Our climate has been stable for the last 12,000 years, but geological history shows that the Earth’s climate and atmosphere have undergone drastic changes in the past. Life itself has affected our atmosphere in amazing ways; in turn, major atmospheric changes occurred in just the right way for us to exist. This history shows how responsive our atmosphere is to incremental changes caused by living things.

Scientists tell us that Earth is 4.5 billion years old. When the earth was only 200,000 years old, something remarkable occurred: Life emerged. The fossil record shows that for 1.5 billion years, anaerobic bacteria (bacteria that digested chemical compounds without using oxygen) were the only life forms on Earth, and they were all in the ocean, which shielded them from the sun’s ultraviolet radiation.

Our atmosphere was about 20 percent carbon dioxide and had no free oxygen. The temperature was in the right range for life, but the sun was gradually burning hotter, threatening the continuation of life on Earth.

Then an Earth-changing event occurred when some of these bacteria developed the ability to use chlorophyll to capture energy from sunlight. These cyanobacteria flourished, emitting oxygen and gradually decreasing the carbon dioxide in our atmosphere. Carbon dioxide traps heat from the sun, and the gradual decrease in carbon dioxide has kept our planet from getting too hot for life.

This change, however, was not a painless one. Rising oxygen levels led to a massive extinction of anaerobic bacteria. As carbon dioxide levels first plummeted, the Earth experienced an extreme ice age (average temp of -58 F) that lasted for millions of years.

The Earth’s temperature warmed eventually, and, somehow, bacterial life endured, eventually giving rise to larger plants and animals. Life was not possible on land, however, until another Earth-changing event occurred. Oxygen in our lower atmosphere drifted up to the stratosphere, where intense ultraviolet radiation caused oxygen to split and recombine, creating ozone, which in turn blocks ultraviolet radiation. By 400 million years ago, the ozone layer had become thick enough to shield the Earth from deadly levels of ultraviolet radiation, and this made it possible for life to colonize the land.

A more recent Earth-changing event was caused by humans. Chlorofluorocarbon, an effective coolant for refrigeration, was created in 1930 and was used around the world in homes and industry. Fortunately for us, a scientist, Mario Molina, discovered that after this chemical escapes into the air, it rises to the stratosphere, where every molecule of chlorofluorocarbon destroys about 100,000 molecules of ozone. Molina discovered that, during our winters, massive holes in the ozone had started developing over the South Pole.

Scientists soon realized that we could destroy the ozone layer if we continued releasing chlorofluorocarbon, and intense ultraviolet radiation would destroy all plant and animal life on land. The use of chlorofluorocarbon was curtailed, and the ozone layer is gradually replenishing. We had a close call with the ozone layer, acting just in time to avoid major problems.

Mankind, however, has been causing another Earth-changing event during the past century, as we have created complex agricultural and industrial systems and a way of life easier than ever before. All of this is causing carbon dioxide levels to rise faster each year.

We should remember that the Earth’s temperature has shifted drastically in response to changes in carbon dioxide levels in the distant past. We also should consider that minor atmospheric changes (increases in chlorofluorocarbon) led to a close call with the ozone layer.

It is important that we understand how our civilization affects the Earth’s atmospheric systems that make our lives possible. Like the cyanobacteria 3.5 billion years ago, we are causing incremental, but powerful changes to our atmosphere, changes that are affecting our climate. We are on the threshold of understanding these life systems enough to learn how to live without throwing our atmosphere out of balance.

We have the ability to change direction to restore balance to our atmosphere and climate. This will require a greater commitment to conserving energy in all our lives, and it will require us to push our politicians to get more serious about reducing greenhouse emissions. This is the gift that our children require of us.

Richard Thomas is a member of Sustain Mid Maine Coalition’s Public Policy Team. Information for this column comes largely from “The End of the Long Summer” by Diane Dumanoski and “Gaia 101,” a course on climate science by Dr. Alder Stone Fuller.