Albedo enhancement over land to counteract global warming: impacts on hydrological cycle Govindasamy Bala • Bappaditya Nag Received: 20 May 2011 / Accepted: 23 November 2011 Ó Springer-Verlag 2011 Abstract A recent modelling study has shown that pre- cipitation and runoff over land would increase when the reflectivity of marine clouds is increased to counter global warming. This implies that large scale albedo enhancement over land could lead to a decrease in runoff over land. In this study, we perform simulations using NCAR CAM3.1 that have implications for Solar Radiation Management geoengineering schemes that increase the albedo over land. We find that an increase in reflectivity over land that mitigates the global mean warming from a doubling of CO 2 leads to a large residual warming in the southern hemi- sphere and cooling in the northern hemisphere since most of the land is located in northern hemisphere. Precipitation and runoff over land decrease by 13.4 and 22.3%, respec- tively, because of a large residual sinking motion over land triggered by albedo enhancement over land. Soil water content also declines when albedo over land is enhanced. The simulated magnitude of hydrological changes over land are much larger when compared to changes over oceans in the recent marine cloud albedo enhancement study since the radiative forcing over land needed (-8.2 W m -2 ) to counter global mean radiative forcing from a doubling of CO 2 (3.3 W m -2 ) is approximately twice the forcing needed over the oceans (-4.2 W m -2 ). Our results imply that albedo enhancement over oceans produce climates closer to the unperturbed climate state than do albedo changes on land when the consequences on land hydrology are considered. Our study also has impor- tant implications for any intentional or unintentional large scale changes in land surface albedo such as deforestation/ afforestation/reforestation, air pollution, and desert and urban albedo modification. 1 Introduction Solar Radiation Management (SRM) geoengineering pro- posals (Royal Society Report 2009) aim to counter the radiative effect of greenhouse forcing by reducing the amount of solar radiation absorbed by the planet. Planetary absorption of solar radiation can be reduced either by deflecting solar radiation in space, in the atmosphere or at the surface. Reflectors in L1 Lagrange point and mirrors in low earth orbit are some examples for space based tech- niques (Angel 2006; Early 1989; NAS 1992; Seifritz 1989). Artificial injection of aerosols in the stratosphere (Crutzen 2006; Robock et al. 2009, 2008) and enhancement of albedo of marine clouds (Bower et al. 2006; Latham 1990, 2002; Latham et al. 2008) are proposed SRM schemes for reflecting solar radiation in the atmosphere. Increasing the land surface albedo via whitening the roofs and pavements in the urban area (Akbari et al. 2009; Oleson et al. 2010) or covering deserts with more reflective polyethylene-alu- minium to increase albedo (Gaskill 2004), making the color of crops lighter (Doughty et al. 2011; Ridgwell et al. 2009) or enhancing the surface albedo of the oceans (Evans et al. 2010; Flannery et al. 1997; PSAC 1965) are a few exam- ples for surface based schemes. Space based schemes and stratospheric injection of aerosols are likely to lead to a more uniform reduction in solar radiation across the planet: these schemes do not Electronic supplementary material The online version of this article (doi:10.1007/s00382-011-1256-1) contains supplementary material, which is available to authorized users. G. Bala (&) B. Nag Divecha Center for Climate Change and Center for Atmospheric and Oceanic Sciences, Indian Institute of Science, Bangalore 560012, India e-mail: [email protected]123 Clim Dyn DOI 10.1007/s00382-011-1256-1
16
Embed
Albedo enhancement over land to counteract global … model...Albedo enhancement over land to counteract global warming: impacts on hydrological cycle Govindasamy Bala • Bappaditya
This document is posted to help you gain knowledge. Please leave a comment to let me know what you think about it! Share it to your friends and learn new things together.
Transcript
Albedo enhancement over land to counteract global warming:impacts on hydrological cycle
Govindasamy Bala • Bappaditya Nag
Received: 20 May 2011 / Accepted: 23 November 2011
� Springer-Verlag 2011
Abstract A recent modelling study has shown that pre-
cipitation and runoff over land would increase when the
reflectivity of marine clouds is increased to counter global
warming. This implies that large scale albedo enhancement
over land could lead to a decrease in runoff over land. In
this study, we perform simulations using NCAR CAM3.1
that have implications for Solar Radiation Management
geoengineering schemes that increase the albedo over land.
We find that an increase in reflectivity over land that
mitigates the global mean warming from a doubling of CO2
leads to a large residual warming in the southern hemi-
sphere and cooling in the northern hemisphere since most
of the land is located in northern hemisphere. Precipitation
and runoff over land decrease by 13.4 and 22.3%, respec-
tively, because of a large residual sinking motion over land
triggered by albedo enhancement over land. Soil water
content also declines when albedo over land is enhanced.
The simulated magnitude of hydrological changes over
land are much larger when compared to changes over
oceans in the recent marine cloud albedo enhancement
study since the radiative forcing over land needed
(-8.2 W m-2) to counter global mean radiative forcing
from a doubling of CO2 (3.3 W m-2) is approximately
twice the forcing needed over the oceans (-4.2 W m-2).
Our results imply that albedo enhancement over oceans
produce climates closer to the unperturbed climate state
than do albedo changes on land when the consequences on
land hydrology are considered. Our study also has impor-
tant implications for any intentional or unintentional large
scale changes in land surface albedo such as deforestation/
afforestation/reforestation, air pollution, and desert and
urban albedo modification.
1 Introduction
Solar Radiation Management (SRM) geoengineering pro-
posals (Royal Society Report 2009) aim to counter the
radiative effect of greenhouse forcing by reducing the
amount of solar radiation absorbed by the planet. Planetary
absorption of solar radiation can be reduced either by
deflecting solar radiation in space, in the atmosphere or at
the surface. Reflectors in L1 Lagrange point and mirrors in
low earth orbit are some examples for space based tech-
niques (Angel 2006; Early 1989; NAS 1992; Seifritz 1989).
Artificial injection of aerosols in the stratosphere (Crutzen
2006; Robock et al. 2009, 2008) and enhancement of
albedo of marine clouds (Bower et al. 2006; Latham 1990,
2002; Latham et al. 2008) are proposed SRM schemes for
reflecting solar radiation in the atmosphere. Increasing the
land surface albedo via whitening the roofs and pavements
in the urban area (Akbari et al. 2009; Oleson et al. 2010) or
covering deserts with more reflective polyethylene-alu-
minium to increase albedo (Gaskill 2004), making the color
of crops lighter (Doughty et al. 2011; Ridgwell et al. 2009)
or enhancing the surface albedo of the oceans (Evans et al.
2010; Flannery et al. 1997; PSAC 1965) are a few exam-
ples for surface based schemes.
Space based schemes and stratospheric injection of
aerosols are likely to lead to a more uniform reduction in
solar radiation across the planet: these schemes do not
Electronic supplementary material The online version of thisarticle (doi:10.1007/s00382-011-1256-1) contains supplementarymaterial, which is available to authorized users.
G. Bala (&) � B. Nag
Divecha Center for Climate Change and Center for Atmospheric
and Oceanic Sciences, Indian Institute of Science,
a Uncertainty is given by the standard error computed from 40 annual means. The standard error is corrected for serial correlation (Zwiers and
von Storch 1995)b Percentage changes are relative to controlc Percentage changes are relative to the absolute value in the control. Land has positive P - E in the control and ocean has negative P - Ed Omega refers to the pressure velocity (negative is upward motion) at the 500 mb pressure levele The first unit is for the mean values in the 19 CO2 case, and the second unit is for the changes given in other columnsf Global-mean change in evaporation is equal to global-mean change in precipitation and hence not shown in the tableg Total soil water in the top six soil layers of the land model to a depth of 36.6 cm
G. Bala, B. Nag: Albedo enhancement over land to counteract global warming
123
is 1.92 ± 0.02 K (Table 2 and Fig. 3) with the ratio of the
land to the ocean mean surface temperature warming
yielding a value of 1.29. The global mean surface tem-
perature change is 2.08 K. (Sutton et al. 2007) showed that
the range of the land/sea warming ratio for the IPCC AR4
models lies between 1.18 and 1.58. The difference in
warming between the land and the ocean has been studied
detail in recent studies (Boer 2011; Joshi et al. 2008;
Lambert et al. 2011). When the land cloud albedo is
enhanced (Geo - 29 CO2), the land/ocean cooling ratio is
higher at 1.43 because radiative forcing is applied only
over land. In the geoengineered case, the residual global
mean temperature change is less than 0.1 K.
In climate change studies, the response of the climate
system to a given forcing is measured in terms of the
feedback parameter which is defined as the change in TOA
net radiative flux per unit change in global-mean surface
temperature as climate change progresses. Previous studies
have demonstrated that the feedback parameter is approx-
imately independent of the forcing mechanisms (Forster
et al. 2000; Hansen et al. 1997, 2005). We estimate the
feedback parameters from the global mean radiative
forcing (Table 1) and equilibrium temperature change
(Table 2): they are 1.59 and 1.44 W m-2 K-1, respec-
tively, for the 29 CO2 and the enhanced albedo cases.
When the ‘‘fast response’’ in global mean surface temper-
ature changes (Table 1) are subtracted from the equilib-
rium temperature change (Bala et al. 2010a), we get values
of 1.72 and 1.57 W m-2 K-1, respectively. In either
method, we find that the parameters differ by only about
10% between the two cases. This suggests that climate
sensitivity (the inverse of feedback parameter) is approxi-
mately constant and the radiative forcing concept is capa-
ble of predicting the global mean temperature change, at
least for the two types of forcings studied here (Forster
et al. 2000; Hansen et al. 1997, 2005).
The temperature changes are larger over land and high-
latitude regions in agreement with the published literature
for 29 CO2 (IPCC 2007) and enhanced albedo cases
(Fig. 3). We notice that the magnitude of temperature
change is larger in the southern hemisphere (SH) in the 29
CO2 case and in the northern hemisphere (NH) in the
enhanced albedo case: warming in NH and SH are 1.9 and
2.2 K, respectively, in the 29 CO2 case and the cooling are
2.3 and 1.8 K in the enhanced albedo case. SH warming is
more in the 29 CO2 case because of large warming in SH
high latitudes in the model (Fig. 3) and NH cooling is more
in the enhanced albedo case because albedo enhancement
is applied only over land which is mostly located in NH.
Because the geoengineered case is approximately the sum
of 29 CO2 and enhanced albedo cases, we find large
residual warming of 0.5 K in SH and a cooling of -0.3 K
in NH in the geoengineered case even though the global
mean change is nearly zero. The associated asymmetry in
the precipitable water change in the geoengineered case
can also be seen in Fig. 3 because precipitable water
changes are tightly controlled by temperature changes
(Allen and Ingram 2002; Held and Soden 2006).
Global mean precipitation increases by 4.20 ± 0.06% in
the 29 CO2 case, decreases by 6.34 ± 0.07% in the
Fig. 2 Radiative forcing calculated using the ‘‘fixed-SST method’’
(Hansen et al. 2005) for doubled atmospheric CO2 content (29
CO2 - 19 CO2), enhanced albedo (Geo - 29 CO2) and geoengi-
neered (Geo - 19 CO2) cases. The hatching indicates regions where
the changes are not significant at the 99% level of confidence.
Significance level is estimated using a Student t test with sample of 40
annual means and standard error corrected for serial correlation
(Zwiers and von Storch 1995)
G. Bala, B. Nag: Albedo enhancement over land to counteract global warming
123
enhanced albedo over land case and decreases by
2.41 ± 0.07% in the geoengineered case. Expressing these
changes as hydrological sensitivity (defined as % change in
global mean precipitation per degree of warming), we find
that the hydrological sensitivity is 2.01% per K for the 29
CO2 case and 3.13% per K for the albedo enhancement
case. These changes are in agreement with earlier studies
(Andrews et al. 2009; Bala et al. 2010a) which showed that
the global hydrological cycle is more sensitive to solar
forcing than to an equivalent CO2 forcing and hence geo-
engineering will lead to a decrease to global mean pre-
cipitation (Bala et al. 2008). When fast responses in
precipitation and temperature (Table 1) are subtracted from
the total equilibrium response (Table 2), hydrological
sensitivity in the 29 CO2 and enhanced albedo cases are
3.00 and 2.93%, respectively, which demonstrates that the
slow response or feedback in precipitation is independent
of the forcing mechanisms (Bala et al. 2010a).
There is large contrast in land versus ocean precipitation
changes. In the case of doubling CO2, percentage changes
in precipitation are more over land than over oceans:
7.40 ± 0.30% over land versus 3.20 ± 0.07% over oceans.
This contrast is amplified in the land cloud albedo
enhancement case: -19.35 ± 0.24% over land versus
-2.08 ± 0.07% over oceans. The geoengineering case is
nearly the sum of the above two cases where the land mean
precipitation decreases by 13.38 ± 0.28% and ocean mean
precipitation increases by 1.05 ± 0.07%. Therefore, we
find that enhancing the albedo over land as a geoengi-
neering technique could lead to a large reduction in rainfall
over land.
The magnitude of precipitation decrease over land in the
geoengineered case is much larger than the magnitude of
precipitation decrease simulated over oceans in the recent
modeling study that investigated the effect of marine cloud
albedo enhancement (13.4% in this study versus 2.9% in
the earlier study). This is expected because the areal extent
of clouds over land available for enhancing the albedo is
less than half available over ocean areas [land occupies
30% of global area and oceans cover 70% of the global
area, total cloud cover over land and oceans in our model
are 53 and 61%, respectively (Table S1)]. Therefore, the
required negative radiative forcing over land in our study to
counter warming from doubling of CO2 is approximately
twice the negative radiative forcing required over oceans in
the earlier study (-8.2 vs. -4.2 W m-2). Accordingly, the
Table 2 Global and annual-mean changes in key climate variables under equilibrium climate change
Variable Region 19 CO2 29 CO2 - 19 CO2 Geo - 29 CO2 Geo - 19 CO2
Surface temperature (K) Global 288.38 ± 0.02a 2.08 ± 0.03 -2.02 ± 0.03 0.06 ± 0.03
a Uncertainty is given by the standard error computed from 40 annual means. The standard error is corrected for serial correlation (Zwiers and
von Storch 1995)b Percentage changes are relative to controlc Percentage changes are relative to the absolute value in the control. Land has positive P - E in the control and ocean has negative P - Ed Omega refers to the pressure velocity (negative is upward motion) at the 500 mb pressure levele The first unit is for the mean values in the 19 CO2 case, and the second unit is for the changes given in other columnsf Global-mean change in evaporation is equal to global-mean change in precipitation and hence not shown in the tableg Total soil water in the top six soil layers of the land model to a depth of 36.6 cm
G. Bala, B. Nag: Albedo enhancement over land to counteract global warming
123
precipitation decline over land in our enhanced albedo case
is 19.4% which is much larger when compared to the
precipitation decline over oceans of 7.3% in the previous
study when marine cloud albedo is increased. In the
enhanced albedo and geoengineered cases, most of the
decrease in precipitation is confined to the tropical land
areas such as central Africa, Amazon, India and Central
America (Fig. 4).
The impact on net water budget can be assessed by
investigating precipitation minus evaporation (P - E). In
our model, runoff over land increases by 9.11 ± 0.61%
when CO2 is doubled. However, in the enhanced albedo
and geoengineeed cases, runoff over land decreases by
28.80 ± 0.51 and 22.31 ± 0.57%. Therefore, we find that
albedo enhancement over land as a geoengineering strategy
could lead to a drying of the continents. As for precipita-
tion, the magnitude of runoff decrease in the geoengineered
case is much larger than the magnitude of runoff increase
in the corresponding case in the earlier study on marine
cloud albedo enhancement (22.3% in this study versus
7.5% in the earlier study). The drying is mostly confined to
tropical land areas: India, Amazon and central Africa
(Fig. 4). The large changes in P - E over the oceans are
likely driven by atmospheric circulation changes. The
changes in soil water content in 29 CO2, enhanced albedo
and geoengineered cases are about 5.02 ± 0.25,
-6.47 ± 0.27 and -1.48 ± 0.13 mm, respectively, which
are associated with an increase in precipitation in the 29
CO2 case and a decrease in the enhanced albedo and
geoengineered cases (Table 2).
In the earlier study on marine cloud albedo enhancement
(Bala et al. 2010b), the increase in precipitation and runoff
over land are associated with the enhanced monsoonal flow
and the associated upward motion over land and sinking
motion over oceans. We find that the reverse mechanism
operates here: there is sinking motion over land and
upward motion over oceans (Table 2; Figs. 3, 5). As for
precipitation and runoff changes over land, we find that the
magnitude of sinking motion over land in the enhanced
albedo case is larger than the magnitude of sinking motion
over oceans simulated in the marine cloud albedo
enhancement case (Bala et al. 2010b) (2.8 vs. 0.63 mb per
day at about 500 mb). The sinking motion over land and
rising motion over oceans extend throughout the tropo-
sphere in the globally averaged vertical profiles in the
enhanced albedo and geoengineered cases (Fig. 5). In these
cases, sinking motion over land is confined to the tropical
land areas such as central Africa, Amazon, India and
Fig. 3 Changes in global and
annual mean temperature,
precipitable water and upward
vertical pressure velocity at 500
mb for doubled atmospheric
CO2 content (29 CO2 - 19
CO2), enhanced albedo (Geo
- 29 CO2) and geoengineered
(Geo - 19 CO2) cases.
Vertical motion in height
coordinates (w, meter/day) can
be obtained from w = -x/(qg)
where x is the simulated
vertical pressure velocity. The
hatching indicates regions
where the changes are not
significant at the 99% level of
confidence. Significance level is
estimated using a Student t test
with sample of 40 annual means
and standard error corrected for
serial correlation (Zwiers and
von Storch 1995)
G. Bala, B. Nag: Albedo enhancement over land to counteract global warming
123
Central America (Fig. 3). The zonal mean profile of
changes in vertical motion at 500 mb clearly shows that the
sinking motion and declines in precipitation and runoff
over land in the enhanced albedo and geoengineered cases
are mostly confined to the tropical latitudes (Fig. 6).
An upper bound for the vertical motion over land or
oceans in the enhanced albedo or geoengineered case can
be estimated using the method adopted in (Bala et al.
2010b):
woT
ozþ Cd
� �¼ Q
where w is the vertical velocity, Cd is the dry adiabatic lapse
rate, (qT/qz) is the environmental lapse rate, and Q is the
diabatic heating rate. For illustrative purposes, we will make
an estimate of the sinking motion over land in the enhanced
albedo case. We use qT/qz = -7.5 9 10-3 K m-1 over
land, and Cd * 1.0 9 10-2 K m-1 (Holton 1992): we use
an environmental lapse rate over land that is between a dry
and moist adiabat. The change in diabatic heating rate in the
atmosphere is the change in net TOA energy flux since
the change in surface net flux is nearly zero (Table S2):
Q = -6.4 W m-2/(Mair*Cpair) where Mair(*104 kg m-2)
is the mass of air above a square meter and Cpair
(*1,000 J kg-1 K-1) is the specific heat capacity of air.
Substitution of the numerical values yields Q * -6.4 9
10-7 K s-1or *-6.4 9 10-2 K day-1, and w * -26 m
day-1 or x * 2.6 mb day-1 in pressure coordinates where
x is the pressure velocity. This value agrees well with the
value shown in Table 2 and Fig. 5 for the mid troposphere
for the enhanced albedo case.
The decrease in the net surface shortwave radiation and
increase in planetary albedo are confined to continental
areas such as central Africa, Amazon, Australia, North
America and Eurasia where the albedo enhancement is
imposed in the enhanced albedo and geoengineering cases
(Fig. 7). The reduction in net surface shortwave radiation
over land is much larger than the reduction over oceans in
these two cases: the global, land and ocean mean changes
in surface absorption of shortwave radiation are -1.98 ±
0.06, -4.92 ± 0.16, and -0.78 ± 0.07 W m-2, respec-
tively, in the enhanced albedo case and -3.10 ± 0.06,
-5.89 ± 0.14, and -1.97 ± 0.08 W m-2 in the geoengi-
neered case (Table S2). Total cloud amount changes do not
correlate well with net shortwave radiation or planetary
albedo changes (Fig. 7) because enhancement of albedo
over land was achieved through changing the cloud droplet
radius rather than cloud amounts. The correlation between
Fig. 4 Changes in global and
annual mean precipitation (P),
evaporation (E) and run-off
(P - E) due to doubled
atmospheric CO2 content
(29 CO2 - 19 CO2), enhanced
albedo (Geo - 29 CO2) and
geoengineered (Geo - 19
CO2) cases. The hatchingindicates regions where the
changes are not significant at the
99% level of confidence.
Significance level is estimated
using a Student t test with
sample of 40 annual means and
standard error corrected for
serial correlation (Zwiers and
von Storch 1995)
G. Bala, B. Nag: Albedo enhancement over land to counteract global warming
123
changes in total cloud fraction and planetary albedo is
between 0.51 and 0.59 for all the three cases and it is
between -0.53 and -0.60 for changes in total cloud
fraction and surface net solar radiation.
4.3 Fast response
When a climate forcing is imposed, the climate system
responds in all time scales in the real world. For our slab
ocean model, the longest time scale is dictated by the
thermal capacity of the mixed layer ocean which is about a
few decades. Fast response refers to rapid adjustments to
the climate system before the global mean surface tem-
perature changes. The rapid adjustments are associated
with fast changes in the atmosphere and land surface since
these components have much smaller heat capacity com-
pared to the mixed layer ocean. Though fast response is a
tiny fraction of the equilibrium climate change (discussed
in the previous section) for many variables such as water
vapour that are tightly coupled to the surface temperature,
it could constitute almost 40% of the total response for few
key variables like precipitation and evaporation on a global
mean basis (Bala et al. 2010a). Further, it has been dem-
onstrated that the climate sensitivity as well as hydrological
sensitivity, defined as the change in global mean precipi-
tation per unit warming, are independent of the forcing
mechanisms when the fast responses are excluded from the
definition of these sensitivities, suggesting that the slow
response or feedback (equilibrium climate change minus
fast response per unit temperature change) is independent
of the forcing mechanism. Therefore, it has been recom-
mended that the fast and slow response be compared sep-
arately in multi-model intercomparisons to discover and
understand robust responses in climate system (Bala et al.
2010a).
In this section, our main interest is to compare the
magnitudes of fast and slow responses and to find out the
fraction contributed by fast response to the total equilib-
rium climate change for key climate variables of interest
namely precipitation, evaporation, omega and runoff over
land (Table 3). The fast response is listed in Tables 1 and
S1 and equilibrium climate change in Tables 2 and S2. We
refer to the difference between equilibrium climate change
and fast response as ‘‘slow response’’ though by convention
this difference normalized by global mean surface tem-
perature is referred to as slow response or feedback. There
is no change in fraction of sea ice extent in prescribed SST
runs and hence the ratio for this variable is not listed in
Table 1. Equilibrium climate change in the geoengineering
case is too small which can lead to unrealistically large
values for the fraction and hence we do not list these
fractions for this case in Table 3.
We find that the fast response in global mean surface
temperature and precipitable water are smaller than slow
response in the 29 CO2 (relative to 19 CO2) and the
enhanced albedo (relative to 29 CO2) cases. (Table 3): fast
response contributes less than 10% to total global mean
surface temperature change and at most 15% to total global
mean precipitable water change in these cases. The non-
zero values in temperature over oceans are due to change in
surface temperature of the sea ice in this model. The global
mean changes in temperature and precipitable water are
primarily driven by changes over land which undergoes
rapid adjustment: in the enhanced albedo case, fast
response in precipitable water over land contributes 32% to
the total response.
Fast response constitutes a major fraction of the equilib-
rium response for precipitation, evaporation, P - E and
omega (vertical pressure velocity) in both 29 CO2 and
enhanced albedo cases (Table 3). The rapid response in
global mean precipitation is about 40% in the 29 CO2 in
agreement with the recent study (Bala et al. 2010a). How-
ever, the changes are vastly different between land and
oceans: the fast responses over land and ocean are 28 and
Fig. 5 Vertical profile of the changes in land-mean and ocean-mean
pressure velocity (omega). Negative values in omega changes
represent increases in upward motion and vice versa. Changes are
shown for doubled atmospheric CO2 content (29 CO2 - 19 CO2),
Ratio of fast response to slow response is shown in last two columns. Values in parenthesis show the ratio of fast response to equilibrium
response
Slow response = equilibrium climate change (Table 2)—fast response (Table 1)a Fast and slow responses have same signs and magnitude of fast response is smaller than slow responseb Fast and slow responses have same signs and magnitude of fast response is larger than slow responsec Fast and slow responses have opposite signs and magnitude of fast response is less than slow responsed Fast and slow responses have opposite signs and magnitude of fast response is larger than slow response
G. Bala, B. Nag: Albedo enhancement over land to counteract global warming
123
forcing compared with the impacts of surface temperature
change (Lambert et al. 2011). Clearly, more theoretical and
modelling studies on climate change and multi-model
intercomparisons are required to further our understanding
of the constraints. However, we believe that the triggering of
sinking motion in the atmosphere for an albedo increase is so
fundamental that all models should show at least qualitative
agreement with our results.
The main goal of our study is to investigate the hydro-
logical consequences of enhancing albedo over land sur-
face. For this purpose, we have used an idealized case of
enhancing the cloud albedo over land. Our simulations are
intended only to elucidate fundamental properties of the
climate system; this study is not intended to realistically
represent future albedo modification over land. In the real
world, surface albedo modifications are proposed for
pavements and roofs of urban areas (Akbari et al. 2009)
and large desert regions (Gaskill 2004), and we can only
infer from our study that there will be large adverse
regional impacts on the hydrology. Our simulations suggest
the likelihood of reduced rainfall over the regions where
albedo is enhanced on a large spatial scale. The implica-
tions of our study are not restricted to intentional albedo
changes alone: it is likely that unintentional albedo changes
from activities such as large scale deforestation and par-
ticulate pollution (and consequent brighter clouds) will also
lead to regional reduction in precipitation and runoff.
Acknowledgments We thank Prof. J. Srinivasan for his helpful
comments on the original manuscript. Suggestions and comments by
Dr. Hugo Lambert and two anonymous reviewers helped us to
improve the manuscript substantially. Financial support for B. Nag
was provided by the Divecha Center for Climate Change, Indian
Institute of Science. Generous computational resources were provided
by the Supercomputer Education and Research Center, Indian Insti-
tute of Science. Technical assistance by S. Krishna, B. Pavana and
Dr. Devaraju in preparing the illustrations in this paper is gratefully
acknowledged.
References
Akbari H, Menon S, Rosenfeld A (2009) Global cooling: increasing
world-wide urban albedos to offset CO2. Climatic Change
94(3–4):275–286
Fig. 8 Schematic diagram illustrating the changes in vertical motion at
500 mb over land and oceans in 29 CO2 (top left panel), enhanced
albedo (top middle panel) and geoengineered (top right panel) cases.
Radiative forcings (positive downward) over land and oceans in each
case are shown at the top. Vertical motion in height coordinates (w,
meter/day) is obtained from w = -x/(qg) where x is the model
simulated pressure velocity. Changes in surface temperature, precip-
itation and precipitation minus evaporation are also shown. Horizontal
arrows show the changes in run off over land (towards right is an
increase). Corresponding changes in a recent study on marine cloud
albedo enhancement (Bala et al. 2010b) are illustrated in bottom panels:
29 CO2 (bottom left panel), enhanced marine cloud albedo (bottommiddle panel) and geoengineered (bottom right panel) cases. It should
be noted that CAM3.1 simulations are used for top panels and bottompanels use CAM3.5 simulations
G. Bala, B. Nag: Albedo enhancement over land to counteract global warming
123
Allen MR, Ingram WJ (2002) Constraints on future changes in
climate and the hydrologic cycle. Nature 419(6903):224–232
Andrews T, Forster PM, Gregory JM (2009) A surface energy
perspective on climate change. J Clim 22:2557–2570
Angel R (2006) Feasibility of cooling the Earth with a cloud of small
spacecraft near the inner Lagrange point (L1). Proc Natl Acad
Sci USA 103(46):17184–17189
Bala G et al (2007) Combined climate and carbon-cycle effects of
large-scale deforestation. Proc Natl Acad Sci USA 104(16):
6550–6555
Bala G, Duffy PB, Taylor KE (2008) Impact of geoengineering
schemes on the global hydrological cycle. Proc Natl Acad Sci
USA 105(22):7664–7669
Bala G, Caldeira K, Nemani R (2010) Fast versus slow response in
climate change: implications for the global hydrological cycle.
Clim Dyn 35(2–3):423–434
Bala G et al (2010b) Albedo enhancement of marine clouds to
counteract global warming: impacts on the hydrological cycle.
Clim Dyn. doi:10.1007/s00382-010-0868-1
Ban-Weiss G, Cao L, Bala G, Caldeira K (2011) Dependence of
climate forcing and response on the altitude of black carbon
aerosols. Clim Dyn. doi:10.1007/s00382-011-1052-y
Betts RA et al (2007) Projected increase in continental runoff due to
plant responses to increasing carbon dioxide. Nature 448(7157):
1037–1041
Boer GJ (2011) The ratio of land to ocean temperature change under