This study maps the global distribution of C₄ vegetation using observations and optimality theory. C₄ plants, which use a different photosynthetic pathway than C₃ plants, respond differently to climate change due to their distinct anatomical and biochemical characteristics. The study finds that global C₄ vegetation coverage decreased from 17.7% to 17.1% of the land surface between 2001 and 2019. This decrease was primarily due to a reduction in C₄ natural grass cover caused by elevated CO₂ favoring C₃-type photosynthesis, and an increase in C₄ crop cover, mainly from corn (maize) expansion. C₄ vegetation contributed 19.5% of global photosynthetic carbon assimilation, which is within the range of previous estimates (18–23%) but higher than the ensemble mean of dynamic global vegetation models (14 ± 13%). The study highlights the critical role of C₄ plants in the global carbon cycle. C₄ plants are one of the three photosynthetic pathways for terrestrial plants and are reported to account for 18–23% of global photosynthesis. They drive wildfire dynamics in tropical and subtropical ecosystems. C₄ plants first evolved in the low atmospheric CO₂ environment of the Oligocene Epoch, roughly 24–35 million years ago. They developed distinct biochemical and anatomical characteristics to enrich CO₂ concentration at the site of Rubisco carboxylation in leaves, thereby reducing photorespiration and enhancing carbon-fixation rates. These characteristics produce different climate sensitivities in C₄ plants compared to more prevalent C₃ plants, and thus are expected to cause a shift in C₄ plant distributions and their contribution to global photosynthesis under contemporary and future climate change. Many previous studies have examined C₄ plant responses to multiple environmental factors. A consensus is that since most C₄ species originated in lower atmospheric CO₂ concentrations, they are expected to benefit less from rising CO₂ concentrations compared to C₃ plants. Meanwhile, higher temperatures are expected and reported to favor C₄ over C₃ photosynthesis, because the affinity of O₂ to Rubisco relative to CO₂ becomes stronger with increasing temperature and also due to differing solubilities of CO₂ and O₂ with increasing temperature. This should produce an advantage for the carbon concentrating mechanism of C₄ species, especially under high temperatures. Hence C₄ species are characteristic of tropical and subtropical ecosystems. Correspondingly, since C₄ photosynthesis is less limited by CO₂ than C₃ photosynthesis, it achieves a higher photosynthetic quantum yield and photosynthetic rates under high light, especially under high temperatures. C₄ species also should have a carbon assimilation advantage in arid environments due to their higher water use efficiency (i.e., less water loss through stomata forThis study maps the global distribution of C₄ vegetation using observations and optimality theory. C₄ plants, which use a different photosynthetic pathway than C₃ plants, respond differently to climate change due to their distinct anatomical and biochemical characteristics. The study finds that global C₄ vegetation coverage decreased from 17.7% to 17.1% of the land surface between 2001 and 2019. This decrease was primarily due to a reduction in C₄ natural grass cover caused by elevated CO₂ favoring C₃-type photosynthesis, and an increase in C₄ crop cover, mainly from corn (maize) expansion. C₄ vegetation contributed 19.5% of global photosynthetic carbon assimilation, which is within the range of previous estimates (18–23%) but higher than the ensemble mean of dynamic global vegetation models (14 ± 13%). The study highlights the critical role of C₄ plants in the global carbon cycle. C₄ plants are one of the three photosynthetic pathways for terrestrial plants and are reported to account for 18–23% of global photosynthesis. They drive wildfire dynamics in tropical and subtropical ecosystems. C₄ plants first evolved in the low atmospheric CO₂ environment of the Oligocene Epoch, roughly 24–35 million years ago. They developed distinct biochemical and anatomical characteristics to enrich CO₂ concentration at the site of Rubisco carboxylation in leaves, thereby reducing photorespiration and enhancing carbon-fixation rates. These characteristics produce different climate sensitivities in C₄ plants compared to more prevalent C₃ plants, and thus are expected to cause a shift in C₄ plant distributions and their contribution to global photosynthesis under contemporary and future climate change. Many previous studies have examined C₄ plant responses to multiple environmental factors. A consensus is that since most C₄ species originated in lower atmospheric CO₂ concentrations, they are expected to benefit less from rising CO₂ concentrations compared to C₃ plants. Meanwhile, higher temperatures are expected and reported to favor C₄ over C₃ photosynthesis, because the affinity of O₂ to Rubisco relative to CO₂ becomes stronger with increasing temperature and also due to differing solubilities of CO₂ and O₂ with increasing temperature. This should produce an advantage for the carbon concentrating mechanism of C₄ species, especially under high temperatures. Hence C₄ species are characteristic of tropical and subtropical ecosystems. Correspondingly, since C₄ photosynthesis is less limited by CO₂ than C₃ photosynthesis, it achieves a higher photosynthetic quantum yield and photosynthetic rates under high light, especially under high temperatures. C₄ species also should have a carbon assimilation advantage in arid environments due to their higher water use efficiency (i.e., less water loss through stomata for