Friday, August 8, 2008 - 8:00 AM

SYMP 22-1: Ecosystems and climates in light of four billion years of history

David Schwartzman, Howard University


The case for a hot early climate now grows more robust, as evidence mounts from a variety of sources including isotope geology and molecular phylogeny. The Earth's climate evolved from a hothouse starting some 4 billion years ago, with thermophilic conditions, only cooling to a near icehouse state starting about a billion years ago. This near icehouse regime created an explosion of biodiversity especially since the rise of the first forests nearly 400 million years ago. New habitats opened up within newly emerging organisms and ecosystems (e.g., within the forest canopy and the digestive tracts of Metazoa,), as well as a result of the progressive multiplication of the physical and chemical environments in the biosphere.
The evolution of the biosphere did not optimize conditions for existing biota, unlike the claim of the original Gaia hypothesis. At least two catastrophes for existing life occurred; the well-known oxygen catastrophe and an earlier temperature catastrophe for thermophilic bacteria. The progressive increase in the biotic enhancement of weathering was a major influence in climatic cooling because of its mediation of the carbon sink with respect to the atmosphere/ocean system. By the late Proterozoic, the rise of atmospheric oxygen likely resulted in an increase in terrestrial biotic productivity. Climate and life coevolved as a tightly coupled system, constrained by abiotic factors (varying solar luminosity, and the crust's tectonic and impact history). Note that the trend of progressively increasing terrestrial biotic productivity since the origin of life is contrasted with the last thousand years of deforestation, and more recent urbanization.
Climate is regulated by the carbon biogeochemical cycle. On time scales of greater than a few hundred thousand years, the biologically mediated carbonate-silicate cycle acts as a homeorrhetic thermostat. But on shorter time scales, positive feedbacks play a stronger role, hence gaian homeostasis will not save us from anthropogenic global warming ecocatastrophe.

Therefore tropical deforestation poses a real threat as a driver of climate change and biodiversity loss. In contrast to the first order trend of climatic cooling for the last 4 billion years, we now witness very rapid anthropogenic global warming.
Schwartzman, D. (1999, 2002) Life, Temperature, and the Earth: The Self-Organizing Biosphere, Columbia University Press, N.Y.