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.