What is the greenhouse effect and global warming?

(Photo: Scanpix/Reuters)

The most recent assessment report from the UN’s climate panel (IPCC) says that the earth’s average temperature has risen by 0.74 degrees in the period from 1906 to 2005and that the average temperature will continue to rise.
Ministry of Climate and Energy of Denmark

The greenhouse effect is a natural mechanism that retains the heat emitted from the earth’s surface. The earth’s average temperature is at the moment around 14 degrees celsius (57 degrees fahrenheit). If the natural greenhouse effect did not exist, the average temperature would be around minus 19 degrees celsius (minus 2 degrees fahrenheit).The greenhouse effect is caused by a range of different gases in the earth’s atmosphere. Water vapour makes the most significant contribution to the greenhouse effect, followed by CO2. The atmospheric content of greenhouse gases – in particular CO2 – and the consequences for the climate are being discussed because the content of these gases in the atmosphere has risen precipitously in a period covering approximately the latest 250 years, and especially the last 50.

At present the concentration of CO2 in the atmosphere is about 385 ppm (millionths). Before industrialisation it was about 280 ppm. Analyses of air contained in ice from the Antarctic ice cap show that there is far more CO2 in the air today than at any time in the last 650,000 years.

The consequence is that the greenhouse effect is becoming stronger, and therefore the earth is becoming warmer. How much warmer has, however, been a matter of dispute. The most recent assessment report from the UN’s climate panel (IPCC) is from 2007. It says that the earth’s average temperature has risen by 0.74 degrees in the period from 1906 to 2005. The warming is stronger over land areas than over the sea, and accordingly it is strongest in the north. At the same time occurrences of heatwaves and violent downpours have also increased, the oceans have risen, and the ice at the world’s poles and on its mountains has begun to melt. All of these effects are predictable in the event of global warming.

The IPCC’s most recent assessment report says that the average temperature will continue to rise, but that the extent and the duration of this rise, and the severity of its consequences, depend on how quickly and how effectively emissions of greenhouse gases can be restricted and, over time, reduced.

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