g03.    How did the atmosphere change after the Precambrian?

Paleozoic climate changed as the earth evolved. The earth turned faster and the days were shorter.  Tidal effects were stronger and there was no green covering of vegetation to better absorb the sun's radiation. (See What Is Geologic Time?) The climate was cooler at the beginning of the Cambrian due to the ice age at the end of the Precambrian. Deposits of Early Ordovician age limestones indicate that there was a warming and the sea encroached on the land. Glacial deposits of Late Ordovician age have been found in the western part of the Sahara Dessert.  These were extensive enough to bring about a decrease in the temperatures of the middle and high latitudes. During the Late Paleozoic, the South Pole was in South Africa; the North Pole was over open ocean.  The equator trended northeastward across Canada and southwestward across Europe. Cooler climates prevailed in Gondwanaland because of its proximity to the South Pole. By the Permian, the southern continent was in an ice age. The northern continent was undergoing arid conditions, which influenced evolution and extinctions of animals and plants.

The 160 years of the Mesozoic seemed to have warm, equable climates.  The paleoequator extended from central Mexico across the northern bulge of Africa. Mountains thrust upward at the end of the Paleozoic inhibited the flow of moist air into the central regions, causing arid conditions. Paleobotanical evidence suggests that tropical conditions prevailed in what are now temperate zones. In late Cretaceous there was a brown algae bloom, absorbing so much CO₂ that there was a deficiency in the atmosphere.  This may have contributed to a temporary cooling. Oxygen isotope ratios from open ocean planktonic calcareous organisms indicate a decline in the ocean temperature about 80 million years ago.  This could have caused a dip in the annual mean temperatures.

Cenozoic climates were typified by ice ages. In the Pleistocene, over 40 million cubic kilometers of snow and ice were dumped on about a third of the land surface of the globe. Climatic zones in the northern hemisphere were shifted southward. Rainfall increased at lower latitudes, making plant life lush. Ice ages also occurred in the Precambrian, late Ordovician, Permo-Carboniferous, and possibly during the Oligocene and Pliocene. Glaciation caused sea level to drop 75 meters: continental shelves were dry land and supported forest growth; the British Isles were joined to Europe; and Siberia and Alaska were connected by a land bridge. The weight of the ice depressed parts of the land from 200-300 meters and the Great Lakes were formed.

e01.    What is biodiversity?

e15.    What was early life like?

e16.    What do computer models of greenhouse changes show?

m35.    How can global warming affect biodiversity?

m38.     Why is ozone being depleted?

m41.    How can matter affect the environment?

m52.    What causes air pollution?

m60.    What impact do tires have on the environment?