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.
During the early Eocene (about 54 to 52 million years ago), climate
was much warmer than today. Tropical temperatures were 5 to
6
Flora fossils indicate that forests dominated the vegetation cover
during the early Eocene (
We simulate both, the early Eocene climate and the pre-industrial
climate, with dense forest on all continents and with bare soil on all
continents, respectively. Based on the differences in temperature and
radiative flux between the respective “forest world” and the
respective “desert world”, we derive the radiative forcing by
forests and the subsequent climate feedbacks. We expect that the
radiative forcing by forest depends on the soil albedo. To isolate the
impact of soil albedo on the radiative forcing by forest, we simulate
the desert world two times. In a first case, we assume a homogeneous
soil albedo of 0.1, which is approximately the albedo of volcanic
rocks and granite bedrock (
For the early Eocene climate and the pre-industrial climate, we
prescribe an atmospheric
The MPI-ESM consists of the atmospheric general circulation model
ECHAM6 (
To simulate the early Eocene climate, we use the maps of orography and
bathymetry by
Following
We perform the same three simulations with the described early Eocene
boundary conditions and with pre-industrial boundary conditions
(Table
In the
We aim to compare the impact of forest on the early Eocene climate
and on the pre-industrial climate. For this purpose, we compare the
forest world simulation to the desert world simulations in both
climates. All simulations run for 400
At the beginning of each simulation, we change the vegetation cover
and the soil albedo drastically. The modification of the land surface
acts as an external forcing on the climate system and perturbs the
radiation balance at the top of the atmosphere (TAO). During the
simulation, the perturbation in the TOA radiation balance,
The model simulations provide pairs of
The linear regression approach is only valid when feedbacks are
constant in time. The slope of the regression line, however,
decreases with time (
For the forest world simulation, the linear regression approach
reveals the radiative forcing and the feedbacks by afforesting
savanna-like vegetation. However, we aim to estimate the radiative
forcing and the feedbacks from afforesting deserts on all continents.
Hence, we modify the linear regression approach in the way that we
combine the forest world simulation with the desert world simulations.
Let
The radiative forcing of afforesting the desert world is expressed by
Net perturbation in the TOA radiation balance consists of a long-wave
component (LW) and a short-wave component (SW). Both components
constitute of a cloud share (cl) and a clear-sky share (cs) leading to
The long-wave clear-sky feedback parameter,
We estimate the Planck feedback parameter,
We quantify the uncertainty of forcings and feedbacks in terms of the
95 % confidence interval which we assess using bootstrapping. We
randomly select 150 pairs of differences in temperature and TOA
radiative flux out of the first 150
Based on the regression approach, we estimate radiative forcing of forestation and climate feedbacks. First, we compare the forest world to the bright desert world and discuss differences in radiative forcing and climate feedbacks between the early Eocene climate and the pre-industrial climate. Second, we compare the forest world to the dark desert world. In this case, forest and soil have about the same albedo which cancels the albedo effect of forest in snow-free regions. We relate the differences in radiative forcing and climate feedbacks between the two climate states to the differences we find in the bright-soil case.
Relative to the bright desert world, the forest world is 4.3 and
5.7
The largest component in net radiative forcing is the short-wave
clear-sky radiative forcing,
The second pronounced component in radiative forcing is the short-wave
cloud radiative effect,
The cloud masking effect is an artefact which results from separating
full-sky radiation into the clear-sky component and the cloud
component. In case a dense cloud cover occurs in the forest world and
in the desert world, full-sky short-wave radiative forcing will be
weak. Clear-sky radiation, however, will be strongly positive due to
the lower surface albedo in the forest world than in the desert
world. Deriving the cloud forcing from the weak full-sky radiative
forcing and the strongly positive clear-sky radiation reveals
a strongly negative cloud radiative forcing even though cloud cover
did not change. Our approach do not allow to disentangle cloud
adjustment and cloud masking. Nevertheless, we assume that
a considerable part of the
Feedbacks stabilise the early Eocene climate stronger than the
pre-industrial climate as the steeper slope of the regression line in
Fig.
We analyse the single components of the net feedback to identify the
reason for the different strength in net feedback in both climate
states. The short-wave clear-sky feedback parameter,
Largest differences in the feedback parameters appear in the
short-wave cloud feedback parameter,
To identify the reason for the different
The regression line in Fig.
We showed above that forest changes temperature by lowering surface
albedo and enhancing cloud cover. To isolate these two effects, we now
set the soil albedo in the desert world to 0.1 which is about the
albedo of forest. In other words, the albedo effect of forest is weak
in snow-free regions. Relative to the dark desert world, forests
reduce temperature by 4.2 and 3.0
The main contributor to the net radiative forcing is
In contrast to the bright-soil case, net feedback is equally strong in
the early Eocene climate as in the pre-industrial climate
(Table
Our results underline that the radiative forcing by changes in vegetation cover and the feedbacks in the climate system depend on the climate state. In the current interglacial climate, forest exerts a positive radiative forcing when soil has a much higher albedo than forest. The major effect is the surface albedo reduction by forest, which is partly offset by a negative cloud adjustment and masking effect. The positive net radiative forcing results in a warming and induces climate feedbacks: The lapse-rate and water-vapour feedback enhances warming on global scale; and the sea-ice albedo feedback amplifies warming in the northern high latitudes.
In the nearly ice-free, warm climate of the early Eocene, forests exert a similar radiative forcing, but climate feedbacks differ considerably. The sea-ice albedo feedback is weaker in the early Eocene climate than in current climate leading to a weaker warming in the northern high latitudes. The positive lapse-rate and water-vapour feedback is stronger than in current climate. Negative cloud-related feedbacks, however, are also stronger and outweigh the stronger positive lapse-rate and water-vapour feedback. In the sum, climate feedbacks stabilise the early Eocene climate stronger than the current climate.
Assuming that soils have an albedo close to the albedo of forest, the major radiative forcing is the cloud adjustment and masking effect which is equally strong in the early Eocene climate as in the pre-industrial climate. The resulting cooling, however, differs in both climates because feedbacks are differently strong. Like in the bright soil case, the positive lapse-rate and water-vapour feedback is stronger in the early Eocene climate, but this time cloud feedbacks and masking effects are of similar strenght in both climates. Hence, the net feedback stabilise the early Eocene climate less leading to a stronger cooling than in the current climate. Polar amplification is still weak in the early Eocene climate due to weak sea-ice related feedbacks.
In our study, plant functional types are considered to be the same for early Eocene climate and for pre-industrial climate. We assume that this simplification will affect the results of our study, at least in the qualitative sense, only little. We prescribe extreme land cover differences between completely forested and completely deserted continents. This difference is likely to cause much stronger effects than the difference in the physiology between current forests and early Eocene forests.
As climate has ever been changing through Earth's history, it is to be assumed that the impact of vegetation on climate is likely to change. Hence, attempts to infer potential changes in the future from processes that occurred in the past are challenging. Studies of past climates, however, help to understand the dynamics of the climate system which is a prerequisite for application of models to explore potential future changes.
We are grateful for comments and help by Bjoern Stevens and Thorsten Mauritsen. Further, we thank Veronika Gayler and Helmuth Haak for technical support. The anonymous reviewer improved the clarity of this paper. This work used computational recources by Deutsches Klima Rechenzentrum (DKRZ) and was supported by the Max Planck Gesellschaft (MPG). The article processing charges for this open-access publication have been covered by the Max Planck Society.
Boundary conditions in the early Eocene climate simulations and in the pre-industrial climate simulations.
Simulations performed with boundary conditions for the early Eocene climate and the pre-industrial climate. The listed vegetation cover is prescribed on all ice-free continents. The values for the land surface albedo refers to snow-free regions. In the desert world, the surface albedo equals the soil albedo. In the forest world, trees cover the soil completely and the albedo of the forest determines land surface albedo.
Net radiative forcing by afforesting a bright desert world in the early Eocene climate and the pre-industrial climate. Further, the net feedback parameter and the equilibrium temperature change are listed. The values are derived from the comparison of the respective forest world with the respective bright desert world. The 95 % confidence interval is given. The transient temperature change refers to the temperature difference averaged over the last 30
Net radiative forcing by afforesting a dark desert world in the early Eocene climate and in the pre-industrial climate. Further, the net feedback parameter and the equilibrium temperature change are listed. The values are derived from the comparison of the respective forest world with the respective dark desert world. The 95 % confidence interval is given. The transient temperature change refers to the temperature difference averaged over the last 30
Vegetation cover in the forest world for the pre-industrial
climate
Evolution of radiative flux at the top of the atmosphere with
temperature changes at the surface in the dark desert world of the
early Eocene climate. At the beginning of the simulation,
savanna-like vegetation is replaced by bare soil with an albedo of
0.1. The first 150 simulated years are shown. The black line is the
regression line fitted to the first 150
The evolution of differences in the TOA radiative flux
between the forest world and the bright desert world with
corresponding differences in near-surface temperature. Global
annual-mean values are considered. Red and blue points relate to
the early Eocene climate and to the pre-industrial climate,
respectively. Dark large points and bright small points show the
first 150 and the last 250
Net radiative forcing and its single components for the comparison of the forest world to the bright desert world. Hatched and plain bars show the radiative forcings for the pre-industrial climate and the early Eocene climate, respectively. The errorbars refer to the 95 % confidence interval.
Difference in cloud cover and planetary albedo between the
forest and the bright desert world averaged over the first year of
the simulations.
Net feedback parameter and its single components for the comparison of the forest world to the bright desert world. Hatched and plain bars show the feedback parameters for the pre-industrial climate and the early Eocene climate, respectively. The errorbars refer to the 95 % confidence interval.
Anomaly of zonal mean temperature change by forest. Global mean temperature difference between the forest world and the desert world are subtracted from differences in zonal mean temperature. Red line and blue line refer to the early Eocene climate and the pre-industrial climate, respectively.
Evolution of the difference in TOA short-wave cloud radiative
flux,
The evolution of differences in the TOA radiative flux
between the forest world and the dark desert world with
corresponding temperature differences. Global annual mean values are
considered. Red and blue points relate to the early Eocene climate
and to the pre-industrial climate, respectively. Dark large points
and bright small points show the first 150 and the last
250
Net radiative forcing and its single components for the comparison of the forest world to the dark desert world. The hatched and the plain bars show the radiative forcings for the pre-industrial climate and the early Eocene climate, respectively. The errorbars refer to the 95 % confidence interval.
Net feedback parameter and its single components for the comparison of the forest world to the dark desert world. The hatched and the plain bars show the feedback parameters for the pre-industrial climate and the early Eocene climate, respectively. The errorbars refer to the 95 % confidence interval.