The Sun is becoming increasingly hotter (or more luminous) with time. However, the rate of change is so slight we won’t notice anything even over many millennia, let alone a single human lifetime. Eventually, however, the Sun will become so luminous that it will render Earth inhospitable to life.
Let’s work through some of the science:
The Sun produces energy through core thermonuclear fusion reactions which converts hydrogen into helium. These reactions generate copious energy that slowly migrates out toward the photosphere and then into space. Astronomers believe that the Sun formed approximately five billion years ago, at which time it initiated these reactions. They have continued ever since.
As the Sun ages, it slowly grows hotter due to the accumulation of residual energy emitted by these core reactions. Initially, the Sun was only about 70% as luminous as it is today. Consequently, the solar constant, the energy Earth receives from the Sun, would have been correspondingly lower. “The Faint Sun Paradox” was borne out of the realization that, though the Sun was cooler in its infancy, the early Earth still contained liquid water.*
Throughout the subsequent billions of years, the Sun’s luminosity increased gradually and will continue to increase in the future. Astronomers estimate that the Sun’s luminosity will increase by about 6% every billion years. This increase might seem slight, but it will render Earth inhospitable to life in about 1.1 billion years. The planet will be too hot to support life.
When stellar astronomers first understood the Sun’s energy generation mechanism, they believed that Earth’s life would survive until the Sun expanded into the red giant stage. Today they know that our time is much shorter, albeit still more than one billion years.
*Astronomers and geologists are still attempting to reconcile this paradox. Some suggest that Earth’s atmosphere was much thicker in its youth and contained greater quantities of heat retentive carbon dioxide. Such a gaseous envelope would have been able to retain more heat onto the planet’s surface, just as Venus’ carbon-dioxide rich atmosphere does today. Also, the Moon was closer and therefore the resultant tidal heating was much greater, as the tidal forces induced by a nearby body are very distance sensitive. These and other factors could explain how a cooler Sun could have sustained a warmer Earth.