1000 Years of Climate Change
The average temperature in the Northern Hemisphere has gotten warmer during the last twenty five years. Is this part of the normal ups and downs of a dynamic climate system? After all, the climate has gone through some pretty dramatic changes in recent history. Who can forget the Little Ice Age of the 1600s? And there was a warm period in Europe during the thirteenth century where average temperatures were about the same as 1950. To get a meaningful context for all this climate information scientists have been collecting field data to extend the climate record as far back as possible. Thomas Crowley has compiled a large number of these local climate records into a history of climate variability for the Northern Hemisphere that now goes back 1,000 years1 . A problem with climate studies has been the role of volcanoes and changes in output from the sun. Volcanoes can cause temporary cooling of the global climate; and solar output is also known to vary. Crowley incorporated volcano effects and solar variablity into a state of the art climate model and found that the model agreed with historical climate from 1000 to 1850 AD, including the Little Ice Age in the 1600s and the Medieval warm period. The weight of circumstantial evidence is now quite strong that the unprecedented warming in the North-ern Hemisphere in the last fifty years is due to greenhouse gases. While the overall change appears modest, 0.8 degrees Celsius, it has already meant large changes in higher latitudes, Canada and Russia for example. By comparison the great drought in the American Midwest in the 1930s, the Dustbowl, coincided with a 1 degree increase in average surface tempera-ture. We have only burned 5 percent of the available fossil fuels, so we are early in the process of global warming. If we continue with business as usual we can expect greenhouse gases to double sometime in the next century. The result of that is anybody's guess, but it will certainly mean further increases in global average temperature and more changes in climate patterns.