(Written April 2007, published in Baddow Life, Issue 16, Summer 2007)

There seems to be a lot of confusion and misinformation around about global warming and how it may be caused by carbon dioxide from burning fossil fuels. Some people have put about exaggerated scare stories about continents being made uninhabitable, whereas others try to deny that there is any problem. So it may help to give a summary of the measurable facts of the matter.

Studies of air trapped in ice caps and more recent records show that the amount of carbon dioxide in the atmosphere has grown by more than a third over the last 200 years, with most of the increase in the last 50 years. The current level is much higher than at any time in the last 650,000 years. Human activity, mostly burning of fossil fuels, releases 24 billion tonnes of carbon dioxide per year, more than enough to account for the measured change. There have been similar increases in other “greenhouse gases” such as methane and nitrous oxide.

Reliable climate statistics are available only for the period from about 1850. These clearly show that the global average temperatures for 2001-2006 are 0.5°C higher than the average for 1950-1975 and 0.7°C higher than in the late 19th century. Reconstructed data show that the global temperature was declining gradually from about 1000 to 1850 AD, but since then has been rising much more sharply, to a level probably higher than at any time for at least 2,000 years.

The sudden rise in both carbon dioxide levels and temperature in the last 100 years, above the relatively stable levels of the last 2,000 years, suggests a cause and effect relationship, but does not prove it. The well known “greenhouse effect” shows that increased carbon dioxide concentration can indeed be a cause of global warming, and models predict a similar rise to what has been observed.

An alternative suggestion has been made that global warming is caused by changes in the brightness of the sun. Indeed it seems likely that historic temperature changes are related to solar radiation. But satellite observations show that the rapid rise in global temperatures since 1980 cannot be explained in this way.

The Intergovernmental Panel on Climate Change recently released its fourth report on the physical science basis for climate change (http://www.ipcc.ch/SPM2feb07.pdf). They note a small warming effect from solar radiation, and a much larger one from greenhouse gas emissions. Other factors, such as sulphate pollution, are judged to have had a cooling effect, but not enough to balance the warming. The report concludes that “Most of the observed increase in globally averaged temperatures since the mid-20th century is very likely due to the observed increase in anthropogenic [human-generated] greenhouse gas concentrations”. This report continues with predictions for the next century, suggesting average temperature rises between 1.8 and 4.0°C depending on the future levels of emissions.

(All data are taken from scientific papers or from data provided by official bodies such as the Met Office and the US National Oceanic and Atmospheric Administration.)

Estimates of how global temperatures have changed over 2000 years. Image from Global Warming Art, http://www.globalwarmingart.com/wiki/Image:2000_Year_Temperature_Comparison_png.