A recent letter to the editor asked the following question: “If and when Earth experiences a next transition to global cooling at, say, a similar slope to perceived current global warming, what quantitative amount of fossil fuels would humans need to burn to offset such cooling? From a practical standpoint, does Earth have enough coal in the ground to save the human race from destruction over, say, 500 years?”
Recognizing that had we not been burning fossil fuels and converting forests to urban and agricultural areas, Earth might naturally be transitioning into a glacial period within the next several thousand years, researchers have asked the same question. After all, the climate system has swung repeatedly between glacial (“Ice Age”) and interglacial (our current climate) over the past 2 million years.
We say the Earth might transition because the evidence is mixed on whether this would happen. Some of the best work on this subject has been done by David Archer at the University of Chicago. (D. Archer and A. Ganopolski, Geochemistry, Geophysics, Geosystems, May 5, 2005). Archer’s model does not predict glaciation within the next few thousand years, even if humans do nothing to perturb the climate system. It comes close, though, and he identifies small, realistic changes to aspects of the model that could lead to an Ice Age within a few thousand years. The consistent message from the model is that we will be close to glaciation in that time, so there is a chance that the 300 gigatons of carbon that we have already added to the atmosphere will help us avoid this catastrophe. Society’s emissions of carbon have increased atmospheric carbon dioxide from 280 parts per million in 1800 to 400 ppm in 2015.
Archer’s model does predict that, if not for human emissions, Earth would likely enter into a glaciation about 50,000 years from now, as summertime solar heating in the Northern Hemisphere falls below a critical threshold. This threshold decreases, however, as CO2 is added to the atmosphere. According to Archer, if we inject another 1,000 gigatons of carbon (about three times what we have added so far), the next glaciation will be pushed forward to 140,000 years from now. This is likely to happen, as the rate of fossil-fuel burning is increasing rapidly and the current lack of plans for emissions reductions appears to guarantee this will continue for the next few decades.
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An even larger release of carbon is possible. The recoverable fossil-fuel reserves contain about 5,000 gigatons of carbon. According to Archer, burning this entire inventory would postpone the next glaciation by at least half a million years. We could do this, too, if we continue to burn fossil fuels for the next 200-300 years.
But is this really good news? Avoiding glaciation tens to hundreds of thousands of years from now overlooks an immediate problem. The climate is warming rapidly and is projected to continue doing so for the foreseeable future. Average global temperatures could increase by 4 to 9 degrees (2 to 5 degrees Celsius) within the next century, according to the 2013 report of the Intergovernmental Panel on Climate Change. That is already enough to trigger increases in sea level, to make hot areas of the globe even more marginal and to change the climate everywhere. And that is just the beginning of what could ultimately happen. If we actually did burn 5,000 gigatons of carbon, atmospheric CO2 concentrations could increase to 1,500-2,000 ppm and the climate could warm by11 to 14 degrees (6 to 8 degrees Celsius). This would make large parts of the world uninhabitable by humans and could eventually melt the polar ice caps completely, raising sea level by about 200 feet.
Such a climate catastrophe would be even worse and far more imminent than another Ice Age. Fortunately, it does not have to happen. We can choose a more temperate and human-friendly climate for our descendants if we make smart choices and transition to non-fossil sources of energy over the coming decades.