  Standard solar models predict a solar luminosity that gradually increased by
about 30% over the past 4.5 billion years. Under the faint sun, Earth should
have been frozen solid for most of its existence. Yet, running water is
observed to have been present since very early in Earth's history. This enigma
is known as the faint sun paradox. We show here that it can be partially
resolved once we consider the cooling effect that cosmic rays are suspected to
have on the global climate and that the younger sun must have had a stronger
solar wind, such that it was more effective at stopping cosmic rays from
reaching Earth. The paradox can then be completely resolved with the further
contribution of modest greenhouse gas warming. When we add the cosmic ray flux
modulation by a variable star formation rate in the Milky Way, we recover the
long term glacial activity on Earth. As to the future, we find that the average
global temperature will increase by typically 10K in the coming 2 Gyr.
