Effects of Elevated CO₂ on Yield and Nutritional Quality of Kale and Spinach: A Meta-Analysis

Elevated atmospheric CO₂ is known to alter plant physiology, yet its specific effects on nutrient-rich leafy vegetables remain insufficiently quantified. This study aimed to examine how eCO₂ influences yield and nutritional quality in kale (Brassica oleracea) and spinach (Spinacia oleracea) through the first meta-analysis focused exclusively on these crops. Following the Collaboration for Environmental Evidence (CEE) guidelines, we systematically reviewed eligible studies and conducted a random-effects meta-analysis to evaluate overall and subgroup responses based on CO₂ concentration, crop type and exposure duration. Effect sizes were calculated using Hedges' g with 95% confidence intervals. The analysis showed that eCO₂ significantly increased biomass in spinach (g = 1.21) and kale (g = 0.97). However, protein content declined in both crops (spinach: g = -0.76; kale: g = -0.61), and mineral concentrations, particularly calcium and magnesium, were reduced, with spinach exhibiting stronger nutrient losses overall. The variability in response across different CO₂ concentrations and exposure times further underscores the complexity of eCO₂ effects. These results highlight a trade-off between productivity and nutritional quality under future CO₂ conditions. Addressing this challenge will require strategies such as targeted breeding programmes, biofortification, precision agriculture and improved sustainable agricultural practices to maintain nutrient density. This research provides critical evidence for policymakers and scientists to design sustainable food systems that safeguard public health in a changing climate.