a, RSL (blue)1 and CO2 (brown, purple)16,17. Age scales offset by 2 kyr (CO2 leads). Fossil original CO2 data16,17 resampled to 1 kyr (brown line) and smoothed (11 kyr running average; brown and purple symbols). CO2 from periods with RSL < −90 m (dashed brown line) are highlighted (purple). The modern data (red) are neither resampled nor smoothed. b, Correlation (orange line) between RSL and fossil CO2 data when RSL > −90 m (brown). Excluded data pairs are purple. The correlation with a slope of 1 m ppmv−1 and an offset of −280 m (orange line) has an R2 of 0.7.