GEOPHYSICAL RESEARCH LETTERS, VOL. 32, L20812, doi:10.1029/2005GL022897, 2005
Radiative effect of surface albedo change from biomass burning
Gunnar Myhre,1 Yves Govaerts,2 Jim M. Haywood,3 Terje K. Berntsen,1,4
and Alessio Lattanzio5
Received 6 March 2005; revised 10 June 2005; accepted 13 September 2005; published 26 October 2005.
[1] The radiative impact of burn scars from biomass is
investigated. Changes in surface albedo derived from
satellite observations over the African continent are
used as a first order indication of this impact. Because
the direct radiative effect of aerosols from biomass
burning is dependent on the underlying surface albedo,
we investigate the interaction of the direct radiative effect
due to biomass burning aerosols with the change in surface
reflection due to the burn scars. The radiative effect of
reduced surface albedo from burn scars is estimated to be
close to 0.1 Wm 2 over a region covering the African
continent. Citation: Myhre, G., Y. Govaerts, J. M. Haywood,
from the mechanism proposed by Hansen and Nazarenko
[2004] of transported aerosols that influence surface albedo
of snow and sea-ice. The surface albedo effect and the effect
of changes in the atmospheric burden of aerosols from
biomass burning both lead to changes in the short wave
radiation budget over Africa. Since the radiative effect of
aerosols is dependent on the surface albedo, we investigate
the albedo effect alone and in combination with the direct
aerosol effect over the African continent.
T. K. Berntsen, and A. Lattanzio (2005), Radiative effect of
surface albedo change from biomass burning, Geophys. Res.
Lett., 32, L20812, doi:10.1029/2005GL022897.
2.1. Surface Albedo Decrease
[4] The seasonal analysis of surface albedo derived from
observations acquired by the Meteosat Visible and InfraRed
Imager (MVIRI) instrument onboard the European geostationary Meteorological Satellite (Meteosat) over Africa
using the method proposed by Pinty et al. [2000] revealed
the high sensitivity of albedo to biomass burning. This is
assessed by looking at the perturbation of surface albedo
variations during the dry season, i.e., reduction to abnormally low values, rather than the expected increase
[Govaerts et al., 2002]. The comparison between pixels
affected and non-affected by this process shows that the
relative reduction in surface albedo in fire prone areas in the
5– 10N region over the African continent typically ranges
between 5 to 8%. Albedo decrease due to fires over the
African continent south of equator is less important and
does not exceed a few percent on the average. Despite the
fact that surface reflectance may locally be reduced very
significantly over a burn scar (e.g., by a factor of approximately 2), relatively small changes in the surface reflectance
are observed at a coarse spatial resolution, in particular when
annual averaging is performed.
1. Introduction
[2 ] Anthropogenic activity has caused a variety of
radiative forcing mechanisms [Intergovernmental Panel on
Climate Change (IPCC), 2001] and over the last years
further mechanisms have been proposed [Forster and Shine,
1999; Ackerman et al., 2000; Koren et al., 2004; Hansen
and Nazarenko, 2004; Boucher et al., 2004; Myhre et al.,
2004]. For example, Hansen and Nazarenko [2004] suggest
that black carbon (BC) aerosols emitted by fossil fuel and
biomass burning activities impact snow and ice albedo and
give a significant positive radiative forcing. Hansen et al.
[1998], Betts [2001], and Myhre and Myhre [2003] show
that surface albedo changes resulting from anthropogenic
land cover changes have significantly impacted the radiative
balance. It has been known for some time that the gases and
aerosols emitted by the process of biomass burning exert
significant local, regional and global radiative forcing
[Penner et al., 1992; Hobbs et al., 1997]. Biomass burning
occurs particularly over the African continent with smaller
contributions from South America and the southern part of
Asia; all of these regions display a distinct seasonal cycle in
emissions with significant interannual variations.
[3] Here we investigate the radiative forcing mechanism
from biomass burning from alteration of surface albedo
caused by burn-scars. This radiative forcing mechanism
with direct impact on the surface albedo is quite different
1
Department of Geosciences, University of Oslo, Oslo, Norway.
European Organisation for the Exploitation of Meteorological
Satellites (EUMETSAT), Darmstadt, Germany.
3
Met Office, Exeter, UK.
4
Also at Centre for International Climate and Environmental Research,
Oslo, Norway.
5
MakaluMedia, Darmstadt, Germany.
2
Copyright 2005 by the American Geophysical Union.
0094-8276/05/2005GL022897$05.00
2. Method
2.2. Model Simulations of Aerosols From
Biomass Burning
[5] The distributions of aerosols from biomass burning
are simulated with a global atmospheric chemistry transport
model, the Oslo CTM2 [Berntsen et al., 2005; Myhre et al.,
2003]. The simulations are performed with meteorological
data from ECMWF for year 2000, with a spatial resolution
of T42 (around 2.8 2.8 degrees) and 40 vertical layers.
The biomass burning aerosols are included as hydrophobic
and hydrophilic organic carbon (OC) and BC with a
conversion rate from the hydrophobic to the hydrophilic
fractions from Cooke et al. [1999]. The emissions from
biomass burning are taken from Cooke and Wilson [1996]
and Liousse et al. [1996], respectively for BC and OC.
Optical properties are modelled based on aircraft measurements during SAFARI-2000 [Haywood et al., 2003]. The
Oslo CTM2 has previously been used to model the aerosol
distribution and radiative effect of biomass burning aerosols
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the radiative transfer calculations. Because we are unable to
precisely quantify the fraction of anthropogenic and natural
emissions, we use the term radiative effect, which include
both anthropogenic and natural biomass burning activities
[e.g., IPCC, 2001]. Monthly mean decreases in surface
albedo as estimated from the satellite observations are used
as input to the radiative transfer calculations.
3. Results
[8] Radiative effects are first presented for the separate
components of changes in surface albedo and in biomass
burning aerosols and, thereafter as a combination of changes
in both mechanisms.
Figure 1. Annual mean absolute percentage changes (a) in
surface albedo and (b) annual mean aerosol optical
thickness (AOT) of aerosols from biomass burning.
during SAFARI-2000 and the results from this model
compared favourably with AERONET data, MODIS aerosol satellite retrievals, and aircraft data [Myhre et al., 2003].
[6] Haywood et al. [2003] and Abel et al. [2003] showed
that the single scattering albedo of fresh biomass burning
aerosol is significantly lower than that of aged biomass
burning aerosol due to condensation of scattering material
and the formation of secondary organic aerosol particles.
We have thus modified the single scattering albedo over
land by reducing the value by 0.05 to 0.85 compared to 0.90
at 550 nm by Myhre et al. [2003].
2.3. Radiative Transfer Calculations
[7] A multi-stream radiative transfer code using the
discrete ordinate method [Stamnes et al., 1988] is adopted
in this study. Rayleigh scattering, gaseous absorption,
scattering by clouds, and scattering and absorption by
aerosols are included [Myhre et al., 2003]. Meteorological
data (including clouds) for the year 2000 from ECMWF are
used in the model calculations which are therefore consistent with the data for surface albedo changes. A temporal
resolution of 3 hours for the meteorological data is used in
3.1. Radiative Effect of the Surface Albedo Changes
[9] Maps of monthly and annual mean surface albedo
decrease due to fire estimated with the method proposed by
Govaerts et al. [2002] for year 2000 is shown in Figure 1a.
Surface albedo change data are expressed in percent (0 –
100%). A decrease in the surface albedo leads to more
absorbed solar radiation at the surface and thus a positive
radiative effect. The pattern of the radiative effect shown in
Figure 2a is very similar to the pattern of surface albedo
change given in Figure 1a. Myhre and Myhre [2003]
showed in an experiment with a constant albedo change
over all land areas that the radiative effect is very sensitive
to surface albedo changes over Africa. For clear sky, this
sensitivity is approximately constant over the whole of
Africa, whereas it is reduced somewhat in the intertropical
convergence zone by the presence of clouds. Table 1 shows
the annual mean radiative effect both for the regional area
shown in Figure 2 and the corresponding global mean. The
global mean radiative effect of albedo change from biomass
burning in Africa is 0.015 Wm 2. The local maximum
radiative effect is close to 2 Wm 2 in the annual mean (see
Figure 2a) and the local maximum in the current horizontal
resolution is nearly 8 Wm 2 in January.
3.2. Radiative Effect of the Biomass Burning Aerosols
[10] Figure 1b shows the modelled annual mean aerosol
optical thickness (AOT) at 0.55mm due to biomass burning
aerosols. The pattern and magnitude in the AOT is in
Figure 2. Annual mean radiative effect of (a) surface albedo change, (b) biomass aerosols, and c) combined effect of
surface albedo change and biomass aerosols.
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Table 1. Annual Mean Radiative Effect (Wm 2) Due to Biomass Burning for the Region Shown in Figures 1
and 2 and the Global Meana
Regional Area
Surface albedo change
Biomass burning aerosols
Combined effect
Global Mean
Annual Mean Radiative
Effect, Wm 2
Percentage
Difference, %
Annual Mean Radiative
Effect, Wm 2
Percentage
Difference, %
0.095
0.84
0.76
14
+2
0.015
0.13
0.12
14
+2
a
The percentage difference shown for surface albedo changes is when aerosols are taken into account, and for aerosols when
reduced surface albedo are taken into account.
reasonable agreement with satellite data and ground based
sunphotometers (AERONET) although a detailed comparison is complicated as several other aerosol components such
as mineral dust exist over the African continent. The global
and annual mean model estimate of the radiative effect of
aerosols from African biomass burning is 0.13 Wm 2,
with peak values stronger than 20 Wm 2 during the subSahelian burning period and stronger than 10 Wm 2 in
the southern African burning period. Figure 2b shows the
annual mean geographical distribution of the radiative
effect. The positive radiative effect of the aerosols in certain
regions is associated with reflective surfaces underlying
the aerosol layer, either desert areas or clouds as also
found during SAFARI-2000 [Myhre et al., 2003; Keil and
Haywood, 2003].
3.3. Radiative Effect of the Combined Surface Albedo
Changes and Aerosols From Biomass Burning
[11] The two previous subsections have considered surface albedo changes and biomass burning aerosols separately. However, both processes are radiatively coupled as a
decrease in surface albedo increases the efficiency of
reflective aerosols. Furthermore, if aerosols reflect or absorb
solar radiation this will lead to a smaller impact on the
surface albedo changes because less solar radiation will
reach the surface. Figure 2c shows the annual mean distribution of the radiative effect of the combined changes in
surface albedo and biomass burning aerosols, which to a
large extent resemble Figure 2b.
[12] The net result of the coupled radiative effect is
0.76 Wm 2 over the studied area around Africa (area
shown in Figures 1 and 2) corresponding to a global mean
forcing of 0.12 Wm 2. To investigate the impact of
surface albedo change on the radiative effect of aerosols
we have performed further calculations. Darkening of the
surface by burnt scars enhances the radiative effect of
the biomass burning aerosols by 2% (see Table 1). On the
other hand including biomass burning aerosols reduces
the radiative effect of surface albedo change by 14% from
0.095 Wm 2 to 0.081 Wm 2.
[13] Biomass burning over the African continent has a
distinct seasonal variation with the sub-Sahelian region
(Region R1) most influenced by burning in the December,
January, and February period and the southern part region
(Region R2) most influenced in from May to September.
Figure 3 shows the seasonal variation in surface albedo
[Govaerts et al., 2002] and the modeled AOT from biomass
burning (Oslo CTM2) and the associated radiative effect for
the sub-Sahelian and the southern part regions. The seasonal
variation is evident with change in surface albedo and
biomass burning aerosols having a corresponding radiative
effect. The figure also reveals a clear seasonal distinction
between the biomass burning in the two African regions.
The small difference in the time evolution of the AOT and
the surface albedo change in the southern hemisphere is
likely that burnt areas are generally smaller in size and more
scattered than in the North hemisphere, affecting also
preferably understorey instead of the top of the canopy.
Consequently, the effect on surface albedo is not necessarily
linearly coupled with the fire intensity and the amount of
corresponding released aerosols.
4. Summary and Conclusions
[14] Biomass burning influences climate and its radiative
budget in many ways. The radiative effect of biomass due to
albedo changes caused by burn scars has been investigated.
Because the radiative effect from biomass burning aerosols
is a function of the surface albedo and because the radiative
effect due to surface albedo changes is a function of the
surface insolation and hence on the aerosol optical depth,
we have also investigated the radiative effect as a combination of the two radiative forcing mechanisms. The direct
effect of biomass burning aerosols dominates over the effect
of change in surface albedo from burn scars. A calculation
of the combined radiative effects enhances the net radiative
effect by just 2% compared to simply adding the results
from separate calculations. These results suggest that the
Figure 3. Annual variation in surface albedo, AOT, and
radiative effect for two regions; R1 (5N–15N and 15W –
45E) and R2 (5S – 30S and 12E – 40E). (a) and
(b) Surface albedo and AOT are shown for R1 and R2,
respectively. (c) and (d) The radiative effect is shown for R1
and R2, respectively.
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interaction of the two mechanisms is relatively weak on a
global basis and simple addition of the radiative forcing
from the two mechanisms is a reasonable approximation.
The African biomass burning leads to annual mean radiative
effect of surface albedo change from burnt scars around
0.1 Wm 2 for the African continent, despite monthly
local maxima which can be as large as 8 Wm 2.
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