Relationship between evolution of plants, consumers, and global climate

Olsen, Paul E. and Johansson, Annika K., (Lamont-Doherty Earth Observatory of Columbia University, Palisades, New York)

To a first approximation, the transitions between major global climate states (i.e. "ice house" vs. "hot house") do not seem to correlate with the largest scale aspects of tectonic history. Ice house conditions came and went during the existence of Pangea during the late Paleozoic while hot house conditions were maintained during the last stages of Pangea, its breakup and its dispersion during the Mesozoic. Hot house conditions gave way to ice house conditions during the Cenozoic during which the continents remained more or less dispersed. In contrast, there is a good correlation between major climate states and the staggered evolution of producing and consuming organisms. Terrestrial plants tend to increase chemical weathering (via root respiration and decay) while herbivores and detritivores tend to suppress chemical weathering (via lowering of root productivity and subsurface decay). Other workers have emphasized the role of the evolution of major plant groups in the development of ice house conditions, and it may be that it is the evolution of consuming animals that brings the Earth back to a hot house state. We propose that the long lag between the evolution of key innovations in plant group and corresponding innovations in consumers is responsible for intervening ice house conditions. If animals were in adaptive equilibrium with producers there would be no net change in weathering rates other than that driven by changes in the solar constant. This scenario fits the observed climatic pattern, the sedimentary record of carbon reservoirs, and the known evolutionary history of plant and consumer groups, but it remains to be tested by examination of appropriate proxies of weathering in the geological record and experimental studies of the details of the differences in weathering rates among different plant groups.