Linkages between CO2, climate, and evolution in deep time.

T he 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 13C of carbonate minerals from fossil soils, the 13C of marine phytoplankton, and the 11B 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, Kürschner et al. (11) present new data that overturn this notion and provide important insights into the climatic linkages during this Epoch. Kürschner et al. (11) use the stomatal method to reconstruct CO2; this method is based on the inverse relationship observed in many plants between stomatal numbers (specifically stomatal index, which is the percentage of stomatal density relative to stomatal density plus epidermal cell density) and CO2 (12, 13) (Fig. 2). In contrast to previous records (8–10), their record shows that CO2 and temperature are coupled: the two major advances of the Antarctic ice sheet are marked by low CO2 and the latest Oligocene and mid-Miocene warm periods, by comparatively high CO2 (Fig. 1). Their study therefore reopens the possibility that CO2 was a prominent force in controlling climate during the Miocene. A major strength of the Kürschner et al. (11) study is their use of three independently calibrated taxa; in contrast, most stomatal-based reconstructions use only one taxon. The similarity in the CO2 estimates across distantly related taxa greatly reduces the likelihood that an additional factor such as water availability or light intensity (12) compromised the stomatal indices and therefore the fidelity of the CO2 signal. This multiple-taxa approach offers an important way forward for improving stomatalbased CO2 reconstructions. Two other points about the climatic linkages are worth noting. First, global climate models (14) and geologic records (5) suggest that a CO2 threshold of 500 ppmv is important for triggering ice-sheet growth. While ice was present throughout the interval of the record of Kürschner et al. (11), CO2 was more than 500 ppmv during the melting phases and 300 ppmv during icesheet expansion (red and blue bands in Fig. 1). These patterns provide additional credence for a CO2–ice threshold of 500 ppmv. Second, the calibration of Earth’s climate sensitivity to CO2 is critical for understanding climate change; for today’s Earth, every doubling of CO2 most likely results in a temperature increase [ T(2 )] of 3°C (1). The CO2 record of Kürschner et al. (11) provides an opportunity to calculate climate sensitivity for the Miocene world. For the cooling event 25–22 Myr ago T(2 ) 1.9– 2.7°C if CO2 is compared with deep-sea paleotemperatures (15) [the range in T(2 ) arises from using a five-point running mean vs. the individual data points of the paleotemperature record];