Climate Dynamics (2006) 26: 55–63
DOI 10.1007/s00382-005-0073-9
Wolfgang Knorr Æ Karl-Georg Schnitzler
Enhanced albedo feedback in North Africa from possible combined
vegetation and soil-formation processes
Received: 26 May 2005 / Accepted: 25 August 2005 / Published online: 27 October 2005
Springer-Verlag 2005
Abstract It has long been recognized that albedo related
vegetation feedbacks amplify climate variability in
North Africa. Recent studies have revealed that areas of
very high albedo associated with certain desert soil types
contribute to the current dry climate of the region. We
construct three scenarios of North African albedo, one
based on satellite measurements, one where the highest
albedo resembles that of soils in the desert transition
zones, and one based on a vegetation map for the ‘‘green
Sahara’’ state of the middle Holocene, ca. 6,000 years
ago. Using a series of climate model simulations, we find
that the additional amplitude of albedo change from the
middle Holocene to the present caused by the very bright
desert soils enhances the magnitude of the June-to-August precipitation change in the region of the present
Sahara from 0.6 to 1.0 mm/day on average. We also find
that albedo change has a larger effect on regional precipitation than changes in either the Earth’s orbit or sea
surface temperatures between 6,000 years ago and today. Simulated precipitation agrees rather well with
present observations and mid Holocene reconstructions.
Our results suggest that there may exist an important
climate feedback from soil formation processes that has
so far not been recognized.
W. Knorr (&)
Department of Earth Sciences, University of Bristol,
Wills Memorial Building, Queen’s Road,
BS8 1RJ Bristol, UK
E-mail: [email protected]
Tel.: +44-117-3315133
Fax: +44-117-9253385
K.-G. Schnitzler
Max-Planck-Institut für Meteorologie, Bundesstr. 53,
20146 Hamburg, Germany
1 Introduction
The albedo of the Sahara and Arabian deserts displays a
large degree of spatial heterogeneity (Pinty et al. 2000),
with important consequences for the present dry climate
of the region (Knorr et al. 2001). This present state
contrasts sharply with the much wetter climate during
the middle Holocene, around 6,000 years before present
(Yu and Harrison 1996; Hoelzmann et al. 2000; Pachur
and Hoelzmann 2000; Prentice et al. 2000). Reconstructions of the vegetation cover of what is now the
Sahara desert show either steppe all the way to its
northern boundary (Hoelzmann et al. 1998), or grassland and shrubland up to 23N or further (Jolly et al.
1998). Termination of this wet period appears to have
been rather abrupt (deMenocal et al. 2000).
It is now widely recognized that the transition was
caused by slow changes in the Earth’s orbit and a
subsequent southward retreat of the North African
monsoon (Kutzbach and Street-Perrott 1985; de NobletDucoudré 2000). The abruptness of the change, however, strongly indicates the presence of positive feedback
processes (Claussen and Gayler 1997; Brovkin et al.
1998; Claussen et al. 1999). The underlying effect that is
assumed responsible for the climate changes follows
largely from the theory of desert formation by Charney
(1975), which states that high albedo associated with
dry, non-vegetated conditions creates a regional minimum in net surface radiation, enhances sinking motion
of air and thus further suppresses precipitation (Sud and
Molod 1988; Lofgren 1995; Zheng and Eltahir 1998). In
addition, changes in sea surface temperatures (SST) have
been found to have a large (Kutzbach and Liu 1997;
Hewitt and Mitchell 1998) or moderate effect (Ganopolski et al. 1998) on North African monsoon strength.
The existence of large, very bright desert areas in
North Africa and the Arabian peninsula and their
impact on regional climate has been recognized more
recently (Knorr et al. 2001). In the Western Sahara,
exceptionally bright areas are observed (see Fig. 1)
Fig. 1 Broadband solar surface albedo used in the climate
simulations. a Present albedo derived from Meteosat satellite
observations (simulations starting with PRE). b Albedo for the
middle Holocene based on reconstructed vegetation and land cover
data (HOL). The albedo used for simulation BAR is the one under
(a) with an upper limit of 0.35
around the area of Mreyye Erg and Aoukar Erg in
Mauretania. The central Saharan area with high albedo
is located northwest of Lake Tchad, known as ‘‘Bilma
Erg’’ in Niger. It is noteworthy that some areas were
found to have present albedo values of 0.5 and higher,
while the albedo of the soil background in areas with
sparse and seasonally changing vegetation cover does
not typically exceed 0.35 (Pinty et al. 2000; Tsvetsinskaya et al. 2002). This suggests that the total albedo
change from mid Holocene to present conditions was
caused not only by the removal of vegetation and the
exposure of underlying, generally brighter soils (cf.
‘‘vegetation induced albedo change’’ in Fig. 2), but also
by some soil formation and degradation processes yet to
be identified.
A possible mechanism for this ‘‘enhanced albedo
change’’ is suggested again in Fig. 2 (Hoelzmann personal. comm.): After all vegetation cover has been removed, soils in arid areas are exposed to four main
known erosion processes, which change topography,
texture and optical properties (including albedo),
namely salinization combined with salt weathering
(Doornkamp and Ibrahim 1990), siltation and surface
crusting (Lal et al. 1989), formation of desert varnish
and duricrusts by aeolian erosion (Mainguet 1999), and
formation of fine sand fields and dunes by aeolian
transport (Schultz 2000). Which process dominates depends on the underlying soil structure and the local
climate. In case of the two bright areas observed in the
Western and Central Sahara, the soil degradation processes were presumably driven by salinization and formation of fine sand planes, both resulting in bright soil
textures (cf. Fig. 2).
Knorr and Schnitzler: Enhanced albedo feedback in North Africa
In order to quantify the climatic effect of this possible
enhanced albedo feedback from soil formation, we
postulate the existence of two types of albedo feedback
mechanisms having operated at the land surface between
the ‘‘green Sahara’’ state of the middle Holocene and the
present climate: one that involves a change from the
vegetated state to one where the albedo of the underlying soils was exposed, and one that includes formation
of the very bright areas observed presently but not found
in places where (sparse) vegetation grows. To describe
the albedo during the middle Holocene, we use a map of
reconstructed vegetation and assign typical values by
vegetation type found in analogous present vegetation
zones. Since we do not know the albedo of the underlying soils during that time, we simply assume that their
albedo did not exceed 0.35 and use the presently observed albedo from satellite data with an upper limit of
that value. This is also the value for exposed soils that
was originally assumed in Charney’s theory of desert
formation, and the maximum desert albedo assumed in
the interactive simulations of Claussen (1997); Kubatzki
and Claussen (1998). For the full present albedo state,
we use the same satellite derived data without the cutoff.
We call the first albedo change ‘‘vegetation induced’’,
and the second ‘‘enhanced albedo change’’ (Fig. 2).
More details are given in the next section.
The purpose of the present study is twofold: first, to
compare the climate effect of the enhanced albedo
change with that of the postulated vegetation induced
albedo change; and second, accepting the additional
magnitude of the enhanced albedo change, to compare
its climate effect with those of orbital and SST changes.
Both is done with an atmospheric general circulation
model (GCM) with appropriately chosen boundary
conditions. It will be the purpose of later studies to
further identify the mechanisms of soil formation that
have been responsible for the formation of the very
bright desert areas and thus to construct a complete
0.55 full desert
exposure of bleached sands
Albedo
56
Enhanced
albedo change
remobilization of sand dunes
0.35 barren land
Vegetation induced
albedo change
lowering of water table
0.25
open shrubland, steppe
savanna
0.15
dry
wet
Fig. 2 Schematic diagram illustrating a possible mechanism for an
enhanced albedo feedback. ‘‘Dry’’ and ‘‘wet’’ refer to the climate
state, ‘‘albedo’’ to the resulting land surface albedo depending on
vegetation and soils
Knorr and Schnitzler: Enhanced albedo feedback in North Africa
57
theory of the full albedo feedback that has operated in
North Africa and on the Arabian Peninsula.
derived from the experiments ‘‘CTL2’’ and ‘‘6K’’ of
Voss and Mikolajewicz (2001). A dominant feature of
those simulations is a cooling of the South Atlantic by
approximately 1C relative to the North Atlantic, and a
cooling of ca. 0.5C of the Indian Ocean for conditions
6,000 years ago. Such rather small changes are difficult
to assess against paleo proxy data of SST. However,
Voss and Mikolajewicz (2001) could show that the
coupled simulation ‘‘6K’’ enhanced precipitation over
North Africa, leading to a better agreement with land
based paleo proxies than for ‘‘CTL2’’. Orbital conditions were set according to the PMIP specifications
(Joussaume and Taylor 1995). ECHAM-4 was run at
T42 resolution (ca. 2.8 by 2.8) over 30 years, with the
first 4 years disregarded to exclude the influence of initial conditions. Experiments PRE and BAR are the same
as ‘‘MSA’’ and ‘‘CTL’’ of Knorr et al. (2001), except
that simulations were longer. Simulations with orbital
conditions of 6,000 years ago are marked by adding
‘+O’, and those using orbital and SST conditions of
6,000 years ago by ‘+OS’ (cf. Table 1).
2 Data and model simulations
We define a series of seven model experiments (see Table 1) with the ECHAM-4 GCM (Roeckner et al. 1996).
The three albedo states are denoted HOL (for mid
Holocene), BAR (postulated ‘‘barren land’’ albedo,
created by vegetation change alone) and PRE (present
albedo). The first three experiments define a series of
simulations to assess the strength of the two albedo
feedback mechanism: HOL minus BAR (vegetation induced), and HOL minus PRE (enhanced). The next two
simulations are designed to assess the climate effect of
orbital and SST changes for present albedo conditions,
and the last two for Holocene albedo conditions.
The ‘‘Meteosat’’ albedo map (used with PRE) was
derived from the 10-day albedo product for 1996 of
Pinty et al. (2000), using the hemispheric albedo product
for 30 solar zenith angle and applying a linear transformation from the Meteosat visible band to solar
broadband spectral characteristics. The median of the 36
10-day values was then taken at each 2 km pixel, after
which albedo was averaged to the T42 grid of ECHAM4. The use of the 30 albedo product instead of another
product that takes account of the diurnal variation in
solar zenith angle leads to a slightly lower, more conservative estimate. This map was inserted into the
standard ECHAM-4 albedo data set (Roeckner et al.
1996). For the second albedo map (BAR) an upper
cutoff value of 0.35 was applied. The third map used
vegetation cover, lake and wetland fractions for the
middle Holocene derived by (Hoelzmann et al. 1998).
Based on modern equivalents found in the Meteosatderived data set, typical values are assigned by land
cover type: 0.05 (lake), 0.15 (wetland and alluvial
plains), 0.13 (forest), 0.2 (xerophytic woods, savanna),
and 0.25 (steppe). The middle Holocene map has a 1 by
1 spatial resolution and extends 10N–31N and 17W–
60E. All three albedo maps were constructed at 1 by 1
spatial resolution (see Fig. 1) before they were converted
to T42 resolution and inserted into ECHAM.
Monthly mean SST for present day conditions are
taken as the average of 1979 to 1994 from the AMIP
project (Gates 1992). For the Holocene conditions of
6,000 years ago, SST anomalies were taken as the
difference between two 100-year average climatologies
Table 1 Description of the
sensitivity experiments
performed with ECHAM-4
3 The climate effect of the enhanced albedo change
The first three GCM experiments differ solely in their
albedo forcing, and the two difference fields of the vegetation induced albedo change (BAR minus HOL, cf.
Table 1) and the enhanced albedo change (PRE minus
HOL) are shown in Fig. (3a, b), respectively. Due to the
averaging to the T42 resolution, the effect of the very
bright areas is somewhat less pronounced than it would
be suggested from Fig. 1. However, whereas most of the
present Sahara shows an albedo change forcing of just
above 0.1 for the vegetation induced case, large areas
exceed 0.2 for the enhanced albedo change. For the
Arabian region, the effect is less pronounced, but the
area above 0.15 differential forcing is greatly increased.
The average value of the Sahara albedo forcing is about
45% higher for the enhanced albedo change (with albedo values of 0.39 for PRE, 0.34 for BAR and 0.23 for
HOL, as inferred from Table 2). For the Sahel, adjacent
to the south, the albedo change is similar in magnitude,
but the difference between the two cases is much smaller
(Table 2, cf. Fig. 3a, b).
Precipitation in today’s transition zone, the Sahel, is
determined by the strength of the African Monsoon
during the northern-hemisphere summer months.
According to Charney (1975), low precipitation over the
Code
Description
Albedo
Orbit
SST
PRE
BAR
HOL
PRE+O
PRE+OS
HOL+O
HOL+OS
Present albedo
Barren land albedo
Holocene albedo
Holocene orbit
Holocene orbit & SST
Holocene albedo & orbit
Holocene albedo, orbit & SST
Meteosat
Meteosat £ 0.35
Holocene
Meteosat
Meteosat
Holocene
Holocene
Present
Present
Present
Holocene
Holocene
Holocene
Holocene
Present
Present
Present
Present
Holocene
Present
Holocene
58
Knorr and Schnitzler: Enhanced albedo feedback in North Africa
Fig. 3 Difference of surface albedo and various climate output
variables. Left column (a, c, e, g) simulations BAR minus PRE for
vegetation induced albedo change. Right column (b, d, f, h)
simulations HOL minus PRE for the enhanced albedo feedback.
(a, b) albedo, (c, d) net surface radiation in Wm–2, (e, f) June-toAugust precipitation in millimeters per day, (g, h) annual
precipitation in millimeters per day
Table 2 Simulated surface radiation balance and simulated and observed precipitation for the months June–August
Simulations
Albedo
Incident SW
Outgoing SW
Incident LW
Outgoing LW
Net Radiation
Precipitation
Obs. Precip.
Sahel
Sahara
PRE
BAR-HOL
PRE-HOL
PRE
BAR-HOL
PRE-HOL
0.29
250.5
–74.7
411.8
–472.4
115.3
4.3
4.1
0.10
48.6
–31.9
–2.8
–10.4
3.5
–1.6
0.12
73.0
–43.2
–5.6
–18.9
5.4
–2.6
0.39
299.7
–116.4
392.8
–505.1
71.0
0.3
0.3
0.11
19.8
–35.6
–8.3
0.5
–23.6
–1.0
0.16
35.1
–56.6
–17.1
0.4
–38.1
–1.6
SW shortwave radiation, LW longwave radiation, both in Wm–2, precipitation in millimeters per day. Sahel is 11...17N, 10E...35W,
Sahara 17...31N, 10E...35W
Knorr and Schnitzler: Enhanced albedo feedback in North Africa
59
Fig. 4 Simulated and observed
annual precipitation in
millimeters. a Fully modern
simulation PRE. b Observed
climatology over land. Contour
lines are every 100 mm
present Sahara desert is induced by sinking motion
created by a regional minimum of net radiation in the
area (cf. above). Knorr et al. (2001) found that changing
albedo from what is here BAR to PRE decreased the
average June-to-August net radiation of the Sahara
from 85 to 71 Wm 2, but had little effect for the Sahel
(same definition as in Table 2), where net radiation is
markedly higher with 115 Wm–2, despite of considerably
lower incoming solar radiation (see Table 2). The same
picture emerges for the two cases of albedo change
considered here: in the Sahara, net radiation decreases
by about 24 Wm 2 for the vegetation induced albedo
change, and by as much as 38 Wm 2 for the enhanced
albedo change. The second value comes close to the
difference between the simulated net radiation between
the present Sahara and Sahel (44 Wm 2, Table 2),
which would be consistent with the notion of a ‘‘green
Sahara’’ during the middle Holocene. The largest contribution to the decrease in net radiation comes from
increased outgoing solar radiation, as would be expected, but over half of it is compensated by increased
incoming solar radiation. As a results, a third to almost
half of the net radiation change is caused by a decrease
in the incoming long-wave radiation created by drier air
and reduced cloud cover.
Contrary to the Sahara, June–August net radiation
changes very little, and that in the opposite direction, for
the area that is defined as Sahel in Table 2. A marked
increase in incoming solar radiation is compensated by
increased outgoing solar and thermal radiation, as well
as some decrease in incoming thermal radiation. As for
the Sahara, precipitation decreases, which is consistent
with the notion that cloudiness and air humidity decrease, too.
The regional north-south contrast is also evident in
Fig. (3c, d): There is a general, large-scale decrease in net
radiation over the entire (present) desert area, and the
additional magnitude from the enhanced albedo change
is distributed rather uniformly. To the south, there is an
increase in net radiation that varies from the western
Sahel and Sudanian (further south), where it is small, to
the eastern part, and peaks over the Ethiopian Plateau.
A comparison between Fig. 3b, d, f reveals a possible
reason for the steep gradient in net radiation change: in
the areas where albedo change is only moderate (ca.
10...12N in the central part), decrease in precipitation
and associated increase in solar incoming radiation
dominates and net radiation increases, whereas north of
a line varying between 12 and 15N, the dominant effect
comes from albedo change – net radiation decreases.
The same is true for the vegetation induced albedo
change (Fig. 3a, c, e), only that precipitation and net
radiation change much less.
The most important climate variable from both a
biogeographic and human perspective is certainly annual average precipitation. It is, therefore, especially
important that this particular quantity is simulated
realistically. Figure 4 shows the simulated value using
full present conditions (PRE) compared to the landbased observed climatology taken over a period of ca.
1930–1960 (Leemans and Cramer 1991, Cramer pers.
comm.), averaged to the same spatial resolution as the
GCM runs. There is a tendency of ECHAM-4 to generate slightly too much rainfall in the western portion of
the Sahara desert, on the Ethiopian plateau and Yemen.
In general, however, the simulations agree very well with
observations, in particular the position of the desert
transition zones (ca. 150 mm/year, see below). It is,
60
Fig. 5 Precipitation for the
climate simulations with present
(dashed lines, PRE) and middle
Holocene albedo (solid lines,
HOL). Red: present orbit and
SST, green: middle Holocene
orbit and present SST, blue:
middle Holocene orbit and
SST, black: observed
climatology. a Isolines of
5 mm/day for June–August. b
Isolines of 150 mm/year.
(Colors as in Fig. 6.)
Knorr and Schnitzler: Enhanced albedo feedback in North Africa
a
b
therefore, appropriate to ask whether the simulated
precipitation change resulting from the enhanced albedo
change (Fig. 3g) is more realistic than the one resulting
from the postulated vegetation-only effect (Fig. 3h).
In the middle Holocene, a large lake existed between
approximately 12 and 18N and 14 and 19E (Pachur
and Rottinger 1997) (see the area of low albedo in
Fig. 2b). At present, precipitation is less than 100 mm/
year in the north, and about 800 mm/year in the south
of this area. We calculate an equilibrium evapotranspiration (McNaughton and Jarvis 1991; Knorr 1997) in
this former lake area of 1980 mm/year. We note that
changes in surface albedo alone can have precipitation
change by 500–600 mm/year for the vegetation induced
case (Fig. 3g), and by 700–800 mm/year for the enhanced albedo effect (Fig. 3h). Even without a detailed
water balance calculation, it seems reasonable to assume
that the additional about 200 mm/year of precipitation
as a result of a larger albedo change from ‘‘dry’’ to
‘‘green’’ Sahara conditions would decrease the net water
loss (evaporation minus precipitation) of such a lake
considerably, making its existence more consistent with
a precipitation change simulated as the result of the
enhanced albedo feedback. The larger amplitude of the
Holocene-to-modern albedo change implied by the satellite data seems, therefore, to generally improve simulations of the regional climate change in North Africa,
at least with ECHAM-4. Note, however, the additional
effect of orbital and SST changes as discussed in the
following section.
4 Comparison between albedo, orbital and SST effects
Accepting the enhanced albedo change as the more
realistic one to describe part of the climate forcing that
lead to the transition from the ‘‘green’’ to the present
desert state in North Africa – acknowledging some
remaining uncertainties about the albedo of 6,000 years
ago – we now compare its climate effect to the effect of
what is commonly assumed to be the main external
drivers of the regional climate change in the area.
Figure 5a shows the observed 5 mm/day isoline for
the months June to August in black (solid line). The
current 5 mm/day isoline corresponds approximately to
the Sahelian–Sudanian transition zone according to
White (1983). The position of the observed line and the
one of the fully modern simulation PRE (red, dashed
line) agree rather well. In the following, we consider the
effect of the various forcing changes in the inverse
direction as in the previous section, going from present
to mid Holocene conditions.
Setting only orbital parameters to 6,000 years ago
(green dashed line, PRE+O), or both orbit and SST
(blue dashed line, PRE+OS) results in only a slight
northward shift of the African monsoon. There is an
eastward expansion of the monsoon as a result of SST,
but not of orbital changes. If, however, only albedo is set
to mid Holocene conditions (red solid line, HOL), the
eastward expansion is as pronounced as for the combined orbital and SST change (PRE+OS). Once the
albedo has been decreased to mid Holocene conditions,
orbital changes have only very little effect on the simulated monsoon strength (compare the red and the green
solid lines, HOL and HOL+O). Only SST changes (blue
solid line, HOL+OS) lead to a further eastward shift of
the monsoon into Yemen, but at the expense of the
monsoon strength in most of North Africa.
It appears that albedo changes dominate and strongly
influence how either SST or orbital changes affect climate: in the case of modern albedo (dashed lines), the
northward expansion of the monsoon seems to be
effectively blocked, with orbital and SST changes having
little effect. A comparison with Fig. 1a suggests that the
northward expansion of precipitation is particularly
suppressed in areas of exceptionally high albedo (around
5W and 15E), in agreement with Knorr et al. (2001),
who found that the southern position of those bright
Knorr and Schnitzler: Enhanced albedo feedback in North Africa
1600
1400
Mid Holocene estimate s
Modern observed
PRE
PRE+O
PRE+OS
HOL
HOL+O
HOL+OS
1200
mm
1000
800
600
400
200
0
5°N
10°N
15°N
20°N
25°N
61
(Hoelzmann et al. 2000). Agreement is generally much
better for all simulations with assumed middle Holocene
albedo. Orbital and SSTs conditions have only little
impact on the results at the low albedo. Only the simulation with the middle Holocene albedo and modern
orbital and SST conditions (HOL) shows slightly less
precipitation at the northern boundary than HOL+O
or HOL+OS. The dry-desert margin, assumed at about
20 mm/year (White 1983), is as far north as 28N in the
other two cases. Agreement between the fully modern
simulation (PRE) and modern observations is also good.
30°N
Fig. 6 Simulations, observations and paleo-estimates for annual
precipitation along 28E
areas created an effective barrier for the African monsoon.
Again following White (1983), we use 150 mm/year
of precipitation as an approximate indicator for the
position of the northern and southern Sahara transition
zones (Fig. 5b). Agreement of the fully modern simulation (PRE, red dashed line) with observations (black
line) is again rather good (cf. Fig. 4). There is a northward shift of the southern boundary of the dry zone
(<150 mm/year) by around 3 latitude when orbital
conditions are set to 6,000 years ago (PRE+O, green
dashed line). If SSTs are changed also, there is no further northward shift (PRE+OS, blue dashed line), but
some precipitation decrease in the west (cooling off the
Guinean coast) and a general eastward shift as seen in
the monsoon season precipitation. An additional albedo
decrease (HOL+OS, blue solid line) has the largest
overall effect and leads to a complete disappearance of
desert conditions on the Arabian peninsula, in qualitative agreement with the assumed existence of wetlands
and lakes in most of the peninsula (Hoelzmann et al.
1998). Precipitation increase, however, is less in the
western part of the Sahara, where HOL and HOL+O
(red and green solid lines) have the largest values. The
effect of orbital changes (red vs. green) is again small at
low albedo (dashed lines) except in the very east of the
area displayed, but stronger than for the monsoon season precipitation at high albedo (solid lines). In general,
there is a northward expansion of wetter conditions by
approximately 10 latitude, while there remain only very
small and scattered areas of very dry desert (below
20 mm, not shown) in the north-eastern part of the
Sahara for all simulations with prescribed mid Holocene
albedo.
So far, we have used a reconstructed albedo map for
the middle Holocene that assumes some sparse vegetation all the way to the northern boundary of the Sahara,
but we have not yet checked whether the simulated
precipitation change is also consistent with paleoreconstructions. Figure 6 shows a comparison of simulated precipitation with Holocene estimates along 28E
based on field studies and identified paleo-lake levels as
well as model results of the paleo-lake water balance
5 Discussion
In this study, it was postulated that two types of albedo
feedback processes have operated on a large scale in
North Africa and the Arabian Peninsula at the wet–dry
transition around 5–6,000 years ago. One process involves solely the removal of vegetation and exposure of
underlying soils that are assumed to have a maximum
albedo of 0.35. This is the same maximum value that was
used in many previous studies. Later, under certain
circumstances, a second soil formation process leads to
much higher albedo under persistent desert conditions.
Evidence for this comes from satellite-derived albedo
data and from the fact that all the very bright soils are
found in the full desert as opposed to its semi-arid
transition zones. While further investigation is clearly
required, this study is aimed at studying the climatic
consequences of the assumed processes.
Comparing with the vegetation induced feedback, it
is shown that the albedo change that appears to be
associated with the enhanced albedo feedback considerably increases the magnitude of the albedo driven
climate change between the middle Holocene ‘‘green’’
state of North Africa and its present desert conditions.
The effect of the enhanced albedo change on both surface net radiation and precipitation is about 50% higher.
This effect is then compared to that of orbital and
estimated SST changes that have occurred between the
middle Holocene and the present. Albedo is found to
have a dominant effect on climate change in the region,
influencing also the magnitude of the other forcing factors. Earlier simulations with ECHAM-3 (e.g. Claussen
and Gayler 1997) without the additional amplitude of
albedo change, had shown a less pronounced change in
the simulated monsoon strength. The comparison has to
be taken with some caution, however, as the simulation
by Claussen and Gayler did include SST changes and
had a different vegetation map than the one used here.
The dominant role of albedo change agrees well with
simulations by Texier et al. (2000), who used the LMD
climate model and the same vegetation map by Hoelzmann et al. (1998). Both studies also agree that SST
changes lead to some eastward shift, but hardly any
northward expansion of the monsoon. Our results further agree with findings by Bonfils et al. (2001), who
have shown that the Sahara precipitation simulated by
62
Knorr and Schnitzler: Enhanced albedo feedback in North Africa
models with higher desert albedo tends to be less sensitive to changes in orbital forcing.
Another important aspect is that the present climate
is well reproduced by the simulations. Use of the satellite-derived albedo has removed a wet bias found for
modern conditions with ECHAM-3 (de Noblet-Ducoudré et al. 2000) and ECHAM-4 (Knorr et al. 2001).
The vegetation map used here to reconstruct the albedo of the middle Holocene is certainly only one possible realisation and subject to some remaining
uncertainties. In particular, it assumes no ‘‘true’’ desert
left, with the entire present Sahara covered by at least
sparse grassland. As this map is based on pollen data, it
may show grassland in areas where it was in fact not
continuous, so that albedo change may in effect be an
upper limit of what has actually occurred. The map may
represent also slightly wetter conditions somewhat prior
to 6,000 years ago. The northward expansion of the
monsoon simulated here suggests that some limited
areas in the north-east of the present Sahara were too
dry to support even arid steppe. The climate simulations
themselves, however, appear to agree well with reconstructed precipitation in just this eastern part of the
Sahara.
It would, therefore, be a logical next step to repeat the
climate simulations with an interactive model that is able
to predict the surface albedo interactively, following
previous studies (e.g. Claussen 1997; Zheng and Eltahir
1998; Zeng et al. 1999; de Noblet-Ducoudré et al. 2000).
This would require further investigation into the
underlying mechanisms of soil formation, their required
conditions, and their timescales. The dominant influence
of albedo change shown here hints that such an interactive model will likely produce an even stronger land
surface feedback. Since the present simulations already
reproduce a climate change in North Africa that is largely consistent with observations—with a possible
exception regarding the albedo in the very northern part
of the desert—such simulations would look especially
promising.
Acknowledgements We would like to thank the European Centre
for Medium Range Weather Forecasting for providing computing
resources, and Philipp Hoelzmann for stimulating discussions. This
work was supported through the German Ministry for Research
and Education’s DEKLIM programme, project Nr. 01 LD 0106.
6 Conclusions
Areas of very bright soil background appear to contribute significantly to the current desert conditions
found in North Africa and the Arabian peninsula. The
dominance of the albedo effect over the effects of orbital
conditions and SST change is such that high albedo is
able to create a strong positive climate feedback. The
nature of the soil formation processes that have created
the high albedo values still needs to be better understood
until they may be included in coupled climate and land
surface model simulations. However, in agreement with
GCM studies, the sensitivity of the North African climate system to the albedo change, and therefore the
potential magnitude of a positive soil-formation feedback process, appears to be large.
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