Rising CO₂ concentrations are expected to enhance crop yields through a direct fertilization effect, but recent field experiments suggest this effect is much smaller than previously thought. Models projecting future crop yields have relied on data from enclosure studies conducted over 20 years ago, which may not accurately reflect real-world conditions. Free-air concentration enrichment (FACE) experiments, which simulate elevated CO₂ levels in open-air field conditions, show that the fertilization effect is significantly less than in controlled environments. This casts doubt on the assumption that rising CO₂ will fully offset yield losses due to climate change. The fertilization effect of CO₂ is primarily due to increased photosynthesis and reduced photorespiration in C₃ plants, but C₄ plants, such as maize and sorghum, may not show a direct response. However, indirect effects, such as improved water use efficiency, may still increase yield. Current crop models, including those used by the IPCC, rely on data from chamber studies that may not accurately represent field conditions. FACE experiments indicate that the fertilization effect is smaller than previously estimated, and that C₄ crops may not benefit significantly from elevated CO₂. Chamber studies, which are commonly used to assess crop responses to elevated CO₂, may not accurately reflect field conditions due to controlled environments and artificial conditions. FACE experiments, which simulate real-world conditions, show that the fertilization effect is smaller than expected. This suggests that current projections of future crop yields may be overly optimistic. The effects of elevated CO₂ on crop yields are also influenced by other factors, such as nitrogen availability and interactions with other environmental stressors like ozone. FACE experiments have shown that the fertilization effect of CO₂ is smaller than previously thought, and that the combined effects of elevated CO₂ and ozone may not be additive. This has important implications for future food security and the ability of crops to adapt to changing environmental conditions. The results of FACE experiments suggest that the fertilization effect of CO₂ on crop yields is much smaller than previously estimated, and that current models may be overly optimistic. This highlights the need for further research to better understand the effects of elevated CO₂ on crop yields and to improve projections of future food security.Rising CO₂ concentrations are expected to enhance crop yields through a direct fertilization effect, but recent field experiments suggest this effect is much smaller than previously thought. Models projecting future crop yields have relied on data from enclosure studies conducted over 20 years ago, which may not accurately reflect real-world conditions. Free-air concentration enrichment (FACE) experiments, which simulate elevated CO₂ levels in open-air field conditions, show that the fertilization effect is significantly less than in controlled environments. This casts doubt on the assumption that rising CO₂ will fully offset yield losses due to climate change. The fertilization effect of CO₂ is primarily due to increased photosynthesis and reduced photorespiration in C₃ plants, but C₄ plants, such as maize and sorghum, may not show a direct response. However, indirect effects, such as improved water use efficiency, may still increase yield. Current crop models, including those used by the IPCC, rely on data from chamber studies that may not accurately represent field conditions. FACE experiments indicate that the fertilization effect is smaller than previously estimated, and that C₄ crops may not benefit significantly from elevated CO₂. Chamber studies, which are commonly used to assess crop responses to elevated CO₂, may not accurately reflect field conditions due to controlled environments and artificial conditions. FACE experiments, which simulate real-world conditions, show that the fertilization effect is smaller than expected. This suggests that current projections of future crop yields may be overly optimistic. The effects of elevated CO₂ on crop yields are also influenced by other factors, such as nitrogen availability and interactions with other environmental stressors like ozone. FACE experiments have shown that the fertilization effect of CO₂ is smaller than previously thought, and that the combined effects of elevated CO₂ and ozone may not be additive. This has important implications for future food security and the ability of crops to adapt to changing environmental conditions. The results of FACE experiments suggest that the fertilization effect of CO₂ on crop yields is much smaller than previously estimated, and that current models may be overly optimistic. This highlights the need for further research to better understand the effects of elevated CO₂ on crop yields and to improve projections of future food security.