Global photosynthesis results in the single largest flux of carbon dioxide between the atmosphere and the biosphere. Long-term changes in photosynthesis could therefore provide a strong feedback to climate change through changing the growth rate of atmospheric CO2. Global photosynthesis cannot be observed, however, and must therefore be inferred through emergent dynamics in multiple proxies. But the historic sensitivity of global photosynthesis derived from such proxies spans an order of magnitude, leading to large uncertainty in estimates of both the historic and expected future changes in photosynthesis. Here, we examine the various proxies of long-term photosynthetic change, and show that they can be reconciled by combining known plant physiology with emergent dynamics of the global carbon cycle. The results suggest that global photosynthesis has increased due to elevated CO2, but with a much lower sensitivity that that implied by some proxies, and a higher sensitivity than that inferred from remote-sensing based estimates.