Land-Cover Change-The Earth's land cover has gone through significant changes during the last few million years. Studies of past land cover and climates provide key insights into natural variation of the Earth's environment and provide data for testing general circulation model simulations of different conditions. During the glacial time, average global climate was about 5°C cooler than today; during the last extended warm interval, 3 million years ago, the climate was 2 to 3°C warmer on average. Credit: USGS