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The natural occurrence of previous climate changes and ice ages is frequently used as an argument against the notion that man’s activities have contributed significantly to the current warming trend. After all, man was clearly not involved in the warming periods that ended each of the previous ice ages. Therefore, why should he be implicated in the present warming trend? This question is raised so frequently within the public discussions of global warming that it is useful to remind ourselves of its scientific basis (for more detail than can be provided here, see the Web site www.realclimate.org).

Early climate changes and ice ages are thought to have been caused by a variety of factors, with a primary one being the well-known variations in the Earth’s orbit (called the Milankovitch cycles). In addition, changes in the sun’s intensity, reflections of incoming sunlight (called the albedo effect) and the insulating effect of greenhouse gases have all played a part.

We also have what is thought to be an accurate record of the concentration of carbon dioxide existing in the atmosphere over the last 650,000 years (from core ice samples collected in Antarctica and Greenland). This record shows that CO2 levels rose and fell between the limits of 180 and 300 parts per million (ppm) during cool and warm periods, respectively, throughout that entire time period.

Over the last 150 years, however, CO2 levels have risen from 280 to 380 ppm. By the end of the 21st century, CO2 levels are expected to exceed 600 ppm at the present rate of fossil fuel consumption. That will be twice the highest levels ever reached during the previous 650,000 years. The current rate of CO2 increase is also unprecedented. During the previous ice ages, it took at least a thousand years for a 30-ppm change in CO2 concentration to occur, while the present level of CO2 rose by 30 ppm in just the last 17 years!

Therefore, what is happening today appears to be driven by a new and different mechanism not previously observed for at least 650,000 years. Since the current rise in CO2 levels started with the onset of the Industrial Age, the activities of man are clearly implicated. This conclusion has been additionally reinforced by measurements of the relative abundance of carbon’s heavier isotopes, carbon-13 and carbon-14, in atmospheric CO2. These isotopic “fingerprints” show that the significant amount of fossil-fuel-derived CO2 found in today’s atmosphere is also unprecedented.

Finally, it is important to note that all of the information provided here has been based on direct physical measurements and not on theory and models.

Eric Grimsrud is a recently retired research scientist and chemistry professor. He lives in Kalispell.

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