Since the formation of the solar system 4.6 billion years ago, the net solar luminosity has increased roughly 40% from its initial value. Assuming that luminosity has increased at a constant rate, estimate the solar constant 100 million years ago, during the Cretaceous period. Assuming current climate conditions and an albedo of a = 0.3, what radiative forcing would be needed to keep Earth in its current state (as of say 1950) if the Sun had the luminosity of the Cretaceous period? What level of atmospheric CO₂ would achieve this radiative forcing?