“What is climate change and why is it happening?
The Earth’s surface heats up as it absorbs short-wave radiation from the Sun. This energy is redistributed across the globe by the circulation of the oceans and atmosphere, and is
radiated back to space at longer (infrared) wavelengths. Atmospheric greenhouse gases such as water vapour, carbon dioxide (CO2), methane (CH4) and nitrous oxide (N2O) trap some of this outgoing heat energy, which warms the troposphere – the lower part of the Earth’s atmosphere. This warmed air radiates energy in all directions. Some of the radiation works its way up and out of the atmosphere, but some finds its way back down to the Earth’s surface. An analogy is made with the effect of a greenhouse, which allows sunshine to penetrate the glass that in turn keeps the heat in, hence the greenhouse effect. The naturally occurring greenhouse effect (see Figure 2.) keeps the surface of the Earth 33°C warmer than it otherwise would be i.e. a frigid minus 18°C.
Incoming solar radiation is always balanced by outgoing terrestrial radiation at an annual
level and across the Earth as a whole. Any factor that alters this balance will create a warming or cooling at the Earth’s surface and in the lower atmosphere, and can affect
climate. Prior to the onset of the Industrial Era around 1750, the background level of the
three principal greenhouse gases had remained constant for a millennium or more. Since the beginning of the Industrial Revolution concentrations have increased directly or indirectly as a result of human activities.
In addition, modern industrial processes have introduced halocarbons (e.g.
chloroflourocarbons – CFCs) and their substitutes (e.g. hydroflourocarbons – HFCs),
perflourocarbons (PFCs) and sulphur hexaflouride (SF6) into the atmosphere. These
substances, some of which are also ozone depleting, are potent greenhouse gases which, in
extremis, may have an atmospheric life of 50,000 years and a global warming potential
(GWP) 22,000 times greater than CO2. By comparison, methane and nitrous oxide are
merely 23 and 296 times more effective than CO2 in their GWP.
There is little doubt that the human influence will continue to change the atmospheric
concentration of greenhouse gases throughout the 21st century. Projections of future
concentrations are expected to be dominated by increases in CO2 from the continued burning of coal and our remaining reserves of oil and gas, and further changes in land use resulting in the loss of the world’s remaining tropical rainforests. In a ‘business-as-usual’ world, CO2 concentrations will be double the pre-Industrial level by the middle of the 21st century. They will double again by the end of the century. At such concentrations global average surface temperature will rise by up to 5.8°C and sea level by almost a metre. In order to stabilise atmospheric concentrations of greenhouse gases at lower levels and reduce the impacts of climate change, emissions need to drop below 1990 levels within a few decades and continue to decrease steadily thereafter until they are but a small fraction of what they are today (IPCC, 2001d).