Design Update: How Wide the Life Zone?

by Guillermo Gonzalez

Two decades ago, researcher Michael Hart provided an important piece of evidence in the argument for divine design. Using long-term climate simulation models, he showed that the region around our sun in which a rocky planet (like Earth) can continuously support life (by maintaining liquid water on its surface) is a very narrow band. According to Hart, that zone (called the CHZ, continuously habitable zone) begins just a little closer to the sun than Earth's current location and ends an even smaller distance beyond our current location.1

Two years ago, however, Hart's conclusion was challenged by James Kasting of Pennsylvania State University. Kasting and his colleagues estimated that the CHZ around our sun is much larger, extending from just a little inside our current orbital location to almost as far out as Mars.2 Those with a Christian world view can still see the miracle of our location, but those with a nontheistic perspective seemed to gain some room for happenstance. I say "seemed" because a closer look at Kasting's estimate shows that they gained nothing, except perhaps the opposite of what they expected, a further accumulation of evidence for design.

This closer look turns up some serious flaws in Kasting's assumptions. First, he failed to adjust his long-term climate calculations to fit the changes brought about when advanced plant life appeared. He made his calculations as if vascular plants had always been on the scene though, in fact, they came only about 350 million years ago. These plants significantly impact the processes, including one called "weathering," that gobble up carbon dioxide and thus keep the planet cool enough for life.3, 4 As we learn more about the role of these plants, we uncover additional indications of design. For instance, we discover that if advanced plants had not come on the scene when they did, Earth's surface would be uninhabitable today, too warm for water retention (nor would we have coal to burn—but that's another story).

A second problem with Kasting's calculations is even more serious. His wide CHZ demands some means to keep the planet warm if it is positioned farther from its star than Earth is from the sun. His answer is to invoke an increasingly heavy carbon dioxide atmosphere (the "greenhouse" effect we hear about in discussions of global warming). While this "solution" would seem to fix the temperature problem, it cancels itself out by introducing other problems, most notably breathing problems for advanced life. This heavy, warm-temperature-sustaining atmosphere would be toxic to oxygen-breathing life forms.

One might be quick to suggest that life could and would gradually adapt. However, as molecular biologist Michael Denton points out in his latest book, Nature's Destiny, biology does have its limits. Nature can only be pushed so far. Biological organisms cannot arbitrarily change to fit any environment.5 For example, the work required to move air in and out of lungs is dependent on the density and viscosity of the air. The denser the atmosphere, the more oxygen an organism expends just in the effort to breathe (i.e., the higher its "oxygen cost"). In addition, the higher the concentration of carbon dioxide in the atmosphere, the faster an animal has to breathe, and the faster an animal breathes, the more energy it uses in breathing. An increase in the CO2 concentration from the present 0.03% to 10% would cause a ten-fold increase in breathing rate (thus in energy expenditure) for humans.6 The smaller animals, with their higher metabolic rates, would be impacted even more rapidly and dramatically.

The higher pressure of a heavy atmosphere might also extinguish the water cycle. As air pressure increases over water's surface, the boiling point increases and the evaporation rate decreases. Increasing the surface pressure on the earth threefold, which Kasting proposes as the temperature fix, would reduce the evaporation rate by about 20%. While some evaporation and precipitation might still occur over the oceans and coastlines, the continental interiors would likely become very dry—no rivers, no lakes, and no snow-capped mountains. How long would life survive under such conditions? Even if liquid water were the only requirement for life, the answer must be "not long."

Kasting's estimate for the size of the CHZ looks less and less plausible and Hart's looks more and more so as we investigate the intricacies of life and of the atmospheric conditions under which it can survive. Scientists are left to wonder how Earth came to exist and persist for so long in the zone where life is possible. The impression of design could hardly be more distinct.

1. M. H. Hart, "The Evolution of the Atmosphere of the Earth," Icarus 33 (1978), pp. 23-39.
2. J. F. Kasting, "Habitable Zones Around Stars: An Update," Circumstellar Habitable Zones (Menlo Park: Travis House Publications, 1996), pp. 17-28.
3. R. A. Berner, "Paleozoic Atmospheric CO2: Importance of Solar Radiation and Plant Evolution," Science 26 (1993), pp. 68-70.
4. K. L. Moulton and R. A. Berner, "Quantification of the Effects of Plants on Weathering: Studies in Iceland," Geology 26 (1998), pp. 895-898.
5. M. J. Denton, Nature’s Destiny (New York: The Free Press, 1998).
6. K. Schmidt-Nielsen, Animal Physiology: Adaptation and Environment (Cambridge: Cambridge Univ. Press, 1990), p. 35.

Dr. Gonzalez is an astronomer at the University of Washington.