Although we don’t fully understand why the Earth has been so habitable for so long, a key mechanism is the evolution of the atmosphere, and one of the drivers of that is photosynthesis. Various types of photosynthetic organisms—large and small, on land and in the oceans—have taken carbon dioxide from the atmosphere and released oxygen, converting the carbon to hydrocarbons and storing it in the rocks of the Earth’s crust. A parallel process, with a similar outcome, is the conversion of carbon from carbon dioxide into carbonate minerals, which is what most shelled organisms do to make their shells. Over time, as the sun has slowly warmed, these two processes have reduced the greenhouse effect enough to keep the Earth from getting too hot.

The idea that life has controlled Earth’s climate to its own benefit forms the basis of the Gaia theory, first proposed by James Lovelock in 1972 and expanded upon by Lovelock and Lynn Margulis in 1974. According to Lovelock and Margulis, the Earth and the living organisms on it form a self-regulating system that ensures not only that conditions remain suitable for life to persist but also that they have been able to do so even as the system’s main source of energy (the sun) has slowly changed in intensity. This self-regulation proceeds through various types of biological processes and climate feedback. (Climate feedbacks are discussed in chapter 1.)

Here’s an example of how this can work. Imagine a vast population of bacteria living in the ocean in a warm climate. Most of them are the old-style bacteria, and a few are the new-improved photosynthetic bacteria that use the sun’s energy to help them convert carbon dioxide to oxygen. Let’s say that they thrive best in slightly cooler conditions, while the regular bacteria prefer it to be warm. As time passes (lots of it), the photosynthetic bacteria gradually consume enough carbon dioxide to cool the climate just a little bit. This is good for them and they thrive, while the regular bacteria shiver and complain (and die). In their increasing numbers, the photosynthetic bacteria cool the climate even more, so they thrive even better, and in time (lots of it), they start to dominate the bacteria population and continue to cool the climate. If they then become too successful and cool the climate too much for their own good, they will no longer flourish, and it will stop getting cooler.

Lovelock’s Gaia theory was not well received by the scientific community in the early days. In fact, it went one worse than that: it was almost universally ignored. One issue was the use of the term “Gaia” (a Greek goddess) and Lovelock’s musing that she was a living being. In 1979, he wrote: “But if Gaia does exist then we may find ourselves and all other living things to be parts and partners of a vast being who in her entirety has the power to maintain our planet as a fit and comfortable habitat for life.” This type of language makes most scientists cringe. Another serious concern was the implication that the organisms of the day had conceived a plan to make the climate amenable and set about doing it.

Some kind of “purpose” on the part of living organisms was not a component of Lovelock’s theory at all, but the early writings on Gaia did not make that clear enough for many. A further concern was that Gaia wasn’t a real scientific theory because it wasn’t testable. Moreover, at that time many scientists who thought about the ancient climate just assumed that it was controlled purely by physical and chemical processes.

The bare surface of the planet had an albedo (reflectivity) that was midway between that of white and black daisies. On Daisyworld, the daisies can survive within the temperature range of 5° to 40°C, but they do best at 22.5°C. Using a numerical model, Watson and Lovelock showed that very early in the planet’s history, while the star was still cool, Daisyworld was too cold for daisies to grow at all. As the planet’s surface temperature eventually reached 5°C, they started to grow; black daisies did best because they had the effect of absorbing sunlight and warming their local environment, while white daisies did poorly because they cooled their local environment. So black daisies ruled. As the star became progressively warmer, and there was less need to enhance that warmth, the white daisies started to compete better with the black daisies, and with continued warming, the white daisies ruled. Eventually the star got so hot that even the white daisies couldn’t reflect enough light away, and the temperature exceeded 40°C, resulting in the death of all daisies.

As soon as the temperature reaches 5°C, the daisies start to grow, and black daisies quickly become dominant. Within a few million years, they have stabilized the temperature at close to the ideal of 22.5°C. From that point on, a combination of white and black daisies keeps the temperature near to that ideal, but when the star becomes too hot for even 100% white daisies to regulate the temperature, things go badly.

Although Daisyworld is a highly simplistic model for climate evolution, it demonstrates that this type of biological control over the climate is possible and also that it doesn’t take purposeful or organized behavior to make it happen. Over the past few decades, the Gaia theory has gradually gained acceptance, and it is now firmly established as a viable explanation for evolution of the atmosphere and for Earth’s 4.57-billion-year history of a (relatively) stable climate. Without mediation by life, there is essentially zero chance that the Earth’s surface temperature would be almost the same now as it was at 4 Ga.

For the most part, biological control of the Earth’s climate to compensate for a warming sun has been achieved—not by black and white daisies but by changes to the proportions of atmospheric gases, or more specifically by reductions in the concentrations of greenhouse gases. This means that most of the trillions of tonnes of carbon that used to be in the atmosphere as carbon dioxide and methane were slowly, methodically, and safely stored in the Earth’s crust in rocks like limestone and as hydrocarbon molecules in coal, oil, and natural gas.

Limestones have been created by a wide range of mostly marine organisms, including corals, bivalves, gastropods, cephalopods, sponges, arthropods, and algae. The organic matter that has become fossil fuels has been derived mostly from microorganisms, green algae, red algae, and later on both primitive and evolved land plants.

This storage of carbon in rocks has been taking place for a very long time. The oldest limestones are older than 3.5 Ga, and there are also carbon-rich black shales that date back to at least 3 Ga. Although most fossil-fuel deposits are in rocks younger than 250 Ma (250 million years), some are much older than that, even Precambrian (older than 540 Ma). That said, most of the really old rocks have been buried sufficiently deeply in the crust that any carbon they might have contained has been converted to graphite at high temperatures.

The carbon in limestone and in fossil-fuel bearing rocks is stored “safely” because these rocks are mostly safe beneath the Earth’s surface. Only a tiny proportion of such rock is naturally eroded each year, releasing some of the carbon to the atmosphere, while a similar amount is being stored away somewhere else, mostly on the seafloor. The problem is that humans are interfering with this natural process in a major way by digging up and burning the fossil fuels and using the limestone to make cement.

Every year, we use a massive amount of fossil fuels (including coal, oil, and natural gas), an amount equivalent to 80 billion barrels of oil. That’s roughly the same as 115 million railway tanker cars, or an oil train long enough to go around the world 17 times! Every single day, that’s equivalent to about 319,700 railway tank cars, enough to stretch from Toronto to Dallas or from Paris to Kiev. Another 2,000-kilometer-long train every day! Almost all of the carbon in that coal, oil, and gas ends up in the atmosphere as carbon dioxide: about 100 million tonnes every day. That very carbon was removed from the atmosphere tens and hundreds of millions of years ago, offsetting the impact of a warming sun, and it was stored in the rocks. By digging it up or pumping it out and putting it back into the atmosphere, as carbon dioxide, we are severely compromising the Earth’s ability to regulate the temperature under a sun that is now much warmer than it was when that carbon was first stored away.

Over the next million years, the amount by which the sun is expected to warm represents a potential temperature change on Earth of about 0.016°C (if everything else is held constant). If humans haven’t been voted off the planet by then, it’s likely that this is something they’ll be able to cope with.

But if we look ahead on a different time scale, things do get more difficult. Modeling future climates out to about 1.5 billion years shows that although the Earth should still support life at that time (because there should still be liquid water), the only place that anyone could survive would be on Antarctica. But that’s so far into the future that it’s not even worth speculating what life on Earth might look like.

Sometime after that, the planet will become so hot that water will be boiled off into space, and then it will be truly uninhabitable for life as we know it.