ACPAtmospheric Chemistry and PhysicsACPAtmos. Chem. Phys.1680-7324Copernicus PublicationsGöttingen, Germany10.5194/acp-17-11209-2017Sulfate geoengineering impact on methane transport and lifetime: results from the Geoengineering Model Intercomparison Project (GeoMIP)VisioniDanieledaniele.visioni@aquila.infn.ithttps://orcid.org/0000-0002-7342-2189PitariGiovanniAquilaValentinaTilmesSimoneCionniIreneDi GenovaGlaucoManciniEvaDepartment of Physical and Chemical Sciences, Università dell'Aquila, 67100 L'Aquila, ItalyCETEMPS, Università dell'Aquila, 67100 L'Aquila, ItalyGESTAR/Johns Hopkins University, Department of Earth and Planetary Science, 3400 N Charles Street, Baltimore, MD 21218, USANational Center for Atmospheric Research, Boulder, CO 80305, USAENEA, Ente per le Nuove Tecnologie, l'Energia e l'Ambiente, 00123 Rome, ItalyDaniele Visioni (daniele.visioni@aquila.infn.it)21September2017171811209112265July201711July20177September201711September2017This work is licensed under the Creative Commons Attribution 4.0 International License. To view a copy of this licence, visit https://creativecommons.org/licenses/by/4.0/This article is available from https://acp.copernicus.org/articles/17/11209/2017/acp-17-11209-2017.htmlThe full text article is available as a PDF file from https://acp.copernicus.org/articles/17/11209/2017/acp-17-11209-2017.pdf
Sulfate geoengineering (SG), made by sustained injection of SO2 in the
tropical lower stratosphere, may impact the CH4 abundance through several
photochemical mechanisms affecting tropospheric OH and hence the methane
lifetime. (a) The reflection of incoming solar radiation increases the
planetary albedo and cools the surface, with a tropospheric H2O decrease.
(b) The tropospheric UV budget is upset by the additional aerosol scattering
and stratospheric ozone changes: the net effect is meridionally not uniform,
with a net decrease in the tropics, thus producing less tropospheric
O(1D). (c) The extratropical downwelling motion from the lower
stratosphere tends to increase the sulfate aerosol surface area density
available for heterogeneous chemical reactions in the mid-to-upper troposphere,
thus reducing the amount of NOx and O3 production. (d) The tropical
lower stratosphere is warmed by solar and planetary radiation absorption by
the aerosols. The heating rate perturbation is highly latitude dependent,
producing a stronger meridional component of the Brewer–Dobson circulation.
The net effect on tropospheric OH due to the enhanced
stratosphere–troposphere exchange may be positive or negative depending on
the net result of different superimposed species perturbations (CH4,
NOy, O3, SO4) in the extratropical upper troposphere and lower
stratosphere (UTLS). In addition, the atmospheric stabilization resulting
from the tropospheric cooling and lower stratospheric warming favors an
additional decrease of the UTLS extratropical CH4 by lowering the
horizontal eddy mixing. Two climate–chemistry coupled models are used to
explore the above radiative, chemical and dynamical mechanisms affecting
CH4 transport and lifetime (ULAQ-CCM and GEOSCCM). The CH4 lifetime may
become significantly longer (by approximately 16 %) with a sustained
injection of 8 Tg-SO2 yr-1 starting in the year 2020, which implies an increase of tropospheric
CH4 (200 ppbv) and a positive indirect radiative forcing of sulfate
geoengineering due to CH4 changes (+0.10 W m-2 in the 2040–2049
decade and +0.15 W m-2 in the 2060–2069 decade).
Introduction
Many geoengineering methods have been proposed in order to temporarily
balance out the direct effect of the increase of anthropogenic greenhouse
gas emissions (). Amongst those, stemming from the
observations of the effects of large volcanic eruptions, is the injection of
sulfate aerosol precursors (e.g., SO2) into the stratosphere
(). The injection above the
tropopause of very large amounts of particles and sulfur gases due to
explosive volcanic eruptions is able to increase the stratospheric aerosol
optical depth by more than 1 order of magnitude. The initial volcanic
SO2 plume quickly nucleates into H2SO4 vapor (),
producing an optically thick cloud of sulfate aerosols
(). The high reflectivity of these
aerosols effectively decreases the amount of solar radiation reaching the
Earth's surface, thus producing a net global cooling. In 1991, for example, the
Pinatubo eruption produced a reduction of the global surface air temperature
from 0.5 K () to 0.14 K using detrended analyses
().
Besides the direct effect on surface temperatures, however, there is the need
for a thorough examination of other effects on atmospheric circulation and
chemical composition of the troposphere and stratosphere brought about by the
increase in lower stratosphere optical thickness (). The
interaction of the H2SO4 particles with solar radiation is twofold: the
aerosols increase the amount of radiation that is reflected and scattered but
they also absorb part of it in the near-infrared wavelengths, increasing the
lower stratospheric diabatic heating rates. This causes a local positive
temperature change () which induces a significant increase
of westerly winds from the thermal wind equation with peaks at midlatitudes
in the midstratosphere (). These dynamical changes tend
to increase the amplitude of planetary waves in the stratosphere and to
enhance the tropical upwelling in the rising branch of the Brewer–Dobson
circulation (). For continuity, a stronger
downward component is found in the lower branch of the Brewer–Dobson
circulation ().
These dynamical changes can bring about modification in the concentration and
growth rate of long-lived species that act as greenhouse gases, such as
N2O and CH4, as observed in the case of the Pinatubo eruption
(). An increase in the downward mid- and high-latitude
fluxes in the lower stratosphere ends up advecting more
stratospheric air below the tropopause, thus decreasing the tropospheric
concentration of these gases. In addition, the horizontal eddy mixing in the
upper troposphere and lower stratosphere (UTLS) is lowered as a consequence of the atmospheric stabilization resulting
from the tropospheric cooling and lower stratospheric warming, thus
decreasing the isentropic transport of CH4 and N2O from the tropical
pipe towards the midlatitudes. This favors an additional decrease of the
UTLS extratropical downward fluxes of CH4 and other long-lived species
(). The overall effect on tropospheric OH due this
enhanced stratosphere–troposphere exchange and perturbed UTLS horizontal
mixing may be positive or negative depending on the net result of different
superimposed species perturbations in the UTLS (CH4, NOy, O3).
Coupled with this perturbation of the stratosphere–troposphere exchange, the
lifetime of long-lived species with tropospheric OH sink can also be modified
by other changes brought about by an injection of tropical stratospheric
aerosols: (a) the surface cooling would directly lessen the amount of water
vapor, thus lowering the tropospheric OH concentration; (b) the tropical
tropospheric UV decrease due to enhanced radiation scattering would reduce
the production of O(1D), thus decreasing OH production from
O(1D) + H2O; (c) the increasing aerosol surface area density (SAD)
would enhance heterogeneous chemistry in the mid-to-upper troposphere, which
reduces the amount of NOx and the rate of O3 production, both
negatively affecting the amount of tropospheric OH. Since CH4 is depleted
by the OH radical, all these changes would mean an increase in methane
lifetime (). The aim of this study is to
evaluate the chemical, radiative and dynamical effects of a sustained
injection of SO2 in the stratosphere on the lifetime and abundance of
CH4.
Summary of main model features. The sixth column includes the stratospheric
aerosol effective radius (reff in µm) at 20 km over the
tropics (2040–2049). Values deduced from SAGE-II observations are
0.22 µm (σ= 0.02 µm) as an average over
1999–2000 for unperturbed background conditions and 0.57 µm
(σ= 0.03 µm) as an average over July 1992–June 1993
for a volcanic perturbation (i.e., Pinatubo) comparable in magnitude to G4
with 5 Tg-SO2 injection (in terms of average stratospheric mass burden of
sulfate). G4 aerosols are injected at the Equator between 16 and 25 km
altitude (uniformly) for GEOSCCM and between 18 and 25 km (Gaussian
distribution) for ULAQ-CCM. MBC indicates the mixing ratio boundary condition and FBC indicates flux boundary condition.
a Latitude by longitude horizontal resolution, number
of vertical layers and model top atmospheric pressure. b Forced
with background aerosols from SAGE-II data for 1999. c The model is
the same as described in , but in this case it was run with
no interactive chemistry. d Quasi-biennial oscillation (QBO) internally generated using a
gravity wave drag parameterization and resolved wave forcing.
e ULAQ-CCM includes aerosol microphysics (RCP4.5
reff= 0.19 µm).
The paper is organized in seven subsequent parts. Section 2 includes a
description of participating models. In Sect. 3, a model evaluation for long-lived
species stratospheric abundance and transport is presented using
available satellite observations. Section 4 analyzes the sulfate
geoengineering (SG) induced perturbations on stratospheric species transport,
while Sect. 5 discusses the effects on tropospheric chemistry and CH4
direct and indirect radiative forcing components, with the overall main
conclusions discussed in Sect. 6.
Model experiments
The characteristics of the experiment follow the description of experiment G4
in the Geoengineering Model Intercomparison Project (GeoMIP)
(). The G4 experiment consists of a constant yearly
injection of SO2 in the tropical lower stratosphere. The SO2 injection
is handled by the single models in the same way they simulate the Pinatubo
eruption in terms of injection height. The background anthropogenic forcing
corresponds to the one from the Representative Concentration Pathway 4.5
(RCP4.5) (). Starting from 2020, 8 (or
5) Tg-SO2 yr-1 are injected in the stratosphere with a sudden stop
after 50 years. An additional 20 years of model simulations are performed (up to
2090) in order to assess the termination effects of the sulfur injection. The
choice of the different amounts of injected SO2 follows two reasons: for
some of the analyses, we have decided to use the same simulations used in
with 5 Tg-SO2 yr-1. However, two experiments
with varying sea surface temperatures (SSTs) have also been carried out with
ULAQ-CCM to identify possible changes due to these dynamics-driving
mechanisms; for this reason, an injection of 8 Tg-SO2 yr-1 was
performed with ULAQ-CCM in order to use the CCSM-CAM4 surface temperatures
that resulted from a 8 Tg-SO2 yr-1 injection. The main features of
the participating models are summarized in Table .
One of these models (CCSM-CAM4) is an atmosphere–ocean coupled model and it
has been used (without interactive chemistry) to calculate the surface
temperature evolution from 2010 to 2090 for a reference RCP4.5 case and a
geoengineering G4 perturbed case with 8 Tg-SO2 yr-1 injected
continuously from 2020 to 2070 (). One of the other two
models (ULAQ-CCM) has assimilated surface temperatures calculated in the
CCSM-CAM4 atmosphere–ocean coupled model for the reference RCP4.5 and the
perturbed G4 cases (i.e., two different datasets for surface temperatures),
whereas the third model (GEOSCCM) has run the G4 case with RCP4.5 SSTs
assimilated from the CESM atmosphere–ocean coupled model. Both models
prescribe CH4 mixing ratios at the surface (except in one numerical
experiment of ULAQ-CCM where emission fluxes are used, as discussed below)
and do not include changes in emission fluxes due to surface temperature
modifications. A more detailed description of these numerical models can be
found in and .
In order to properly assess the different contributions to CH4 changes
discussed before, three different experiments have been carried out with the
ULAQ-CCM model: experiments (a) and (b) use appropriate surface temperatures for
RCP4.5 and G4 cases (as previously explained), with surface CH4 treated
under mixing ratio boundary condition (MBC) and flux boundary condition (FBC)
approaches for (a) and (b), respectively. Experiment (c), on the other hand,
uses the same SST for both RCP4.5 and G4 cases (as in GEOSCCM), with the
purpose of highlighting the impact of SST changes on the G4-RCP4.5 large-scale
transport perturbations. The full list of numerical experiments
completed with the three models is presented in Table .
Summary of numerical experiments with ensemble size. The amount of injected SO2 (per year) is specified between brackets in the G4 column.
ModelRCP4.5G4Used forCCSM-CAM422 (8 Tg-SO2)SSTs for the ULAQ-CCM simulationGEOSCCM33 (5 Tg-SO2)Assessing CH4 changes due to transportULAQ-CCM (a)22 (8 Tg-SO2)Assessing CH4 changes due to transportULAQ-CCM (b)22 + 1a+ 1b+ 1c (8 Tg-SO2)Assessing CH4 changes due to chemistryULAQ-CCM (c)22 (5 Tg-SO2) + 1d (8 Tg-SO2)Assessing CH4 changes due to transport and chemistry
a FBC sensitivity case (sn1) with
temperature and winds from RCP4.5 in the chemistry module and continuity
equations of chemical tracers. b FBC sensitivity case (sn2) with
temperature from RCP4.5 in the chemistry module. c FBC sensitivity
case (sn3) with winds from RCP4.5 in the continuity equations of chemical
tracers. d MBC sensitivity case for experiment (c) using the same
sulfur injection as in experiments (a) and (b).
The ULAQ-CCM sensitivity cases run with the FBC approach will help in
assessing the role of temperature and wind changes in the CH4 lifetime
perturbation under geoengineering conditions.
Model evaluation
Both ULAQ-CCM and GEOSCCM have already been extensively reviewed in the past,
both on their general features () or for issues related to
this study, such as the extratropical UTLS (), or surface
UV (). The shortwave radiative transfer module of the
ULAQ-CCM was carefully evaluated in the AeroCom intercomparison exercise of
.
In order to properly evaluate the models regarding the specific points of
this paper, however, further evaluations have been done with different sets of
observations. A list of these is available in Table . CH4
measurements are taken by the Halogen Occultation Experiment (HALOE), which
is onboard the Upper Atmosphere Research Satellite (UARS), launched in
1991 (). Climatologies are formed for the period
1991–2005 based on extended data from . HALOE
measurements range from 15 to 60–130 km altitude (depending on the species)
and cover 80∘ S to 80∘ N in latitude within 1 year. In
all intercomparisons, the HALOE climatological mean and the interannual
standard deviation (1σ) are shown. CH4 and N2O profiles are
estimated by Aura Tropospheric Emission Spectrometer (TES) thermal infrared radiances at
λ= 8 µm with the version 5 retrieval algorithm, where
CH4 is corrected using co-retrieved N2O estimates
(). Climatological mean and interannual standard
deviations for both species are calculated for the period 2004–2010.
Climatological mean and interannual standard deviation of N2O between
2001–2005 are based on the Odin/SMR product (). A further
discussion regarding TES and HALOE differences can be found in
, together with a more in-depth evaluation of ULAQ-CCM
CH4 predictions.
Summary of CH4 and N2O satellite observations used in this study.
CH4 diagnostics largely reflect the skill of the transport representation
in the models. We examined climatological zonal profiles at selected
latitudes, months and pressure levels for both model outputs and
observations (Fig. ). The climatologies refer to the years
1990–2010, in order to include the range of HALOE and TES observations. Both
ULAQ-CCM and GEOSCCM compare well with observations and are normally in the
±1σ deviation interval, relative to the climatological zonal mean.
Some spread between models appears, more evidently in the polar regions at
100 hPa. This might be due to a combination of insufficient advective
high-latitude downwelling and too-strong eddy mixing in the Southern
Hemisphere during the autumn season in ULAQ-CCM. GEOSCCM values are generally
closer to observations than those of ULAQ-CCM. Otherwise, models perform
quite similarly, and overall these diagnostics do not reveal major weaknesses
in the simulations.
Evaluation of zonal and annual mean CH4 mixing ratios ULAQ-CCM
(red) and GEOSCCM (blue) simulations averaged over 1991–2010. Observations
are taken from HALOE (black dots, average 1991–2005) ()
and TES Aura (black triangles, average 2004–2010).
A more in-depth evaluation of transport properties in the models can be found
in the Supplement regarding the correlation between CH4 and N2O and the
mean age of air. The correlation between CH4 and N2O can be used to
investigate transport properties relative to model and observations
(). Figures S1 and S2 in the Supplement show CH4 vs.
N2O correlations between 100 and 1 hPa. In Table S1 in the Supplement, we
present Pearson correlation coefficients relative to the different latitude
bands. All these panels show a compact correlation and a good agreement with
the observations; the relative Pearson coefficients in Table S1a and b are
always significant. Panels regarding polar regions (Figs. S1a, d and S2a, d)
present a larger spread with a slightly lower (but still significant) Pearson
coefficient between 90 and 60∘ S. In the lower stratosphere at tropical and midlatitudes,
there is a strong compact relationship between CH4 and N2O related to
the slope equilibrium (): the mixing happens on a faster timescale
than the chemical loss and transport to the surface. At polar latitudes,
the correlation is affected by vortex edge, which represents a mixing barrier
during the winter–spring season (Fig. S3).
Another important diagnostic for the evaluation of the model transport is
based on the mean age of air (AoA). In particular, the latitudinal gradient
between tropics and midlatitudes can be used to assess tropical ascent
independently of quasi-horizontal mixing (). Following
, tropical mean AoA profiles combine the effect of ascent rate
and horizontal mixing. The agreement of model and observations only shows
that the combined effects of ascent and mixing produce a realistic mean AoA
in the models. Figure S3c identifies how ascent contributes to the overall
tropical transport.
The horizontal gradient of mean age is able to reveal some characteristics of
the Brewer–Dobson circulation (BDC) (), namely the ascent
rate. In fact, differences between midlatitude and tropical values exclude
horizontal mixing, since that equally affects both the tropics and
midlatitudes. In GEOSCCM and ULAQ-CCM, the horizontal gradient is smaller than
observations by up to 21 km, indicating a fast ascent, but still included in
the range of observed variability. The analysis of the relationship between
mean AoA and N2O (Fig. S3d) evaluates the lower stratospheric transport
and our use of the well-measured N2O in Figs. S1 and S2. The model values
of mean AoA and N2O shown represent the climatological mean (1980–2005)
in the range 10–100 hPa and 10∘ S–10∘ N, while observed
values of mean age of air are the same as in Fig. S3a and observed values of
N2O are the SMR/Odin climatological mean (2001–2005). The correlation for
N2O > 150 ppbv looks compact, ad the slope of the model curves is
similar to the observed curve; model values of N2O and mean AoA are in the
same range as the observations. Figure S3e presents the evaluation of
latitudinal sections of N2O at 50 hPa against SMR/Odin data. For tropical
values, GEOSCCM and ULAQ-CCM agree very well with the observations; overall
model values fall inside the 2σ interannual variability. At northern
midlatitudes, ULAQ-CCM overestimates SMR; in the Southern Hemisphere, GEOSCCM
values are larger than SMR and ULAQ-CCM values lower.
Tropical stratospheric vertical mass fluxes
(20∘ S–20∘ N) of (a) CH4 and
(b) N2O for GEOSCCM (blue) and ULAQ-CCM (red) results; the
vertical mass fluxes are defined as [ρw*], where w* is the zonal
mean residual vertical velocity and ρ is the zonally averaged mass
concentration of CH4 and N2O, respectively. A model evaluation is made
with flux data obtained with w* from MERRA reanalysis and CH4, N2O
mixing ratios from HALOE and SMR results (black) (kg km-2 yr-1).
CH4 and N2O fluxes are averaged over 1991–2005 and 2001–2005,
respectively, to keep consistency with the adopted HALOE and SMR mixing ratio
values.
In order to properly asses the temperature of the polar stratosphere and its
interannual variability, the models must correctly simulate the vertical
propagation of planetary waves from the troposphere to the stratosphere.
Since it is possible to use the correlation between winter polar temperatures
and eddy heat fluxes in the lower stratosphere as a proxy for planetary wave
propagation, we looked at the correlation between the meridional heat flux at
100 hPa (40 to 80∘ for the two hemispheres) and the 50 hPa polar
temperatures (60 to 90∘ for the two hemispheres), following
. Table S2 in the Supplement compares the coefficients of
the linear fit between the two quantities for ULAQ-CCM, GEOSCCM and the ERA40
reanalysis. The positive slope is found in both models and reanalysis, with a
greater similarity in the Northern Hemisphere with respect to the Southern
Hemisphere; this difference was already shown in .
In Fig. , the vertical mass fluxes are evaluated by looking at
the CH4 and N2O measurements combined with the vertical velocities
measured by MERRA, defining the flux as [ρw*]. A good agreement
between measurements and models is found in the 5 to 100 hPa profile, with
GEOSCCM underestimating the vertical flux between 50 and 30 hPa. Figure S4
in the Supplement shows a latitudinal section of the heat fluxes in order to
further evaluate the transport skill of the two models.
Perturbation of stratospheric species transport
Absorption of solar near-infrared (NIR) and planetary radiation by the
geoengineering aerosols produces an increase of diabatic heating rates in the
tropical lower stratosphere, resulting in local warming, changes in the
latitudinal distribution of zonal winds, changes of the equatorial quasi-biennial oscillation (QBO)
() and a strengthening of the stratospheric Brewer–Dobson
circulation (BDC) (). Enhanced tropical upwelling (about
5–10 % increase in vertical velocities in the lower stratosphere) and
extratropical descent tend to move CH4 poor air more efficiently towards
the extratropical UTLS, as well as for other stratospheric long-lived
species. The net impact on tropospheric OH and CH4 lifetime depends on the
net result of superimposed species perturbations in the UTLS (CH4, NOy,
O3, SO4), in addition to tropospheric chemistry perturbations due to
changes in water vapor content, UV radiation and heterogeneous reactions on
sulfate aerosols that affect the NOx balance.
The 5–10 % increase of stratospheric tropical upward mass fluxes of both
CH4 and N2O, as shown in Fig. a, b, is predicted by the
models in geoengineering conditions as a consequence of the increasing
tropical midstratospheric upwelling, with a larger anomaly in GEOSCCM with
respect to both MBC experiments run with the ULAQ-CCM (cases a and c in
Table , with 8 and 5 Tg-SO2 injected, respectively). The
choice to only include MBC experiments when discussing vertical mass flux
anomalies is made in order to better highlight transport anomalies, because
in the FBC experiment the anomaly would be largely masked by the increasing
amount of tropospheric CH4. The larger GEOSCCM anomaly could be explained
by the QBO modification produced by geoengineering aerosols, since the
prolonged lower stratospheric westerly phase produces a better tropical
confinement (). This effect is
absent in the ULAQ-CCM model, which does not have an internally generated
QBO, but specifies the QBO with observed equatorial zonal wind data using a
nudging procedure ().
The UTLS horizontal mixing anomalies (Fig. c, d) are larger in
case (a) of ULAQ-CCM with respect to ULAQ-CCM (c) and GEOSCCM. In the latter,
two model simulations, RCP4.5 SSTs are used for both the baseline and the
geoengineering perturbed experiments, whereas ULAQ-CCM (a) is driven in the
latter experiment by G4 surface temperatures (from CCSM-CAM4). In this case,
the larger decrease of the UTLS horizontal mixing can be explained by the
increased atmospheric stabilization caused by the sea surface cooling, which
is not present in GEOSCCM and ULAQ-CCM (c). The ULAQ-CCM (c) results do not
change significantly in a sensitivity simulation made by increasing the
stratospheric sulfur injection from 5 to
8 Tg-SO2 yr-1 (see Table ), pointing out the
important role of the decreasing horizontal mixing resulting from sea surface
cooling as in ULAQ-CCM (a).
G4-RCP4.5 anomalies of (a, b) vertical and (c, d) horizontal mass fluxes of (a, c) CH4 and (b, d) N2O (years 2040–2049 time average). Vertical mass fluxes in
panels (a, b) (defined as in Fig. ) are averaged over
the tropics (20∘ S–20∘ N) in the 5–50 hPa vertical
layer, with GEOSCCM results in blue and ULAQ-CCM results in red and magenta
for cases (a) and (c) as in Table 1, respectively (kg km-2 yr-1).
Horizontal mass fluxes in panels (c, d) (defined as vρ, with
v and ρ the 3-D meridional wind component and mass concentration of
CH4 and N2O, respectively) are averaged (in absolute values) over the
extratropics (90–20∘ S and 20–90∘ N) in the 50–150 hPa
vertical layer, with model results as in panels (a, b)
(kg m-2 yr-1).
The time series of model calculated CH4 and N2O changes in the UTLS is
presented in Fig. for ULAQ-CCM and GEOSCCM. If we compare the
ULAQ-CCM case (c) with GEOSCCM, the results of the two models are similar for
N2O and are consistent with changes of lower stratospheric heating rates
and BDC (due to aerosols and O3). The N2O anomalies are of
the order of -1 ppbv in both models (that is about -0.3 %), while
those of CH4 are of the order of -5 ppbv in the ULAQ model and about a
factor of 2 smaller in GEOSCCM. This is due to missing chemical processes in
the upper troposphere in GEOSCCM, where tropospheric OH is kept fixed at
RCP4.5 values.
As already discussed in Fig. , the UTLS anomalies G4-RCP4.5 are
rather different for ULAQ-CCM (a), mostly as a consequence of the changing
SSTs in G4, with decreased horizontal mixing in the UTLS and enhanced
isolation of the tropical pipe. The negative anomaly of N2O (a
quasi-passive tracer) increases up to 2–4 ppbv after 2030, whereas the
negative CH4 anomaly increases up to approximately 10 ppbv between 2030
and 2050. A clear sign inversion is predicted after 2050 for the CH4
anomaly in geoengineering conditions as a consequence of a negative OH trend
resulting from superimposed effects of NOx and O3. A positive trend of
stratospheric O3 is, in fact, predicted in G4 with respect to RCP4.5 due
to the lowering chlorine–bromine loading in the atmosphere in the 21st
century ().
Time series of globally averaged changes of CH4(a) and
N2O (b) in the 50–150 hPa vertical layer for GEOSCCM (blue)
and ULAQ-CCM (red and magenta, for cases (a) and (c) as in Table 1,
respectively) (decadal averages). Units are ppbv.
The zonally averaged changes of N2O and CH4 are presented in
Fig. , with a comparison of model results from GEOSCCM and
ULAQ-CCM (a). The midstratospheric changes are quite comparable between the
two models, whereas the UTLS negative anomalies in ULAQ-CCM (a) are
significantly larger for the reasons discussed above in
Figs. –, they are fully comparable when
considering GEOSCCM and ULAQ-CCM (c) results, as shown in Fig. S5. Again,
this points out the sea surface cooling role on the UTLS horizontal mixing
in sulfate geoengineering conditions. Further remaining differences between
GEOSCCM and ULAQ-CCM (c) regarding horizontal mixing can be explained by a
different treatment of QBO, which is modified in GEOSCCM with a prolonged
e-shear in the G4 simulation. Interhemispheric asymmetries in the lower
stratospheric mixing ratio anomalies of ULAQ-CCM (a) and their differences
with respect to GEOSCCM can be explained by a combination of vertical and
horizontal mass flux changes and will be addressed later on in the
discussion.
To better understand the differences between the cases with fixed SSTs and
the one with changing SSTs, in Fig. we show the anomalies in
sea surface temperatures used in ULAQ-CCM (a) and (b) against the ones used
in ULAQ-CCM (c); surface temperatures are taken from the CCSM-CAM4
atmosphere–ocean coupled model for RCP4.5 and G4 simulations (with an injection
of 8 Tg-SO2), as described in Table . The zonally averaged
surface temperature anomalies G4-RCP4.5 are presented in Fig. a
for the various decades from 2020 to 2090. A strong interhemispheric
asymmetry is evident, with a negative anomaly more pronounced in the Arctic
region by approximately 1.5 K with respect to the latitude range
50–70∘ S. The geoengineering cooling impact on Arctic sea ice is
the main driver for the larger negative temperature anomaly in the Northern
Hemisphere high latitudes, which favors a more pronounced atmospheric
stabilization in the Northern Hemisphere winter–spring months with respect to
the Southern Hemisphere. The time series of the globally averaged surface
temperature anomalies is shown in Fig. b for the RCP4.5 and G4
cases: the slow oceanic response coupled to the atmospheric perturbation of
long-lived species delays the surface temperature
return in G4 to RCP4.5 values by more than one decade.
Zonal mean mixing ratio anomalies G4-RCP4.5 for (a, b) GEOSCCM and (c, d) ULAQ-CCM, CH4(a, c) and
N2O (b, d) (time average 2040–2049). ULAQ-CCM results are for
case (a) in Table . Units are ppbv. In panels (a, c), the
contour line increment is 10; in panels (b, d), the contour line
increment is 2.
Atmospheric lifetimes (years) calculated in the ULAQ-CCM (case b in
Table 1), relative to five species with stratospheric photolysis and O(1D)
reaction sink (i.e., N2O, CFC-11, H1301, CFC-12, CFC-114). The first column
shows year 2000 values (as an average over the 1996–2005 decade); the second
column shows a model mean from the report on lifetimes.
Subsequent columns show the calculated lifetime anomalies due to sulfate
geoengineering (average 2030–2069). Inside the square brackets we highlight
the physical and chemical effects driving the lifetime changes: changing
stratospheric transport in the fourth column and changing stratospheric O3
in the fifth column (due to the aerosol-induced NOx loss). Results in the
rightmost two columns are obtained through G4 sensitivity experiments (sn1,
sn3) explained in Table .
(a) Zonally averaged surface temperature changes G4-RCP4.5
(K) in the ULAQ-CCM (cases a and b), using sea surface temperatures from the
atmosphere–ocean coupled model CCSM-CAM4 (decadal time averages from
2020 to 2089; see legend for the different colors). (b) Time series
of the globally averaged surface temperatures (K) from 2020 to 2090
(RCP4.5 in red and G4 in blue). (c) Annually averaged surface
temperature anomalies G4-RCP4.5 (K) from the atmosphere–ocean coupled model
CCSM-CAM4 (time average 2030–2069). Shaded areas are not statistically
significant within ±1σ.
The decreased horizontal fluxes of long-lived species discussed in
Fig. for the ULAQ-CCM simulations with changing SSTs are a
direct consequence of the atmospheric stabilization. As shown in
Fig. c, the increased atmospheric stability in sulfate
geoengineering conditions may be partially counterbalanced by the increased
longitudinal variability of the induced cooling, in particular in the
Northern Hemisphere, which may enhance the amplitude of planetary waves.
Regions of oceanic warming in the sub-Arctic are a consequence of the
increasing amount of sea ice in G4 and related enhanced transport of colder
and saltier waters towards the subpolar regions (). This
favors cold sea water downwelling and thus positive anomalies of SSTs with
respect to reference RCP4.5 conditions, mainly in the North Atlantic region
(where the decrease of sea ice would produce less saltier waters, followed by
less downwelling, leading to cooler SSTs).
Lastly, we show the anomalies of vertical and horizontal fluxes in
Figs. and , respectively, for ULAQ-CCM (a) and
for GEOSCCM. For ULAQ-CCM (a), a 5 % increase of the midstratospheric
tropical upward fluxes is predicted in G4 with respect to the reference
RCP4.5 case, with a pronounced interhemispheric asymmetry. The Southern
Hemisphere increase of downward mass fluxes is much larger than in the
Northern Hemisphere, both in absolute and relative units. The stratospheric
mean meridional circulation is more efficiently perturbed in the Southern
Hemisphere due to the more effective atmospheric stabilization in the
Northern Hemisphere (see above; Fig. ). A 5–10 % decrease of
the extratropical horizontal mass fluxes is also predicted, as expected from
the discussion above for Fig. . The isolation of the tropical
pipe is increased in a dynamical regime with increased tropical upwelling and
enhanced atmospheric stabilization. The importance of SST changes due to
geoengineering is highlighted by the much smaller interhemispheric
difference shown by GEOSCCM for the downward fluxes, as well as in
ULAQ-CCM (c) (not shown), while the increase in the tropical upward fluxes in
Fig. is comparable to the ULAQ-CCM results. Furthermore, due to less
atmospheric stabilization, GEOSCCM shows much smaller changes in
extratropical horizontal fluxes (Fig. ). This is further
highlighted in Fig. S6, where the horizontal mass flux anomalies are also
shown for ULAQ-CCM (c). In this figure, the difference between the two
ULAQ-CCM simulations regarding the horizontal mass flux anomalies is clearly
visible, with ULAQ-CCM (c) having latitudinal means 1 order of magnitude
smaller compared to ULAQ-CCM (a) and much more comparable to GEOSCCM in the
extratropics.
(a) Latitude dependent CH4 (solid line) and N2O
(dashed line) vertical mass flux anomalies G4-RCP4.5 from the ULAQ-CCM (a)
and GEOSCCM calculations, in red and blue, respectively (vertical average
5–50 hPa; time average 2040–2049). Units are kg km-2 yr-1.
Panels (b, c) show the corresponding latitude averaged mass flux
anomalies (absolute and percent values, respectively): SH from 90 to
20∘ S; tropics from 20∘ S to 20∘ N; NH from 20 to
90∘ N. The vertical flux anomalies ΔΦV are defined
as Δ[w*ρCH4] and Δ[w*ρN2O],
where w* is the zonal mean residual vertical velocity,
ρCH4 and ρN2O are the mass concentrations of
CH4 and N2O, respectively, and Δ denotes the G4-RCP4.5
difference.
Another highlight of the different effects of transport and chemical effects
on lifetimes is shown in Table , where atmospheric lifetime
anomalies are shown for five species with stratospheric photolysis and
O(1D) reaction, as calculated in ULAQ-CCM (b). The net lifetime changes
G4-RCP4.5 result from the superposition of two effects: perturbation of
species transport and sulfate-aerosol-induced changes in O3 via NOx
depletion from heterogeneous chemical reactions. The increased tropical
upwelling moves these long-lived species more efficiently at higher altitudes
in the midstratosphere where the photolysis sink is enhanced, thus
decreasing the lifetimes. On the other hand, the chemically induced ozone
increase (due to the NOx sink by sulfate aerosols) tends to increase the
overhead column, with a decreased midstratospheric UV flux. As a
consequence, the photolysis rates decrease, thus prolonging the lifetimes. As
shown in , however, the net effect on ozone of the aerosol-induced
NOx depletion is not constant in time due to the decreasing
amount of Cl–Br species during the 21st century.
Perturbation of tropospheric chemistry
Stratosphere–troposphere exchange of geoengineering sulfate enhances the
aerosol SAD in the upper troposphere, thus favoring NOx depletion through
heterogeneous chemical reactions (i.e., hydrolysis of N2O5 and
BrONO2) (). Again, this implies less OH production and
a longer CH4 lifetime (mostly via
NO + HO2→ NO2+ OH). Figure compares the
G4-RCP4.5 anomalies of sulfate aerosol mass and surface area density in the
UTLS, as calculated in ULAQ-CCM (c) and GEOSCCM. The ULAQ-CCM model results
are taken from numerical experiments (c) in Table 1 in order to make a more
meaningful comparison with GEOSCCM (same injection of
5 Tg-SO2 yr-1; SSTs in G4 with respect to RCP4.5).
A combination of isentropic SO4 transport above the tropopause and
tropical upwelling/extratropical descent produces aerosol accumulation in the
extratropical lower stratosphere with a clear maximum of mass density in the
Northern Hemisphere (>2 µg m-3 at ∼ 12–14 km
altitude). Larger values in the ULAQ-CCM of both SAD and mass density in the
tropical upper troposphere are due to a more efficient gravitational settling
of the particles. An important difference between the two models is that
ULAQ-CCM includes an aerosol microphysics code for predicting the particle
size distribution, which, on the other hand, is assigned in GEOSCCM. A
comparison of the simulated stratospheric distribution of the SO4 SAD is
shown in Fig. S7 in order to highlight the ability of both models to
correctly simulate the tropical aerosol confinement, and a further discussion
of the differences between the two models in this aspect, together with
profile evaluation using SAGE II data, is presented in .
The two models predict an increase of SAD ranging between 2 and
10 µm2 cm-3 in the extratropical upper troposphere, and
this increase is the major diver for tropospheric NOx changes in
geoengineering conditions. Enhanced heterogeneous NOx conversion to
HNO3 on the aerosol surface ends up limiting the efficiency of reaction
NO + HO2→ NO2+ OH, thus reducing OH and upper
tropospheric O3 production, with a consequently longer CH4 lifetime.
Figure shows the ULAQ model calculated anomaly of UTLS NOx
in experiment (b) of Table , with values ranging between -0.02
and -0.2 ppbv in the upper troposphere (10 to 30 % reduction).
As in Fig. but for horizontal mass flux anomalies
G4-RCP4.5 (vertical average 50–150 hPa; time average 2040–2049). Units are
kg m-2 yr-1. The horizontal flux anomalies ΔΦH
are defined as Δ[vρCH4] and Δ[vρN2O], where v is the 3-D meridional wind component.
G4-RCP4.5 anomalies of sulfate aerosol surface area
density (a, b) and mass density (c, d) in the upper
troposphere and lowermost stratosphere from ULAQ-CCM (a, c) and
GEOSCCM (b, d) (time average 2040–2049). ULAQ-CCM results are from
numerical experiments (c) in Table 1. Units are
µm2 cm-3 for the surface area and µg m-3 for
the mass density. In panels (a, b), the contour line increment is 0.5
for values less than 12 and 2.0 from 14 to larger values. In
panels (c, d), the contour line increment is 0.1 for values less than
2.5 and 1.0 from 3.0 to larger values.
G4-RCP4.5 anomalies of NO + NO2 mixing ratios in the upper
troposphere and lowermost stratosphere from experiment (b) of the ULAQ-CCM
(time average 2040–2049). Panels (a, b) are for absolute (ppbv) and
percent NOx changes, respectively. The contour line increments are
0.025 ppbv and 5 % in panels (a, b), respectively.
The tropospheric OH balance is also affected also by the UV amount available
for O(1D) production from O3 photolysis
(H2O + O(1D) → 2OH) and indirectly from the upper
tropospheric O3 reduction due to the decreased chemical production from
NO + HO2 and NO + RO2. Upper tropospheric ozone, however, is also affected
by perturbed stratosphere–troposphere (strat–trop) fluxes and lower stratospheric ozone depletion in
geoengineering conditions (). High-latitude
stratospheric ozone depletion produces significant UVB increase at
the surface (). On the other hand, the enhanced radiation
scattering in the tropical lower stratosphere overbalances the UVB increase
due to tropical stratospheric ozone losses, ending up in a net decrease of
tropical tropospheric UVB, which means again less OH production and longer
CH4 lifetime (regulated essentially by tropical OH). Figure
shows the percent anomalies of UVB as calculated in GEOSCCM and ULAQ-CCM (c)
for the two components that are explicitly online in the models (O3 and
sulfate aerosols). A 1.5 to 2.0 % UVB decrease is predicted by the models
equatorward of 40∘ latitude in both hemispheres (-1.60 % for
GEOSCCM and -1.94 % for ULAQ-CCM). The sulfate geoengineering impact on
tropospheric UV penetration and heterogeneous chemistry changes has been
widely discussed in , along with their effects on surface ozone
concentration.
G4-RCP4.5 percent anomalies of surface UVB as a function of
latitude from ULAQ-CCM (c) (red) and GEOSCCM (blue) (2040–2049). UVB
changes are shown for the two components that are explicitly online in the
models (i.e., O3 and aerosols) and for their net. ULAQ-CCM results are
taken from numerical experiment (c) in Table in order to make a
more meaningful comparison with GEOSCCM, as in Fig. 9.
Solar radiation reflection by geoengineering aerosols increases the planetary
albedo and cools the surface, with a tropospheric water vapor decrease as a
response to this cooling: less OH is produced by reaction
H2O + O(1D), thus prolonging the CH4 lifetime. The combination
of this climate–chemistry effect with the others discussed above (NOx, UV,
strat–trop O3 transport) produces the net OH perturbation in G4 with
respect to RCP4.5 (Fig. a) and the resulting CH4 change
(Fig. b). The calculated average tropospheric anomaly of CH4
is +190 ppbv, i.e., 10.6 % with respect to the RCP4.5 base case
average mixing ratio in the years 2040–2049. The stratospheric anomalies are
consistent with those discussed in Fig. c, obtained with the
same G4 perturbation, but using the MBC approach (ULAQ-CCM a).
ULAQ-CCM calculated G4-RCP4.5 anomalies of (a) OH
concentrations and (b) CH4 mixing ratios (time average
2040–2049) from experiment (b) in Table . Units are
106 molec cm-3 for OH and ppbv for CH4. The contour line
increment is 0.1 × 106 molec cm-3 for OH and 25 ppbv
for CH4.
Any attempt to assess the long-term atmospheric response of CH4 to OH
changes needs the surface mixing ratio to be allowed to respond freely to
tropospheric perturbations of its main sink process (i.e., oxidation by OH),
which determines the CH4 lifetime. The usual modeling approach of adopting
an assigned time-dependent mixing ratio as a surface boundary condition (MBC)
can still be used to calculate climate–chemistry-induced changes in CH4
lifetime, but this cannot provide information on the tropospheric mass
changes of CH4 induced by the OH perturbations. In addition, to obtain a
correct estimate of the lifetime perturbation, the MBC approach would
necessitate the use of correction factors, due to the missing feedback of
lower tropospheric CH4 changes on HOx chemistry ().
CH4 surface emissions, sinks, global mass burden and lifetime in
the ULAQ-CCM for experiment (b) (year 2000).
The alternative approach of using a surface flux boundary condition (FBC)
would, in principle, resolve these issues. Table summarizes CH4
surface emissions, sinks, global mass burden and lifetime in ULAQ-CCM (b)
for the year 2000. The major atmospheric sink of CH4 is the reaction with OH
and this determines the CH4 lifetime, except for an additional smaller
contribution from soil deposition and an additional stratospheric sink due to
CH4 reactions with O(1D) and Cl. The calculated OH abundance is then
critical in the determination of a realistic global burden and lifetime of
CH4. Tropospheric OH concentrations have been evaluated in
using climatological values from . In
the same published work, a comparison of calculated tropospheric CH4
mixing ratios is made with observations from TES/Aura radiances.
(a) Time series of CH4 global mean atmospheric lifetime
(years, left scale, bars) calculated in the ULAQ-CCM FBC case (experiment b
of Table ), with bars referring to decadal averages (gray for
RCP4.5 and white for G4). Superimposed are globally averaged CH4 surface
mixing ratios (ppmv, right scale) for the corresponding RCP4.5 and G4
simulations (black solid and red curves, respectively). The dotted curve
shows globally averaged CH4 surface mixing ratios for the RCP4.5 MBC case
(experiment a in Table ), i.e., using prescribed fixed mixing
ratios at the surface (). (b) Time series of
G4-RCP4.5 radiative forcing of CH4 (mW m-2). Black, purple and blue
curves show the direct and indirect effects (purple and blue curves are for
CO2 and stratospheric H2O from CH4 oxidation, respectively). Dashed
blue curve is for stratospheric H2O changes resulting from G4-RCP4.5
temperature anomalies at the tropopause tropical layer (TTL).
The ULAQ-CCM calculated time series of CH4 lifetime and surface mixing
ratio is presented in Fig. a, for both reference RCP4.5 and
perturbed G4 cases, using the FBC approach (experiment b in
Table ). A simple approach was used for the time evolution of
CH4 emission fluxes: the geographical distribution was fixed at year 2000
values, but the net global value was linearly scaled to the ratio of RCP4.5
recommended surface mixing ratios in future years (dotted line in
Fig. a) with the year 2000 recommended value (1754 ppbv). An
in-depth study of future climate change effects on CH4 natural emissions
or future changes on the geographical distribution of anthropogenic emissions
is beyond the purposes of the present study. The lifetime change G4-RCP4.5
shown in Fig. a increases up to 1.7 years in 2070 during the
time period of geoengineering implementation, then slowly decreases in the
so-called termination period (2070–2090) down to 1.2 years in 2090.
Similarly, the surface mixing ratio change increases up to 250 ppbv in 2070
and then slowly decreases in the termination period down to 150 ppbv in
2090. These slow decreases are due to the long time needed for atmospheric
CH4 to return to baseline RCP4.5 values. In addition, sea surface
temperatures need a few decades to recover to RCP4.5 values
(Fig. a, b), thus triggering a prolonged perturbation of the
stratospheric circulation.
A summary of gas-phase radiative forcing (RF) components related to the CH4 perturbation is
presented in Fig. b. Direct stratospheric aerosol RF obviously
dominates in sulfate geoengineering (∼-1.2 W m-2), as
discussed in , using independent estimates available in
the literature. Among gas species, CH4 produces the largest indirect RF
(∼+0.1 W m-2), in addition to contributions from O3
(negative) and stratospheric H2O (positive), with the latter due to slight
warming of the tropopause tropical layer (TTL) (see ).
Small indirect CH4 contributions come from increasing amounts of CO2
and H2O in the CH4 oxidation chain. This chemical increase of
stratospheric H2O, however, is normally smaller than the one driven by the
geoengineering aerosol warming at the TTL cold point (as shown in
Fig. b).
Atmospheric lifetimes (years) calculated in the ULAQ-CCM
(experiment b in Table ), relative to three species that include
an OH reaction sink (i.e., CH4, HCFC-22, CH3CCl3). CH4 is
predicted with the FBC approach; the other two species with specified surface
mixing ratios (unchanged between G4 and RCP4.5). The first column shows year 2000
values (as an average over the 1996–2005 decade); the second column shows a model
mean value from the report on lifetimes. Subsequent columns
show the calculated lifetime anomalies due to sulfate geoengineering (average
2030–2069). Inside the brackets we highlight the physical and
chemical effects driving the lifetime changes (see text).
Table summarizes our calculations for OH-dependent species
lifetimes under geoengineering conditions. The ULAQ-CCM calculated lifetimes
under year 2000 conditions are fully comparable with the values in the
report on lifetimes. G4-RCP4.5 anomalies averaged between
2030–2069 range between +1.33 years for CH4 and +0.5 years for
CH3CCl3. The FBC approach was used for CH4 in order to properly
evaluate its feedback on HOx chemistry. The rightmost three columns in
Table show the different contributions to the lifetime changes,
through G4 sensitivity experiments (sn1, sn2, sn3) explained in
Table . The major contribution to the CH4 lifetime change (but
also for HCFC-22 and CH3CCl3) comes from the presence of aerosols with
their feedback on NOx–HOx–O3 photochemistry, as discussed before
in Figs. , and (temperature and
winds are kept unchanged with respect to RCP4.5 in the G4-sn1 sensitivity
case, in the chemistry module and continuity equations of chemical tracers).
The effects of tropospheric cooling with decreased water vapor (due to solar
radiation scattering by the stratospheric aerosols) and strengthening of the
BDC with enhanced strat–trop downward flux (due to heating rates by the
stratospheric aerosols) tend to partially or completely cancel each other.
The impact of tropospheric cooling on OH-driven lifetimes is limited by the
fact that the lowered H2O and OH production is partially counterbalanced
by a less efficient reaction of NO + O3→ NO2+ O2 in a
colder troposphere (see Fig. S8). This decreases NO2 and the NOx sink
to HNO3, which implies an OH increase, mostly in the upper troposphere. In
addition, OH formation from NO + HO2 reaction is enhanced if the NO loss on
O3 is less efficient.
Visual representation of the photochemical and transport effects of
sulfate geoengineering on CH4, as studied in this paper. Effects connected
to perturbed CH4 emissions due to surface cooling are not shown because
they were not explicitly considered in this study. These effects are
essentially a decrease in wetland areas connected to reductions in rainfall
and halting of permafrost thawing.
The strengthening of the Brewer–Dobson circulation affects essentially the
upper tropospheric amount of SO4, CH4, NOy and O3. This results
in a negative anomaly for geoengineering SO4 and for CH4 (due to the
enhanced lower stratospheric tropical confinement; see Figs.
and c) and a positive anomaly for NOy and O3 (due to the
enhanced strat–trop downward flux). The induced OH anomaly is negative from
CH4 (a net HOx source) and O3 (which is an OH sink in the upper
troposphere). On the other hand, it is positive from SO4 and NOy (due
to the increasing NOx amount, their negative or positive anomaly will
produce). This positive NOx anomaly induced in the upper troposphere by
the enhanced stratospheric circulation mostly regulates the net positive OH
change in the ULAQ-CCM with decreasing lifetimes (fifth column in
Table ).
Conclusions
In the present work, we have described how an injection of 5–8 Tg of SO2
per year would modify the large-scale transport and lifetime of CH4, using
two climate–chemistry coupled models, ULAQ-CCM and GEOSCCM. Both models use
prescribed SST coming from two atmosphere–ocean coupled models: CCSM-CAM4
for ULAQ-CCM and CESM for GEOSCCM. The model evaluation has shown that both
models correctly simulate the vertical profiles for the chemical species
under analysis (N2O as a quasi-passive tracer and CH4), the mean age of
air and the vertical velocity w*. Furthermore, the latitudinal heat fluxes
have been compared with ERA40 reanalysis in order to evaluate the skill of
the models in correctly simulating the meridional transport.
We have shown that changes in the BDC due to lower stratospheric aerosol
heating reduce the amount of CH4 in the extratropical UTLS. This is both
because of the strengthening of the downward branches of the BDC which brings
more stratospheric air (poorer in CH4) down in the upper troposphere and
because of a greater isolation of the tropical pipe that reduces the amount
of horizontal mixing. However, in order to properly assess the magnitude of
the transport perturbation (whether it is horizontal mixing or vertical
fluxes), the addition of the feedback of the ocean has proven crucial. Cooler
oceans allow for a further atmospheric stabilization of the atmosphere, and
the cooling of the sub-Arctic regions produces important hemispheric
asymmetries that are not found in fixed SSTs simulations. This points to a
important limitation of pure CCM studies, with prescribed time-dependent SSTs
consistent with a given RCP scenario. The large-scale transport effects of
sulfate geoengineering on trace species can only be captured on all their
nonlinear aspects using coupled atmosphere–ocean global circulation model (AOGCM)
simulations, which may quantify the SG-induced changes on SSTs. These can, in turn, be used as input for the
aerosol–chemistry–radiation–dynamics fully interactive CCM experiments.
Furthermore, we have shown that the changes in CH4 lifetime and
concentration take place because of a reduction of atmospheric OH, mostly due
to three overlapping factors: (1) reduction in tropospheric water vapor
caused by the surface cooling; (2) decrease in O(1D) caused by a decrease
in tropical tropospheric UV (because part of the incoming solar radiation is
scattered by the stratospheric aerosols, which also deplete stratospheric
ozone); (3) decrease in NOx production caused by the enhancing of
heterogeneous chemistry (see visual summary in Fig. ). Changes
in stratospheric large-scale transport and strat–trop exchange may also
contribute to perturb the tropospheric amount of OH, with a net effect whose
sign results from simultaneous changes of CH4, NOy, O3 and SO4. All
of these effects may cause a CH4 lifetime increase of more than 1 year in
the central decades of the experiment, leading, in turn, to an increase in
methane mixing ratio of over 200 ppbv.
Overall, these changes produce a positive radiative forcing of more than
+0.1 W m-2 in our radiative transfer model calculations, a result that
it is still 1 order of magnitude smaller than the direct negative radiative
forcing of the aerosols, which has been estimated to be
-1.2 ± 0.5 W m-2 for a 5 Tg SO2 yr-1 injection,
considering simulations from a vast array of models ().
In addition, gas species concentration changes (especially ozone) would also
affect air quality and surface UV concentrations, which might have
implications on human health, as already noted in and
. As discussed in the present study, as well as in
, and , the
stratospheric ozone depletion induced by geoengineering solar radiation
management techniques directly impacts the tropospheric UV budget. The health
impact of a surface UV enhancement (located only at mid-to-high latitudes in the
case of sulfate geoengineering) may be partly counterbalanced by the
decreased tropospheric OH concentration and O3 production.
Our analysis is limited to an atmospheric perturbation produced by sulfate
geoengineering on photochemistry and large-scale transport; other important
changes that would happen under this hypothetic scenario are the ones in
natural surface emissions of CH4 that would occur following changes in
surface temperatures. Natural emissions would be reduced under sulfate
geoengineering for three main reasons: (1) a reduction in surface
temperatures that would, in turn, be connected with a highly probable reduction
in rainfall, compared with the predicted increase under most future warming
scenarios (); this would reduce the amount of
CH4 produced by wetland areas, thus affecting the atmospheric methane
concentration; (2) the increased surface deposition of sulfate under SG
conditions would itself produce changes in emissions from wetlands
(); (3) SG could help avert one of the possible risks of
global warming, i.e., the emission of methane from permafrost thawing
(). It remains to be investigated how much these effects,
together, could offset the photochemical CH4 increase resulting from our
study.
Data from model simulations are available from the corresponding author.
The Supplement related to this article is available online at https://doi.org/10.5194/acp-17-11209-2017-supplement.
The authors declare that they have no conflict of
interest.
This article is part of the special issue “The Geoengineering
Model Intercomparison Project (GeoMIP): Simulations of solar radiation
reduction methods (ACP/GMD inter-journal SI)”. It does not belong to a
conference.
Acknowledgements
Figures , S1, S2 and S3 were produced with the ESMValTool
(). GEOSCCM simulations performed by Valentina Aquila
were supported by the NASA High-End Computing (HEC) Program through the NASA
Center for Climate Simulations (NCCS) at Goddard Space Flight Center.
Irene Cionni acknowledges funding received from the European Union's Horizon
2020 research and innovation programme under grant agreement no. 641816
(CRESCENDO). The authors would like to thank Renaud de Richter and
Peer Johannes Nowack for their insightful
comments regarding the conclusions of this study. Edited by: Ben Kravitz Reviewed by: two
anonymous referees
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