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Abstract:
We evaluate the radiative forcing of forests and the feedbacks triggered by forests in a warm, basically
ice-free climate and in a cool climate with permanent high-latitude ice cover using the Max Planck Institute
for Meteorology Earth System Model. As a paradigm for a warm climate, we choose the early Eocene, some
54 to 52 million years ago, and for the cool climate, the pre-industrial climate, respectively. To isolate firstorder
effects, we compare idealised simulations in which all continents are covered either by dense forests or
by deserts with either bright or dark soil. In comparison with desert continents covered by bright soil, forested
continents warm the planet for the early Eocene climate and for pre-industrial conditions. The warming can be
attributed to different feedback processes, though. The lapse-rate and water-vapour feedback is stronger for the
early Eocene climate than for the pre-industrial climate, but strong and negative cloud-related feedbacks nearly
outweigh the positive lapse-rate and water-vapour feedback for the early Eocene climate. Subsequently, global
mean warming by forests is weaker for the early Eocene climate than for pre-industrial conditions. Sea-ice related
feedbacks are weak for the almost ice-free climate of the early Eocene, thereby leading to a weaker high-latitude
warming by forests than for pre-industrial conditions. When the land is covered with dark soils, and hence,
albedo differences between forests and soil are small, forests cool the early Eocene climate more than the preindustrial
climate because the lapse-rate and water-vapour feedbacks are stronger for the early Eocene climate.
Cloud-related feedbacks are equally strong in both climates. We conclude that radiative forcing by forests varies
little with the climate state, while most subsequent feedbacks depend on the climate state.
