Global temperatures during the early Eocene period were estimated using deep-sea temperature data. By analyzing model simulations, researchers found that changes in atmospheric CO2 levels had a stronger impact on deep-sea and surface temperatures compared to sea-surface temperatures. This suggests that global deep-sea temperature changes can be used to estimate global surface air temperature changes accurately. The best match between model simulations and paleo-proxy data was found with an atmospheric CO2 level of 1,680 ppm during the early Eocene Climatic Optimum.