Linkages between CO 2 , climate, and evolution in deep time

Abstract

The most recent assessment report from the Intergovernmental Panel of Climate Change (IPCC) clearly illustrates the links between atmospheric CO2 and climate (1) and the impacts of climate change on life (2). The geologic record contains a treasure trove of “alternative Earths” that allow scientists to study how the various components of the Earth system respond to a range of climatic forcings. These past examples illustrate how ecosystems function, and often they provide constraints for predicting the magnitude and impact of future climate change. Multiple independent methods for reconstructing ancient levels of atmospheric CO2 have been developed over the past two decades: these include the distribution of stomatal pores in fossil leaves, the δ13C of carbonate minerals from fossil soils, the δ13C of marine phytoplankton, and the δ11B of marine carbonate (3). Records of paleo-CO2 from these methods as well as calculations of CO2 from geochemical models (4) generally correlate well with independent records of temperature. Over the past 450 million years (Myr), CO2 was low when extensive, long-lived ice sheets were present (≈330–290 Myr ago and 35 Myr ago to the present day) and moderately high to high at other times (5, 6). However, some intervals in Earth's past fail to show any consistent relationship. One conspicuous example is the Miocene (23.0–5.3 Myr ago), an Epoch where multiple advances of the Antarctic ice sheet are juxtaposed with a period of global warmth ≈15 Myr ago (7). Most CO2 records during this period are low [<300 ppm by volume (ppmv)] and do not covary with temperature (8–10) (Fig. 1). These records imply that other radiative forcings such as changes in paleogeography or meridional heat transport were disproportionately more important than CO2 at this time. In this issue of PNAS, Kurschner et al. (11) present new data that overturn this notion and provide important insights into the climatic linkages during this Epoch.