Taking action to reduce greenhouse gas emissions imposes costs now in order to avoid potentially very large costs from more severe climate change in the future. Steve Polasky, Professor of Ecological/Environmental Economics University of Minnesota, reviews major sources of uncertainty and how that alters the choice of optimal climate change policy. He discusses current debates on how best to frame climate change policy; whether it should be framed as setting limits on greenhouse gas concentrations to avoid potentially catastrophic damages or as an application of benefit-cost analysis. (#29839)