The movement of carbon through Earth’s atmosphere, hydrosphere, crust, and mantle may not immediately fascinate everybody, but we will appreciate those cycles as we understand how they help make our existence possible. Scientists have learned how Earth’s fine-tuned carbon cycles have compensated for the Sun’s increasing brightness to keep our planet from overheating or overcooling and, thus, making long-term life possible and comfortable.

Carbon is a fundamental requirement for all physical life. Only one element in the periodic table possesses the necessary chemical bonding complexity and the necessary chemical bonding stability to permit the existence and operation of the essential building block molecules (for example, proteins, DNA, RNA, lipids) of life. For physical life to exist anywhere in the universe it must be carbon-based.

Physical life also requires carbon to exist at the just right abundance levels. As I have previously written, too little or too much carbon rules out life. Compared to a typical rocky planet of its size and distance from its host star, Earth possesses about 1,200 times less carbon. For this reason alone, life-bearing planets must be extremely rare in the universe.

For many reasons that I describe in Improbable Planet, the existence of advanced life requires at least a 3-billion-year previous history of abundant, diverse microbial life.1 For this history to be possible, carbon must move throughout the planet’s atmosphere, hydrosphere, crust, and mantle in highly prescribed ways. Scientists have long appreciated the exquisite fine-tuning of the movement of carbon in the atmosphere, hydrosphere, and crust. But only recently have they begun to understand the fine-tuned nature of carbon movement in Earth’s deep crust and mantle and how this understanding yields yet more evidence for the design of Earth for advanced life and for human life in particular.

Necessity of Carbon Cycles
Carbon-based molecules in Earth’s atmosphere act like huge blankets that keep Earth’s surface warm. The two most abundant of these molecules are carbon dioxide (CO2) and methane (CH4). Both are greenhouse gases in that they trap the Sun’s heat and make Earth’s surface warmer than it otherwise would be.

In combination with the greenhouse gas water vapor, and to a much lesser degree nitrous oxide, carbon dioxide and methane in Earth’s atmosphere warm Earth’s surface by 33°C (59°F). The Sun, however, does not provide Earth with a constant heat. Its luminosity has gradually increased over the 9-billion-year period during which the Sun’s nuclear furnace fuses hydrogen into helium (see figure). For Earth’s surface temperature to remain optimal for life, it is necessary for the greenhouse warming generated by water vapor, carbon dioxide, and methane in Earth’s atmosphere to decrease at a rate that continually compensates for the Sun’s increasing luminosity.

Figure: Sun’s Luminosity throughout Its History
Image credit: Hugh Ross

The quantity of water vapor in Earth’s atmosphere is determined by Earth’s surface temperature and the proportion of Earth’s surface covered by liquid water. Given that the percentage of Earth’s surface covered by liquid water has changed little during the past 2.4 billion years, at the optimal surface temperature for life the quantity of water vapor in Earth’s atmosphere is roughly constant. Hence, the maintenance of an optimal surface temperature for life requires that the quantities of carbon dioxide and methane in Earth’s atmosphere decrease at rates (throughout life’s history on Earth) that compensate for the Sun’s increasing luminosity. That needed compensation has occurred and is occurring through two carbon cycles, the shallow carbon cycle and the deep carbon cycle.

Shallow Carbon Cycle
The shallow carbon cycle refers to the movement of carbon throughout Earth’s atmosphere, hydrosphere (Earth’s oceans, seas, lakes, rivers, icefields, and glaciers), and crust. This cycle has been well understood both qualitatively and quantitatively for over a decade. Some life-forms remove carbon from the atmosphere, for example, through photosynthesis. Other life-forms add carbon to the atmosphere, for example, through the respiration of oxygen. When life-forms die, the decay of their bodies normally releases carbon back to the atmosphere. When dissolved silica in the oceans reacts with alkali metal cations to produce nonkaolinite clays, carbon dioxide is released into the oceans and atmosphere.

In terms of removing carbon from the atmosphere, the two most important shallow carbon cycle components are silicate weathering and the burial of organic carbon. Rain falling on exposed silicates (the main constituent of continental landmasses) acts as a catalyst to chemically transform silicates into carbonates and sand. This reaction removes carbon dioxide from the atmosphere. Different life-forms will expose less or more silicates to falling rain and thus regulate the rate of silicate weathering. (Read a complete description of silicate weathering here.)

Rapid burial of living and dead organisms by floods, slides, volcanic eruptions, and earthquakes prevents the carbon in the tissues and organs of life from decaying and thereby releasing carbon dioxide and methane to the atmosphere. Tectonic forces operating in Earth’s crust, over time, convert buried organic material into limestone, coal, and petroleum. The same tectonic forces will, on occasion, expose underground limestone, coal, and petroleum to the surface where they can decay and release carbon dioxide and methane to the atmosphere.

Geophysicists and geochemists now possess good measurements of the quantities of carbon pulled from the atmosphere and all the quantities of carbon added to the atmosphere by all the components of the shallow carbon cycle throughout the 3.8-billion-year history of life on Earth. While there are considerable variations in the quantities of carbon pulled or added to Earth’s atmosphere throughout the past 3.8 billion years, on average slightly more carbon is removed from Earth’s atmosphere than the amount that is added.

The extra amount of carbon pulled from Earth’s atmosphere compensates for much, but not all, of the surface temperature increase induced by the Sun’s ongoing luminosity increase. It is now evident that the deep carbon cycle compensates for the remainder. However, for life to be sustained on Earth for a long enough time period for microbial life to chemically transform Earth’s habitats into environments suitable for plant and animal life—and for this plant and animal life to further transform Earth’s habitats so that they can sustain human life—the combination of Earth’s shallow carbon cycle and Earth’s deep carbon cycle must be fine-tuned in many different ways to perfectly compensate for the Sun’s increasing luminosity.

In part 2 of this two-part series I will describe all the known components of the deep carbon cycle and how they must be fine-tuned to make possible the long-term survival of life on Earth and the existence of human beings. I will conclude with a discussion of how our increasing knowledge and understanding of the shallow and deep carbon cycles are yielding progressively more potent physical evidences for the super-intelligent, supernatural designs of Earth by God for the specific benefit of humans and human civilization.

Featured image: Shallow Carbon Cycles
Image credit: United States Department of Energy

Check out more from Reasons to Believe @Reasons.org