Mark Z. Jacobson, professor of civil and environmental engineering at Stanford University and director of its Atmosphere and Energy program, says that producing all new energy with wind power, solar power, and hydropower by 2030 is feasible, and that existing energy supply arrangements could be replaced by 2050.[12] Barriers to implementing the renewable energy plan are seen to be "primarily social and political, not technological or economic".[13] Jacobson says that energy costs today with a wind, solar, and water system should be similar to today's energy costs from other optimally cost-effective strategies.[14] The main obstacle against this scenario is the lack of political will.[15] His conclusions have been disputed by other researchers.[16] Jacobson published a response that disputed the piece point by point[17] and claimed that the authors were motivated by allegiance to energy technologies that the 2015 paper excluded.[16]