ESDEarth System DynamicsESDEarth Syst. Dynam.2190-4987Copernicus PublicationsGöttingen, Germany10.5194/esd-9-739-2018Two drastically different climate states on an Earth-like terra-planetTwo drastically different climate states on an Earth-like terra-planetKalidindiSirishasirisha.kalidindi@mpimet.mpg.deReickChristian H.RaddatzThomasClaussenMartinhttps://orcid.org/0000-0001-6225-5488Max Planck Institute for Meteorology, Bundesstraße 53, 20146 Hamburg, GermanyInternational Max Planck Research School on Earth System Modelling, Bundesstraße 53, 20146 Hamburg, GermanyCenter for Earth System Research and Sustainability, Universität Hamburg, Bundesstraße 53, 20146 Hamburg, GermanySirisha Kalidindi (sirisha.kalidindi@mpimet.mpg.de)7June20189273975615September201710October201730April20183May2018This work is licensed under the Creative Commons Attribution 4.0 International License. To view a copy of this licence, visit https://creativecommons.org/licenses/by/4.0/This article is available from https://esd.copernicus.org/articles/9/739/2018/esd-9-739-2018.htmlThe full text article is available as a PDF file from https://esd.copernicus.org/articles/9/739/2018/esd-9-739-2018.pdf
We study an Earth-like terra-planet (water-limited terrestrial planet) with
an overland recycling mechanism bringing fresh water back from the high
latitudes to the low latitudes. By performing model simulations for such a
planet we find two drastically different climate states for the same set of
boundary conditions and parameter values: a cold and wet (CW) state with
dominant low-latitude precipitation and a hot and dry (HD) state with only
high-latitude precipitation. We notice that for perpetual equinox conditions,
both climate states are stable below a certain threshold value of background
soil albedo while above the threshold only the CW state is stable. Starting
from the HD state and increasing background soil albedo above the threshold
causes an abrupt shift from the HD state to the CW state resulting in a
sudden cooling of about 35 ∘C globally, which is of the order of the
temperature difference between present day and the Snowball Earth state.
When albedo starting from the CW state is reduced down to zero the
terra-planet does not shift back to the HD state (no closed hysteresis). This
is due to the high cloud cover in the CW state hiding the surface from solar
irradiation so that surface albedo has only a minor effect on the top of the
atmosphere radiation balance. Additional simulations with present-day Earth's
obliquity all lead to the CW state, suggesting a similar abrupt transition
from the HD state to the CW state when increasing obliquity from zero. Our
study also has implications for the habitability of Earth-like terra-planets.
At the inner edge of the habitable zone, the higher cloud cover in the CW
state cools the planet and may prevent the onset of a runaway greenhouse
state. At the outer edge, the resupply of water at low latitudes stabilizes
the greenhouse effect and keeps the planet in the HD state and may prevent
water from getting trapped at high latitudes in frozen form. Overall, the
existence of bistability in the presence of an overland recycling mechanism
hints at the possibility of a wider habitable zone for Earth-like
terra-planets at low obliquities.
Introduction
Recent advancements in observational astrophysics like with the Kepler
mission led to the discovery of a vast number of potentially habitable
planets (Kopparapu et al., 2014). Habitable planets are planets which can
maintain liquid water on their surface (Hart, 1978; Kasting et al., 1993;
Kopparapu et al., 2014). The habitability of a planet is influenced by
several factors like stellar flux, orbit, and planetary properties
(Schulze-Makuch et al., 2011). In the case of Earth-like planets, it has
been shown that the width of the habitable zone strongly depends on the
water cycle and the carbonate–silicate cycle (Kasting et al., 1993;
Kopparapu et al., 2014; Zsom et al., 2013). This is because these cycles
control the atmospheric concentration of water vapour, carbon dioxide, and
surface pressure of a planet. In our study, we focus on the role of the water cycle.
The water cycle on a planet strongly depends on the amount of surface water
present, which is controlled by a complex interplay among processes like
atmospheric circulation, precipitation, and cloud formation. Depending on the
amount of surface water, habitable planets fall into two classes:
aqua-planets – planets covered with global oceans – and terra-planets – planets
with vast deserts or vegetated surfaces and a limited amount of
water (Herbert, 1965; Abe et al., 2005, 2011; Leconte et al., 2013).
In the present study, we investigate the climate of an Earth-like
terra-planet for low obliquities.
Over recent years, there has been a growing interest in the study of
terra-planets due to three reasons: first, the absence of oceans on a
terra-planet helps to isolate the effects of land surface processes and thus
aids in better understanding of the land–atmosphere coupling (Aleina et al.,
2013; Rochetin et al., 2014; Becker and Stevens, 2014). Second,
terra-planets with optically thin atmospheres (like present-day Earth's
atmosphere) can maintain their inner edge of the habitable zone much closer
to their parent star compared to aqua-planets (Zsom et al., 2013). The
reason is that limited atmospheric access to water results in a dry climate
with water confined to the high latitudes for low obliquities (Abe et al.,
2005; Abe et al., 2011; Leconte et al., 2013). In such dry climates, the
water vapour feedback is severely muted and the greenhouse warming is
substantially lowered, which allows dry planets to maintain habitability even
at higher stellar fluxes (Zsom et al., 2013). Third, terra-planets at low
obliquities can support a wider habitable zone compared to that of
aqua-planets (Abe et al., 2011) because the dry atmosphere of terra-planets
limits the escape of hydrogen molecules and shows a higher resistance to the
runaway greenhouse effect. Also, the dry atmosphere inhibits the formation
of clouds, ice, and snow and thus helps the planet to resist complete
freezing (Abe et al., 2011; Leconte et al., 2013). However, the habitable
areas on such a dry planet are confined to the edges and bottom of frozen ice
caps (Leconte et al., 2013; Zsom et al., 2013). Whether at such edges liquid
water can exist sufficiently permanently for life to evolve and persist is
unclear (Zsom et al., 2013). Recycling mechanisms similar to those which
occur on the present-day Earth like ocean circulation and surface run-off
must exist to maintain a long-lasting liquid water inventory. Leconte et
al. (2013) argue that on dry planets, mechanisms like gravity-driven ice
flows and geothermal flux can maintain sufficient amounts of liquid water at
the edges and bottom of large ice caps. The liquid water thus formed can
eventually flow back to the low latitudes to be re-available for
evaporation. However, there is no climate modelling study on an
Earth-like terra-planet which actually implements either implicitly or
explicitly such a recycling mechanism bringing fresh water back from high to
low latitudes. In our study, we consider an Earth-like terra-planet with an
unlimited subsurface water reservoir as a way to mimic the recycling
mechanism. Even though the water reservoir in our case is unlimited, it is
not similar to an aqua-planet because it includes additional resistances
which restrict atmospheric access to water (i.e. soil and plant resistances
along with aerodynamic resistance).
Evidence from palaeoclimate modelling studies reveals that planets within
the habitable zone can support multiple climate states but all of these
states may not satisfy the stable liquid water on their surface. For example, our
Earth can exist in two different climate states – the present-day warm
state with abundant surface liquid water and the Snowball Earth state with
no surface liquid water (Marotzke and Botzet, 2007; Voigt and Marotzke,
2010; Voigt et al., 2011). Recent studies on the habitability of
aqua-planets also indicate that the presence of multistability can further
complicate the interpretation of habitability of Earth-like planets
(Linsenmeier et al., 2015; Boschi et al., 2013; Lucarini et al., 2013). With
our study we demonstrate that under certain conditions Earth-like
terra-planets exhibit two drastically different climate states and both
these climate states can support habitable areas with long-lived surface liquid water.
It should be noted that the present paper is mainly descriptive in nature
and is not meant to give a detailed explanation of the mechanisms leading to
the emergence of the two climate states and why transition happens between
them. This is still under investigation. The paper is organised as follows:
Sect. 2 describes our model and our terra-planet configuration and gives an
overview on the simulations performed for this study. Section 3 discusses the
two drastically different terra-planet climate states at perpetual equinox
conditions. Section 4 describes the transition between the two climate states.
In Sect. 5 hysteretic behaviour is discussed. Section 6 explores the role of
the snow albedo feedback for the emergence of the different climate states.
Section 7 is about the terra-planet climate at present-day obliquity. Section 8
draws some general conclusions from our study, in particular for the
habitability of terra-planets.
Model and simulation set-upModel
We use the ICOsahedral Non-hydrostatic (ICON) general circulation model
jointly developed by the MPI for Meteorology and the German Weather Service (DWD)
and run it in terra-planet configuration, i.e. with a single globally
extended continent. The model has a horizontal resolution of R2B04
equivalent to a resolution of an evenly distributed rectangular grid of
about ∼ 160 km and 47 layers in the vertical. The atmosphere
model uses a non-hydrostatic dynamical core on an icosahedral-triangular
Arakawa C grid (Zängl et al., 2015), and the model atmospheric physics is
similar to ECHAM6 physics (Stevens et al., 2013). The radiative transfer
calculations are based on the Rapid Radiative Transfer Model (Mlawer et al.,
1997; Iacono et al., 2008). Convection is parameterized by the mass flux
scheme of Tiedtke (1989) with modifications to penetrative convection by
Nordeng (1994). Cloud cover is calculated based on relative humidity
(Lohmann and Roeckner, 1996). For a complete description of the model
physics and parameterizations, the reader is referred to Stevens et al. (2013).
The ICON version used in this study inherits the land physics from
ECHAM5 (Roeckner et al., 2004) extended by a layered soil hydrology
(Hagemann and Stacke, 2015).
Terra-planet configuration
The terra-planet configuration is designed to be highly symmetric with no
orography and glaciers. The rotation rate and the solar constant are the
same as for the present-day Earth. We consider two situations: perpetual
equinox (zero obliquity) and present-day Earth's obliquity (23.5∘).
Background concentrations of CH4, N2O, and aerosols are fixed to
zero, while water vapour is prognostic and CO2 concentration in the
atmosphere is fixed to 348 ppmv. The ozone distribution is assumed to be
zonally uniform and meridionally symmetric.
Land surface properties are assumed to be homogenous: the total soil depth
in our study is about 10 m (metres), of which the layers below a depth of
1.2 m are forced to be homogenously
filled permanently by at least 90 % with water
all over the globe. We refer to these bottom two layers of the
soil in our study as “subsurface reservoir”, which should not be confused
with a geological reservoir operating at timescales much longer than the
soil hydrological timescales. The root depth is fixed to 6 m such that the
roots always have access to this subsurface reservoir. Leaf area index (LAI)
is set to a value of 3. This value controls how much surface in a grid cell
participates in transpiration, while the rest exhibits bare soil
evaporation. LAI and root depth are not considered to be a representation of
vegetation but only as a technical means to parameterize the atmospheric
access to water like other hydrological parameters of the model, e.g. soil
porosity or hydraulic conductivity. The albedo of snow ranges between 0.4 and 0.8
depending on the surface temperature. Surface roughness is fixed to 0.05 m.
By the introduction of the subsurface reservoir we implicitly equip our
planet with a very efficient recycling mechanism shuffling water back from
sink regions (P-E> 0) to source regions (P-E< 0). This
can be understood as follows. In sink regions water either piles up as snow
or is lost as run-off. But since neither snow height nor run-off affect the
climate in our simulations, the global amount of water relevant for the
physical climate stays constant for a stationary state. Accordingly,
considering only this “effective” water, the amount of water added to the
subsurface reservoir equals the water lost in the sink regions. For this
reason, we can interpret the permanent refilling of the subsurface reservoir
to mimic a very efficient recycling of water from sink to source regions.
Representing overland water recycling with a homogenously filled subsurface
reservoir is indeed an idealization. In fact, recycling may occur at
different speeds and is thereby less or more effective than what we consider
in our study. Based on the speed of recycling, the water level of this
subsurface reservoir would vary from what is considered in our study. It
should be noted that the choice of water level of the subsurface reservoir
considered in our study (1.2 m) is not arbitrary, but a result of a sequence
of simulations, in which we explored the continuum between an aqua-planet and a terra-planet (see Appendix A).
Our terra-planet set-up closely resembles that of recent 3-dimensional
studies (Abe et al., 2011; Leconte et al., 2013) on Earth-like terra-planets
except that those studies did not consider an overland water recycling
pathway bringing back water from high to low latitudes that we mimic by the
prescribed subsurface water reservoir.
Simulations
We study the effect of background surface albedo (α) on the climate
in a series of simulations with α varying from 0.02 to 0.24 at
perpetual equinox (0∘) (Z2 to Z24 simulations in
Table 1) and at present-day conditions (23.5∘) (S7 to S24 simulations
in Table 1). All these simulations start from the same initial
atmospheric state with a homogenous temperature (290 K) and moisture content
(25 kg m-2). Due to the small thermal inertia of the land
surface and the atmosphere, the simulations reach a steady state within
10 years. We continue the simulations for additional 30 years, of which the
last 10 years are used in the analysis of the mean climate. To see the
transition between different climate states at the perpetual equinox
condition more clearly, we perform an additional simulation (TRANS). For
this, we first simulate the planet in the stable hot and dry (HD) state for
30 years for α= 0.14 and then increase α abruptly to
α= 0.14 + 0.01 corresponding to the cold and wet (CW) state and
continue the simulation for another 30 years. Then, to check for hysteresis,
we continue the TRANS simulation by switching the albedo back to 0.14 and
continue lowering α stepwise until zero. Additionally, to test the
sensitivity of the climate states to model parameterizations, we performed
simulations with a different convection scheme at perpetual equinox
conditions (T7 to T24 simulations). Finally, to investigate the role of
snow albedo feedback on the terra-planet climate states, the terra-planet is
simulated with dark snow (DS simulations in Table 1) (i.e. we assume snow
albedo to be same as background surface albedo). The details of all
terra-planet simulations are listed in Table 1.
Time series of global mean surface temperature (∘C) and
precipitation (mm day-1) for different background soil albedo values in
the Z7–Z24 simulations.
Annual mean meridional profiles of (a) surface temperature,
(b) precipitation, (c) precipitable water, (d) soil
moisture (averaged over the top three layers), (e) cloud fraction, and
(f) planetary albedo for the two terra-planet states: HD (α= 0.14)
and CW (α= 0.15) in the simulations Z14 and Z15 averaged over a
period of 10 years.
Drastically different climate states
Figure 1 shows the time evolution of global mean surface temperature and
precipitation for different terra-planet simulations at zero obliquity
(Z7 to Z24 simulations). We notice that the terra-planet exists in two
drastically different climate states: HD state for
α< 0.15 and a CW state for α≥ 0.15. This
is different from findings in previous studies (Abe et al., 2005, 2011;
Leconte et al., 2013) on low-obliquity terra-planets in which only the HD
state was found. The mean climate in the two states is remarkably different (Fig. 2).
Surface temperature
The annual mean surface temperature in the CW state is below freezing point
almost everywhere, except in the low latitudes where it is around
10 ∘C. High cloud cover (Fig. 2e) and a high planetary albedo
(Fig. 2f) lower the surface absorption of incoming radiation and result in
very low temperatures in the CW state. However, in the HD state,
the global mean surface temperature is about 35 ∘C higher than in
the CW state.
Precipitation
In addition to temperature, the other striking difference between the two states is
the location of precipitation bands on the planet. In the HD state, no
precipitation occurs in the low-latitude region between 40∘ S and
40∘ N. Only latitudes higher than 40∘ receive some
amount of rainfall (Fig. 2b) (compare Abe et al., 2005, 2011; Leconte et
al., 2013). In contrast, in the CW state, precipitation is mainly
concentrated in the low latitudes with a banded structure similar to the
present-day equatorial Inter-Tropical Convergence Zone (ITCZ). The reason
for the absence of a CW state in previous studies is the lack of an effective
mechanism that can recycle the water trapped at the high latitudes in the
form of snow and ice back to the low latitudes.
Vertical profiles of cloud liquid water content (g kg-1) and
cloud area fraction averaged over the low-latitude region (30∘ S–30∘ N)
for the two terra-planet states in the simulations Z14 and Z15.
Feedbacks that keep the HD state dry and the CW state wet
Figure 2c and d show the distribution of water on the planet. In the HD state,
the very high temperatures in the low latitudes raise the water
vapour saturation limit and the moisture-holding capacity of the atmosphere,
allowing the planet to store a substantial amount of its water in the
atmosphere (Fig. 2c). In such an atmosphere, rain occurs in the form of
virga rain, i.e. almost all of this rain evaporates on its way to the
surface. Therefore, in such a case, rain does not contribute to the
moistening of the soils in the low latitudes. This, along with huge amounts
of net radiation at the surface in the HD state, keeps the uppermost soil
layers in the low latitudes very dry. Dry uppermost soil layers imply small
evaporation, leading to no precipitation, which in turn leads to even less
evaporation. This self-reinforcing mechanism in the HD state always
maintains very dry upper soil layers in the low latitudes (Fig. 2d). On the
whole, suppressed precipitation at low latitudes along with a nevertheless
permanent export of moisture away from the low latitudes result in water being mainly present at high latitudes in the HD state. Conversely, in
the CW state, the lower annual mean temperatures in the low latitudes
facilitate condensation and precipitation and minimize the moisture content
of the atmosphere (Fig. 2c). Evaporation of falling rain indeed also occurs
in the CW state but is not strong enough to prevent the rain from reaching
the surface. Thus, continuous precipitation at low latitudes in the CW state
keeps the upper soil layers in the low latitudes always wet (Fig. 2d)
thus providing sufficient water by evaporation for a stable precipitation
regime in the low latitudes.
Feedbacks that keep the HD state hot and CW state cold
The vertical distribution of cloud cover and cloud water content in the low
latitudes for the two terra-planet states is displayed in Fig. 3. For the
HD state, the cloud cover in the low latitudes is exclusively composed of
high level clouds (Fig. 3). The reason is that the higher water vapour
saturation limit of the atmosphere in the HD state raises the height at
which condensation and cloud formation occur. High clouds with very low
liquid water content are more transparent to shortwave radiation; at the
same time they reduce outgoing longwave radiation and thereby warm the
planet. Moreover, hotter temperatures in the HD state lead to a moister
atmosphere (Fig. 2c) and in turn stronger greenhouse warming. Overall, high
clouds in the low latitudes along with higher water vapour greenhouse
warming keep the planet always hot and stabilize the HD state. Instead, in
the CW state, the cloud cover in the low latitudes is mainly comprised of
low-level clouds (Fig. 3) due to a lower water vapour saturation limit of the
atmosphere. Low clouds with high liquid water content cool the planet as
they increase the planetary albedo. Also, lower temperatures in the CW state
lead to a drier atmosphere (Fig. 2c) and a weaker water vapour greenhouse
warming. On the whole, low clouds together with weaker greenhouse warming
always keep the planet cool and stabilize the CW state.
Annual mean behaviour of (a) and (b) meridional stream
function in 1010 kg s-1; (c) vertical profiles of meridional
transport of water at 10∘ N for the two terra-planet states in the
simulations Z14 and Z15 averaged over a period of 10 years. The black arrows
in (a) and (b) denote the northern Hadley circulation centre.
Annual mean behaviour of northward transport of total, latent, and
sensible energy in petawatts (PW) for the two terra-planet states in the
simulations Z14 and Z15 averaged over a period of 10 years.
Mean circulation and energy transport
The mean circulation pattern for the two climate states is shown in Fig. 4a
and b. We notice that in both the states, the circulation pattern resembles the
present-day three-cell hemispheric structure. But when comparing the two
states, the width and intensity of the circulation are very different: in
the HD state, the Hadley cell is more vigorous and becomes slightly wider with
height (when measured around 500 hPa) compared to that in the CW state. The
neutrally stable atmospheric conditions found in the HD state require a
larger mass flux to transport away heat and to stabilize the equatorial
temperatures and hence support a more vigorous circulation (Held and Hou,
1980; Caballero et al., 2008; Mitchell, 2008). Additionally, we notice
that the circulation centre of the Hadley cell is very different in the two
states (depicted by black arrows in Fig. 4a and b). The HD state has its
circulation centre much closer to the surface at about 750 hPa, while in the
CW state the centre is much higher at 500 hPa.
Observations and modelling studies in the literature also report intense low-centred circulations for present-day Earth's climate in the tropical eastern
pacific during the northern hemispheric summertime (Zhang et al., 2004, 2008;
Nolan et al., 2007), for a Snowball Earth state (Pierrehumbert, 2005) and
for Titan (Mitchell et al., 2006, 2009). These circulations have the
tendency to export larger amounts of moisture out of the low latitudes
compared to high-centred circulations. We also notice this increased
transport of moisture away from the low latitudes with the more vigorous low-centred Hadley cell in the HD state compared to that in the CW state
(Fig. 4c). The reason is that in the CW state the existence of deep convection allows
moisture to reach much higher elevations before being exported to high
latitudes (Nolan et al., 2007). At higher elevations, more moisture can
condense and be lost as precipitation due to low temperatures and thus much
less water remains for being exported away. Instead, in the HD state the
lack of precipitation allows more moisture to be exported.
Figure 5 shows the northward transport of energy (transport due to latent
and sensible energy) by atmospheric circulation in the two climate states.
For the CW state, latent energy transport is equatorward in the low
latitudes and poleward in the mid-latitudes. In contrast, in the HD state
latent energy is exported poleward at all latitudes (latent energy curves in
Fig. 5). The reason for the opposite sign in the low latitudes is that in
the CW state (as in the case of present-day Earth) intense precipitation
dries out the atmosphere and thus keeps most of the water within the low
latitudes, limiting its poleward export (Fig. 4c) – hence at these latitudes
the net flow is always equatorward. Instead, in the HD state, by the absence
of precipitation, moisture is retained in the air so that it is exported poleward.
The sensible energy transport in the HD state is considerably larger as
compared to that in the CW state (Fig. 5) despite a lower equator-to-pole
temperature gradient. This implies that the atmospheric circulation is more
efficient in transporting the sensible energy to the high latitudes.
Nevertheless, the net northward transport of energy is dominated by the
sensible energy transport. Further, the larger northward transport of energy
in the HD state results in a smaller equator-to-pole temperature gradient
compared to the CW state.
Transition to the CW state
Starting from the HD state (α= 0.14), we abruptly increase the
albedo to α= 0.14 + 0.01 (simulation TRANS) and thereby
initiate a shift to the CW state. The full transition from the HD state to
the CW state takes about 4 years. The changes in annual mean surface
temperature, precipitation, snow cover, and mean circulation during the
course of transition are shown in Fig. 6. One can distinguish three transitional
stages.
Stage 1. After the abrupt increase in α, small precipitation
clusters appear in the low-latitude region and the terra-planet starts to
cool down due to an increase in cloud cover and planetary albedo. Snow cover
at high latitudes starts to increase slowly. The mean meridional circulation
structure changes – small circulation cells appear very close to the
equator. This happens for a period of 1 year.
Stage 2. The precipitation clusters aggregate into a band of deep convection
around the equator accompanied by a sharp increase in precipitation. At this
point, we still see precipitation bands even at high latitudes around
50∘. However, the precipitation intensity at high latitudes is
smaller (around 6 mm day-1 lower) compared to the precipitation at the
equator. Surface temperature drastically decreases and the circulation
structure is now associated with two cells, a shallow cell close to the
equator and a deep cell slightly away from the equator.
Stage 3. Precipitation bands at the high latitudes start moving equatorward
and precipitation intensity decreases compared to the previous stage.
Surface temperature decreases further. Snow cover further increases and
starts moving towards the equator. The circulation is now less intense with a
circulation centre around 500 hpa.
Finally, the CW state is reached. Precipitation only occurs at low
latitudes. Surface temperature is below freezing almost everywhere on the
planet except at the low latitudes. Snow cover reaches down to 40∘ latitude.
Hysteresis
To further study the bifurcation structure, we investigated the hysteretic
behaviour. Figure 7 shows the global mean surface temperature plotted as a
function of α. The spontaneous transition from the HD to the CW state
and the associated abrupt cooling is seen as path 1. Starting from the
CW state and lowering back α stepwise below the threshold value
until zero does not lead back to the HD state. The reason is that the high
cloud cover present in the CW state hides the surface from solar
irradiation. Therefore, a reduction of α has only a minor effect on
the top-of-the-atmosphere radiation balance so that thereby it is impossible
to heat the planet sufficiently strongly to switch back to the HD state.
This is true even when repeating these simulations with snow albedo equal
to background soil albedo (DS experiments; no figure shown). Thus, the
planet remains in the CW state, indicating that under the chosen conditions
for the terra-planet set-up the hysteresis is not closed (Fig. 7).
Does snow albedo feedback play a role in the emergence of the two climate states?
Changes in snow cover can lead to multiple climate states in the Earth
system with drastically different global mean surface temperatures like
present-day Earth and Snowball Earth (Budyko, 1969; Sellers, 1969). In
this case, the large temperature difference between the two states is caused
by the positive snow albedo feedback. In our study, we also notice such a huge
difference in global mean surface temperature between the two terra-planet
climate states (Sect. 3). In order to test whether the snow albedo feedback
is responsible for the huge temperature difference in our study, we
performed additional simulations in which we changed the snow albedo to be
equal to the background albedo of soil (DS simulations in Table 1). With the
darker snow, we still find the two climate states (Fig. 8) and the
spontaneous transition between them. The existence of the bifurcation even
in the simulations with dark snow implies that the snow albedo feedback is
not the cause of the existence of the two states. However, the snow albedo
feedback does enhance the drastic temperature change in the bright snow
simulations by around 12 ∘C compared to the dark snow simulations.
Preliminary analysis indicates that a combination of cloud and hydrological
feedbacks leads to the bistability (refer to Sect. 3.3 and 3.4).
Terra-planet with seasonality
So far, we considered a planet without seasonality because obliquity was set
to zero. Next, we investigate the climate on a terra-planet changing with a
seasonal orbit taking present-day Earth's obliquity of 23.5∘
(S simulations in Table 1). We find that with seasonality the HD state is
absent and the terra-planet always stays in the CW state (Fig. 9). This is
contrary to previous studies on terra-planets, which show that terra-planets
always exist in a warm state for obliquities lower than 30∘ (Abe
et al., 2005, 2011). We hypothesize that for Earth-like obliquity, the
bistability is lost due to the seasonal migration of the rain bands towards
the low latitudes. The seasonal migration of rain bands facilitates seasonal
rain in the dry low-latitude region. Since soil moisture has a memory
lasting for several weeks to months, this causes the top soil layers in the
low latitudes to remain wet even during the dry season. Thus there is always
soil moisture in the originally dry low-latitude region to allow for
continuous evaporation and precipitation. This probably destroys the HD
state so that the planet is always self stabilized in the CW state. A more
detailed study on the reasons behind the loss of bistability for non-zero
obliquities is ongoing but beyond the scope of the present paper.
Annual mean surface temperature (a), precipitation (b),
snow cover fraction (c), and meridional stream function (d)
for different stages of the transition (TRANS simulation) from the HD state to
the CW state.
Global mean surface temperature as a function of background soil
albedo (α). Simulated HD states are denoted by points a–d and
simulated CW states by e–j. Path 1 denotes the spontaneous transition from HD
to CW when increasing α, and paths 2–4 denote the reverse state development
upon stepwise lowering of background albedo starting from the threshold value
until zero. Obviously, the hysteresis does not close for the terra-planet at
zero obliquity considered here.
Annual mean meridional profiles as in Fig. 2 but with dark snow (snow
albedo is the same as background soil albedo – DS simulations).
Annual mean meridional profiles as in Fig. 2. The red lines represent
the HD state as in Fig. 2 (Z14 simulation in Table 1 with an obliquity of
0∘). The purple lines correspond to the simulation with same initial
conditions as in the HD state in Fig. 2 but with an obliquity of 23.5∘
(S14 simulation in Table 1).
Discussion and conclusions
So far terra-planets have been investigated for a wide range of planetary
properties like mass, rotation rate, atmospheric composition, and orbit (Abe
et al., 2005, 2011; Leconte et al., 2013). Here, we focus on climate
simulations of a terra-planet with low obliquity (< 30∘),
flat orography, and otherwise Earth-like conditions. An important difference
from previous studies concerns the treatment of atmospheric access to water.
Past studies on possible climates of terra-planets prescribed a limited
water inventory, which for low obliquities leads to trapping of the water at
high latitudes, stabilizing the planet in a state with no precipitation at
low latitudes, similar to what we call the hot and dry state in our study. By
contrast, in the present study we assume an unlimited subsurface water
reservoir, which is still different from an aqua-planet configuration
(also with an unlimited water reservoir), because resistances between soil
and atmosphere restrict the atmospheric access to soil water in addition to
the restriction from aerodynamic resistance. For such an Earth-like
terra-planet with restricted water access we find two drastically different
climate states, a HD state characterized by a hot climate with
precipitation confined to high latitudes and a CW state which
is closer to present-day Earth's climate with precipitation mainly occurring
at low latitudes and an intense cycling of water there. Compared to the
other studies mentioned above, we only find this additional CW state
because by prescribing the subsurface water reservoir we implicitly assume a mechanism restoring water from the high latitudes to
low latitudes
for our terra-planet, refilling the subsurface water reservoir sufficiently
effectively to maintain the very active low-latitude water cycle in this
state. The difference in global mean temperature between these two climate
states is 35 ∘C (with the same boundary conditions), which is of
the same order of magnitude as the temperature difference between present-day and Snowball Earth climate (Pierrehumbert et al., 2011; Micheels and
Montenari, 2008; Fairchild and Kennedy, 2007; Hoffman and Schrag, 2002).
Similar to the abrupt transition to a Snowball Earth state, for perpetual
equinox conditions our terra-planet simulations also show an abrupt
transition, namely from the HD state to the CW state. These two states exist
for low background surface albedo α, while for high α only
the CW state is possible. The abrupt transition occurs when in the HD state
α is increased beyond a particular threshold value. While the
transition to the Snowball Earth is driven by the snow albedo feedback,
preliminary analysis indicates that in our study the transition is triggered
by a reorganization of the hydrological cycle and amplified by cloud
feedbacks. Moreover, we notice that in our set-up the terra-planet exhibits
an open hysteresis: even with background surface albedo reduced to zero it
does not return
to the HD state. Additional terra-planet simulations
with an obliquity like the real Earth all result in a CW state, hinting at
another bifurcation from the HD to the CW state when the obliquity is
increased to non-zero values.
Concerning the global water cycle, the HD and CW states share one important
similarity, namely a strong atmospheric moisture flux from low to high
latitudes. The planetary boundary layer at low latitudes is extremely dry in
the HD state with relative humidity of about 15 %. Clearly, trade winds
can maintain such a dry boundary layer only if the water supply at the
surface is sufficiently limited. However, without any evapotranspiration at
the surface the Hadley cell would dry out completely, losing the greenhouse
effect that sustains the high temperatures of the HD state. Furthermore, in
the CW state, to keep the rain along the equator one also needs a
considerable water supply in the low latitudes. In summary, both climate
states are associated with a strong atmospheric transport of water from low
to high latitudes, which has to be balanced by evapotranspiration at the low
latitudes. Therefore, allowing for both states to be potentially realized
under the same boundary conditions, on a real planet, mechanisms must exist
that can continuously restore water back to low latitudes. For present-day
Earth this happens via the oceans. For our terra-planet water is stored in
frozen form at high latitudes like in the past glacial states of our Earth.
The resupply of water may happen via processes like melting of glaciers,
transport of ice by gravity flows, and melting at the bottom of large ice
caps due to high pressure and geothermal heat flux. This provides liquid
water, which may be brought back to the low latitudes by rivers (Abe et al.,
2011; Leconte et al., 2013). Note that the huge differences in climate
between the two states is primarily a consequence of the completely
different functioning of the global hydrological cycle, so that the assumed
recycling mechanism can be considered as an additional degree of freedom to
the internal dynamics, extending the range of possible terra-planet climates.
Our findings may have some relevance for estimates of the habitable zone for
such Earth-like terra-planets. In the HD state liquid water is confined to
the mid-latitudes (40–50∘) in both hemispheres,
whereas in the CW state, there is sufficient precipitation and high enough
temperatures for permanent liquid water at low latitudes (40∘ S–40∘ N). Thus in both climate states life can potentially persist.
At the outer edge of the habitable zone, the assumed resupply of water from
high to low latitudes stabilizes the greenhouse effect, keeps the planet in
the HD state, and may prevent a situation with all water accumulated at the
high latitudes in the frozen form. At the inner edge of the habitable zone,
by this resupply the planet can maintain precipitation and high cloud cover
at the equator in the CW state. Thereby, the planetary albedo is increased,
which cools the planet and may prevent the runaway greenhouse state with all
the water well mixed in the atmosphere in the gas phase. On the whole, our
study thus suggests that the presence of a mechanism which recycles water
from the high latitudes back to the low latitudes results, as described, in
the two drastically different climate states and may extend the habitable
zone of Earth-like terra-planets at low obliquities.
The ICON model source code and the simulations data used in
the analysis are available on request from the authors.
Sequence of simulations that led to the terra-planet bistability
The bistability found in this study should not be mistaken as a result of a
random experiment or a bug in the land surface model. The bistability was
found by systematically modifying our global climate model starting from a
“swamp simulation” until a “terra-planet configuration”. The respective
simulations showed climate states that were plausible from the configuration
changes. In the swamp configuration our model is able to successfully
reproduce the climate of an aqua-planet. Starting from this set-up, we
sequentially changed different parameters like surface heat capacity,
surface roughness, background soil albedo, snow albedo, and the level of the
global subsurface water reservoir as well as the land surface schemes (soil
hydrology) in our land surface model (Table A1). The bistability emerged
when the water level of the subsurface reservoir was lowered to 1.2 m. In
Table A1 the sequence of simulations is listed and in Fig. A1 we show the
resulting time evolution of temperature and the zonal structure of precipitation.
Summary of simulations performed to illustrate the procedure by which
we found bistability on our terra-planet.
SimulationsBackgroundSurfaceHeatSnowWater reservoir depthsoilroughnesscapacityalbedoalbedoAqua-planet0.07OceanOcean0.0750 m slab ocean, no heat transportSwamp10.07OceanOcean0.07Constant ground water table at a depth of 0.3 mSwamp20.07LandLand0.07Constant ground water table at a depth of 0.3 mHD0.07–0.14LandLandDynamic (0.4–0.8)Constant ground water table at a depth of 1.2 mCW0.15–0.24LandLandDynamic (0.4–0.8)Constant ground water table at a depth of 1.2 m
(a) Time series of global mean surface temperature (∘C)
and (b) annual mean meridional profile of precipitation (mm day-1)
for different simulations performed to illustrate the procedure by which we
found bistability on our terra-planet.
Sensitivity of the climate states to model convection scheme
To confirm that the two climate states of the terra-planet are not an
artifact of convective parameterizations, we performed an additional
simulation with a different convection scheme. By default, the model uses
the Nordeng convection scheme (Nordeng, 1994). We have replaced the default
settings and simulated terra-planet simulations with the Tiedtke convection
scheme (Tiedtke, 1989) and we find the two drastically different climate
states irrespective of the convection scheme (Fig. B1).
Annual mean meridional profiles as in Fig. 2 but with Tiedtke convection
scheme plotted using T simulations in Table 1.
The authors declare that they have no conflict of interest.
Acknowledgements
We thank the reviewers for their valuable input to the
paper. We also thank Reiner Schnur, MPI-M, for the technical support and
Jürgen Bader, MPI-M, for his valuable suggestions. This work was
supported by the International Max Planck Research School on Earth System
Modelling (IMPRS-ESM) and the Max Planck Society (MPG). The computational
resources were provided by the Deutsches Klima Rechenzentrum (DKRZ).
The article processing charges for this open-access publication
were covered by the Max Planck Society.
Edited by: Valerio Lucarini
Reviewed by: Dorian Abbot, Jun Yang, and two anonymous referees
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