F or more than 2 million years our earth has cycled in and out of Ice Ages, accompanied by massive ice sheets accumulating over polar landmasses and a cold, desert-like global climate. Although the tropics during the Ice Age were still tropical, the temperate regions and sub-tropical regions were markedly different than they are today. There is a strong correlation between temperature and CO2 concentrations during this time.

Historically, glacial cycles of about 100,000 years are interupted by brief warm interglacial periods-- like the one we enjoy today. Changes in both temperatures and CO2 are considerable and generally synchronized, according to data analysis from ice and air samples collected over the last half century from permanent glaciers in Antarctica and other places. Interglacial periods of 15,000- 20,000 years provide a brief respite from the normal state of our natural world-- an Ice Age Climate. Our present interglacial vacation from the last Ice Age began about 18,000 years ago.

Over the last 400,000 years the natural upper limit of atmospheric CO2 concentrations is assumed from the ice core data to be about 300 ppm. Other studies using proxy such as plant stomata, however, indicate this may closer to the average value, at least over the last 15,000 years. Today, CO2 concentrations worldwide average about 380 ppm. Compared to former geologic periods, concentrations of CO2 in our atmosphere are still very small and may not have a statistically measurable effect on global temperatures. For example, during the Ordovician Period 460 million years ago CO2 concentrations were 4400 ppm while temperatures then were about the same as they are today.

Do rising atmospheric CO2 concentrations cause increasing global temperatures, or could it be the other way around? This is one of the questions being debated today. Interestingly, CO2 lags an average of about 800 years behind the temperature changes-- confirming that CO2 is not the cause of the temperature increases. One thing is certain-- earth's climate has been warming and cooling on it's own for at least the last 400,000 years, as the data below show. At year 18,000 and counting in our current interglacial vacation from the Ice Age, we may be due-- some say overdue-- for return to another icehouse climate!