Atmospheric carbon dioxide linked with Mesozoic and early Cenozoic climate change

Carbon isotopes of fossil plants and model simulations suggest that atmospheric carbon dioxide levels were variable during the period 200 to 60 million years ago. The large decreases in the partial pressure of CO 2 coincide with glaciations, providing evidence against climate–CO 2 decoupling during the Mesozoic. The relationship between atmospheric carbon dioxide (CO 2 ) and climate in the Quaternary period has been extensively investigated, but the role of CO 2 in temperature changes during the rest of Earth’s history is less clear 1 . The range of geological evidence for cool periods during the high CO 2 Mesozoic ‘greenhouse world’ 2 , 3 of high atmospheric CO 2 concentrations, indicated by models 4 and fossil soils 5 , has been particularly difficult to interpret. Here, we present high-resolution records of Mesozoic and early Cenozoic atmospheric CO 2 concentrations from a combination of carbon-isotope analyses of non-vascular plant (bryophyte) fossils and theoretical modelling 6 , 7 . These records indicate that atmospheric CO 2 rose from ∼420 p.p.m.v. in the Triassic period (about 200 million years ago) to a peak of ∼1,130 p.p.m.v. in the Middle Cretaceous (about 100 million years ago). Atmospheric CO 2 levels then declined to ∼680 p.p.m.v. by 60 million years ago. Time-series comparisons show that these variations coincide with large Mesozoic climate shifts 8 , 9 , 10 , in contrast to earlier suggestions of climate–CO 2 decoupling during this interval 1 . These reconstructed atmospheric CO 2 concentrations drop below the simulated threshold for the initiation of glaciations 11 on several occasions and therefore help explain the occurrence of cold intervals in a ‘greenhouse world’ 3 .