Between 8 and 6 million years ago, there was a global increase in the biomass of plants using C-4 photosynthesis as indicated by changes in the carbon isotope ratios of fossil tooth enamel in Asia, Africa, North America and South America. This abrupt and widespread increase in C-4 biomass may be related to a decrease in atmospheric CO2 concentrations below a threshold that favoured C-3-photosynthesizing plants. The change occurred earlier at lower latitudes, as the threshold for C-3 photosynthesis is higher at warmer temperatures.