Greenhouse gas emissions resulting from human activity are considered to be an important driver of the current global warming. Accurate understanding of this cause-effect relationship is essential to propose realistic scenarios of the future evolution of Earth’s climate, but the human monitoring history of these variables may be very tenuous to properly take into account the relative importance of their large time-scale dynamics into the climate system. Geological records offer an opportunity to address this time-scale issue as they provide track of the long-term dynamics of atmospheric greenhouse gas contents and surface temperature throughout Earth’s history. A team of researchers of the Laboratoire de géologie de Lyon : terre, planètes et environnement (LGL-TPE) published the results of their researches in Scientific Reports.
CO2 is considered the main greenhouse gas involved in the current global warming and the primary driver of temperature throughout Earth’s history. However, the soundness of this relationship across time scales and during different climate states of the Earth remains uncertain. Here we explore how CO2 and temperature are related in the framework of a Greenhouse climate state of the Earth. We reconstruct the long-term evolution of atmospheric CO2 concentration (pCO2) throughout the Cretaceous from the carbon isotope compositions of the fossil conifer Frenelopsis.
We show that pCO2 was in the range of ca. 150–650 ppm during the Barremian–Santonian interval, far less than what is usually considered for the mid Cretaceous. Comparison with available temperature records suggest that although CO2 may have been a main driver of temperature and primary production at kyr or smaller scales, it was a long-term consequence of the climate-biological system, being decoupled or even showing inverse trends with temperature, at Myr scales. Our analysis indicates that the relationship between CO2 and temperature is time scale-dependent at least during Greenhouse climate states of the Earth and that primary productivity is a key factor to consider in both past and future analyses of the climate system.