Abstract
Plants influence extreme heat events by regulating land-atmosphere water and energy exchanges. The contribution of plants to changes in future heat extremes will depend on the responses of vegetation growth and physiology to the direct and indirect effects of elevated CO2. Here we use a suite of earth system models to disentangle the radiative versus vegetation effects of elevated CO2 on heat wave characteristics. Vegetation responses to a quadrupling of CO2 increase summer heat wave occurrence by 20 days or more—30–50% of the radiative response alone—across tropical and mid-to-high latitude forests. These increases are caused by CO2 physiological forcing, which diminishes transpiration and its associated cooling effect, and reduces clouds and precipitation. In contrast to recent suggestions, our results indicate CO2-driven vegetation changes enhance future heat wave frequency and intensity in most vegetated regions despite transpiration-driven soil moisture savings and increases in aboveground biomass from CO2 fertilization.
Highlights
Plants influence extreme heat events by regulating land-atmosphere water and energy exchanges
We analyze a suite of Earth system models (ESMs) from the Coupled Model Intercomparison Project Phase 5 (CMIP5)[13] in a set of idealized experiments to quantify the contribution of the vegetation response to CO2 to projected heat wave changes and to better understand its role in shaping regional-scale and intermodel differences in projected future heat extremes
Though enhanced CO2 often has a limited impact on leaf area index (LAI, defined as one-sided leaf area per unit ground surface area), in nutrient-limited regions, and in mature forests[21], CO2 fertilization can lead to enhanced LAI during the early stages of plant development[21,22] and in regions that are water-limited[23]
Summary
Plants influence extreme heat events by regulating land-atmosphere water and energy exchanges. Vegetation responses to a quadrupling of CO2 increase summer heat wave occurrence by 20 days or more—30–50% of the radiative response alone—across tropical and mid-to-high latitude forests These increases are caused by CO2 physiological forcing, which diminishes transpiration and its associated cooling effect, and reduces clouds and precipitation. In contrast to recent suggestions, our results indicate CO2-driven vegetation changes enhance future heat wave frequency and intensity in most vegetated regions despite transpiration-driven soil moisture savings and increases in aboveground biomass from CO2 fertilization. Given the impact of CO2 physiological forcing and CO2 fertilization (hereafter collectively referred to as CO2 vegetation forcing) on surface moisture and energy fluxes, each may contribute to projected changes in future heat wave events, but the net effects on projected heat waves remain unresolved. Observed growing season water reductions range from usapvitnog~s 2f.r1o5mmmCOd2a-yi−nd[1] uincedsomtraengsrpaisrsaltainond ecosystems (exposed to 100% increase day−1 in some temperate deciduous tree in CO2)[30], to ~0.6 species
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