliquity varies with a dominant period of 41 Kyr. Its variations are due to torques from Jupiter (because it is large) and other planets. This rate of change corresponds to 0 . 13 /Kyr, which means, e.g., that the Tropic of Cancer—the northernmost latitude at which the Sun may appear directly overhead—has moved 1.4 km in the last 100 yr.
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MIT12_009S11_lec12_16 - 5 5.1 Natural climate change:...

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