 Once the core hydrogen is exhausted in five billion years, the Sun will expand into a subgiant phase and slowly double in size over about half a billion years. It will then expand more rapidly over about half a billion years until it is over two hundred times larger than today. With the increased surface area of the Sun, the amount of energy emitted will increase. Unless Earth's orbital radius increases significantly, the global temperature of the Earth will climb because of the rising luminosity of the Sun. As global surface temperatures continue to rise, more water will start to evaporate away into space. The current atmospheric escape rate of hydrogen in the oceans is approximately one meter of oceans every billion years, but as the Sun continues to warm, the rate will increase to at least one meter of ocean every million years. The gradual loss of Earth's water will ultimately cause plate tectonics to gradually slow down and grind to a halt as water is a lubricant for tectonic activity and with all water gone, the crust will dry out and become too hard to be subducted into the mantle. With the cessation of plate tectonics, no more carbon will be released into the atmosphere via volcanic activity nor will be subducted into the ocean floor, therefore causing the carbon cycle to cease altogether. However, geoscientist James Kasting says the oceans may evaporate much earlier, in about a billion years.
