The impact of vegetation response to CO2 on energy fluxes at Earth's surface and top of atmosphere

Increasing concentrations of atmospheric CO₂ alter the Earth's energy budget not only by interaction with radiation, but also through adjustments of the climate state. One set of such adjustments is mediated by the response of vegetation to an increase in CO₂: Changes in vegetation cover and leaf area index (LAI) as well as CO₂-induced plant stomatal closure alter surface fluxes of radiation, heat, and water. These changes in surface fluxes in turn affect atmospheric temperature, water vapor, and cloud cover, further affecting the Earth's energy balance. Past studies have shown that stomatal closure leads to a positive radiative forcing at the top of atmosphere (TOA), largely caused by adjustments of clouds to reduced transpiration. However, the full set of adjustments including the role of LAI changes and the surface energy balance have not been considered in investigations of the Earth's energy budget.

We aim to provide a detailed analysis of energy budget changes at the land surface and TOA by utilizing idealized simulations performed with the Max-Planck-Institute Earth System Model, and place these results in the context of a larger ensemble by analysing transient C4MIP simulations with a similar coupling. In our simulations we prescribe an abrupt doubling of CO₂ concentration only seen by the land model, while its atmospheric counterpart continues to experience pre-industrial conditions. This isolates the response of vegetation and changes to the climate system resulting from it, by omitting the direct response of the atmosphere and radiation to increased CO₂. To estimate which changes originate from an increase in LAI, an additional experiment is run with prescribed LAI per vegetated area.

Preliminary results show persistent decreases in near-surface relative humidity and low cloud cover over land, especially pronounced in the extra-tropics. As a result, incident shortwave radiation at the land surface increases by 0.85 Wm^-2 in the global average. Together with a decreased latent heat flux, this is compensated by a greater sensible heat flux and moderate temperature increase, causing more longwave emission. Accordingly, the outgoing radiation at the TOA shows a decrease in the shortwave component, but an increase in longwave radiation. The simulation with prescribed LAI shows a much higher radiative forcing of 0.33 Wm^-2 compared to 0.13 Wm^-2 in the experiment with dynamically evolving LAI, suggesting that adjustments in LAI could compensate significant parts of the forcing through CO₂-induced stomatal closure. However, this signal is less robust compared to the persistent changes and will require additional, dedicated experiments to be investigated thoroughly. These findings show a long term effect of stomatal closure on surface energy fluxes and suggest that considering LAI response to increased CO₂ could alter estimates of radiative forcing, highlighting the need for further study.