Since its creation 4.5 billion years ago the Earth’s climate has continually been changing. Climate sceptics believe the current rise in temperature is a natural cycle and point to historical dates like the mini ice age in the seventeenth century when the Thames froze and the warm period in the medieval ages as evidence of this. They argue that these fluctuations occurred before there was a sharp increase in carbon dioxide emissions from man so the two are not related.

However current global warming is different to previous temperature fluctuations in history for several reasons.

1) Present global warming is man made. In 1995, the world's climate experts in the IPCC (Intergovernmental Panel on Climate Change) concluded in their report that "The balance of evidence suggests that there is a discernible human influence on the global climate". In the IPCC 4th report released this year they stated that "the understanding of anthropogenic warming and cooling influences on climate has improved since the Third Assessment Report (TAR), leading to very high confidence that the globally averaged net effect of human activities since 1750 has been one of warming." The cause is because of a 31% increase in carbon dioxide emissions from fossil fuels, a 151% increase in methane and a rise in nitrous oxide from agriculture that far exceeds pre-industrial levels. The high levels of carbon dioxide settle in the atmosphere and act as a barrier that traps the sun’s energy as heat when it is reflected back off the Earth. This trapped heat builds up and slowly warms up the planet.

2) The rise in the Earth’s temperature is increasing at a faster rate than previous warm periods. In the last 10,000 years, the Earth's average temperature hasn't changed by more than one degree Celsius, meaning that during the last Ice Age temperatures were only one to five degrees Celsius cooler than they are today. However scientists predict that because of global warming the Earth will heat up by 1.5 to 6.2 degrees Celsius in the next 100 years.

Links:

IPCC 2007 Report: http://www.ipcc.ch/SPM2feb07.pdf

Bill Totten Blog: http://billtotten.blogspot.com/2007/04/frequently-asked-questions-about-global.html

An instrumental record of global average temperatures from 1860 to 2000