The U.S. Department of Energy is developing an earth system model to tackle the twin challenges of improving the ability to simulate and predict significant, long-term changes in environmental variables important to the energy-sector while also ensuring the model will execute efficiently on the exascale systems DOE will be deploying in the 2021 time frame, all of which will be GPU-based. The Energy Exascale Earth System Model project combines 8 national laboratories and other partners towards this effort. A challenge is to provide a software development path that targets GPUs while also making use of existing CPU-based systems that still form the majority of compute cycles available for testing and development. Our approach so far is to use a mix of directives and template-based approaches in Fortran and C++, respectively. Efficiently using GPUs also requires some reformulation of the problem to emphasize high resolution in either the fluid dynamics or certain paramaterizations. The current status of progress in these areas will be described.
