Unit 1: Many Planets, One Earth // Section 4: Carbon Cycling and Earth's Climate
How did early Earth transition from a hell-like environment to temperatures more hospitable to life? Early in the Archean (ancient) eon, about 3.8 billion years ago, the rain of meteors and rock bodies from space ended, allowing our planet's surface to cool and solidify. Water vapor in the atmosphere condensed and fell as rain, creating oceans. These changes created the conditions for geochemical cycling—flows of chemical substances between reservoirs in Earth's atmosphere, hydrosphere (water bodies), and lithosphere (the solid part of Earth's crust).
At this time the sun was about 30 percent dimmer than it is today, so our planet received less solar radiation. Earth's surface should have been well below the freezing point of water, too cold for life to exist, but evidence shows that liquid water was present and that simple life forms appeared as far back as 3.5 billion years ago. This contradiction is known as the "faint young sun" paradox (Fig. 7). The unexpected warmth came from greenhouse gases in Earth's atmosphere, which retained enough heat to keep the planet from freezing over.
Figure 7. The faint, young sun and temperatures on Earth
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The Archean atmosphere was a mix of gases including nitrogen, water vapor, methane (CH4), and CO2. (As discussed in section 6, "Atmospheric Oxygen," free oxygen did not accumulate in the atmosphere until more than two billion years after Earth was formed.) Volcanoes emitted CO2 as a byproduct of heating within the Earth's crust. But instead of developing a runaway greenhouse effect like that on Venus, Earth's temperatures remained within a moderate range because the carbon cycle includes a natural sink—a process that removes excess carbon from the atmosphere. This sink involves the weathering of silicate rocks, such as granites and basalts, that make up much of Earth's crust.
As illustrated in Figure 8, this process has four basic stages. First, rainfall scrubs CO2 out of the air, producing carbonic acid (H2CO3), a weak acid. Next, this solution reacts on contact with silicate rocks to release calcium and other cations and leave behind carbonate and biocarbonate ions dissolved in the water. This solution is washed into the oceans by rivers, and then calcium carbonate (CaCO3), also known as limestone, is precipitated in sediments. (Today most calcium carbonate precipitation is caused by marine organisms, which use calcium carbonate to make their shells.) Over long time scales, oceanic crust containing limestone sediments is forced downward into Earth's mantle at points where plates collide, a process called subduction. Eventually, the limestone heats up and turns the limestone back into CO2, which travels back up to the surface with magma. Volcanic activity then returns CO2 to the atmosphere.
Figure 8. The geochemical carbon cycle
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Source: © Snowball Earth.org.
Many climatic factors influence how quickly this process takes place. Warmer temperatures speed up the chemical reactions that take place as rocks weather, and increased precipitation may flush water more rapidly through soil and sedimentary rocks. This creates a negative feedback relationship between rock weathering and climatic changes: when Earth's climate warms or cools, the system responds in ways that moderate the temperature change and push conditions back toward equilibrium, essentially creating a natural thermostat.
For example, when the climate warms, weathering rates accelerate and convert an increasing fraction of atmospheric CO2 to calcium carbonate, which is buried on the ocean floor. Atmospheric concentrations of CO2 decline, modifying the greenhouse effect and cooling Earth's surface. In the opposite instance, when the climate cools weathering slows down but volcanic outgassing of CO2 continues, so atmospheric CO2 levels rise and warm the climate.
This balance between CO2 outgassing from volcanoes and CO2 conversion to calcium carbonate through silicate weathering has kept the Earth's climate stable through most of its history. Because this feedback takes a very long time, typically hundreds of thousands of years, it cannot smooth out all the fluctuations like a thermostat in one's home. As a result, our planet's climate has fluctuated dramatically, but it has never gone to permanent extremes like those seen on Mars and Venus.
Why is Venus a runaway greenhouse? Venus has no water on its surface, so it has no medium to dissolve CO2, form carbonic acid, and react with silicate rocks. As a result volcanism on Venus continues to emit CO2 without any carbon sink, so it accumulates in the atmosphere. Mars may have had such a cycle early in its history, but major volcanism stopped on Mars more than 3 billion years ago, so the planet eventually cooled as CO2 escaped from the atmosphere. On Earth, plate tectonics provide continuing supplies of the key ingredients for the carbon-silicate cycle: CO2, liquid water, and plenty of rock.