Wind and wave energy are not strictly renewable sources of energy, says Axel Kleidon of the Max Planck Institute for Biogeochemistry in Jena, Germany. "Efforts to satisfy a large proportion of our energy needs from the wind and waves will sap a significant proportion of the usable energy available from the sun. In effect, he says, we will be depleting green energy sources. His logic rests on the laws of thermodynamics, which point inescapably to the fact that only a fraction of the solar energy reaching Earth can be exploited to generate energy we can use."