ACPAtmospheric Chemistry and PhysicsACPAtmos. Chem. Phys.1680-7324Copernicus PublicationsGöttingen, Germany10.5194/acp-16-1479-2016Stratospheric sulfate geoengineering could enhance the terrestrial
photosynthesis rateXiaL.lxia@envsci.rutgers.eduhttps://orcid.org/0000-0001-7821-9756RobockA.https://orcid.org/0000-0002-6319-5656TilmesS.Neely IIIR. R.Department of Environmental Sciences, Rutgers University, New
Brunswick, NJ, USANational Center for Atmospheric Research, Atmospheric Chemistry
Division, Boulder, CO, USANational Centre for Atmospheric Science and the Institute of Climate
and Atmospheric Science, University of Leeds, Leeds, UKL. Xia (lxia@envsci.rutgers.edu)10February20161631479148918August201521September201520January201621January2016This work is licensed under a Creative Commons Attribution 3.0 Unported License. To view a copy of this license, visit http://creativecommons.org/licenses/by/3.0/This article is available from https://acp.copernicus.org/articles/16/1479/2016/acp-16-1479-2016.htmlThe full text article is available as a PDF file from https://acp.copernicus.org/articles/16/1479/2016/acp-16-1479-2016.pdf
Stratospheric sulfate geoengineering could impact the terrestrial carbon
cycle by enhancing the carbon sink. With an 8 Tg yr-1 injection of
SO2 to produce a stratospheric aerosol cloud to balance anthropogenic
radiative forcing from the Representative Concentration Pathway 6.0 (RCP6.0)
scenario, we conducted climate model simulations with the Community Earth
System Model – the Community Atmospheric Model 4 fully coupled to
tropospheric and stratospheric chemistry (CAM4–chem). During the
geoengineering period, as compared to RCP6.0, land-averaged downward visible
(300–700 nm) diffuse radiation increased 3.2 W m-2 (11 %). The enhanced diffuse radiation
combined with the cooling increased plant photosynthesis by
0.07 ± 0.02 µmol C m-2 s-1, which could
contribute to an additional 3.8 ± 1.1 Gt C yr-1 global gross
primary productivity without explicit nutrient limitation. This increase
could potentially increase the land carbon sink. Suppressed plant and soil
respiration due to the cooling would reduce natural land carbon emission and
therefore further enhance the terrestrial carbon sink during the
geoengineering period. This potentially beneficial impact of stratospheric
sulfate geoengineering would need to be balanced by a large number of
potential risks in any future decisions about the implementation of
geoengineering.
Introduction
Stratospheric sulfate injection is one of the most discussed geoengineering
strategies for manipulating the climate system to counteract anthropogenic
global warming (e.g., Crutzen, 2006; Wigley, 2006). Regularly injected
sulfate aerosol precursors could produce aerosols that would stay in the
stratosphere for 1–2 years, depending on the particle size and emission rate
(Rasch et al., 2008a; Niemeier et al., 2011).
This would reduce incoming solar radiation and therefore reduce the
temperature (e.g., Rasch et al., 2008a; Robock et al., 2008; Jones et al.,
2010; Berdahl et al., 2014). As explained in the initial design of the
Geoengineering Model Intercomparison Project (GeoMIP) experiment (Kravitz et
al., 2011), reducing the solar constant is another way to simulate sulfate
injection geoengineering, and is easier to implement in a climate model. It
was used in earlier geoengineering simulations (e.g., Govindasamy and
Caldeira, 2000), and also can be thought of as a model of satellites in space
blocking sunlight, as proposed by Angel (2006). Although the two methods
could both potentially cool the surface, if they could ever be implemented,
they would produce different climate responses, including stratospheric ozone
depletion, troposphere ozone change, downward ultraviolet radiation, and
downward diffuse radiation (e.g., Niemeier et al., 2013; Kalidindi et al.,
2015; Nowack et al., 2015). Climate changes due to sunshade geoengineering
and sulfate injection geoengineering have been extensively studied (Rasch et
al., 2008b, 2009; Robock, 2008), including enhanced stratospheric ozone
depletion (Tilmes et al., 2008; Heckendorn et al., 2009; Pitari et al.,
2014) and possible drought in summer monsoon regions (Robock
et al., 2008; Bala et al., 2008; Jones et al., 2013; Tilmes et al.,
2013). There are also a couple
studies of its impact on the ecosystem – mainly focusing on the net primary
productivity (Glienke et al., 2015; Kalidindi et al., 2015), the carbon cycle
(Tjiputra et al., 2015), and on agriculture (Pongratz et al.,
2012; Xia et al., 2014). However, diffuse radiation
perturbations and their biological consequences are only mentioned in a few
previous studies (e.g., Robock, 2008; Robock et al., 2009; Glienke et al.,
2015), and need to be comprehensively studied.
Volcanic eruptions as a natural analog of sulfate injection geoengineering
provide evidence that sulfate aerosols in the stratosphere cool the surface
and dramatically change the partitioning of downward direct and diffuse solar
radiation (Robock, 2000, 2005). After the Mt. Pinatubo eruption in 1991 there
was a sharp slowing of the CO2 atmospheric concentration growth rate.
This was mainly due to a strong terrestrial biosphere sink in the middle
latitudes of the Northern Hemisphere that balanced the stronger oceanic
CO2 outgassing due to a simultaneous El Niño and increasing
anthropogenic emission (Keeling et al., 1995; Ciais et al., 1995). Cooling
due to volcanic eruptions (Robock, 2000) might be one explanation of the
unusual biospheric sink, since the cooling benefits tropical plant growth and
reduces the release of CO2 by soil respiration and wildfires (Keeling et
al., 1995; Nemani et al., 2003). On the other hand, increased diffuse
radiation promotes plant productivity (Gu et al., 1999, 2002, 2003; Roderick
et al., 2001; Cohan et al., 2002; Farquhar and Roderick, 2003; Mercado et
al., 2009). In total, in 1992 and 1993, an additional
1.2–1.5 Gt C yr-1 was captured by terrestrial vegetation (Mercado et
al., 2009). Global dimming (reduction of downward shortwave radiation due to
tropospheric pollution after World War II) is another example of how diffuse
radiation promotes terrestrial vegetation growth (e.g., Wild, 2009; Mercado
et al., 2009). With the geographically varying changes in diffuse radiation
fraction (0 to +30 %) due to global dimming (1950–1980), the
terrestrial carbon sink increased by 0.4 Gt C yr-1 (Mercado et al.,
2009). A recent study also showed that Amazon fires of 1998–2007
increased the annual mean diffuse radiation by 3.4–6.8 % due to biomass
burning aerosols, which would benefit the net primary productivity by
1.4–2.8 % in the Amazonian forests and balance 33–65 % of the
annual carbon emissions from biomass burning (Rap et al., 2015). Long-term
sulfate injection geoengineering would produce a permanent sulfate aerosol
cloud in the stratosphere, and this long-term diffuse radiation enhancement,
together with the cooling effect, would likely play an important role in the
terrestrial carbon budget.
Model and experiment design
We used the full tropospheric and stratospheric chemistry version of the
Community Earth System Model 1 – Community Atmospheric Model 4 (CESM1
CAM4–chem) with horizontal resolution of
0.9∘× 1.25∘ lat–long and 26 levels from the surface
to about 40 km (3.5 mb) (Lamarque et al., 2012; Tilmes et al., 2015a, 2016)
to simulate two solar radiation management schemes: a specific sulfate
injection scenario and a solar constant reduction scenario. Since the
experiments are branched from the Climate Chemistry Model Initiative (CCMI)
runs in which CAM4–chem participates, we used the same configuration as the
reference run. Therefore we used the Community Land Model (CLM) version 4.0
with prescribed satellite phenology (CLM4SP) instead of the version of CLM
with a carbon–nitrogen cycle, coupled with CAM4–chem. This model calculates
vegetation photosynthesis under the assumption of prescribed phenology and no
explicit nutrient limitations (Bonan et al., 2011). With the satellite
phenology option, although nitrogen limitation is not explicitly included,
there is some inherent nitrogen limitation because nitrogen availability
limits the leaf area index in the satellite measurements used in CLM4SP, and
the model has been validated with gross primary productivity (GPP)
observations. Dynamic vegetation is not turned on in this study. The ocean
model does not include any biogeochemical calculations in this study.
The specific sulfate injection scenario is G4 Specified Stratospheric Aerosol
(G4SSA), which uses a prescribed stratospheric aerosol distribution to
simulate a continuous annual tropical emission into the stratosphere (at
60 mb) of 8 Tg SO2 yr-1 from 2020 to 2070, which produces a
radiative forcing of about -2.5 W m-2. The steady-state aerosol
surface area density has the highest value of
33.2 µm2 cm-3 in the tropics at 50–60 mb and gradually
decreases to 10–12 µm2 cm-3 at the poles (Tilmes et
al., 2015b). Starting on 1 January, 2070, the sulfate injection reduces
gradually to zero on 31 December, 2071 (Tilmes et al., 2015b). The G4SSA
simulation continues after the end of sulfate injection from 2072 to 2089 in order to
study the termination effect. Using specified stratospheric aerosols,
tropospheric aerosols are not changed, and therefore we cannot evaluate how
the geoengineered stratospheric sulfate aerosols would be transported into
the troposphere and affect tropospheric chemistry. Using a fixed
stratospheric aerosol distribution to compare the effect of geoengineered
stratospheric aerosols in different models is similar to what has been done
to investigate the impact of volcanic eruptions in chemistry climate model
comparison projects in the past. For more details on the prescription of
stratospheric aerosols in CAM4–chem, see Neely III et al. (2015). The
reference simulation is the Representative Concentration Pathway 6.0 (RCP6.0)
(Meinshausen et al., 2011) from 2004 to 2089. We have run three ensemble
members for both G4SSA and RCP6.0.
The solar constant reduction scenario is the G3 solar constant reduction (G3S)
which reduces the solar constant to balance the forcing of the Representative
Concentration Pathway 4.5 (RCP4.5) (Meinshausen et al., 2011) and keeps the
temperature close to 2020 values. This solar reduction geoengineering
scenario is from 2020 to 2069, and its reference run is RCP4.5 from 2004 to
2089. The reason we used different reference runs (RCP4.5 and RCP6.0) for the
two experiments (G3S and G4SSA) is that they come from different phases of
GeoMIP. G3S was initiated before G4SSA at the start of GeoMIP and the
reference run for the first phase of GeoMIP was RCP4.5. G4SSA
participates in both GeoMIP and CCMI. Since RCP6.0 is the standard reference
run for CCMI, to encourage more climate chemistry modeling groups to
participate in G4SSA and generate robust understanding of how atmospheric
chemistry responds to sulfate injection geoengineering, Tilmes et
al. (2015b) proposed that G4SSA be based on
RCP6.0. Since the anthropogenic forcing is very similar for RCP4.5 and RCP6.0
between 2020 and 2070, we expect very little difference between the two
experiments. The basic principle that solar dimming does not affect
stratospheric ozone or produce diffuse radiation, in the way that stratospheric aerosols
do, is well illustrated by the G3S results. Both G3S and RCP4.5 have only one
ensemble member each.
Global-average (a) temperature, (b) precipitation,
(c) low cloud coverage, and (d) surface downward solar
radiation under G4SSA sulfate injection geoengineering (blue lines) and under
RCP6.0 (red lines). Land-average (e) surface downward visible direct
radiation, (f) diffuse radiation, (g) surface evaporation,
(h) canopy transpiration, and (i) vegetated land top 10 cm
soil water (liquid water and ice) content under G4SSA (blue lines) and RCP
6.0 (red lines). The three red lines and blue lines indicate three ensemble
members of RCP6.0 and G4SSA. Sulfate injection starts at 2020 and ends at
2070.
ResultsClimate and radiation response
Under the RCP6.0 scenario, the anthropogenic greenhouse gas radiative forcing
increases global average surface air temperature from 288.5 to 290.2 K
during the period of 2004–2089 (Fig. 1a). The higher temperature enhances
the hydrological cycle, and therefore global precipitation as well as land
average evaporation (Fig. 1b, g) increase. Global soil water content (top 10 cm,
including liquid water and ice) slightly increases with global warming
(Fig. 1i). The global surface downward solar radiation gradually decreases by
about 1 W m-2 during the period 2004–2089 (Fig. 1d) as the total
cloud coverage increases, particularly low-cloud coverage, which increases by
0.7 % (Fig. 1c). However, the land-average visible direct solar radiation
shows an upward trend (Fig. 1e) due to the effects of gradual tropospheric
aerosol reductions under RCP6.0. The downward total solar radiation averaged
over land (not shown) also has a slight increasing trend from 2004 to 2089,
which is opposite to the globally averaged surface solar radiation trend.
There are two reasons for this: the reduction in aerosol emissions mainly
affects the continents and the increase of cloud coverage is mainly over the
ocean. Averaged visible diffuse radiation (300–700 nm) over land decreases
in RCP6.0 (Fig. 1f) due to the decrease of aerosol emission in the RCP6.0
scenario (Meinshausen et al., 2011). Under this global warming scenario,
vegetated-land-averaged canopy transpiration decreases mainly due to
increasing CO2 (Fig. 1h) (Reddy et al., 1995).
With 1.6 W m-2 less total surface solar radiation (Fig. 1d), G4SSA
successfully cools the surface by 0.8 ± 0.2 K as compared to RCP6.0
(Fig. 1a). This cooling slows down the hydrology cycle with less average
precipitation (-0.07 mm day-1, -2.5 %) (Fig. 1b), less ground
evaporation (Fig. 1g), and less global low-cloud coverage (Fig. 1c), which is
consistent with previous studies (e.g., Niemeier et al., 2013; Tilmes et al.,
2013; Jones et al., 2013; Kalidindi et al., 2015). Furthermore, there is no change in
the soil water content under G4SSA and RCP6.0 scenarios (Fig. 1i). Visible
diffuse radiation over the land increases significantly (Fig. 1f) as the
sulfate aerosols in the stratosphere (3.0 Tg S equilibrium loading; Tilmes
et al., 2015b) scatter solar radiation. Therefore, while the total surface
solar radiation reduces by 1.6 W m-2, the visible diffuse solar
radiation increases by 3.2 W m-2 over land under all sky
conditions. Kalidindi et al. (2015) showed that with a 20 Tg sulfate aerosol
(SO4) stratospheric loading to balance the radiative forcing of
2 × CO2, broadband diffuse
radiation would increase by 11.2 W m-2 compared with the reference
run. However they used a very unrealistic stratospheric aerosol distribution,
with a very small effective radius of 0.17 µm and uniform
geographical distribution. In fact, 3 months after the eruption of Mt. Pinatubo in
1991, broadband diffuse radiation increased from 40 to 140 W m-2 under
clear-sky conditions at the Mauna Loa observatory (Robock, 2005), but only
the edge of the Pinatubo cloud was over Mauna Loa, and the maximum effect was
even greater. The photosynthesis rate of a northern hardwood forest (Harvard
Forest) increased 23 % in 1992 compared with an unperturbed year (1997)
(Gu et al., 2003). Therefore, under this sulfate injection geoengineering
scenario, which is equivalent to one 1991 Pinatubo eruption every 2.5 years
(Bluth et al., 1992) with the assumption that all sulfate aerosol will reach
the stratosphere, diffuse radiation enhancement is expected to enhance the
terrestrial photosynthesis rate and potentially increase the land carbon
sink. Furthermore, the drier, cooler, and more diffuse radiation environment
under G4SSA reduces the canopy transpiration comparing with RCP6.0 (Fig. 1h)
(Kanniah et al., 2012), which may indicate that less CO2 is released
back to the atmosphere by plant respiration.
Solar constant reduction climate intervention (G3S) efficiently cools the
surface as well. Since there is less radiative forcing reduction due to the
experiment design, the annual global-averaged temperature reduction
(gradually from 0 to 0.8 ∘C) is less than the reduction in G4SSA.
Precipitation and ground evaporation also reduce under G3S. However, G3S has
no effect on diffuse radiation compared with RCP4.5, since there are no
additional aerosols injected into the atmosphere. The overall trend of
surface visible diffuse radiation in both G3S and RCP4.5 slowly decreases
because of decreasing emissions (the tropospheric aerosol removal effect in
RCP4.5, not shown). Although the two experiments have different radiative
forcing reductions: 2.5 W m-2 for G4SSA and 0–1.5 W m-2 for
G3S, we expect linear changes in temperature and precipitation corresponding
to the radiative forcing change (Irvine et al., 2010; Kravitz et al., 2014).
We focus on the diffuse radiation effect in this study, which is included in
G4SSA and excluded in G3S due to the experiment design. Therefore, it is
reasonable to compare the two experiments with regard to their diffuse radiation
effect on photosynthesis.
Diffuse radiation and climate change impacts on vegetation
photosynthesis rate
Diffuse radiation is more advantageous for plant productivity than direct
radiation (e.g., Gu et al., 2002) because diffuse radiation provides more
homogeneous distribution of radiation within the canopy and more light can be
absorbed by shaded leaves without exceeding the photosynthetic capacity of
the plants. Increased diffuse radiation within a certain range will promote
plant net production productivity and therefore enhance the carbon sink
(Niyogi et al., 2004; Misson et al., 2005; Oliveira et al., 2007). However,
if the aerosol load exceeds a certain level it will suppress photosynthesis
(Chameides et al., 1999; Cohan et al., 2002). Knohl and
Baldocchi (2008) and Mercado et al. (2009) estimated that
the tipping point of the diffuse radiation effect is a ratio of 0.40–0.45
between diffuse radiation and total solar radiation; this is the maximum
ratio with a positive effect on plant photosynthesis. Under our sulfate
injection climate intervention scenario, the ratio of diffuse radiation and
total solar radiation increases from 0.296 to 0.333. Therefore, the increase
of diffuse radiation in our study would have a positive impact on plant
photosynthesis.
Land average photosynthesis rate without explicit nutrient
limitation (a) under sulfate injection geoengineering (G4SSA) (blue
lines) and RCP6.0 (red lines) and (b) under solar constant reduction
geoengineering (G3S) (blue line) and RCP4.5 (red line).
Without explicit nutrient limitation, simulated land average photosynthesis
would continuously increase in the future due to the stronger CO2
fertilization effect as the CO2 concentration increases from 377 ppm
(2004) to 632 ppm (2089) (Fig. 2a) (e.g., Allen et al., 1987; Leakey et al.,
2009). However, this model-simulated increase may not be realistic, since the
actual photosynthesis rate is limited by the amount of soil nutrients such as
nitrogen and phosphorus (e.g., Vitousek and Howarth, 1991; Davidson et al.,
2004; Elser et al., 2007). Under the G4SSA scenario, global-averaged
photosynthesis increases to
0.07 ± 0.02 µmol C m-2 s-1 compared with that in
the RCP6.0 scenario (Fig. 2a). This enhancement is due to the combination of
the climate changes, such as cooling, and diffuse radiation enhancement.
Different types of plants show maximum photosynthesis rates at certain
optimal temperatures, depending on CO2 concentrations (e.g., Sage and
Kubien, 2007). Figure 3 shows that the photosynthesis rate in different
regions responds to G4SSA differently and temperature plays an important
role. In general, the cooling effect from solar radiation management would
increase photosynthesis in tropical regions where there is likely to be
extreme heat stress under the global warming scenario, and slow down
photosynthesis in high latitude regions, since the temperature has not
exceeded the optimal temperature even under the global warming scenario. In
the tropics, the photosynthesis rate change has an increasing trend (Fig. 3),
because the cooling effect of G4SSA benefits photosynthesis more when global
warming gets severe. Furthermore, the large variation of the photosynthesis rate
change in the tropics (Fig. 3) might be related to the strong sensitivity of
tropical forest to precipitation change (Phillips et al., 2009; Tjiputra et al., 2015).
Figure 2b shows the photosynthesis rates in G3S and RCP4.5. Without the
diffuse radiation effect, the land-averaged photosynthesis rate has no
significant change under solar radiation management (G3S). The cooling effect
on photosynthesis has been canceled out by combining increases in tropical
regions and decreases in temperate regions (Fig. 4b). Therefore, the increase
of the photosynthesis rate in Fig. 2a under the G4SSA scenario is primarily
caused by the enhancement of diffuse radiation.
Regional-averaged annual photosynthesis rate difference of G4SSA
minus RCP6.0 from 2020 to 2069 when sulfate injection geoengineering
was applied.
Without explicit nutrient limitation, the increase of the photosynthesis rate
is almost entirely over vegetated land during years 2030–2069 of G4SSA
compared with RCP6.0 (Fig. 4a) as a combination impact of climate factors
controlling plant photosynthesis (Fig. 5). The strongest increase is in the
Amazon rainforest with a value of 1.42 µmol C m-2 s-1
(26.3 %) (Fig. 4a), where multiple layers of the canopy, especially the
tallest canopy, would receive more diffuse radiation, and the cooling helps
plant growth during the entire year. Those two positive impacts of diffuse
radiation and surface temperature changes from G4SSA are countered by the
negative impacts from the regional reductions of soil water content (not
shown here) and the global reduction of total solar radiation (Fig. 5b and
c). In a previous study, precipitation was found to be the largest climate
factor controlling GPP during 1998–2005 (Beer et al., 2010). Considering that the global forest carbon
sink was 2.41 ± 0.42 Gt C yr-1 during the period of 1990–2007,
and the Amazon rainforest contributes ∼ 25 % (Pan et al., 2011),
increasing its photosynthesis rate by 4.2 ± 5.9 % would potentially
help to bring more carbon out of the atmosphere. Since, in reality, most
Amazonian soils are highly weathered and relatively nutrient poor, this
simulated increase might be overestimated (Davidson et al., 2004). However, in our study, the prescribed plant
phenology has some inherent nutrient limitation, and therefore the
overestimation should not be substantial. In high latitude and high altitude
regions, although increasing diffuse radiation still increases the
photosynthesis rate, temperature reduction has a negative impact on
photosynthesis (Fig. 5a), which is consistent with a previous study (Glienke
et al., 2015), and the stronger temperature reduction in high latitude
regions would reduce the photosynthesis rate (Fig. 4a). Over high altitude
regions, such as the Rocky Mountains and the Himalayas, increased snow cover
(not shown here) contributes to the reduction of photosynthesis under G4SSA
as well. The expected reduction in the stratospheric ozone column in high
latitudes, due to increased heterogeneous reactions promoting
ozone-destroying cycles, increases UV radiation (e.g., Pitari et al.,
2014), which will not be further investigated in this study.
Furthermore, changes in tropospheric chemistry and stratosphere–troposphere
exchange due to G4SSA could modify the surface ozone concentration
regionally, which may be another potential impact on the photosynthesis rate.
Further investigation of those issues is needed.
(a) Photosynthesis rate differences between G4SSA and
RCP6.0 during years 2030–2069 (sulfate injection period, excluding the first
10 years). (b) Photosynthesis rate anomaly between G3S and RCP4.5
during years 2030–2069 of solar reduction. Hatched regions are areas with
p> 0.05 (where changes are not statistically significant based on a
paired t test).
Without the diffuse radiation effect, the photosynthesis rate differences
between G3S and RCP4.5 are not significant in more regions (Fig. 4b) than for
the differences between G4SSA and RCP6.0. The Amazon rainforest still has the
largest photosynthesis increase, with a maximum value of
1.24 µmol C m-2 s-1, but the average photosynthesis
change in the Amazon region is only 0.7 ± 5.7 %. The two climate
interventions (G4SSA and G3S) have different assumptions and different
reference runs (RCP6.0 and RCP4.5) and they have different levels of cooling,
different precipitation changes, and different CO2 concentrations. We
cannot, therefore, evaluate how much the enhancement of diffuse radiation
contributes to the increase of photosynthesis. When comparing the global-averaged photosynthesis change (Fig. 2) with the cooling effect, the diffuse
radiation change does increase the carbon uptake significantly with a
p value less than 0.002.
Diffuse radiation and climate change impacts on the terrestrial
carbon sink
We have calculated the additional carbon sink due to the increase of
photosynthesis. Using the land area (1.5 × 108 km2) in
CLM, for G4SSA, the global land average photosynthesis rate increases
0.07 ± 0.02 µmol C m-2 s-1 compared with RCP6.0.
Therefore, the increase of the photosynthesis rate without explicit nutrient
limitation would increase GPP by 3.8 ± 1.1 Gt C yr-1 from
terrestrial vegetation. Mercado et al. (2009) estimated that after the 1991
eruption of Mt. Pinatubo the land carbon sink increased by 1.13 in 1992 and
1.53 Gt C yr -1 in 1993, which was the result of both diffuse
radiation and the cooling effect. The diffuse radiation effect was the
dominant factor in 1992 (1.18 Gt C yr-1), while it was much less
significant in 1993 (0.04 Gt C yr-1).
Correlation coefficient of the monthly photosynthesis rate anomalies
in JJA during years 2030–2069 (G4SSA minus RCP6.0, Fig. 3a) and
(a) surface temperature anomalies, (b) top 10 cm soil
water (including liquid water and ice) anomalies, (c) surface
downward solar radiation anomalies, and (d) surface visible diffuse
radiation anomalies during years 2030–2069.
Discussion
Our result of increasing of gross primary productivity due to enhanced
stratospheric aerosols has uncertainties and needs to be further evaluated
with new experiments using multiple Earth system models. Since the
carbon–nitrogen cycle in CLM4 is turned off, leaf area index (LAI) cannot be
diagnosed by the climate changes due to G4SSA and hence the photosynthesis
response may be biased. However, even if we use CLM4CN with the
carbon–nitrogen cycle modeled, the photosynthesis response would still be
imperfectly modeled, since there is a high bias in the LAI simulation and
structural errors in the leaf photosynthesis process (Lawrence et al.,
2012). Also, without dynamic vegetation, our study keeps a prescribed plant
functional type during the whole simulation, and cannot simulate plant type
change under a different climate.
Another source of uncertainty is the use of only one climate model. Jones et
al. (2013) and Glienke et al. (2015) showed that there is a large range of
simulated net primary productivity (NPP) changes as the CO2
concentration increases or under solar reduction geoengineering using
different land models, which is mainly due to the availability of a nitrogen
cycle. With a nitrogen cycle, there is a much smaller CO2 fertilization
effect on plant growth. We expect that with the carbon–nitrogen cycle turned
on, the upward trend of the photosynthesis rate under both G4SSA and RCP6.0
in Fig. 2a will be reduced. Furthermore, models respond to different climates
at the same atmospheric CO2 concentration differently. Eight models
participating in the GeoMIP G1 (instantaneously quadrupling of the CO2
concentration (abrupt4xCO2) while simultaneously reducing the solar constant
to balance the forcing) (Kravitz et al., 2011) showed different and even
opposite trends of NPP changes between abrupt4xCO2 and G1 because of
different behaviors in GPP and respiration (Glienke et al., 2015). In G1, GPP
as well as NPP reduced compared with abrupt4xCO2 using CCSM4 (CAM4
coupled with CLM4CN). However, G1 has a much stronger temperature reduction
and no diffuse radiation change. Considering the inconsistent responses of
models to geoengineering-induced climate changes even with the same CO2
concentration, multiple model study is necessary to better understand how
photosynthesis and NPP would change under sulfate injection geoengineering.
Sulfate injection geoengineering could potentially change the terrestrial
carbon sink since it might increase GPP compared with a global warming
scenario due to the diffuse radiation and other climate changes. However, to
further investigate this issue, we need to consider other mechanisms that
sulfate injection geoengineering would trigger. The cooling effect would also
suppress plant and soil respiration. After the eruption of Mt. Pinatubo, the
terrestrial carbon sink increased due to both the cooling effect (Ciais et
al., 1995; Keeling et al., 1995) and the diffuse radiation fertilization
effect (Jones and Cox, 2001; Lucht et al., 2002). Mercado et al. (2009)
estimated that the cooling effect and diffuse radiation equally contributed
to the enhancement of the terrestrial net primary productivity changes in
1992, since the cooling effect suppresses soil respiration and reduces carbon
emissions. In 1993, the cooling effect actually enhances the land carbon sink
more than the diffuse radiation. Since heterotrophic respiration (the decomposition of soil organic carbon) might be more sensitive to temperature change than GPP (Jenkinson et al., 1991) that would further enhance the terrestrial carbon sink due to cooling from sulfate injection geoengineering. Therefore, if
we include the reduction of plant and soil respiration due to the cooling
effect, land processes would capture even more carbon in sulfate injection
geoengineering scenarios. However, current land models tend to simulate soil
organic carbon decomposition under climate changes in a simple way, which
might not be able to accurately predict the temperature sensitivity of global
soil organic carbon decomposition as well as the terrestrial carbon cycle
change under future climate changes (Davidson and Janssens, 2006).
In our simulations, the CO2 concentration is prescribed in both G4SSA
and RCP6.0, but we expect that the CO2 concentration of G4SSA might be
lower than the global warming scenario due to the diffuse radiation and the
cooling effects, since this CO2 concentration change has been observed
after volcanic eruptions due to enhanced land carbon sinks (Keeling et al.,
1995; Ciais et al., 1995). The predicted CO2 concentration increase rate
based on industrial emissions in the early 1990s was 1.7 % yr-1,
but the observed CO2 concentration after 1991 declined instead of
increasing. However, the atmospheric CO2 concentration is also highly
impacted by another carbon reservoir, the ocean. The ocean covers most of
Earth, and CO2 feedbacks from geoengineering will also occur in the
ocean, including responses dependent on the ocean surface temperature, ocean
biological processes, and changing ocean dynamics (Tjiputra et al., 2015).
For example, an El Niño will cause the ocean to temporarily emit more
CO2 to the atmosphere. Although idealized geoengineering experiments
have not shown any significant effect on El Niño (Gabriel and Robock,
2015), a longer period of geoengineering might impact ocean circulation. The
ocean model we used simulates dynamical and temperature responses, but does
not include a biochemical and carbon cycle. Such responses will need to be
included for an integrated assessment of the impacts of geoengineering on the
global carbon budget.
Although there have been many reasons to be hesitant about the implementation
of geoengineering (Robock, 2012, 2014), sulfate injection climate
intervention may have a great potential to increase land GPP, reduce the
terrestrial carbon source, and change the ocean carbon cycle. More studies
are needed to further understand the details of each process.
Conclusions
With our experimental design, simulated stratospheric sulfate geoengineering
with 8 Tg yr-1 injection of SO2 would change the partitioning of
solar radiation with an increase of surface diffuse radiation of about
3.2 W m-2 in visible wavelengths over land. This enhanced diffuse
radiation combining with other climate changes, such as cooling, soil water
content change, and total solar radiation reduction, increased plant
photosynthesis rates significantly in temperate and tropical regions, and
reduced the photosynthesis rate in high latitude and mountain regions.
Overall, the increase of the land-averaged photosynthesis rate is
0.07 ± 0.02 µmol C m-2 s-1, which could
contribute to an additional 3.8 ± 1.1 Gt C yr-1 global carbon
sink. These results are affected by the experimental design, since the
carbon–nitrogen cycle and dynamic vegetation are not included. Further
investigation is needed to fully understand the contribution of enhanced
diffuse radiation due to sulfate geoengineering on the terrestrial carbon
sink.
Acknowledgements
This work is supported by U.S. National Science Foundation (NSF) grants AGS-1157525 and GEO-1240507. Computer
simulations were conducted on the National Center for Atmospheric Research
(NCAR)
Yellowstone supercomputer. NCAR is
funded by the NSF. The CESM project is supported by the NSF and the Office of Science (BER) of the US Department of Energy.
We thank Jean-Francois Lamarque, Daniel Marsh, Andrew Conley, Louisa K. Emmons, Rolando R. Garcia, Anne K. Smith, and
Douglas E. Kinnison for the CAM4–chem development. We thank Peter Lawrence
and Danica Lombardozzi for helping us understanding how CLM4 calculates
photosynthesis. R. R. Neely III was supported by NSF via NCAR's Advanced
Study Program. We thank the reviewers, who helped to substantially improve
this work. Edited by: B. Kravitz
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