Let’s be optimistic and assume that we manage to avoid a self-inflicted nuclear holocaust, an extinction-sized asteroid, or deadly irradiation from a nearby supernova. That leaves about 6 billion years until the sun turns into a red giant, swelling to the orbit of Earth and melting our planet. Sounds like a lot of time.
But don’t get too relaxed. Doomsday is coming a lot sooner than that.
The Earth is, in some ways, in a precarious spot in the solar system. There’s a range of orbital distances inside which a planet can have both liquid surface water (which is believed to be necessary for life) and enough atmospheric CO2 to carry on photosynthesis. This range is called the photosynthesis habitable zone. The Earth orbits barely within the sun’s zone. Some scientists estimate that the inner edge lies just 7.5 million kilometers away, which is only 5 percent of the distance between the Earth and the sun.
And that inner edge is moving out. Our sun is a massive ball of gas held together by its own gravity. At its center, the intense pressure and heat fuse hydrogen nuclei together to form helium. It takes four hydrogens to make one helium nucleus. As the number of nuclei in the core of the sun decreases by three with each helium nucleus formed, the outward pressure of the core also decreases (because the pressure is proportional to the number of nuclei per volume). In response, the outer layers of the sun squeeze the core harder, increasing its pressure, temperature, and fusion rate, leading to a 10 percent growth in brightness every billion years.
The Earth responds to this increasing brightness, in part, by decreasing the thickness of its warming blanket, which is provided by atmospheric CO2. Rising temperatures on Earth speed up reactions between water and silicate-rocks, which in turn draws CO2 out of the atmosphere. Sort of like changing your winter down comforter for a cotton sheet in the summer, this helps to keep the surface temperature of the planet habitable.
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