Healing Forests: Natural causes of global warming

By Charlie Nelson
April 2003
updated November 2003

Global warming scenarios (2011)

Claims that man-made pollution has caused unprecedented global warming have been undermined by research that shows the earth was warmer in the Middle Ages (The Age, 7 April 2003).

A review of 240 scientific studies has shown that today's temperatures are neither the warmest nor are they producing the most extreme conditions.  The review, by a Harvard University team, examined the findings of temperature proxies such as tree rings, ice cores, and historical accounts to estimate historical temperatures.

The findings prove that the world had a medieval warm period between the ninth and fourteenth centuries, with world temperatures higher than today's.  They also confirm the existence of a little Ice Age that set in about 1300, during which the world cooled dramatically.  Since 1900, the world has begun to warm up, but has still to reach the balmy temperatures of the Middle Ages.

The study, to be published in the journal Energy and Environment will probably be welcomed by sceptics of the greenhouse effect.

Contradicting this research is one by academics from the University of East Anglia, UK, and the University of Virginia, US.  Based on analysis of the trunks of ancient trees and ice cores from Greenland and Antarctica, they claim that the Earth appears to be warmer since 1980 than at any time in the last 18 centuries. 

But think about the prospect of simultaneous natural and human-caused warmings.

Natural variations in temperature could be caused by variations in the earth's orbit, or variations in the sun's output, or several other factors including sulphur dioxide emitted by volcanoes (sulphur dioxide has a cooling effect).  The precise contributions of these natural causes of temperature variations are unknown and unpredictable at present.  Sunspots are a significant factor, as shown in our paper on the subject (download, pdf file, 270k).

Human caused warming is by increased concentrations of greenhouse gasses such as carbon dioxide.  The science of the greenhouse effect is not in doubt - the planet Venus is affected by a greenhouse effect.  What is in doubt is the precise relationship between the concentration of greenhouse gasses and temperature rise (and the associated lags).  Humans have also emitted sulphur dioxide which has a cooling effect and has tended to mask the warming effect of rising carbon dioxide for much of the last 100 years until the mid-1970's when clean air acts became operative.

Imagine a scenario of a natural increase in temperature of, say 4 degrees Celsius combined with a human caused increase of 4 degrees, both over the next century!  This is not a scenario that we can afford to be complacent about.

All the more reason to take action against rising greenhouse gasses!

Global warming scenarios

Satellite data confirms warming trend

Sunspots and global warming (pdf file, 270k)