Vegetation feedback may reduce climatic aridity over the contiguous United States in a 2xCO2 situation

This study examines the potential impact of vegetation feedback on the changes in aridity in association with the doubling of atmospheric CO2 concentrations over the contiguous United States. A series of global climate models, equipped with a dynamic vegetation model has been simulated. In addition, the values of Thornthwaite's moisture index (Im) are also examined to quantify the changes in climate aridity. As CO2 increases, warm and dry atmospheric conditions lead to a significant decrease in the Im, suggesting that there is an increase in aridity over most of the analysis domain, regardless of the presence of vegetation feedback. This is mainly attributed to the water supply from precipitation to be less than the higher climatic demands for water through strong warming. It is found that by including vegetation feedback in the model, changes in vegetation greenness significantly alleviates the decrease in the Im, especially in the sub-humid region including most of the southern, midwestern, and northwestern parts of the United States. Over these regions, the greening through vegetation feedback strengthens moisture transfer from soil to atmosphere through an enhancement of evapotranspiration. The moistening further contributes to decreasing the warming effect (1–2°C) and potential evapotranspiration (3–10 mm month-1), and increasing precipitation (4–10 mm month-1). Summing up, an increase in vegetation greenness makes the climate less arid than that of the exclusion of vegetation feedback because it enhances the water supply and reduces the water demands. The present results indicate that vegetation feedback may prevent an increase in climatic aridity related to CO2 doubling, which is clearly applicable in vulnerable areas to climate change.