Can thermodynamics help us to understand the role of life in the Earth system and its evolution?
|Location||International Geological Congress,oslo 2008|
|Holding Date||21 September 2008|
Life has greatly shaped the past evolution and the present state of the Earth system. Major geochemical cycles, especially the cycles of carbon and oxygen, have been driven far away from their respective, abiotic states by life, resulting in the high oxygen and low carbon dioxide concentrations in the present-day atmosphere. To understand why these changes have occurred, that is, why these biotic effects are to be expected, I view these changes from a perspective of non-equilibrium thermodynamics. Practically all processes are thermodynamic and irreversible in their nature, and therefore produce entropy. In steady state, the produced entropy within the Earth system is balanced by the entropy export to space. Entropy production serves as a general "currency" that allows us to compare energy, water, carbon and other biogeochemical processes and transformations in a quantitative way and relate this "currency" to how far the associated steady state is maintained away from thermodynamic equilibrium.
The proposed principle of Maximum Entropy Production (MEP) states that sufficiently complex systems evolve to states at which entropy production is maximized given the prevailing constraints, which then corresponds roughly to a steady state that is maintained furthest away from thermodynamic equilibrium. Here I describe how this view applies to the planetary energy balance, the cycles of water and carbon, and the interactions with life. The cycles of water and carbon are strongly coupled to the energy balance: differences in heating result in atmospheric motion, which in turn drives the water cycle away from equilibrium, affecting cloud cover and the planetary albedo. The water cycle drives the geologic carbon cycle through its effects on carbonate-silicate weathering on land and cation export to the oceans. Life plays a central role in modifying these interactions, e.g. by affecting the intensity of weathering or the precipitation of carbonates in the ocean. From this I derive what a direction towards states of higher entropy production would imply for Earth system evolution. This thermodynamic perspective of life and Earth system functioning provides a promising basis for a simple yet fundamental understanding of how the Earth with life evolved in time.