the Earth has always had and always will have the same albedo and greenhouse effect that it has today. If that were really that case, then the Earth’s average temperature would have been below freezing when the Sun was beginning its main sequence life about 4.6 billion years ago. At that time the Sun was about 25% less luminous than it is today. The fact that oceans existed on the Earth almost from the Earth’s beginning is evidence that the Earth couldn’t have had and average temperature below freezing. One possibility is that the Earth’s primitive atmosphere produced a strong greenhouse effect that raised the Earth’s temperature well above the freezing point. During the next 5 billion years, the Sun will increase in brightness by about 60%. If the Earth stays the same, its average temperature will rise to about 325 K, or about 125° F. This would make the tropical regions and perhaps the entire Earth uninhabitable. Nevertheless, these changes will occur so gradually that hundreds of millions of years will pass before they become a matter for great concern. We have much more reason to worry about short-term changes in the Sun’s energy output.