a,d, Curves indicate the estimated percentage of emissions from each age cohort of gas- and oil-fired units (a) and coal-fired units (d). b,c, The operating capacity of gas- and oil-fired units (b) and coal-fired units (c) where the youngest units are at the bottom. The dominance of young Chinese coal-fired units and US gas-fired units is apparent. Note that 0 years old means the power units began operating from 2010 in this study. See Supplementary Fig. 1 for the definition of regions.