Solar activity and climate

Solar irradiance variation has been a main driver of climate change over geologic time, but its role in the recent warming has been found to be insignificant.

Earth formed around 4.54 billion years ago by accretion from the solar nebula. Volcanic outgassing probably created the primordial atmosphere, which contained almost no oxygen and would have been toxic to humans and most modern life. Much of the Earth was molten because of frequent collisions with other bodies which led to extreme volcanism. Over time, the planet cooled and formed a solid crust, allowing liquid water to exist on the surface.

Three to four billion years ago the Sun emitted only 70% of its current power. Under the present atmospheric composition and this past solar luminosity, water would have been uniformly frozen. Evidence suggests water's presence, in the Hadean and Archean eons, leading to what is known as the faint young Sun paradox. Hypothesized solutions to this paradox include a vastly different atmosphere, with much higher concentrations of greenhouse gases than currently exist.

Over the following approximately 4 billion years, the Sun's energy output increased and atmospheric composition changed. The Great Oxygenation Event around 2.4 billion years ago was the most notable alteration. Over the next five billion years the Sun's ultimate death as it becomes a red giant and then a white dwarf will have large effects on climate, with the red giant phase likely ending any life on Earth.

Solar activity before the 1970s is estimated using proxy variables, such as tree rings and the number of sunspots.

Since 1978, solar irradiance has been measured by satellites with significantly greater accuracy. These measurements indicate that the Sun's total solar irradiance fluctuates over the ~11 years of the solar cycle but has not increased since 1978.

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