But their study of levels over the last 60 million years suggests that the gas was once even more abundant than it is today.
Carbon dioxide is the main gas caused by human activity that has been linked to global warming.
Based on current trends, by 2030 the total climatic impact of rising levels of all greenhouse gases will be equal to that caused by the doubling of pre-industrial CO2 concentrations. By 2100, the effect would be trebled. The research, by Dr Paul Pearson of the University of Bristol and Professor Martin Palmer, of Imperial College, London, is reported in the journal Nature. They used a new technique to establish CO2 levels in the ancient atmosphere, analysing the shells of planktonic organisms that once lived near the surface of the ocean. This enabled them to establish past seawater acidity, which in turn was dictated by the amount of atmospheric CO2.