Our research focuses mostly on three areas:
1. Earth system theory deals with the thermodynamic description of how processes convert energy within the Earth system, the identification of their interactions with boundary conditions and the resulting thermodynamic limits, the description of the emergent behaviour and relating it to established concepts in Earth system science.
2. Land-atmosphere interactions applies thermodynamics-based theory to the processes that transfer energy, water and mass in the land-atmosphere system to understand and predict land-atmosphere interactions, the emergent climatic state over land and its sensitivity to change, and the role of the terrestrial biosphere in these interactions.
3. Renewable energy in the Earth system uses the limits derived from thermodynamics to evaluate resource estimates for different forms of renewable energy and the consequences of their use in the Earth system.
The following provide brief examples of our research in these three areas.
Planetary sequences of energy conversion. A planetary thermodynamic view of the Earth system, with its cascades of energy conversions (left, solid lines) and its effects (right, dashed lines). This view sets the basics of how thermodynamics and the laws of thermodynamics apply to Earth system processes, from solar radiation to motion, geochemical cycling, biotic and human activity. After Kleidon 2010, 2012, 2016.
Classification of planets. The figure shows a classification scheme for the evolutionary state of planets based on which kinds of free energy a planet generates from the stellar forcing. A type I planet in which only radiative exchange takes place and no free energy is generated from radiation (such as Mercury); a type II planet in which heat engines drive the climate system of a planet (such as Venus); a type III and type IV planet with a thin vs. a thick biosphere that generates chemical free energy using sunlight and that differ by the extent to which biotic effects alter the thermodynamic state of the planet (such as early and present-day Earth); and a hypothetical type V planet in which an energy-intensive technological species (such as humans on Earth) contributes substantially to the free energy generation of the planet through technology (such as photovoltaics). After Frank et al. (2016).
Using thermodynamics to predict surface energy balance partitioning. We use thermodynamics to describe how the surface and the atmosphere exchange energy and mass. Turbulent heat fluxes are viewed as the outcome of a convective heat engine driven by the surface heating by solar radiation. The predicted fluxes are compared to observations to test whether the surface-atmosphere system operates at its thermodynamic limit. This comparison shows that turbulent exchange appears to operate very close to its thermodynamic limit. The figure shows the comparison of predicted turbulent fluxes from thermodynamics (Jopt, black line) to observations (Jobs, black circles). After Kleidon and Renner (2018)
Using thermodynamics to predict global climate change. Our thermodynamic approach can then be used to better understand the physical consequences of global climate change. The diagram shows an application to the question why land surfaces warm more than ocean surfaces with global warming. The diagram shows the global warming due to 4xCO2 on land (red) and ocean (blue) on the left, and the ratio on the right. Our approach can reproduce the climate model response rather well (Open symbols -- our approach; solid symbols -- climate model predictions). From Kleidon and Renner (2017).
Renewable energy generation by the Earth system. Thermodynamics allows us to quantify how much renewable energy is generated within the Earth system, and how much of it can at most be used. The large influx of solar radiation is successively diminished by sequences of energy conversions, as shown in the Figure. The different forms of energy that are generated in these sequences can then form a source of renewable energy. Yet, the more energy is being converted, the lower the potential of the renewable energy source.
Limits to wind energy utilization. Typically, only a fraction of the energy generated by the Earth system can be used. This figure shows results from idealized climate model simulations. What the simulations show is that more wind energy use (top) results in lower wind speeds (bottom), thereby setting a limit to how much wind energy can at most be utilized. The maximum levels of generation can be understood by simplified approaches that describe maximum levels of energy conversions. After: Miller and Kleidon (2016)