The greenhouse effect
Carbon dioxide and other heat-trapping gases are naturally present in the atmosphere.
These gases trap heat radiated from the earth’s surface. The effect is the warming of the atmosphere and the earth’s surface, and is called the ‘greenhouse effect’
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Since the industrial revolution, several million tonnes of heat trapping (or greenhouse) gases have been released into the atmosphere, accumulating steadily and trapping more and more heat. Around the start of the industrial revolution, the amount of greenhouse gases (mainly carbon dioxide, methane and nitrous oxide) were a fairly constant 280 parts per million. Today, the overall amount of GHGs has exceeded 430 ppm; more than a 35 percent increase from pre-industrial levels22.
In June 1988, James Hansen, a scientist with NASA, told politicians in the United States that he was almost 99 percent sure that the reason for record high temperatures that year was not from ‘natural variations’, but from the growing concentrations of atmospheric carbon dioxide.
The Mauna Loa atmospheric measurements (see table below) are the longest continuous record of atmospheric carbon dioxide concentrations available in the world. Considered to be one of the most favourable locations for measuring carbon dioxide concentrations, the data shows the highest concentrations of carbon dioxide ever to be recorded. Our greenhouse gas emissions have not stopped at the level seen in the graph, but are continuing to increase at the rapid rate of 2.5 ppm each year2 – an alarmingly high rate.
Parallel to the increase in greenhouse gas levels, and as anticipated by scientists2, global mean temperatures have increased. In effect, the earth has warmed by 0.76 ºC since the 1900s2. Each decade, the temperature has increased by about 0.2 ºC.
This time series, from the Climatic Research Unit (source: jpl, NASA), shows the combined global land and sea surface temperatures from 1850 to 2007. The y-axis depicts departures from the long-term average (called ‘temperature anomaly’).
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| source: naturalpartiot.org | source: wikipedia | © Kaavya Nag |
Not surprisingly then, all of the ten warmest years on record have occurred since 19902.
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According to the World Meteorological Organisation (WMO), the top ten warmest years on record have all occurred in the last twelve years. Arctic sea ice was also at a record low level. In September 2007, the Northwest Passage (picture on the right) in the Arctic was ice-free for the first time in satellite record history. (NASA) Scientific evidence and modeling suggests that a 2 ºC rise in average global temperatures represents a ‘tipping point’. Unless the level of greenhouse gases is stabilized, the associated severity of impacts will continue to escalate, and over the next few decades, we would face unavoidable economic and ecological costs2. To prevent the planet from warming to more than this temperature, concentrations of greenhouse gases must not exceed 550 ppm of CO2 equivalent. |
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