1997: Warmest Year Since 1400?
June 30, 1998
But why stop at 1400? Because that is just about the farthest back in the recorded past for which this statement is true. Go back just a few hundred years more to the period 1000 - 1200 AD and you find that the climate was considerably warmer than now. This era is known as the Medieval Warm Period. A 1996 Science article showed that the temperature in around 1000 AD was about 1 degree C warmer than it is today, (Keigwin). And a 1994 report in the journal Climate Change shows that this warm period was global in extent, (Grove and Switsur).
But by 1300 it began to cool, and by 1400 we were well into the Little Ice Age. It is no surprise that temperatures in 1997 were warmer than they were in the Little Ice Age.
So the statement, "1997 is the warmest year since 1400," is deceptive. If 1997 had been compared with the years around 1000 AD, 1997 would have looked like a rather cool year.
Incidentally, no one knows what caused that warm period in the middle ages. But one thing we do know is that carbon dioxide from cars and fuel burning was not altering global climate in 1000-1200 AD. The Medieval Warm Period must have had a natural origin. The 1990 report by the UN Intergovernmental Panel on Climate Change notes, "there is no evidence that [the Medieval warm period] was accompanied by an increase in greenhouse gases."