labs_title

Caldeira Lab Research:Paleoclimate and geochemical cycles

The role of terrestrial plants in limiting atmospheric CO2 decline over the past 24 million years

Mark Pagani, Ken Caldeira, Robert Berner & David J. Beerling

The Earth has naturally maintained low CO2 concentrations over the past 24 million years. Negative feedback mechanisms inhibited the CO2 concentrations from falling lower than about 190 ppm. Here we examine the function terrestrial plants have with respect to sustaining CO2 concentrations.


Beerling, D.J., Berner, R., Caldeira, K., and Pagani, M., The role of terrestrial plants in limiting atmospheric CO2 decline over the past 24 million years, Nature 460, 85-89; doi:10.1038/nature081333, 2009

The Earth’s CO2, tectonic and climatic history over the past 50 Myr: a, Proxy records of long-term changes in atmospheric CO2 concentration. Shaded bands represent a range of alkenone-based atmospheric CO2 estimates. Open circles represent CO2 estimates from boron isotope–pH reconstructions. b, Long-term δ18O record from benthic foraminifera. p.p.m., parts per million. PDB, Pee Dee belemnite standard.

Abstract

Environmental conditions during the past 24 million years are thought to have been favorable for enhanced rates of atmospheric carbon dioxide drawdown by silicate chemical weathering. Proxy records indicate, however, that the Earth’s atmospheric carbon dioxide concentrations did not fall below about 200–250 parts per million during this period. The stabilization of atmospheric carbon dioxide concentrations near this minimum value suggests that strong negative feedback mechanisms inhibited further drawdown of atmospheric carbon dioxide by high rates of global silicate rock weathering. Here we investigate one possible negative feedback mechanism, occurring under relatively low carbon dioxide concentrations and in warm climates, that is related to terrestrial plant productivity and its role in the decomposition of silicate minerals. We use simulations of terrestrial and geochemical carbon cycles and available experimental evidence to show that vegetation activity in upland regions of active orogens was severely limited by near-starvation of carbon dioxide in combination with global warmth over this period. These conditions diminished biotic-driven silicate rock weathering and thereby attenuated an important long-term carbon dioxide sink. Although our modelling results are semi-quantitative and do not capture the full range of biogeochemical feedbacks that could influence the climate, our analysis indicates that the dynamic equilibrium between plants, climate and the geosphere probably buffered the minimum atmospheric carbon dioxide concentrations over the past 24 million years.