Physicist Axel Kleidon of the Max Planck Institute for Biogeochemistry in Jena, Germany has shown that it is a mistake to consider the wind and waves to be truly renewable energy sources. Build enough wind farms to replace fossil fuels, he says, and we would reduce the energy available in the atmosphere and actually accelerate climate change. We know from thermodynamics that only a modest fraction of the solar energy reaching Earth is available as “free energy” we can use, taking the form of winds, ocean currents, and lifting of evaporated water. The rest becomes heat, which is not available to do work. Some of the free energy harnessed by wind and wave generators will be lost as heat, since they can not be perfectly efficient. So by building wind and wave farms, we will be converting part of the sun's useful energy into thermal energy. The effects of this would probably show up first in wind farms themselves, where the gains expected will be less than predicted as the energy of the Earth system is depleted. Kleidon’s calculations show that the amount of energy which we can harness from the wind is reduced by a factor of 100 if you take into account the depletion of free energy by wind farms. In addition, sucking that much energy out of the atmosphere will change precipitation, turbulence and the amount of solar radiation reaching the Earth's surface. The magnitude of the changes will be comparable to the changes to the climate caused by doubling atmospheric concentrations of carbon dioxide. Even current photovoltaic solar cell designs will contribute to global warming, because they convert only a small fraction of the light that hits them, and absorb most of the rest, converting it to heat that warms the environment.