Exploring Clues to Our Past
The climate of the earth has gone through many warm periods and cool periods during its history. So today's problem of climate change is nothing new to the earth. However, the climate changes in the past were due to conditions imposed by Mother Nature and those changes seem to have cycled back and forth between warm and cool periods. Today's changes worry us because they are being caused by human activities. These manmade environmental changes could have such an influence on the earth that the cycle might stop. Could we experience a warming trend that would make our planet's climate like Venus (430 degrees Celsius), or a cooling trend that would make a martian-like climate (-49 degrees Celsius)? One of the factors that make these inhospitable temperatures on Mars and Venus is the gases in their atmosphere. These gases influence the heat budget, which determines their global climate.
About 100 million years ago, a long slide began from a warm to a cold climate. At that time, carbon dioxide levels were much higher than today because of high levels of volcanic activity. This volcanic activity, which is tied with sea floor spreading, began to decrease and, as atmospheric levels of carbon dioxide began to diminish, the earth's climate gradually cooled. The uplifting of continents and mountain ranges, caused by collisions of the earth's plates, enhanced formation of glaciers. As our global climate cooled further, the earth experienced ice ages.
Ice ages occur when global temperatures fall and sheets of ice build up from snowfall accumulations over long periods of time. During the last two ice ages, which each lasted about 90,000 years, ice sheets up to 1-2 miles thick spread over half of North America. Records show that the global temperatures during these ice ages were between 5 and 9 degrees Celsius cooler that today's temperatures. Between the ice ages were interglacial periods, which were marked by global warming and lasted about 10,000 years. Maximum global temperatures during these warming periods were about 1.5 degrees Celsius warmer than today.
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