http://www.envirotruth.org/big_chill.cfm

.....The last ice age ended about 10,000 years ago. Temperatures rose to the 'Holocene Maximum' of about 5,000 years ago when it was about 3°F higher than now, dropped in the time of Christ, and then rose to the 'Mediaeval Climate Optimum' of about 600 ad to 1100 ad, when temperatures were about 2°F higher than now. This was a golden age for northern European agriculture and led to the rise of Viking civilisation. Greenland, now a frozen wasteland, was then a habitable Viking colony. There were vineyards in the south of England. Then temperatures dropped to 'The Little Ice Age' in the 1600s, when the Thames froze over. And they have been rising slowly ever since, although they are still much lower than 1,000 years ago. We are now living in a rather cool period.

What caused these ups and downs of temperature? We do not know. Temperature changes are a fact of nature, and we have no idea if the postulated 0.5°F heating over the last 100 years is caused by man's activities or is simply part of a natural cycle. What we can say, though, is that if Europe heats up by 2°F it would do it a power of good. We can see this from records of 1,000 years ago. Moreover, increased carbon dioxide makes plants grow more quickly, so improving crops and forests....