Abstract
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 first-order 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 pre-industrial 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.
Highlights
In present-day climate, forests tend to warm the high latitudes by masking the bright snow cover leading to a lower surface albedo than with bare soil or grass (Bonan, 1992; Betts and Ball, 1997; Bonan, 2008), and this warming can be amplified by the sea-ice albedo feedback (Claussen et al, 2001; Fraedrich et al, 2005; Brovkin et al, 2009)
Mainly the contrasting vegetation cover causes the difference in surface albedo because the climate response to the radiative forcing is still weak leading to negligible changes in snow cover and sea-ice cover
We have compared the biogeophysical effect of forests in a warm, nearly ice-free climate and in a cool climate with permanent ice-cover at high latitudes
Summary
In present-day climate, forests tend to warm the high latitudes by masking the bright snow cover leading to a lower surface albedo than with bare soil or grass (Bonan, 1992; Betts and Ball, 1997; Bonan, 2008), and this warming can be amplified by the sea-ice albedo feedback (Claussen et al, 2001; Fraedrich et al, 2005; Brovkin et al, 2009). Forests tend to cool the climate by enhancing transpiration and evaporation which leads to an increased cloud cover and a higher planetary albedo. Similar to the studies on present-day climate, they find that trees warm the late-Cretaceous high latitudes relative to bare soil by masking the snow cover and triggering the ice-albedo feedback.
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call
