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.
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
Using the GENESIS climate model with a mixed-layer ocean,
Here, we choose the early Eocene climate (54–52 Ma) which was a warm,
presumably nearly ice-free climate. Tropical temperatures were 5 to 6 K
higher than today
In their modelling studies,
To disentangle direct biogeophysical effects and triggered feedbacks, we use
the linear regression approach proposed by
For the early Eocene climate and the pre-industrial climate, we prescribe an
atmospheric CO
The MPI-ESM consists of the atmospheric general circulation model ECHAM6
The JSBACH model simulates fluxes of energy, water, momentum, and
CO
To simulate the early Eocene climate, we use the maps of orography and
bathymetry by
Boundary conditions for the early Eocene climate simulations and for pre-industrial conditions simulations.
Following
To achieve an initial equilibrium climate, we simulate the Eocene climate
starting from the equilibrium climate by
To evaluate the equilibrium Eocene climate, which we use as initial state for
all Eocene simulations, we compare the simulated climate against the marine
and terrestrial temperature reconstructions used by
Annual mean 2 m temperature for the early Eocene simulation in
shaded colours. Stars show reconstructed annual mean sea surface temperature (SST)
and near-surface temperature of the early Eocene derived from
The simulated annual mean temperatures show a good agreement with the
temperature estimates in the tropics and subtropics (Fig.
In the northern high latitudes, terrestrial temperature reconstructions are
5 to 11 K higher than the simulated temperature. Especially, the marine
temperature reconstruction by
In the southern high latitudes, the simulation agrees with marine and
terrestrial temperature estimates. The only exception is the SST
reconstruction by
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 refer 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 forests determines land surface albedo.
The cold bias in the high latitudes is a common problem when simulating the
early Eocene climate as
Starting from our early Eocene background climate, we perform three
simulations (Table 2). In the
Consistent with the Eocene simulations, we perform three simulations with
pre-industrial boundary conditions. All pre-industrial simulations start from
the equilibrium climate by
Vegetation cover in the forest world for the pre-industrial climate
The simulations – dark desert world, bright desert world, forest world – last
for 400 years. After that period, climate has approached, but not yet
reached, an equilibrium at the end of the simulations. To estimate the impact
of forests on climate from these (still unequilibrated) simulations, we
quantify the radiative forcing by forests and the subsequent climate
feedbacks by using the linear regression approach by
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 (TOA). During the simulation, the perturbation in
the TOA radiation balance,
From the model simulations, we calculate global annual mean values for
The linear regression further allows one to estimate the equilibrium temperature
change by deforestation. At the intersection of the regression line with the
The feedback parameter is evaluated from the slope of the regression line
which could change with time. For the first decades after a perturbation,
however,
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 fitted regression line.
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 net perturbation in the TOA radiation balance consists of a long-wave
component (LW) and a short-wave component (SW). Both components consist 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 radiative forcings and climate 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 years of our simulation. Each pair of our simulation can be selected several times. We repeat the resampling 10 000 times. From each time, we estimate the feedbacks and forcings. Then, we sort the resulting 10 000 values. Truncating the upper and lower 2.5 % provides the 95 % confidence interval.
Relative to the bright desert world, the forest world is 4.2 and 5.7 K
warmer at the end of the early Eocene simulations and at the end of the
pre-industrial simulations, respectively (Table
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. The first 150 years are shown and considered for the
regression and the correlation coefficient,
The largest component in net radiative forcing is the short-wave clear-sky
radiative forcing,
Net radiative forcing by afforesting a bright desert world for 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 using the linear regression approach (Sect.
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 error bars refer to the 95 % confidence interval.
Difference in cloud cover and planetary albedo between the forest
world and the bright desert world averaged over the first year of the
simulations. Differences in planetary albedo result from differences in
surface albedo and in cloud cover.
The second pronounced component in radiative forcing is the short-wave cloud
radiative effect,
Beside cloud adjustment,
Net radiative forcing by afforesting a dark desert world for the early
Eocene climate and for pre-industrial conditions. 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 using the linear regression approach (Sect.
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 error bars refer to the 95 % confidence interval.
Feedbacks stabilising the climate are stronger for the early Eocene climate
than for the pre-industrial case 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 strengths in net feedback in both climate states. The
short-wave clear-sky feedback parameter,
The largest differences in the feedback parameters appear in the short-wave
cloud feedback parameter,
To identify the reason for the different sign in
Over the continents,
A state-dependent cloud feedback is also suggested by
In the dark desert world, soils have about the same albedo as forests.
Nonetheless, forests reduce temperature by 4.2 and 3.0 K until the end of
the early Eocene simulations and until the end of the pre-industrial
simulations, respectively (Table
Zonal mean temperature difference between the forest world and the desert world. Red line and blue line refer to the early Eocene climate and the pre-industrial climate, respectively. The average over the last 30 years of the simulation is considered.
The individual feedbacks are of different strengths for the early Eocene
climate and for pre-industrial conditions (Fig.
Evolution of the difference in TOA short-wave cloud radiative flux,
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. The first 150 years are shown and considered for the regression
and the correlation coefficient,
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 error bars 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 error bars refer to the 95 % confidence interval.
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. To this end, we have chosen simulations of the early Eocene climate and the pre-industrial climate. We have separated the effect of forests in the radiative forcing and the feedbacks induced by implementing a forest cover on desert continents. To assess the sensitivity of our results to the soil albedo, we have assumed either a high soil albedo or a low soil albedo. When soils have a much higher albedo than the forest cover, we find a positive radiative forcing by forests in the current interglacial climate. Our analysis reveals that the major cause is the surface albedo reduction by forests which is partly offset by an increased cloud cover by forests leading to a higher planetary albedo. The positive net radiative forcing results in a global warming and induces climate feedbacks: the lapse-rate – water-vapour and cloud feedbacks enhance warming on a 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, we find a similar radiative forcing by forests as in the pre-industrial interglacial climate. Climate feedbacks, however, differ considerably. The sea-ice albedo feedback is weaker for the early Eocene climate than for the pre-industrial climate leading to a weaker warming by forests in the northern high latitudes. The positive lapse-rate and water-vapour feedback is stronger than for pre-industrial conditions. Negative cloud-related feedbacks, however, are also stronger than for pre-industrial conditions and nearly outweigh the stronger positive lapse-rate and water-vapour feedback. In total, climate feedbacks stabilising the global climate are stronger for the early Eocene climate than for the pre-industrial case, and forests warm the Eocene climate to a lesser degree than the pre-industrial climate.
If soils have an albedo close to the albedo of the forest cover, then our simulations suggest that forests cool the climates by enhancing cloud cover leading to an increased planetary albedo. Like in the bright soil case, the corresponding radiative forcing is equally strong for the early Eocene climate as for pre-industrial conditions, but the triggered climate feedbacks differ leading to a different amount of cooling: a stronger positive lapse-rate and water-vapour feedback for the early Eocene climate results in a stronger global cooling than in pre-industrial climate, and a much weaker snow and ice-albedo feedback leads to a weaker high-latitude cooling for the early Eocene climate than for pre-industrial conditions.
In the real world, the values of surface albedo are within the range of the values prescribed in our sensitivity study. In most regions the surface albedo is much closer to the low value of 0.1 than to the high value. Only in some desert regions with desiccated palaeo lakes, like the Bodélé depression in North Africa today, values are as high as 0.4. We assume that this is valid also for the Early Eocene climate. Therefore, we assume that our results for dark soil are applicable to the real world qualitatively albeit with a smaller amplitude of values.
Even though we have presented results from only one model, we assume that our
main conclusion – that radiative forcing varies little with the climate
state, while subsequent feedbacks depend on the climate state – is valid in
general. A weak snow/ice albedo feedback in an almost ice-free climate is
what we expect other models to reproduce. A stronger water-vapour feedback in
a warmer climate is consistent with previous studies using another model
In our study, plant functional types are considered to be the same for the early Eocene climate as for the pre-industrial climate. We assume that this simplification will only weakly affect the results of our study, at least in the qualitative sense. We prescribe extreme land cover differences between completely forested and completely deserted continents. This difference presumably causes much stronger effects than the difference in the physiology between current forests and early Eocene forests. The topic of changing plant functional types with climate will be subject to further studies.
We are grateful for comments by Bjorn Stevens and Thorsten Mauritsen, and we thank Veronika Gayler and Helmuth Haak for technical support. Comments by two anonymous reviewers which improved this paper are greatly appreciated. This work used computational resources by Deutsches Klima Rechenzentrum (DKRZ) and was supported by the Max Planck Gesellschaft (MPG). The article processing charges for this open-access publication were covered by the Max Planck Society. Edited by: M. Crucifix