[jdm61]

It is measured using core samples of ice, dirt, rock,etc from the time period we are looking at. You should see if you can find some figures on what it was 65 million years ago when the oxygen content of the atmosphere was much higher than it is now. In the late Cretaceous period, you had temperate climates in places in Alaska and Northern Canada where you know have sub-arctic tundra. The question is not whether climate changes......it does. We are not that far out of the 'little ice age" that lasted for several hundred years and only about 12,000 froma MAJOR ice age that saw glaciation all the way to the Ohio River valley. The question is what effect, if any, we are having and how much.