Plants with the C₄ photosynthesis pathway typically respond to climate change differently from more common C₃-type plants, due to their distinct anatomical and biochemical characteristics. These different responses are expected to drive changes in global C₄ and C₃ vegetation distributions. However, current C₄ vegetation distribution models may not predict this response as they do not capture multiple interacting factors and often lack observational constraints. Here, we used global observations of plant photosynthetic pathways, satellite remote sensing, and photosynthetic optimality theory to produce an observation-constrained global map of C₄ vegetation. We find that global C₄ vegetation coverage decreased from 17.7% to 17.1% of the land surface during 2001 to 2019. This was the net result of a reduction in C₄ natural grass cover due to elevated CO₂ favoring C₃-type photosynthesis, and an increase in C₄ crop cover, mainly from corn (maize) expansion. Using an emergent constraint approach, we estimated that C₄ vegetation contributed 19.5% of global photosynthetic carbon assimilation, a value within the range of previous estimates (18-23%) but higher than the ensemble mean of dynamic global vegetation models (14 ± 13%; mean ± one standard deviation). Our study sheds insight on the critical and underappreciated role of C₄ plants in the contemporary global carbon cycle.