Do We Know About
Good weather records extend back less than 150 years in most places. In that time, the Earth's global average temperature has increased by approximately 0.7 degrees centigrade or 1.3 degrees Fahrenheit. Some of this warming is natural, however a large part is the result of human induced greenhouse warming.
Since the end of the last ice age occurred over 10,000 years ago, the planet has continued to undergo changes in climate. Warming during medieval times and cooling during the "Little Ice Age" a few centuries ago dominate the last millenia. From paleo records, we know that the climate of the past million years has been dominated by the glacial cycle, a pattern of ice ages and glacial retreats lasting thousands of years.
The image to the left shows the changes
in ice cover over the Northern Hemisphere. Eighteen-thousand years ago,
at the peak of the last ice age, scientists estimate that nearly 32% of
the earth's land area was covered with ice, including much of Canada,
Scandinavia, and the British Isles. These glaciers developed because the
earth was in the midst of an ice age. Today ice coversage about 10% of
the Earth's land surface. For more on the history of climate changes,
please see this link.