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What is climate change and what causes it?

The world's climate has always fluctuated due to a range of geological, biological, atmospheric, oceanographic and astronomic factors .  But there is wide scientific consensus that climate variations observed over the last 150 years can only be explained by the impacts of human activities acting in conjunction with natural factors.

The latest report by the International Governmental Panel on Climate Change (IPCC) external site   says that:

"It is extremely likely that human influence has been the dominant cause of the observed warming since the mid-20th century."  

The pace of change is not even across the world, with some places more affected than others. Left unchecked these changes will accelerate, with significant consequences for nature, economy and society here in Scotland.

The role of the greenhouse effect

A proportion of the sun's radiation that penetrates the Earth's atmosphere is absorbed by the Earth's surface, heating it up. This is 'short-wave' radiation.   Some of this energy is then re-irradiated or 'bounced back' at longer wavelengths back towards space.  Some of this out-going long wave radiation is absorbed by so-called 'greenhouse gasses' - mainly water vapour in clouds, and by some trace gasses including carbon dioxide - and re-irradiated in all directions, some towards the surface again.  The result is to trap part of the thermal energy in the lower atmosphere, which heats up.

This process is responsible for the relatively high mean surface temperature on the Earth of about 15 degrees Celsius. Without this effect the mean surface temperature would be about -18 degrees C!   This process is called the natural greenhouse effect.

The higher the concentration of greenhouse gasses in the atmosphere the greater the warming. Human activities, especially the greenhouse gas emissions from fossil fuel burning, are warming the planet more that that which would occur naturally.  This phenomenon is sometimes called the enhanced greenhouse effect.

Vegetation and land use change

Global mean surface temperatures are increasing as greenhouse gasses, mainly CO2 , methane and nitrous oxide - build up in the atmosphere trapping heat.  Fossil fuel combustion (plus a smaller contribution from cement manufacture) is responsible for more than 75% of human-caused CO2 emissions.  Land use change, particularly large scale clearance of tropical rainforest is responsible for the remainder.

Degradation of other natural carbon stores is a growing concern. Peatland habitats are vast stores of carbon when they are in good condition.  In poor condition (particularly when they have been drained to make way for alternative land uses) they become sources of carbon dioxide and other greenhouse gasses, further enhancing the greenhouse effect. 



Last updated on Wednesday 19th October 2016 at 14:01 PM. Click here to comment on this page