The Dec. 21 letter “Just a warm cycle” contained some accurate information, but an incomplete conclusion. As the writer notes, there have been global temperature fluctuations during the past 2,000 or 3,000 years of 1 to 3 degrees Fahrenheit that would have occurred in the absence of human beings. Some causes can be volcanic eruptions, atmospheric pressure change cycles, solar cycles and probably changes in oceanic circulations.
Over a longer period, more significant climate changes (more than 10 degrees Fahrenheit) resulting in ice ages are due to periodic changes in the Earth’s orbit around the sun (approximately 100,000-year cycles) and changes in greenhouse gas content in the atmosphere.
About 18,000 years ago, ice covered northern sections of the U.S. as far south as central Illinois; the ice over northern Minnesota was about a mile thick. Then the orbital cycle changes caused the ice to begin to melt, sea levels gradually rose about 400 feet, and the climate stabilized except for the fluctuations noted above.
Now we have reached the point in the climate change cycle at which we should begin to cool down so that the glaciers begin to grow and return to their previous state. However, that’s not likely to happen because of the increasing level of greenhouse gases. Greenhouse gases (mostly water vapor, carbon dioxide and methane) trap Earth’s heat and keep us warm. If there were no greenhouse gases, Earth’s average temperature would be more than 50 degrees colder and Earth would be mostly unsuitable for human beings. (Most people have noticed that temperatures don’t fall as much on a cloudy winter night as they do on a clear night due to the effect of the water vapor: a very important greenhouse gas.)
Adding more carbon dioxide to the atmosphere will prevent the return of the glaciers during the next few thousand years. Millions of years ago, the carbon dioxide content of the atmosphere was much higher, Earth was warmer, dinosaurs roamed the planet and glaciers did not form. However, melting all of the remaining ice over Greenland and Antarctica would raise the mean sea level about 225 feet, flooding a large portion of Earth.
We humans now have two choices: We can continue to raise the carbon dioxide (and methane) content of the atmosphere, thereby disrupting the ice age cycles, but also melting the remaining glaciers and flooding our coastal areas, or we can keep the carbon dioxide (and methane) content low enough to prevent the melting of the glaciers and permit the ice age cycles to continue. Of course, there must be a “Goldilocks” level of carbon dioxide content that would disrupt the ice age cycles but leave our current glaciers about the same as they are. Carbon dioxide is currently at 400 parts per million in the atmosphere, and I’m wondering what this “Goldilocks” level is.
The writer is a retired meteorologist with degrees in physics and meteorology. The length limit was waived to permit a fuller response to the issue.