Positive feedbacks between the terrestrial carbon cycle and climate represent an outstanding area of uncertainty in simulations of future climate change. Coupled climate-carbon cycle models have simulated widely divergent feedback magnitudes, and attempts to explain model differences have had only limited success. In this study, we demonstrate that the response of vegetation primary productivity to climate changes is a critical controlling factor in determining the strength of simulated carbon cycle-climate feedbacks. This conclusion sheds new light on coupled climate-carbon cycle model results, and highlights the need for improved model representation of photosynthesis processes so as to better constrain future projections of climate change.