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Is the Sun getting hotter or colder?
The Sun is halfway through its lifespan. If it's burning through fuel, that must mean it's shrinking and getting colder, right?
While it may seem counterintuitive, the Sun is actually burning brighter and brighter with time. The Sun is basically a fusion reactor that produces energy by converting hydrogen into helium. As it loses more and more hydrogen, less fusion energy is output.
It's sounding like the Sun should be getting colder, but that's where the twist comes in. Less energy means the atoms exert less pressure against the forces of gravity and the weight of the atoms atop them. This causes volume to decrease, which results in an increase in density and, consequently, temperature, as the remaining atoms are squeezed together more tightly. The higher temperature accelerates the rate of fusion, generating more energy. The result? The Sun's brightness has been increasing by 1% every 110 million years.
Since the Sun is getting hotter, it's just a matter of time before the Earth slips out out of the habitable zone. How much time? Astronomers use a simple formula to estimate the center of the habitable zone: √(L)×1.34 AU, where 1.34 is the current center of our habitable zone and L is the ratio of a star's luminosity relative to the Sun's current luminosity.
Of course, this is a simplification, so pinning down a precise time would be inexact. But what we do see is that, when the Sun will be 10% brighter than it is now - in 1.1 billion years - the center of the habitable zone will be 1.41 AU and the inner boundary will be 1.06 AU, making the Earth a steamy, uninhabitable greenhouse.
Will CO2 levels continue to rise?
Is the Earth then fated to become a runaway greenhouse like Venus, in which the greenhouse effect becomes so strong that liquid water can no longer exist on its surface?
The Earth's carbon cycle is a very complex system with numerous sources and sinks. Photosynthesis is the sink everyone is familiar with - plants clean our air by "breathing in" CO2 and expelling oxygen - but another sink is the weathering of rocks. Similarly to how rust forms on metal by reacting with water and oxygen, rocks react with water and carbon dioxide. While this weathering isn't a huge carbon sink at present, as temperatures rise, so too will the rate of weathering.
Plantlife depends on carbon dioxide to survive, and just like with a predator-prey relationship, there has historically been a balance between the amount of CO2 in the atmosphere and the amount of plantlife on Earth. If polar bear populations rise too high, seal populations will decline, leading to a decline in the polar bear population, until balance is once again achieved. But if you introduce a new predator into the system, there's no guarantee that the polar bear population will rebound. Similarly, it's predicted that in about 600 million years, weathering will have decreased the CO2 levels below a point at which most plantlife can survive any longer.
With less CO2 in the atmosphere, shouldn't the greenhouse effect settle down and temperatures drop? Actually, water vapor presently accounts for the largest percentage of the greenhouse effect. By one billion years from now, the increasing brightness of the Sun will have raised global surface temperatures to about 47 °C, resulting in more water vapor in the atmosphere, a stronger greenhouse effect, and a feedback loop that will boil away our oceans by about 1.1 billion years from now. Earth will become an uninhabitable, runaway greenhouse.