The steady collapse of the growing helium-ash core which the Sun accumulates as it fuses hydrogen into helium. This causes the Sun and all stars to leave the 'zero-age' Main Sequence soon after they are born, and steadily increase in luminosity. For where the Sun is today, by about 500 million years from now it will be about twice as luminous. This translates into a very large change in the surface temperature of the Sun. The process is non-linear, because once enough greenhouse gases like water vapor and CO2 get into the atmosphere, the surface heating increases faster that the increase in solar luminosity. From what I have read, in about a few hundred million years, the biosphere will be tens of degrees warmer than it is now on average. Polar ice caps will be a thing of the past....forever.
All answers are provided by Dr. Sten Odenwald (Raytheon STX)
NASA IMAGE/POETRY Education and Public Outreach program.