Radiative forcing by forest and subsequent feedbacks in the early Eocene climate

Abstract

Using the Max Planck Institute for Meteorology Earth System Model, we investigate the forcing of forests and the feedback triggered by forests in the pre-industrial climate and in the early Eocene climate (about 54 to 52 million years ago). Other than the interglacial, pre-industrial climate, the early Eocene climate was characterised by high temperatures which led to almost ice-free poles. We compare simulations in which all continents are covered either by dense forest or by bare soil. To isolate the effect of soil albedo, we choose either bright soils or dark soils, respectively. Considering bright soil, forests warm in both, the early Eocene climate and the current climate, but the warming differs due to differences in climate feedbacks. The lapse-rate and water-vapour feedback is stronger in early Eocene climate than in current climate, but strong and negative cloud feedbacks and cloud masking in the early Eocene climate outweigh the stronger positive lapse-rate and water-vapour feedback. In the sum, global mean warming is weaker in the early Eocene climate. Sea-ice related feedbacks are weak in the almost ice-free climate of the early Eocene leading to a weak polar amplification. Considering dark soil, our results change. Forests cools stronger in the early Eocene climate than in the current climate because the lapse-rate and water-vapour feedback is stronger in the early Eocene climate while cloud feedbacks and cloud masking are equally strong in both climates. The different temperature change by forest in both climates highlights the state-dependency of vegetation's impact on climate

Stability of the vegetation–atmosphere system in the early Eocene climate

“…The different regional extent of multiple vegetation-atmosphere states in the early Sundquist, E. T., and Broecker, W. S., American Geophysical Union, Washington/DC, 357-375, 1985, 1554 Early Oligocene ice-sheet expansion on Antarctica -stable isotope and sedimentological evidence from Kerguelen Plateau southern Indian-Ocean, Geology, 20, 569-573, 1992. 1553 Zachos, J., Pagani, M., Sloan, L., Thomas, E., andBillups, K.: Trends, rhythms, andaberrations in global climate 65 Ma to present, Science, 292, 686-693, 2001.…”

“…For our early Eocene simulations, we use the same boundary conditions as Port et al (2015). Orography and bathymetry are based on the maps by Bice and Marotzke (2001) which Heinemann et al (2009) interpolated from the original resolution of 2 • times 2…”

“…Our simulations start from the simulations by Port et al (2015). They explore the impact of extreme land cover on the early Eocene climate and the pre-industrial climate by simulating and a "desert world" and a "forest world" for both climates, respectively.…”

“…At the end of the simulations by Port et al (2015), climate differs significantly between the forest worlds and the desert worlds. We extend those simulations by further 1000 years.…”

“…Using the Earth system model of the Max Planck Institute for Meteoroloy (MPI-ESM), we simulate the early Eocene climate and the pre-industrial climate. We start simula- tions from two different vegetation states, all ice-free continents are either completely covered with dense forests or with bare-soil deserts, respectively (Port et al, 2015). From these initial states, the model system is allowed to freely evolve, with dynamically interacting atmosphere, ocean, and vegetation.…”