There has been substantial discussion by politicians, scientists and the general public about the likely effects of the changing composition of the atmosphere of the Earth. The CO2 concentration of the Earth's atmosphere continues to increase in response to land-use change and the burning of fossil fuels. There is no question that this increase is directly linked to the ever increasing global human population and our demand for energy. This is one of the most easily quantified aspects of global environmental change.

It is well accepted that the enhanced CO2 concentrations associated with global change will directly influence the process of photosynthesis in most species of plants. Early observations suggested that increased rates of photosynthesis would lead to increased growth rates of plants. There was a proliferation of experimental studies investigating the effects of CO2 on plants. As more data were collected these became increasingly difficult to interpret. The majority of studies were conducted under artificial conditions in growth rooms, glasshouses or open-top chambers. The great deficiency of most studies was that vegetation was exposed for relatively short periods of time. There seemed to be no way forward that would permit the study of the longterm effects of exposure of vegetation to enhanced CO2 concentrations.

In the early 1990s a few groups of scientists faced with this problem suggested using sites with naturally enhanced atmospheric levels of CO2 as natural experiments. There followed a number of scientific programmes in several locations around the world where vegetation was exposed to enhanced CO2.