The early Paleogene greenhouse climate is punctuated by a series of extreme global warming events known as hyperthermals that are associated with massive additions of carbon to the ocean-atmosphere system. However, no existing proxies have suitable resolution to capture the change in atmospheric carbon dioxide (pCO 2 ) across these events. Here, we reconstruct a nearly continuous record of pCO 2 during the early Paleogene based on changes in terrestrial carbon isotope discrimination calculated from published high-resolution marine and terrestrial carbon isotope records. We calculate relatively stable baseline pCO 2 = 569 + 250/−146 ppmv with significant increases in pCO 2 at each of four hyperthermals. These background levels are significantly higher than most existing proxy estimates, but still lower than levels commonly assumed within carbon cycle models. Based on the pCO 2 levels we calculate across each hyperthermal, we show that these events are associated with carbon additions most likely dominated by terrestrial organic matter oxidation or mantle-derived CO 2 . By matching the new high-resolution pCO 2 data with global temperature data we calculate Earth-system sensitivity of ~0.8 to 1.6 KW−1 m2 across these hyperthermals. The slightly elevated ESS during the PETM and H2 suggests positive feedbacks through other greenhouse gases, changes in vegetation and/or oxidation of organic matter/methane may have amplified the temperature response to CO 2 addition.