A new model suggests that the rate at which ecosystems respond to climate change is a critical and overlooked component to reduce uncertainty about future projections. When a model assumes that ecosystems will respond very slowly to climate change, it tends to project widespread decreases in net primary productivity in the western U.S. as soon as 2100. When it assumes a rapid adjustment, a model often predicts an increase in net productivity. The gap between these two scenarios shows how much work we still have to do to improve our understanding of big-picture patterns.A new model suggests that the rate at which ecosystems respond to climate change is a critical and overlooked component to reduce uncertainty about future projections. When a model assumes that ecosystems will respond very slowly to climate change, it tends to project widespread decreases in net primary productivity in the western U.S. as soon as 2100. When it assumes a rapid adjustment, a model often predicts an increase in net productivity. The gap between these two scenarios shows how much work we still have to do to improve our understanding of big-picture patterns.