Earth's atmosphere

I am writing with regard to two letters (C&EN, Oct. 23, pages 4 and 6). One, by Oswald Bergmann, asks if global warming should be self-limiting according to Stefan-Boltzmann law. The simple answer is yes. This has been described by Thomas Spiro and William Stigliani in their textbook "Chemistry of the Environment." Earth, as a blackbody, can maintain a stable temperature by radiating out the same amount of energy it receives from the sun, a zero net energy balance.

When the greenhouse effect is enhanced—for example, by anthropogenic emissions—the outgoing energy drops, causing the Earth net energy balance to be positive momentarily, which leads to a temperature increase. More energy will radiate out, according to the Stefan-Boltzmann law, until the net energy balance is restored to zero. So Earth's temperature will stabilize at another, increased value. The Stefan-Boltzmann effect acts like a negative feedback to keep Earth's temperature from changing in one direction only. However, Earth is not a perfect blackbody. It has other internal mechanisms that regulate its climate, in addition to solar receipt and the greenhouse effect. Some of those mechanisms are positive feedbacks that amplify small perturbations.

Suppose a slightly enhanced greenhouse effect increases the temperature only slightly (and would stabilize quickly via Stefan-Boltzmann law). This may cause the total area covered by snow to decrease, which leads to increased surface absorption of solar radiation since snow reflects more sunlight than anything else. More sunlight absorbed causes the temperature to go up further, leading to more melting, etcetera. Understanding and quantifying the effects of such feedback mechanisms are among the top priorities of current climate-change research.

The other letter, by William Levinson, suggests that efforts to reduce or limit the human-enhanced greenhouse effect would be useless because Earth has natural climate changes. It is true that temperatures have fluctuated, and Earth has experienced cycles of ice ages and warm periods. However, there is little evidence to suggest variations of solar activity are the major cause of those cycles. Rather, the cycles are regulated by the slight variations of the Earth-sun orbital relationship and are driven by the feedback mechanisms, including the natural greenhouse effect.

Furthermore, polar ice core data show convincingly that atmospheric carbon dioxide concentrations have been correlated or coupled tightly with Earth temperature for at least the past 650,000 years, when Earth experienced six or seven such cycles (Science 2005, 310, 1313). This suggests that the greenhouse effect is a major factor regulating Earth's climate.

Levinson cites the preindustrial Viking settlement of Greenland as an example of natural climate change. It is true that during the so-called medieval warmth, parts of the Northern Hemisphere experienced unusually warm temperatures (only by less than 1 ^oC on average). However, Levinson might be dismayed to know that the slight cooling (again, only a fraction of a degree) following the medieval warmth contributed significantly to the demise of the Greenland Vikings.

Jihong Cole-Dai
Brookings, S.D.