Evidence for a changing climate continues to accumulate but I'm well aware that many people remain skeptical about the role of humans in this process. In this column I thought to review one of the several pieces of evidence for human involvement in the present period of global warming.
Over the past several hundred thousand years our planet has experienced climatic cycles oscillating between thousands of years of warmth with small polar icecaps and thousands of years of cold with large areas of the Earth's higher latitudes covered by thick glaciers and sea ice. These cycles have had a rough periodicity with the glacial maximums occurring about every hundred thousand years. The record of these glacial maximums and the intervening warm epochs has been found to be preserved both in marine sediments and in ice cores from Greenland and Antarctica. The cause of these historical climate cycles are considered to be the result of variations in the amount of solar radiation reaching the Earth because of well-known variations in the Earth's orbit around the sun and in the inclination of the Earth's axis with respect to the sun.
Specifically, the data from marine sediments and ice cores indicates that during the past several hundred millennia it has taken about 10,000 years for the planet to warm up during a warming spell and 50,000 or more years to cool back down. While the warm spell between these periods of change lasts around 30,000 years. In synchrony with these changes in global temperature, the level of carbon dioxide in the atmosphere rose and fell, and during these climatic cycles rising temperatures usually preceded the rising atmospheric carbon dioxide.
The present warm period began about 15,000 years ago and by the beginning of the last millennium the amount of carbon dioxide in the atmosphere had reached its usual historical levels for warm periods, which was 250-280 parts per million (ppm). Then, coincident with the start of the Industrial Age in the mid-Eighteenth Century, something quite unusual began to happen. The carbon dioxide level in the atmosphere started going up again, not only well before there was any evidence of a further rise in global temperature but, more strikingly, at rates many times the rate it had risen in the past. The concentration of atmospheric carbon dioxide at the present time is close to 390 ppm and it is rising at a rate of more than 2 ppm per year. Given that we have not experienced a prolonged period of massive volcanism during the last 200 years the most likely sources for much of this excess carbon dioxide are, of course, the burning of fossil fuels and the clearing of forests. Now, some 200 years after this unusually rapid rise in atmospheric carbon dioxide began, we've begun to experience a rise in the global mean surface temperature, fulfilling the decades-old predictions of climate scientists.
Is this sufficient evidence that humans are playing a significant role in global warming?