The greenhouse effect has been contributing to changes in the Earth’s temperature for millions of years. Here are a couple of examples.

The Earth is currently in a 100,000 year cycle of warming and cooling, commonly known as ice ages. These long-term cycles are related to change in the earth’s path as it revolves around the sun. Some ice ages were so severe that ice sheets stretched to the tropics. Times of such extreme cold are known as the "snowball Earth" condition, where the oceans would have been totally ice covered. During the last ice age, layers of snow and ice covered much of Canada and the northern portions of the United States, reaching as far south as Des Moines.

Glacial periods are punctuated by warm periods such as the one we are living in now. The temperature difference between the warm period we are currently experiencing and the last period of glaciation is only about 10 to 15 degrees. So a relatively small change in temperature can have an enormous impact on climate.

Although not triggered by carbon dioxide, ice ages are modified by changes in carbon dioxide. Over the last 800,000 years, the rise and fall of the Earth’s temperature closely mirrors the rise and fall in atmospheric levels of carbon dioxide.

Left to itself, the Earth will continue the glaciation cycle of the past with a very slow cooling of the Earth over the coming tens of thousands of years. But we are currently experiencing something much different. Instead of very gradual cooling, the Earth is rapidly warming.

Another example of the power of carbon dioxide is mass extinctions. Millions of years ago, sudden changes in climate caused mass extinctions of life on Earth. The most prominent one is the Permian-Triassic Extinction of 248 million years ago when 90% to 96% of life on earth perished.

Scientists believe many of these mass extinctions in Earth’s distant past were caused by giant volcanic eruptions. These behemoths of the past make current volcanic eruptions pale in comparison. These eruptions are believed to have continued for thousands of years with lava covering vast areas of the planet.

As these volcanoes erupted, they spewed enormous amounts of carbon dioxide into the atmosphere which caused a large increase in global temperature resulting in the extinction of many species.

So we can see that the Earth has undergone huge temperature and climate swings in the past driven by carbon dioxide and the greenhouse effect. The only difference is that today’s carbon dioxide emissions are coming from oil derricks and coal mines rather than massive volcanic eruptions.

Don Hofstrand is a retired agricultural economist from Iowa State University Extension. During the last few years of his work life, he focused on renewable energy and climate change. He and his wife live in Mason City.

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