.

For each of these gases, there are natural processes that release it into the atmosphere (sources), and other natural processes that remove it again (sinks).

The relatively stable atmospheric concentrations that prevailed in the pre-industrial world tell us that these sources and sinks were in balance (more or less) at that time. Clearly, this natural balance has been disturbed over the past 200 years or so – a period marked by an explosive growth in the human population.

At the end of the 18th century, there were fewer than 1 billion people on the planet; there are over 6.3 billion today, and official estimates suggest that the upward trend is likely to continue for some time to come.