The earth's climate has changed noticeably within man's recorded history and much more dramatically during that longer period whose record we must examine in the geology of earth's crustal rock. To plan an intelligent use of our resources, we must frame our plans in a total environment; and the earth's climate is perhaps the determinant factor in this environment. We must understand how the climate is going to change and whether man's activities can influence climate. It is clear that if our activities are of sufficient scale to cause the climate to deteriorate, then they might also be made to improve it, at least for some minority of the earth's population. Understanding is also important on the part of those whose interest might be restraining such experiments.