Default publications post thumbnail

Pressure’s On: Accounting for Earth’s Habitability

Here’s an SAT question for you: Carbon dioxide is to global warming as atmospheric pressure is to ___________. Answer: global warming. While carbon dioxide grabs today’s headlines, atmospheric pressure levels in Earth’s history likely played a critical role in whether the planet would ultimately be suitable for large-bodied animals (including humans).

As far back in Earth’s history as scientists can look, they detect an abundance of life. However, Earth experienced at least three potentially devastating changes since life first arose. For one, the brightness of the Sun (which determines how much heat Earth receives) has increased more than 15 percent over the last 3 to 4 billion years. Two, water initially covered the entire surface of Earth but now continents comprise 30 percent of the surface. And three, Earth’s atmosphere started out devoid of oxygen, yet oxygen now comprises 20 percent of the atmosphere (see GOE or Die for how this change could have made Earth uninhabitable, but didn’t).

In fact, the latter two changes help compensate for the increasing luminosity of the Sun, ensuring Earth’s habitability. As continents appeared, weathering of the land masses removed carbon dioxide (one greenhouse gas) from the atmosphere. Also, the oxygenation of the atmosphere removed atmospheric methane (a second greenhouse gas). Thus, as the brightness of the Sun increased, these two processes cooled Earth’s surface both gradually (weathering) and rapidly (oxygenation).

How does atmospheric pressure affect these processes?

To understand the impact that atmospheric (air) pressure exerts on a planet’s temperature, consider Venus and Mars. Venus, closer to the Sun, is naturally warmer than Earth. However, it has a carbon dioxide-rich atmosphere with a pressure more than 90 times greater than Earth’s. These conditions result in a surface temperature hot enough to spontaneously ignite this magazine (although there would be no oxygen for combustion). On the other hand, Mars holds a carbon dioxide-rich atmosphere with a pressure around 1 percent of Earth’s. Consequently, the maximum surface temperature on Mars still causes water to freeze. In other words, air pressure dramatically affects a planet’s temperature.

While scientists have attained a small (but growing) body of evidence revealing the composition of Earth’s early atmosphere, nothing is known about its pressure. However, recent advances hope to change this lack of knowledge.

The key to understanding the ancient atmosphere is finding geologic formations that were affected by the atmospheric pressure. For example, lava traps air pockets as it cools. The size of the bubbles decreases as the pressure increases. Similarly, the size of raindrops decreases with increasing atmospheric pressure. Obviously, ancient lava bubbles and raindrops no longer exist, but other geological processes will fill the lava bubbles with other minerals and fossilize the craters made from raindrops hitting mud. Such remains in ancient rocks preserve a record of ancient air pressure.

An astrobiologist from the University of Washington and his colleagues found geological formations in Australia and Africa containing such signatures from rocks dating around 2.7 billion years old.1 They hope to determine the ancient air pressure to a satisfactory precision level. In so doing, their research stands to impact two questions of apologetic significance.

One objection that both naturalists and young-earth creationists raise against an old-earth creation model (like Reasons to Believe’s) is: Why did God wait so long to introduce humanity? A higher ancient atmospheric pressure would provide one more response to this objection. Higher pressure means thicker air, which is tougher to breathe. A factor of two increase in air pressure would significantly impair our ability to work and care for this creation.2 Evidently God caused the pressure to drop to current levels before introducing humanity onto Earth.

A second question can be stated this way: Given that the Sun continues to get brighter, aren’t humanity’s days numbered? A steady decrease of carbon dioxide has helped offset the increased energy received from the Sun over the last 3 to 4 billion years. However, photosynthesis requires a certain level of carbon dioxide in the atmosphere—levels not much below current values. Thus, further compensation requires some other mechanism or we’re toast.

Microscopic creatures might be the answer. Decreasing the atmospheric nitrogen (the dominant component) would result in a decreasing atmospheric pressure, and less atmospheric pressure means that more energy escapes from Earth’s surface. Research by a group of Caltech scientists suggests that this mechanism might extend Earth’s habitability by a billion years.3 The mechanism for reducing the atmospheric pressure from nitrogen likely involves bacteria and other single-celled organisms.

Such research demonstrates that complex interactions of astronomical, geological, atmospheric and biological processes must work in concert to maintain Earth’s habitability. One can reasonably conclude that once the atmospheric conditions permitted, God introduced humanity onto the scene. Scientific advance continues to provide additional support for the idea that a supernatural Designer fashioned Earth as a habitat for humanity.



1 Michael Shirber, “Measuring the Weight of Ancient Air,” Astrobiology Magazine, October 6, 2008,

2 Michael J. Denton, Nature’s Destiny (New York: The Free Press, 1998): 127–30.

3 King-Fai Li et al., “Atmospheric Pressure as a Natural Climate Regulator for a Terrestrial Planet with a Biosphere,” Proceedings of the National Academy of Sciences, 106 (June 16, 2009): 9576–79.