Several lines of evidence suggest that the warming climate plays a vital role in driving certain types of extreme weather. The impact of this warming and extreme weather on forest carbon assimilation capacity is poorly known. Filling this knowledge gap is critical towards understanding changing the amount of carbon that forests can hold. Here, we used a perfect-deficit approach to identify forest canopy photosynthetic capacity (CPC) deficits and analyze how they correlate to climate extremes, based on data measured by the eddy covariance method at 26 forest sites integrating 146 site-years. We found that droughts severely affect the carbon assimilation capacities of evergreen broadleaf forest and deciduous broadleaf forest. In addition, the carbon assimilation capacities of Mediterranean forests are highly sensitive to climate extremes, while marine forest climates tend to be insensitive to climate extremes. Our estimates suggest an average global reduction of forest canopy photosynthetic capacity of 6.3petagramsof carbon per growing season over 2001-2010, with evergreen broadleaf forests contributing 51.7% of the total reduction. ; JRC.H.7-Climate Risk Management