Vegetation Response to Rising CO2 Amplifies Contrasts in Water Resources Between Global Wet and Dry Land Areas

Rising atmospheric CO2 impacts on vegetation physiological processes can alter land feedbacks on precipitation and water resources, but understanding of regional differences in these changes is uncertain. We investigate the impact of rising CO2 on land water resources for different wetness levels using four Earth system models. We find an overall tendency of runoff to increase across all wetness levels. However, runoff increases in wet regions are much larger than those in dry regions, especially in wet seasons. This substantial amplification of contrasts between wet and dry regions increases at 3% per 100 ppm increase in CO2 relative to the historical period, reaching 18% for a quadrupling of CO2, quantified by a new wetting contrast index (WCI). Physiological effects suppress evapotranspiration more in wet than dry regions, which has a larger contribution than radiative forcing to the amplification of runoff contrast, reshaping the spatial distribution of future land water resources. Plain Language Summary Increasing atmospheric CO2 concentration is expected to intensify the global water cycle and reshape the regional distribution of water resources. Using specialized simulations from four Earth system models, this study shows that physiological effects (stomatal closure and leaf area increase), rather than radiative forcing, of rising CO2 are the main driver of amplified regional wetness contrasts, with stronger suppression of evapotranspiration and so larger runoff increases in wet regions than in dry regions. These results underscore the importance of accounting for the physiological forcing of rising CO2 in hydrological change projections, and imply the urgency for future water resource assessments and managements to adapt for an increasing imbalance in regional water resources. Key Points We quantify land water resources change in response to rising CO2 for different wetness levels using a new wetting contrast index We find a substantial amplification of runoff contrast across wetness levels because the runoff increases more in wet than dry regions Vegetation responses to rising CO2 contribute to amplified runoff contrast by suppressing evapotranspiration more in wet than dry regions