There are obstacles in better understanding the climate impacts associated with new materials that could be used for Stratospheric Aerosol Injections (SAI), like the lack of an integrated framework ...that combines climate modeling across scales, laboratory studies and small‐scale field experiments. Vattioni et al. (2023, https://doi.org/10.1029/2023gl105889) explored one aspect of using alternative, non‐sulfate materials for SAI. They investigated how uncertain the response of stratospheric ozone would be to alumina injections for SAI. In their study, they quantify chlorine activation rates in the presence of alumina, and then cascade these uncertainties into estimates of ozone depletion, concluding that alumina might have less detrimental impacts on stratospheric chemistry than sulfate, but with large uncertainties. Their results provide a useful basis upon which future research endeavors combining indoor and outdoor experiments and modeling may be structured to produce robust assessments of SAI impacts, benefits and uncertainties, together with clarifying what kind of research needs to be prioritized.
Plain Language Summary
We could use tiny particles injected into the higher atmosphere to reflect a small portion of incoming sunlight and thereby cool the planet. But doing so comes with risks and uncertainties: for instance, one might wonder how do we select which kind of particles to use. Sulfate is present in nature, for instance during the aftermath of volcanic eruptions followed by an observable surface cooling. However, we know that mimicking that effect would come with some drawbacks, for example, it heats the upper layer of the atmosphere and affects ozone. Alumina, supposedly, would impact atmospheric chemistry less than sulfate and so might be considered “preferable,” but not being naturally present in the atmosphere, there are lots of things we don't know. For example, Vattioni et al. (2023, https://doi.org/10.1029/2023gl105889) demonstrate that even potential implications for atmospheric chemistry are highly uncertain when looking at alumina particles as a candidate for Stratospheric Aerosol Injections (SAI). Therefore, their study is a good opportunity to think more broadly about intended SAI‐associated climate impacts and unwanted side effects, and how to better coordinate research activities in this space.
Key Points
Vattioni et al. (2023, https://doi.org/10.1029/2023gl105889) demonstrated large uncertainties in the projected impacts of alumina particles in the stratosphere
We use the results to discuss more broadly how to better think about the climate impacts and side effects of Stratospheric Aerosol Injection
We propose the idea of a “living assessment” of Stratospheric Aerosol Injections that can constantly integrate useful experimental results with modeling work
Sulfur‐based stratospheric aerosol intervention (SAI) can cool the climate, but also heats the tropical lower stratosphere if done with injections at low latitudes. We explore the role of this ...heating in the climate response to SAI, by using mechanistic experiments that remove the effects of longwave absorption of sulfate aerosols above the tropopause. If longwave absorption by stratospheric aerosols is disabled, the heating of the tropical tropopause and most of the related side effects are strongly alleviated and the cooling per Tg‐S injected is 40% bigger. Such side‐effects include the poleward expansion of eddy‐driven jets, acceleration of the stratospheric residual circulation, and delay of Antarctic ozone recovery. Our results add to other recent findings on SAI side effects and demonstrate that SAI scenarios with low‐latitude injections of absorptive materials may result in atmospheric effects and regional climate changes that are comparable to those produced by the CO2 warming signal.
Plain Language Summary
We explore the effects of the injection of sulfur into the tropical stratosphere to lower global average temperatures from those projected under a high‐emission greenhouse gas scenario to the levels of a mid‐emission scenario. We found that some detrimental effects of this injection are of a similar magnitude to those from climate change itself in some regions. This includes a strong warming 15 km above the tropics, which alters large‐scale weather patterns in the atmosphere. In comparison to the mid‐emission scenario, we find enhanced surface warming in the polar regions and modification in regional precipitation patterns over land, therefore not completely alleviating the warming of the high‐emission scenario in high northern latitudes. We show that the tropical stratospheric heating is responsible for a large portion of these side effects on tropospheric climate.
Key Points
Many side effects of sulfur‐based stratospheric aerosol intervention are caused by heating of the tropical lower stratosphere
Some regional patterns of change tied to atmospheric circulation can be of the same magnitude as those that are CO2‐driven
The absorptivity of the aerosol particles increases their lifetime but decreases their cooling efficiency per Tg‐S per year by 40%
The impacts of Stratospheric Aerosol Injection (SAI) on the atmosphere and surface climate depend on when and where the sulfate aerosol precursors are injected, as well as on how much surface cooling ...is to be achieved. We use a set of CESM2(WACCM6) SAI simulations achieving three different levels of global mean surface cooling and demonstrate that unlike some direct surface climate impacts driven by the reflection of solar radiation by sulfate aerosols, the SAI‐induced changes in the high latitude circulation and ozone are more complex and could be non‐linear. This manifests in our simulations by disproportionally larger Antarctic springtime ozone loss, significantly larger intra‐ensemble spread of the Arctic stratospheric jet and ozone responses, and non‐linear impacts on the extratropical modes of surface climate variability under the strongest‐cooling SAI scenario compared to the weakest one. These potential non‐linearities may add to uncertainties in projections of regional surface impacts under SAI.
Plain Language Summary
The injection of reflective aerosols, or their precursors, into the lower stratosphere (Stratospheric Aerosol Injection, SAI) has been proposed as a temporary measure to offset some of the adverse impacts of climate change whilst atmospheric concentrations of greenhouses are being stabilized and, ultimately, reduced. The impacts of SAI on the atmosphere and surface climate would depend on when and where the sulfate aerosol precursors are injected, as well as on how much surface cooling is to be achieved. Here we analyze SAI impacts on stratospheric climate and ozone in a set of Earth system model simulations under varying magnitudes of the SAI‐induced global mean cooling. We demonstrate that unlike some of the direct surface climate impacts from the reflection of solar radiation by sulfate aerosols, the SAI‐induced changes in stratospheric circulation, chemistry and climate are more complex, with the model simulations pointing toward more non‐linear behavior of the high latitude circulation and ozone under higher SAI scenarios. These potential non‐linearities may add to uncertainties in projections of regional surface impacts under SAI.
Key Points
Impacts of Stratospheric Aerosol Injection (SAI) depend on how much surface cooling is to be achieved
High latitude circulation, ozone and modes of extratropical variability can vary non‐linearly with the SAI‐induced global surface cooling
These potential non‐linearities may add to uncertainties in projections of regional surface impacts under SAI
Stratospheric aerosol injection (SAI) of reflective sulfate aerosols has been proposed to temporarily reduce the impacts of global warming. In this study, we compare two SAI simulations which inject ...at different altitudes to provide the same amount of cooling, finding that lower‐altitude SAI requires 64% more injection. SAI at higher altitudes cools the surface more efficiently per unit injection than lower‐altitude SAI through two primary mechanisms: the longer lifetimes of SO2 and SO4 at higher altitudes, and the water vapor feedback, in which lower‐altitude SAI causes more heating in the tropical cold point tropopause region, thereby increasing water vapor transport into the stratosphere and trapping more terrestrial infrared radiation that offsets some of the direct aerosol‐induced cooling. We isolate these individual mechanisms and find that the contribution of lifetime effects to differences in cooling efficiency is approximately five to six times larger than the contribution of the water vapor feedback.
Plain Language Summary
Stratospheric aerosol injection (SAI)—the artificial introduction of reflective droplets, called aerosols, into the middle atmosphere–could reflect a small portion of sunlight and cool the planet in order to temporarily reduce the impacts from global warming. Injecting the aerosols at higher altitudes would be more expensive, but it would also be more efficient because the aerosols would last longer before falling out of the atmosphere. Additionally, injecting at a lower altitude would cause more water vapor to enter the middle atmosphere; since water vapor is a greenhouse gas, this would increase the greenhouse effect, meaning more aerosols would be needed to cool the surface to a desired temperature. In this study, we directly compare high‐altitude SAI to low‐altitude SAI to determine how much more efficient it is to inject at a higher altitude, and we break down the different factors that effect efficiency to see which has the biggest effect.
Key Points
We compare two stratospheric aerosol injection strategies which inject SO2 at different altitudes to meet the same temperature target
The low altitude strategy requires two thirds more injection to provide the same amount of cooling
We isolate and quantify the different factors which cause the high altitude injection strategy to cool the surface more efficiently
The problem of reducing the impacts of rising anthropogenic greenhouse gas on warming temperatures has led to the proposal of using stratospheric aerosols to reflect some of the incoming solar ...radiation back to space. The deliberate injection of sulfur into the stratosphere to form stratospheric sulfate aerosols, emulating volcanoes, will result in sulfate deposition to the surface. We consider here an extreme sulfate geoengineering scenario necessary to maintain temperatures at 2020 levels while greenhouse gas emissions continue to grow unabated. We show that the amount of stratospheric sulfate needed could be globally balanced by the predicted decrease in tropospheric anthropogenic SO2 emissions, but the spatial distribution would move from industrialized regions to pristine areas. We show how these changes would affect ecosystems differently depending on present day observations of soil pH, which we use to infer the potential for acid-induced aluminum toxicity across the planet.
As part of the Geoengineering Model Intercomparison Project a numerical experiment known as G6sulfur has been designed in which temperatures under a high-forcing future scenario (SSP5-8.5) are ...reduced to those under a medium-forcing scenario (SSP2-4.5) using the proposed geoengineering technique of stratospheric aerosol intervention (SAI). G6sulfur involves introducing sulfuric acid aerosol into the tropical stratosphere where it reflects incoming sunlight back to space, thus cooling the planet. Here, we compare the results from six Earth-system models that have performed the G6sulfur experiment and examine how SAI affects two important modes of natural variability, the northern wintertime North Atlantic Oscillation (NAO) and the Quasi-Biennial Oscillation (QBO). Although all models show that SAI is successful in reducing global mean temperature as designed, they are also consistent in showing that it forces an increasingly positive phase of the NAO as the injection rate increases over the course of the 21st century, exacerbating precipitation reductions over parts of southern Europe compared with SSP5-8.5. In contrast to the robust result for the NAO, there is less consistency for the impact on the QBO, but the results nevertheless indicate a risk that equatorial SAI could cause the QBO to stall and become locked in a phase with permanent westerly winds in the lower stratosphere.
The Montreal Protocol has succeeded in limiting major ozone-depleting substance emissions, and consequently stratospheric ozone concentrations are expected to recover this century. However, there is ...a large uncertainty in the rate of regional ozone recovery in the Northern Hemisphere. Here we identify a Eurasia-North America dipole mode in the total column ozone over the Northern Hemisphere, showing negative and positive total column ozone anomaly centres over Eurasia and North America, respectively. The positive trend of this mode explains an enhanced total column ozone decline over the Eurasian continent in the past three decades, which is closely related to the polar vortex shift towards Eurasia. Multiple chemistry-climate-model simulations indicate that the positive Eurasia-North America dipole trend in late winter is likely to continue in the near future. Our findings suggest that the anticipated ozone recovery in late winter will be sensitive not only to the ozone-depleting substance decline but also to the polar vortex changes, and could be substantially delayed in some regions of the Northern Hemisphere extratropics.
Simulated stratospheric temperatures over the period 1979–2016 in models from the Chemistry‐Climate Model Initiative are compared with recently updated and extended satellite data sets. The ...multimodel mean global temperature trends over 1979–2005 are −0.88 ± 0.23, −0.70 ± 0.16, and −0.50 ± 0.12 K/decade for the Stratospheric Sounding Unit (SSU) channels 3 (~40–50 km), 2 (~35–45 km), and 1 (~25–35 km), respectively (with 95% confidence intervals). These are within the uncertainty bounds of the observed temperature trends from two reprocessed SSU data sets. In the lower stratosphere, the multimodel mean trend in global temperature for the Microwave Sounding Unit channel 4 (~13–22 km) is −0.25 ± 0.12 K/decade over 1979–2005, consistent with observed estimates from three versions of this satellite record. The models and an extended satellite data set comprised of SSU with the Advanced Microwave Sounding Unit‐A show weaker global stratospheric cooling over 1998–2016 compared to the period of intensive ozone depletion (1979–1997). This is due to the reduction in ozone‐induced cooling from the slowdown of ozone trends and the onset of ozone recovery since the late 1990s. In summary, the results show much better consistency between simulated and satellite‐observed stratospheric temperature trends than was reported by Thompson et al. (2012, https://doi.org/10.1038/nature11579) for the previous versions of the SSU record and chemistry‐climate models. The improved agreement mainly comes from updates to the satellite records; the range of stratospheric temperature trends over 1979–2005 simulated in Chemistry‐Climate Model Initiative models is comparable to the previous generation of chemistry‐climate models.
Plain Language Summary
A previous analysis by Thompson et al. (2012, https://doi.org/10.1038/nature11579) showed substantial differences between satellite‐observed and model‐simulated stratospheric cooling trends since the late 1970s. Here we compare recently revised and extended satellite temperature records with new simulations from 14 chemistry‐climate models. The results show much better agreement in the magnitude of stratospheric cooling over 1979–2005 between models and observations. This cooling was predominantly driven by increasing greenhouse gases and declining stratospheric ozone levels. An extended satellite temperature record and the chemistry‐climate models show weaker global stratospheric cooling over 1998–2016 compared to 1979–1997. This is due to the reduction in ozone‐induced cooling from the slowdown of ozone trends and the onset of ozone recovery since the late 1990s. There are larger differences in the latitudinal structure of past stratospheric temperature trends due to the effects of unforced atmospheric variability. In summary, the results show much better consistency between simulated and satellite‐observed stratospheric temperature trends than was reported by Thompson et al. (2012, https://doi.org/10.1038/nature11579) for the previous versions of the satellite record and last generation of chemistry‐climate models. The improved agreement mainly comes from updates to the satellite records, while the range of simulated trends is comparable to the previous generation of models.
Key Points
There is substantial improvement in the comparison between modeled and observed stratospheric temperature trends over the satellite era
Observations and models show weaker stratospheric cooling since ~1998 when ozone‐depleting substances have been declining in the atmosphere
Larger differences exist between modeled and observed stratospheric temperature trends at high latitudes partly due to internal variability
We perform the first multi-model intercomparison of the impact of nudged
meteorology on the stratospheric residual circulation using hindcast
simulations from the Chemistry–Climate Model Initiative ...(CCMI). We examine
simulations over the period 1980–2009 from seven models in which the
meteorological fields are nudged towards a reanalysis dataset and compare
these with their equivalent free-running simulations and the reanalyses
themselves. We show that for the current implementations, nudging
meteorology does not constrain the mean strength of the stratospheric
residual circulation and that the inter-model spread is similar, or even larger,
than in the free-running simulations. The nudged models generally show
slightly stronger upwelling in the tropical lower stratosphere compared to
the free-running versions and exhibit marked differences compared to the
directly estimated residual circulation from the reanalysis dataset they are
nudged towards. Downward control calculations applied to the nudged
simulations reveal substantial differences between the climatological lower-stratospheric tropical upward mass flux (TUMF) computed from the modelled
wave forcing and that calculated directly from the residual circulation.
This explicitly shows that nudging decouples the wave forcing and the
residual circulation so that the divergence of the angular momentum flux
due to the mean motion is not balanced by eddy motions, as would typically
be expected in the time mean. Overall, nudging meteorological fields leads
to increased inter-model spread for most of the measures of the mean
climatological stratospheric residual circulation assessed in this study. In
contrast, the nudged simulations show a high degree of consistency in the
inter-annual variability in the TUMF in the lower stratosphere, which is
primarily related to the contribution to variability from the resolved wave
forcing. The more consistent inter-annual variability in TUMF in the nudged
models also compares more closely with the variability found in the
reanalyses, particularly in boreal winter. We apply a multiple linear
regression (MLR) model to separate the drivers of inter-annual and long-term
variations in the simulated TUMF; this explains up to ∼75 %
of the variance in TUMF in the nudged simulations. The MLR model reveals a
statistically significant positive trend in TUMF for most models over the
period 1980–2009. The TUMF trend magnitude is generally larger in the nudged
models compared to their free-running counterparts, but the intermodel range
of trends doubles from around a factor of 2 to a factor of 4 due to nudging.
Furthermore, the nudged models generally do not match the TUMF trends in the
reanalysis they are nudged towards for trends over different periods in the
interval 1980–2009. Hence, we conclude that nudging does not strongly
constrain long-term trends simulated by the chemistry–climate model (CCM) in the
residual circulation. Our findings show that while nudged simulations may,
by construction, produce accurate temperatures and realistic representations
of fast horizontal transport, this is not typically the case for the slower
zonal mean vertical transport in the stratosphere. Consequently, caution is
required when using nudged simulations to interpret the behaviour of
stratospheric tracers that are affected by the residual circulation.
Abstract
Stratospheric aerosol geoengineering has been proposed as a potential solution to reduce climate change and its impacts. Here, we explore the responses of the Hadley circulation (HC) ...intensity and the intertropical convergence zone (ITCZ) using the strategic stratospheric aerosol geoengineering, in which sulfur dioxide was injected into the stratosphere at four different locations to maintain the global-mean surface temperature and the interhemispheric and equator-to-pole temperature gradients at present-day values (baseline). Simulations show that, relative to the baseline, strategic stratospheric aerosol geoengineering generally maintains northern winter December–January–February (DJF) HC intensity under RCP8.5, while it overcompensates for the greenhouse gas (GHG)-forced southern winter June–July–August (JJA) HC intensity increase, producing a 3.5 ± 0.4% weakening. The residual change of southern HC intensity in JJA is mainly associated with stratospheric heating and tropospheric temperature response due to enhanced stratospheric aerosol concentrations. Geoengineering overcompensates for the GHG-driven northward ITCZ shifts, producing 0.7° ± 0.1° and 0.2° ± 0.1° latitude southward migrations in JJA and DJF, respectively relative to the baseline. These migrations are affected by tropical interhemispheric temperature differences both at the surface and in the free troposphere. Further strategies for reducing the residual change of HC intensity and ITCZ shifts under stratospheric aerosol geoengineering could involve minimizing stratospheric heating and restoring and preserving the present-day tropical tropospheric interhemispheric temperature differences.
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