The evolution of Earth's climate on geological timescales is largely driven by variations in the magnitude of total solar irradiance (TSI) and changes in the greenhouse gas content of the atmosphere. Here we show that the slow ∼50 Wm increase in TSI over the last ∼420 million years (an increase of ∼9 Wm of radiative forcing) was almost completely negated by a long-term decline in atmospheric CO . This was likely due to the silicate weathering-negative feedback and the expansion of land plants that together ensured Earth's long-term habitability. Humanity's fossil-fuel use, if unabated, risks taking us, by the middle of the twenty-first century, to values of CO not seen since the early Eocene (50 million years ago). If CO continues to rise further into the twenty-third century, then the associated large increase in radiative forcing, and how the Earth system would respond, would likely be without geological precedent in the last half a billion years.