Changes

According to geologists, the Earth has experience numerous [[ice age]]s during which the temperature dropped so far that [[glacier]]s could cover [[North America]] and [[Europe]].  

According to geologists, the Earth has experience numerous [[ice age]]s during which the temperature dropped so far that [[glacier]]s could cover [[North America]] and [[Europe]].  

There is a close correlation between the average temperature of the Earth's atmosphere and the amount of [[carbon dioxide]] in the atmosphere. During warm periods, the carbon dioxide level increases, and during cool periods it decreases. Today the carbon dioxid concentration is 380 ppm. The pre-industrial level was 280 ppm.  

There is a close correlation between the average temperature of the Earth's atmosphere and the amount of [[carbon dioxide]] in the atmosphere. During warm periods, the carbon dioxide level increases, and during cool periods it decreases. Today the carbon dioxid concentration is 380 ppm. The pre-industrial level was 280 ppm.