The "greenhouse effect" is the reason why the earth remains comfortably warm and suitable for life as we know it. But only small amounts of carbon dioxide (CO2) and water are necessary to fulfil this vital task. Scientists believe that increases in greenhouse gases — CO2 (from fossil fuels), methane (from bacterial action on plant material and natural gas from commercial gas and oil fields), nitrous oxide (from nitrogen-based fertilisers), ozone and chlorofluorocarbons (CFCs) — in the last 180 years are upsetting the natural balance.
The July 13 New Scientist reported on studies carried out at the University of Chicago in 1985 which calculated the combined effect of these greenhouse gases and projected their growth in concentration. They showed that by the year 2030 the minor contributions of gases other than CO2 would add up, and that the total effect would be as if the natural concentration of CO2 had doubled. Computer models predicted that this would imply a global warming of some 3° C above the level of the mid-19th century. Of this projected 3° rise in temperature, about 0.5° has already happened.
An increase in the amount of atmospheric CO2 results in more water evaporating from the ocean and entering the atmosphere. This water vapour contributes to the greenhouse effect.
Already in the 1890s scientists observed that the increased burning of fossil fuels had an effect on global temperature. Before the industrial revolution, the CO2 concentration was below 280 parts per million; in 1957, it was 315 ppm; it is now 360 ppm.
From measurements of air bubbles trapped in polar ice sheets, scientists believe that the natural concentration of CO2 in the atmosphere is about 270 ppm — something that had stayed constant for at least 10,000 years. This means that the increase in the background level of CO2 concentration alone is already 33%.
While questions remain about how rapidly the world is likely to warm in the 21st century, there is scientific consensus that the earth is likely to warm by 2° C by the year 2100, with a corresponding rise in sea level, if the build-up of greenhouse gases in the atmosphere continues at its present rate.
The New Scientist quotes a 1996 Intergovernmental Panel on Climate Change (IPCC) report which predicts that global warming by the year 2100 will be in the range of 1-3.5°. Sea levels are predicted to rise as a result by 15-90 centimetres. This will place 9 million people at risk from flooding.
"The 'best' warming rate predicted by the IPCC study is 0.2° per decade. To put that into perspective the predicted rate of change is about 50 times faster than the warming which bought an end to the last major ice age which, up to that point, was itself the largest sustained natural change in climate for the past 100,000 years."