2. Why is there so much concern about current climate change?

2.1 The enhanced greenhouse effect

Our climate is always changing: it always has and it always will. As discussed for the previous question, these changes result from interactions between a number of astronomic, atmospheric, oceanographic, biological, geological and social factors. They contribute to a natural greenhouse effect.

Current concern centres on the rate of change in the concentration of greenhouse gases in the atmosphere – the enhanced greenhouse effect. This is due to human activity – especially our use of coal, gas and oil (fossil fuels) and deforestation. These activities amount to a shortcircuiting of the terrestrial and, especially, the geological parts of the carbon cycle.

When we burn fossil fuels (coal, oil, gas) we take carbon out of the geological reservoir and emit it into the atmosphere. This dramatically reduces the time normally taken for that carbon to re-enter the atmosphere (from tens of millions of years to a few years). Similarly, deforestation accelerates the rate of transfer of carbon from plants back into the atmosphere.

Furthermore, the geological organic carbon reservoir (including fossil fuels) is very large – more than 10 000 times larger than the atmospheric reservoir. The effect of taking even relatively small amounts of carbon from the geological organic carbon reservoir and transferring it to the atmospheric carbon reservoir is very marked. Imagine you have two (very big!) bags, one containing 10 000 000 marbles, and the other with 1000. If you take 1000 marbles from the larger bag it reduces in size by 0.01% (a hundredth of 1%). If you add those marbles to the smaller bag it doubles in size (increases by 100%). So you have made a negligible difference to the size of the bigger bag, but a major difference to the smaller bag.

Once in the atmosphere carbon stays there for typically 100 years before being taken up by either the terrestrial or marine part of the carbon cycle. While in the atmosphere the carbon combines with oxygen to form CO₂ – a greenhouse gas.