The Earth has been warming since the end of the little ice age, which was not a firm, fixed date but was about 300 years ago. The temperature record from central England that started in the late 1600's shows a lot of sharp increases and decreases overlying a steady increase with a rate that has not changed. Thomas Jefferson was a close observer, and he noted the change that happened during his lifetime. Here are Jefferson's comments (which I copied from Newsmax.com):

Thomas Jefferson Noted Global Warming

Climate change crusaders insist that the earth is warming largely due to the emission of greenhouse gases by motor vehicles and factories.

But Thomas Jefferson wrote about global warming back in the early 19th century, before there were any emissions from cars, coal-fired power plants, and other developments of the Industrial Age.

In a letter to Philadelphia physician and professor Nathaniel Chapman dated Dec. 11, 1809, nine months after he left the presidency, Jefferson wrote: “The change which has taken place in our climate is one of those facts which all men of years are sensible of and yet none can prove by regular evidence. They can only appeal to each other’s general observation for the fact.

“I remember that when I was a small boy, say sixty years ago, snows were frequent and deep in every winter, to my knee very often, to my waist sometimes, and that they covered the earth long. And I remember while yet young to have heard from very old men that in their youth the winters had been still colder, with deeper and longer snows. In the year 1772, thirty-seven years ago, we had a snow two feet deep in the Champain parts of this state, and three feet in the counties next below the mountains . . .

“While I lived at Washington, I kept a Diary, and by recurring to that I observe that from the winter of 1802-03 to that of 1808-09 inclusive, the average fall of snow of the seven winters was only 14½ inches, and that the ground was covered but sixteen days in each winter on average of the whole. The maximum in any one winter during that period was 21 inches fall, and 34 days on the ground, the minimum was 4½ inches fall and two days on the ground . . .

“Williams in his history of Vermont has an essay on the change in the climate of Europe, Asia and Africa.”

It’s clear, then, that the earth was warming during Jefferson’s time. It’s also clear that the climate change could not be attributed to man’s activities.

The advocates of the AGW hypothesis initially tried to finagle data to show that the Medieval Warm Period and the Little Ice Age never existed, but since the CRU emails were released, have had to acknowledge that they did. Prior to that occurrence last year they had claimed that natural processes had nothing to do with temperature change, and that since the late 1970's change had been the result of man's activities. Acknowledging the previous naturally occurring temperature changes does serious harm to the cataclysmic AGW hypothesis, and supporters are now in a major spin mode.