Earth Cooled

The end of the Cretaceous suddenly reversed the climate. The consensus among scientists holds that an enormous meteor impacted Earth 65 million years ago. The meteor ejected a gigantic cloud of debris and dust and ignited widespread fires, which pumped ash into the atmosphere. The debris, dust, and ash blocked out much of the Sun's light, chilling the climate. So severe was the reversal in climate that the dinosaurs and a large number of marine species, unable to cope with the new conditions, perished.

Roughly 4.5 billion years ago, active volcanoes spewed carbon dioxide into the atmosphere, causing the greenhouse effect. Earth's early climate was so hot that all water vaporized, trapping heat in the atmosphere and intensifying the greenhouse effect.

The sudden reversal in climate has had a lasting, if erratic, effect on Earth. During the past 55 million years, temperatures have declined 20 degrees F (11 degrees C). Around 35 million years ago, the climate grew cold enough for glaciers to form on Antarctica. Yet, by 4 million years ago, the glaciers had melted and forests had returned to Antarctica. The forests were transitory: within one million years glaciers had once more spread across Antarctica and had begun to grow in the northern hemisphere.

The warmth of the Cretaceous returned briefly 130,000 years ago, when the climate was again warmer than it is today. The water from melting glaciers flowed to the oceans, raising the sea level 60 ft. (18 m.) higher than it is today. Approximately 30,000 years later, the climate cooled yet again, and glaciers once more spread across the continents, plunging Earth into its most recent Ice Age. Between 16,000 and 10,000 years ago, the glaciers in retreat, temperatures rose nearly 15 degrees F (8 degrees C). Toward the end of this era, and extending to 6,000 years ago, rain was plentiful. Africa had no deserts. Rather, Lake Chad, swollen with rain, was 10 times its current size. Salt Lake, in what is today Utah, was likewise several times larger than it is today. Variations in temperature were greater than they are today, because the tilt of Earth's axis was extreme. If Earth did not tilt on its axis, there would be no seasons, because the northern and southern hemispheres would receive the same amount of sunlight year round. However, Earth tilts on its axis 23.5 degrees. As a consequence, the northern and southern hemispheres receive differing amounts of sunlight during the course of a year, accounting for the seasons. The greater the tilt, the greater the amount of sunlight falls in summer, and the greater the darkness in winter. About 10,000 years ago, Earth tilted on its axis 25 degrees. The northern and southern hemispheres received 7 percent more sunlight in summer than they do today, and 15 percent more sunlight in summer than in winter. By contrast, the differential in sunlight between summer and winter is only 8 percent today.

Temperatures peaked 7,000 years ago at 2-3 degrees F (1-1.5 degrees C) above current temperatures. The climate remained warm and wet another 3,000 years. There was then no desert in the American Southwest, which received enough rainfall to sustain the growth of trees. Between 6,000 and 4,000 years ago temperatures rose 5 degrees F (2.7 degrees C) , melting parts of the glaciers that remained in Antarctica and Greenland. The water from these glaciers flowed to the ocean, raising sea level 300 ft (91 m.). The end of the rainy epoch 4,000 years ago turned the climate arid. Rainfall in the American Midwest fell 25 percent, even as July temperatures peaked 4 degrees F (2 degrees C) above current July temperatures. Deserts formed in the American Southwest, Africa, Asia and Australia. Along the Atlantic coast of North America, the climate remained warm and humid.

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