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Energy and the Earth system

Quantification of drivers of chemical disequilibrium in Earth’s atmosphere

Among widely used and debated signs of life on a planet, the strong chemical disequilibrium associated with Earth's atmospheric composition is an unambiguous sign for the widespread presence of life on Earth Lovelock (1965, 1975). This disequilibrium is reflected particularly in the coexistence of methane and oxygen, which would be depleted by chemical reactions to carbon dioxide and water if it were not continuously replenished. The high concentration of these compounds, among others, makes the thermodynamic state of the Earth's atmosphere unique when compared to other planets and moons.

In this project we attempt to understand and quantify the relative importance of different drivers in shaping this disequilibrum. To quantify the extent of disequilibrium, we need to take into account the rates at which chemical free energy is generated by exchange fluxes and dissipated by chemical reactions. In order to estimate the relevance of biotic activity on the overall disequilibrium, we quantify the relative contributions by photochemical, electrochemical, geochemical and biotic processes. We illustrate this methodology using an idealized setting of a simple reaction involving methane (see Figure 2) and by using an atmospheric chemistry model. Our thermodynamic analysis of atmospheric disequilibrium provides a quantitative, holistic basis for future work to understand the effects that life has on transforming the whole planetary atmosphere. This, in turn, should help us to better understand habitable conditions and to allow us for possible remote detection of life on other planetary bodies. In addition, the methodology developed here provides the basis for future studies on quantifying free energy generation rates and disequilibria associated with geochemical cycling in general.

Natural limits to renewable energy within the Earth system

Renewable energy is seen as the solution to avoid the greenhouse gas emissions associated with the consumption of fossil fuels, and some studies suggest near infinite reservoirs of renewable energy sources that can easily be used without impacts. For instance, in Vance (2009) it is claimed that the high wind speeds of the jet streams contain 100 times as much energy as humans use today. However, the natural limits of renewable energy needs to be evaluated within the larger context of how Earth system processes generate free energy. We continued our work on the natural limits of renewable forms of energy and:

• estimated the maximum wind power than can be extracted from jet streams with a climate model Miller, Gans, Kleidon (2011). The estimate is 200-times less than previous estimates and shows substantial climatic impacts and little potential to meet human energy demands. This low estimate is consistent with the well-established concept of geostrophic flow, that is, the high wind speeds of the jet streams result from the near absence of friction and not from a strong power source;
• worked on establishing a global estimate of renewable energy sources and their interrelation (see Figure);
• demonstrated that the commonly used method to establish wind power estimates of 1/2 rho v^3 represents the transport of kinetic energy, and not its generation rate (Gans, Miller, Kleidon (2010), Miller, Gans, Kleidon (2011)). When estimating natural limits of wind power, the effect of kinetic energy removal by turbines on the flow need to be accounted for;
• showed with climate model simulations that meeting the current primary energy demand with solar power leads to substantially less climatic impacts than meeting this demand by wind power (Miller et al., in prep.);
• estimating maximum rates of work that can be extracted from direct and diffuse solar radiation (Gans et al., in prep.).

Publications