The increase in atmospheric CO2 concentration is the combined consequence of fossil fuel burning and deforestation. Long term increases in the CO2 concentration are of great concern because this gas is a 'greenhouse gas'. CO2 is one of a group of gases (including water vapour and methane) that absorb the long wave or 'heat' radiation leaving the Earth for deep space and radiate it back to the Earth. This absorption and re-radiation acts like a blanket and keeps the Earth warmer than it would otherwise be.

The Greenhouse effect is a natural phenomenon, present for at least the last 3-4 billion years. What is new is that human activities are adding increasing concentrations of greenhouse gases to the atmosphere, CO2 being just one. Humans are enhancing the Greenhouse effect and a global warming is forecast as a result. It seems certain that this global warming will change the Climate System. However, the severity and nature of this change is not clear. We will return to the topic of the enhanced Greenhouse effect later.

The concentration of CO2 in the contemporary atmosphere has been carefully measured since 1958. The longest duration of observations available - from the volcano site of Mauna Loa in Hawaii - show two very interesting features. The first is the steady upward trend in the CO2 concentration. This is the consequence of fossil fuel burning and changes in landcover, principally the clearing and burning of tropical forests. The second feature is the annual cycle of CO2 variation within the longer term (decades) trend. This cycle of CO2 represents the landcover of the Earth taking up CO2 during intense photosynthesis, and then returning this gas to the atmosphere as a direct photosynthetic product or through plant decomposition.

The figure above is truly an amazing graph. The annual cycle of variation in CO2 is a remarkable record of the entire biosphere 'breathing'. Analysis of the significance of the changes in CO2 concentration shows two things. First, the amount of CO2 moving in and out of the biosphere through photosynthesis of the landcover represents billions of tonnes of carbon.

The second point is that the fluctuations are almost entirely the consequence of changes in the activity of terrestrial vegetation 'landcover' in the northern hemisphere and in the forested regions. It is estimated that forests account for more than 70% of the annual flux of CO2 into and out of the biosphere. The contribution of the Southern Hemisphere is much smaller than that of the Northern hemisphere. Can you think why? The answer will follow a little further into this story.