Acknowledgements
This work was supported by the Climate Change Prediction Program and Scientific Discovery for Advanced Computing (SciDAC) programme within the US Department of Energy's Office of Science, Los Alamos National Laboratory (LANL) Director's Fellowship, LANL Institute for Geophysics and Planetary Physics, and NASA Grant NNX08AV21G. LANL is operated under the auspices of the National Nuclear Security Administration of the US Department of Energy under Contract No. DE-AC52-06NA25396.