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By Root 715 0

rather than all ending up in one place. Water, for instance, cycles through rivers, oceans, ice caps, the atmosphere, and us. An H2O molecule falling in a snowstorm on the rocky peak of Mount Kenya may have been exhaled in the dying gasps of Queen Elizabeth I: Water, driven by energy, is always circulating. Nitrogen, oxygen, phosphorus, sodium, iron, calcium, sulfur, and other elements are also perpetually on the move. Carbon is perhaps the most important cycle of all, because of the thermostatic role played by its molecular state; particularly in its gaseous form as CO2, but also in combination with other elements, such as with hydrogen as CH4 (methane). It was the failure of the carbon cycle that doomed Venus and Mars, yet here on Earth various feedbacks have kept the system in relative balance for billions of years—even altering the strength of the greenhouse effect to offset the sun’s increasing output of radiation over geological time.

Over million-year timescales, the carbon cycle balances out between the weathering of rocks on land, which draws carbon dioxide out of the air, and its emission from volcanoes. Carbon is deposited in the oceans and then recycled through plate tectonics, as oceanic plates subduct under continental ones, providing more fuel for CO2-emitting volcanoes. The process is self-correcting: If volcanoes emit too much carbon dioxide, the Earth’s atmosphere heats up, increasing weathering rates and drawing down CO2. If carbon dioxide levels fall low enough for weathering to cease—as perhaps was the case during the early “snowball Earth” episodes, when global-scale ice caps put a stop to the weathering of rocks—volcanic emissions continue uninterrupted, allowing CO2 to build up until a stronger greenhouse effect melts the ice and allows balance to be restored. The system is stable but not in stasis: The geological record shows tremendous swings in temperature and carbon dioxide concentrations over the ages, though always within certain boundaries.

Perhaps one of the strongest arguments against the Gaia concept is the fact that even if the planet in general remains habitable, things do sometimes go badly wrong. Over the last half-billion years since complex life began there have been five serious mass extinctions, the worst of them wiping out 95 percent of species alive at the time. Most appear to have been linked to short circuits in the carbon cycle, where volcanic super-eruptions led to episodes of extreme global warming that left the oceans acidic and depleted in oxygen, and the land either parched or battered by merciless storms. And yet, over millions of years, new species evolved to fill the niches vacated by extinguished ones, and some kind of balance was restored. Over the last million years, recurrent ice ages demonstrate how regular cycles can lead to dramatic swings in temperature, as orbital changes in the Earth’s motion around the sun lead to small differences in temperature, which are then amplified by carbon-cycle and ice-albedo (reflectivity) feedbacks. Our planet may be self-regulating, but it is also extraordinarily dynamic.

GOD SPECIES OR REBEL ORGANISM?

Life is now an important component of most of the planet’s major cycles. The majority of carbon is locked up in calcium carbonate (limestone) rocks, laid down in the oceans by corals and plankton. The appearance of photosynthesis was perhaps one of life’s most miraculous innovations, allowing microbes—and later, green plants—to use atmospheric carbon dioxide as a source of food. Water is an essential part of the process: In cellular factories called chloroplasts, plants split water into hydrogen and oxygen, combining the hydrogen with carbon from the air to form carbohydrates, and releasing oxygen as a waste product. The process opened up an opportunity for the evolution of animals, which could eat the carbohydrates as a food source and recombine them with oxygen (forming CO2 and water), thereby generating energy and closing the loop.

Evolution of life is a critical part of the process of planetary self-regulation, because