Our Earth-systems analysis reveals that plant responses to CO2 across a wide spectrum of time scales had far-reaching consequences for the evolution of climate and the biota. It challenges the widespread belief that destabilizing positive geophysiological feedbacks between plants, CO2, and climate on geological time scales are rare when compared with negative stabilizing feedbacks (3, 8–10). We suggest that the PFLs recognized here (Fig. 1) led to reinforcement (28), causing rates of evolution to accelerate on a grand scale. Falling CO2 concentrations are now linked to the unparalleled innovation of land plants in the Paleozoic (4, 6, 7) that, in turn, accelerated the diversification of terrestrial tetrapod and insect faunas (4, 29) and caused a marked rise in atmospheric oxygen (3), fuelling the spectacular ecological radiation of gigantism in insects (29). These consequences contrast sharply with positive feedbacks between plants and CO2 in the short-term carbon cycle that are expected to accelerate anthropogenic climate change over the 21st century (30) and that may lead to the deletion of evolutionary processes in the tropics (31).
Acknowledgements
We thank P. Falkowski, J. Ehleringer, I. Handoh, L. Hickey, C. P. Osborne, and F. I. Woodward for helpful comments and discussion on the manuscript. D.J.B. gratefully acknowledges funding of this work through the Leverhulme Trust (F/00118/G), and R.A.B. acknowledges U.S. Department of Energy Grant DE-FB02-01ER15173.
Footnotes
Abbreviations: NFL, negative feedback loop; PFL, positive feedback loop.
To whom correspondence should be addressed. E-mail: d.j.beerling@sheffield.ac.uk .
© 2005 by The National Academy of Sciences of the USA
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