Figure 4.2 ~ Millions of years ago

The world’s climate has become colder and more variable over the last million years. The vertical axis plots 8 power of 18 0, the excess of 18 0 relative to 16 0 in samples taken from deep-sea sediments that date to different times over the last six million years. The concentration of 18 0 (18 power 0) in seawater increases during cold periods, because water contains the lighter isotope of oxygen. 18 0 evaporates more readily and is thus trapped in glacial ice. Other data indicate that during cold periods, the world was drier and the C)2 concentration of the atmosphere was lower (Redrawn from Opdyke et al. 1995)

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Dinesh ^club

Using a variety of proxy measures of past temperature, rainfall, ice volume, and the like, derived mostly from cores of ocean sediments, lake sediments, and ice caps, paleoclimatologists have recently constructed a stunning picture of climatic deterioration over the last three million years. The earth’s mean temperature has dropped several degrees, and the amplitudes of fluctuations in rainfall and temperature have increased (fig., 4.2). for reasons that are still poorly understood, glaciers wax and wane in concert with changtes in ocean circulation, carbon dioxide, methane, and dust content of the atmosphere, and changes in the average amount and distribution of precipitation. Different cyclical patterns of glacial advance and retreat involving all these variables have prevailed. A 32,700 year cycle dominated the early part of the period, a 41,000 year cycle between about 2.7 million and 1 million years ago, and a 95,800 year cycle the last million years. - page 132

4 years ago.

Dinesh ^club

a href="https://www.ipernity.com/doc/308605/49463484">

4 years ago.

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Figure 4.2 ~ Millions of years ago

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