Regional climate modelling confirms the enhancement of cloud cover over EU forests diagnosed with satellite records

Forests can significantly influence local climate both by altering the carbon cycle (biogeochemical effects) and changing the surface energy budget (biophysical effects). While the former effect is well established in international climate policies and accounted for in mitigating strategies, the latter is not included in the negotiations. This is because the high level of uncertainties and the spatial variability of biophysical effects have made it impractical to offer clear advice on which policymakers could act. That said, the impact of these effects is non-negligible and ignoring them may lead to biased and non-optimal land-based climate policies. 

One such effect that is seldom studied is how changes in forest cover can alter the cloud regime, which can potentially have repercussions on the hydrological cycle, the surface radiation budget and possibly on the planetary albedo itself. Following a recent study (Duveiller et al. 2021) that provides a global scale assessment of this effect derived from satellite remote sensing observations, we conducted a similar experiment using a climate model simulation to explore if such patterns could be reproduced. We performed a simulation at a convection-permitting grid spacing of 5 km over the larger European domain using the regional climate model (RegCM4) coupled with CLM4.5. We assessed the signal of forest cover on the cloud regime by applying a space-for-time substitution over a local moving window across the simulated cloud fractional cover for the period 2004–2014, fully in-line with the methodology applied by Duveiller et al. on the satellite records.

Results show that afforestation generally leads to an increase in low cloud cover over most of the domain, confirming the results obtained by Duveiller et al. with the observation-based assessment. We found that the impacts of deforestation on cloud cover using these two different datasets shows a similar magnitude and seasonal pattern. At the local scale the observations and climate model results agree on the potential cloud cover increase/decrease caused by afforestation/deforestation. 
Results showed the capability of a fully coupled land-atmosphere regional climate model to detect the magnitude and the main patterns of potential indirect effects of forest cover change on the local cloud cover. Overall, this indirect biophysical effect would add further climatic value to forests beyond that of carbon sequestration and local surface cooling by evaporation.
The need for a comprehensive view on the climate impacts of forests is particularly timely and relevant for Europe. Our assessment provides further guidance that could assist land planners by indicating where afforestation measures could trigger positive feedbacks on cloud cover. This would further add value to the design of ambitious nature-based policies such as the European Green Deal.