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The Effects of Geoengineering on the Southern Ocean

Judy Twedt

Summary Paper of Final Project Ocn 558: Climate Modeling

Cecelia Bitz and Luanne Thompson

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1. Introduction:

A. Motivation

Amidst the uncertainties of climate warming and the inertia of the international community,

geoengineering-- the deliberate alteration of Earth's climate- via stratospheric sulfate injections has

been proposed as a viable means of 'buying time' and quickly cooling the climate to counteract a

climate emergency. Sulfate geoengineering has particular appear because it mimics a known

phenomenon- natural volcanic eruptions- and can be implemented without large-scale international

consensus. An alternative strategy, hypothetically feasible, is the shut-off of greenhouse gas emissions

and removal of greenhouse gasses from the atmosphere (hereafter GHGrem).

One reasonable expectation of any geoengineering project is that it protect polar ice sheets from

melting. In this study, I model and compare GHGrem with sulfate geoengineering simulations conducted

by Kelly McCusker to determine whether either are viable means of cooling the Southern Ocean and

protecting the Antarctic Ice sheet. Stratospheric sulfate geoengineering causes changes in atmospheric

circulations (Rasch et al., 2007). These changes are communicated to the ocean to a large extent

through the winds (Farneti and Delworth, 2010). To capture these interdependencies and evaluate the

response of the Southern Ocean, it is crucial that the model contain ocean and sea-ice dynamics.

Simulations with slab-ocean models which dominate studies of this type are not able to simulate the

ocean's response to geoengineering. This study begins to fill the need for an evaluation of

geoengineering strategies with a fully coupled model.

The outline of this paper is as follows: First, I summarize the salient results from McCusker's sulfate

geoengineering simulations and motivate the GHGrem experiment. Then I describe the model and

GHGrem simulations. Next, I examine the results of the greenhouse gas removal, and compare the

Southern Ocean response under both geoengineering methods. In particular, I will compare changes in

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surface temperature, the vertical temperature profile, and the surface wind stress between the end of

the 20th century, the RCP 8.5 scenario, and the two geoengineering simulations.

B. Sulfate-Geoengineering Simulations

Stratospheric sulfates cool the Earth by reflecting short wave solar radiation and brightening the planet.

They do not reduce the greenhouse warming effect, but partially shade the Earth from solar radiation.

There is a natural analog in volcanic eruptions, which have caused temporary cooling of the Earth

system. This cooling subsides as the volcanic aerosols precipitate out from the atmosphere. Kelly

McKusker has conducted multiple simulations of sulfate injection scenarios, branching from the rcp8.5

emissions scenarios and adding sulfate concentrations to the stratosphere in 2035. Her results show

that stratospheric sulfates effectively brighten the earth and reduce the mean global temperature, but

not uniformly. In particular, the high southern latitudes show a recalcitrant response to the sulfate

injections. The southern ocean stubbornly warms, as the rest of the planet cools. In addition to the

warm Southern Ocean, results of the sulfate geoengineering simulations show a coincident increase in

the southern winds, caused by temperature variations in the atmosphere arising from continued

increase in GHG emissions.

C. An Alternative Cooling Strategy: Greenhouse Gas Removal

Instead of increasing the earth's albedo with sulfates, Earth could be cooled by removing CO2 and other

greenhouse gasses, thereby reducing greenhouse warming. As a geoengineering project, this is purely

hypothetical, but physically possible. Simulating GHGrem is instructive in two ways. By comparing this

cooling with sulfate engineering, we relate different atmospheric inputs to changes in the Southern

Ocean surface temperatures. Secondly, this comparison illuminates the dependence of the meridional

overturning circulation and Southern Ocean upwelling on the wind-blown Antarctic Circumpolar current.

The results of these simulations show that sulfate engineering and greenhouse produce quite different

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outcomes on the Southern Ocean winds and surface temperature. Studies of this kind should be used

guide expected outcomes of actual geoengineering scenarios, and to provide the necessary information

to make careful judgment in the case of climate emergencies.

2. Model

We used the Community Climate System Model 4 (CCSM4) comprised of atmosphere, ocean, land, land-

ice, and sea-ice models. The Community Atmosphere Model (CAM4) uses a lin-Roof finite volume core

with 26 layers in the vertical. The horizontal grid is 288x200, with a uniform resolution of 0.9 x 1.25. The

Parallel Ocean Program version 2 (POP2) uses a nominal 1 grid with spherical coordinates in the

Southern Hemisphere, but in the Northern Hemisphere the pole is displaced into Greenland. The

horizontal resolution is a uniform 1.11 in the zonal direction. The meridional direction is non-uniform:

ranging from 0.27 at the equator to 0.54 at 33 N/S, and constant from thereon. There are 60 vertical

levels (Gent, et. al., 2011).

The sulfate geoengineering simulations branched from the IPCC representative concentration pathway

(rcp) 8.5 emissions scenario. A sulfate burden was prescribed to cancel the warming signal, based on

previous work by Rasch et. al. (2008). In the first three years the stratospheric burden was ramped at 8

Tg/yr sulfate (SO4), followed by a ramping of 0.67 Tg/yr for the remainder of the simulation. This was

calculated to counteract the increased radiative forcing from the rcp 8.5 scenario.

The GHGrem simulation also branched from the rcp 8.5 scenario; in 2035 the greenhouse gas

concentrations were abruptly returned to their 1988 levels and left constant thereafter. We chose 1988

because this was the year in which CO2 concentrations reached 350 ppm. This has been deemed by

Hansen (2008) and others as a target concentration, to prevent further climate change. The five

prescribed concentrations were for carbon dioxide, methane, nitrous oxide, CFC-11, and CFC-12 (figure

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19). It's important to note that the CFC-11 levels concentrations were decreasing in 2035 following the

Montreal Protocol; by forcing their concentrations to a fixed 1988 value, we maintain this concentration

that would otherwise have continued to decrease. The hybrid simulation was run until 2083.

3. Analysis and Results

Comparisons are with the 2045-2054 climatology for the rcp 8.5 scenario and the two geoengineering

simulations. The 20th Century control period is from 1970-1999. In this period, the geo2035 simulations

have not quite reached equilibrium. I chose this climatology because the viability of maintaining a

sulfate burden more than 20 years is highly unlikely. The GHGrem simulations has a quick response of

approximately six years before reaching an equilibrium state.

Spatial Surface Temperature Trends

Under the rcp 8.5 scenario, surface temperatures increase on average 1.8 ℃ , with amplified (2-4+

degrees) warming over the polar regions (figure 1). The ensemble average of the quick ramp of sulfates

(geo2035) effectively cancels the average global warming due to greenhouse gasses. The spatial

distribution of this cooling, however, is uneven (figure 2). The Arctic cools below the 20th century

average while the Antarctic continues to warm. In particular, the surface waters of the Weddell Sea are

up to 2 ℃ warmer, as are the surface waters above off East Antarctica, south of the Indian Ocean. The

asymmetric temperature changes roughly cancel each other out in the global mean. Under sulfate

injections, the Arctic would cool but the Antarctic would continue to warm.

Removing greenhouse gasses cancels most of the global mean temperature increase, but is on average

~0.6 ℃ warmer than the 20th century control (figure 3). The comparison with geo2035 shows greater

cooling in the Antarctic, and variable cooling in the Arctic (figure 4 ). Compared to the geo2035 average,

the Southern Ocean is much cooler over west Antarctica from ~ 30 W to 150 E. This is surprising, given

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that the greenhouse gas removal wasn't strong enough to cancel the global temperature increase, and

the geo2035 simulation did cancel the global temperature increase. This suggests that circulation

anomalies by greenhouse gasses have a significant effect on the Southern Ocean surface temperatures.

This effect is not cancelled by sulfate injections. The comparison with the 20th century control (figure 3)

shows that the GHG removal cancels the warming of surface waters near West Antarctica, and actually

cools the Ross Sea below the end of the 20th century climatological average.

Vertical Temperature Trends

Changes in the atmospheric circulation are communicated to the ocean via surface winds. The

circulation anomalies in these geoengineering simulations can be deduced from vertical temperature

change profiles. Under the RCP 8.5 scenario, there is general warming of the troposphere due to

greenhouse gasses absorbing outgoing long wave radiation, but cooling of the stratosphere (figure 5).

The changes are greatest at the equator, but there is also asymmetry between the vertical profile of

warming in the polar regions. In particular, the high northern latitudes have more tropospheric warming

than the high southern latitudes.

When sulfates are added to the RCP 8. 5 greenhouse burden, the stratosphere warms and the

troposphere cools (figure 6). The warming in the stratosphere in roughly symmetric above the equator,

in the 100-30 mb region. Cooling in the troposphere is highly asymmetric, with cooling from 90 N to

approximately 30 S. Stratospheric warming follows the structure of sulfate distributes, and is higher in

regions of greater concentration, due to sulfates absorbing short wave solar radiation. The injection of

sulfates dramatically changes the structure of the vertical temperature profile. The maximum of the

vertical temperature profile with sulfates has shifted from the equatorial troposphere to the equatorial

stratosphere. Rather than canceling circulation changes, the sulfates create new circulation anomalies.

This is consistent with results of a study by Rasch et. al. (2008) who studied the role of particle size of

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sulfates in the lower troposphere using CAM3 and a slab ocean model. They also found that sulfates

cooled the troposphere, but did not cancel temperature changes in the stratosphere.

Removing greenhouse gasses, on the other hand, cancels the changes in the lower troposphere (figure

7) . Compared to the 20th century control, there is still some warming of ~ 1 ℃ above the tropics, in the

200-300mb region. The GHGrem stratosphere is cooler than the 20th century control, with uneven

changes above the polar regions. Here, as well as in the RCP 8.5 case, the stratosphere above the south

pole cools at lower elevations than the stratosphere above the north pole.

Neither GHGrem nor sulfate engineering cancel the vertical profile of temperature in the atmosphere;

however, GHGrem reduces the temperature gradient relative to the sulfate injection scenario. This

critical difference causes wind stress anomalies, and calls for a much more thorough discussion but is

beyond the scope of this paper. The key point is that the GHGrem begins to return the vertical

temperature profile to a state similar to the 20th C control. The sulfate scenario alters the temperature

structure substantially in the stratosphere, and nudges the temperature profile even further from the

20th century control period profile. These changes are felt on Earth's surface and reflected in changes to

the winds.

Surface Wind Stress Trends

Here I compare changes in wind stress under rcp 8.5 forcing and the two geoengineering strategies. The

Antarctic Circumpolar Current (ACC) is increased under the rcp 8.5 scenario, with increases in surface

stress up to 0.04 N/m^2. (figure 8). Sulfate geoengineering offsets some of this change; the Antarctic

winds from ~50 S to 60 S, however, are still increased relative to the 20th Century control (figure 9).

By comparison, the GHGrem simulation caused a reduction in the ACC from 120W to 160E over the Ross

Sea but did not cancel the changes south of the Indian Ocean, in East Antarctica (figure 10). Relative to

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the Sulfate simulation, there is a net reduction in wind stress encircling the 50 to 60 degree latitude

band (figure 11).

More interestingly, in the GHGrem case, the two regions of temperature change in the SO relative to

the 20th century control correspond to the two regions of change in wind stress (figures 3 and 10). The

was residual warming from 30 to 60 E, south of the Indian Ocean is the same region that experienced

continued increase in wind stress with GHGrem. By contrast, the region that cooled the most was from

~ 170 W to 170 E, in the Ross Sea. This is the region that saw the biggest reduction in wind stress relative

to the 20th century control. This suggests a very close relationship between changes in surface wind

stress and temperature. In the Southern Ocean, subsurface waters are warming than surface waters.

The prevailing westerlies cause divergence from the continent, and induce warm water upwelling. This

warm water upwelling relates the surface temperature and wind stress anomalies.

Ekman Transport

In their recent review, Marshal and Speer (2012) state that winds and sea-surface fluxes near Antarctica

decrease the density of surface layers in the Antarctic Circumpolar Current (ACC). As dense water sinks,

it draws warmer, saltier water from the surrounding ocean. This is supplied from deeper layers along

tilted, exposed density surfaces. The warm, upwelling waters melt ice on the shelf and in the open

ocean, and control the northern extent of the cryosphere (Marshall and Speer, 2012). This northward

induced Ekman flux is counteracted to some extent by an eddy-induced southward flux (Ferneti and

Delworth, 2010; Abernathey et. al., 2011). In this study, I focus only on change in the Ekman transport.

To compare the effects of wind stress on warm-water upwelling, I calculated the curl of the wind stress.

I used a first order difference scheme that effectively becomes a second order difference scheme when

used with centered u,v coordinates. The curl equation in spherical coordinates is taken only in the

direction:

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With the model grid, this translates to:

Figures 12 - 15 show the zonally averaged GHGrem and sulfate engineering curls from 78 S to 43 S at

four different times in the run: years 5, 8, 10, and 25. The regions of upwelling are indicated by negative

values, and are south of the zero line in all the plots. I focus on two differences between the simulations:

the magnitude of the upwelling and the location of the zero line. In all the years plotted, there is greater

upwelling in the sulfate run than in the greenhouse gas removal run. After year 8, the location of the

sign change splits: the sulfate run Ekman transport shifts southward and the greenhouse gas removal

Ekman transport shifts northward. The southward shift of the Ekman transport in the sulfate run is

consistent with expectations that the Antarctic westerlies shift southward under greenhouse warming

(Abernathey et. al., 2011). The results of these geoengineering simulations show that stratospheric

sulfates induce poleward shifts in Ekman transport relative to the greenhouse gas removal. These

preliminary plots also suggest that changes to the strength of the upwelling occur faster than changes to

the spatial extent of the upwelling. In year five of the simulations, GHGrem shows less upwelling than

the sulfate injections, yet the northward extend of the upwelling is equal. By year 8, the Ekman

transport of the sulfate simulations has shifted south relative to the GHGrem simulations. Further

analysis is required to probe the significance of these time scales.

4. Discussion and Future Work

This was preliminary work, and opened up many more questions for future investigation. Next, I would

like to compare the ocean temperature profile at depth between the sulfate, GHGrem, and 20th century

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control runs and plot a full time series of changes in ocean temperature, at various depths. This would

give an indication of changes that occur on longer time scales. We are currently adding a second

ensemble member to the 1988 greenhouse gas removal simulation to examine variability in the

responses, and simulating an abrupt return to pre-industrial forcing, also as a 2035 branch run. Held et.

al., (2009) investigated the response of the climate to an abrupt return to preindustrial forcing with the

Geophysical Fluid Dynamics Laboratory's Climate Model version 2.1. They found that the climate

responded in less than five years to the abrupt change in radiative forcing. This study focused on the

spatial structure of the fast response; it would be prudent to consider the slowly evolving response of

geoengineering strategies, driven by deep ocean thermodynamics, as well.

This experiment provided a cursory comparison of the effects of geoengineering via sun shading with

stratospheric sulfates, and via greenhouse gas removal. Using an fully coupled model, we found that

sulfate geoengineering cools the planet on average, but does not counteract, and may even exacerbate,

circulation anomalies. These anomalies cause a strengthening and poleward shift of the surface

westerlies over the Southern Ocean, and increase Ekman transport. This, in turn, upwells warm

subsurface waters which further melt the surrounding sea ice. Rapid removal of greenhouse gases, by

contrast, reduces the winds over a large portion of the Southern Ocean relative to the sulfate

engineering simulation, and over the Ross Sea relative to the 20th Century control. An abrupt return to

350 ppm of CO2 didn't not fully cancel greenhouse warming on average, but it did largely cancel the

warming over the Southern Ocean and reduced the temperature over the Ross Sea below the

temperatures of the end of the 20th century control period.

Faced with projected climate emergencies, we need scrupulous analyses of the expected outcomes and

possible unintended consequences of geoengineering. This study explored one aspect of sulfate and

GHGrem engineering. There are countless others. Damage to the ozone hole was not considered here,

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nor were changes to biogeochemical cycles such as ocean acidification. Furthermore, geoengineering

with stratospheric sulfates has very short-lived cooling effects. If performed in the absence of a major

coordinated effort to reduce greenhouse gas emissions, it would cause severe temperature change and

climate shock after shut-off. Thus, geoengineering considerations should not supplant efforts to reduce

greenhouse gas emissions. Since geoengineering may become a part of our response to climate change,

coordinated modeling efforts will be needed to provide the information necessary for sound judgments.

Acknowledgements

I thank Kelly McCusker and Cecelia Bitz for motivating this work, and for the opportunity to collaborate

on this study. Many of the ideas presented were developed through conversations, emails, and

unpublished works by Kelly McCusker.

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References:

Abernathey, R., Marshall, J., and Davier Ferreira, 2011: The Dependence of Southern Ocean Meridional Overturning on Wind Stress, J.Phys. Oceanography, 41, 2261-2277. Farneti, R., and Thomas Delworth, 2010: The Role of Mesoscale Eddies in Remove Oceanic Response to Altered Southern Hemisphere Winds, J. Phys. Oceanography,40, 2348 - 2354. Gent, Peter R., and Coauthors, 2011: The Community Climate System Model Version 4. J. Climate, 24, 4973–4991. Held, I. et. al., 2009: Probing the Fast and Slow Components of Global Warming by Returning Abrutly to Preindustrial Forcing, J. of Climate, 23, 2418 - 2427. Marshall, J., and Kevin Speer, 2012: Closure of the meridional overturning circulation through Southern Ocean upwelling, Nature Geoscience, advance online publication, DOI: 10.1038/NGEO1391 Rasch, P., Crutzen, P., and Danielle Coleman, 2008: Exploring the geoengineering of climate using stratospheric sulfate aerosols: The role of particle size. Geophys. Research Letters, 35, L02809.

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Figures

Note: Most comparisons are made between the geogengineering 2045 - 2054 climatology and the 1970 - 1999 (20th C) climatology. For Surface Temperature, Vertical Temperature, and Wind Stress changes the order of the first three plots presented is:

a. rcp 8.5 - 20th C b. sulfate geoengineering - 20th C c. GHGrem - 20th C.

1. Surface Temperature Profiles

Figure 1. Surface Temperature Increase under rcp 8.5 warming

Figure 2 . Temperature Change under Sulfate Geoengineering

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Figure 3. Temperature change under GHGrem engineering

Figure 4. Comparison of GHGrem and sulfate engineering temperature change

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2. Vertical Temperature Profiles

Figure 5. RCP 8.5 vertical temperature change

Figure 6. vertical temperature change under sulfate engineering

Figure 7. Vertical temperature change under GHGrem

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3. Surface Wind Stress

Figure 8. Surface Stress Change with rcp 8.5

Figure 9. Surface Stress Change with Sulfate engineering

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Figure 10. Surface stress Change with GHGrem

Figure 11. Surface Stress Comparison: GHGrem - sulfates

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4. Ekman Transport

Figure 12. Zonal Avg Ekman Pumping in 2040

Figure 13. Zonal Avg Ekman Pumping in 2043

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Figure 14. Zonal Avg Ekman Pumping in 2035

Figure 15. Zonal Avg Ekman Pumping in 2060

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5. Greenhouse Removal

Figure 16. Time series of Greenhouse Gas Concentrations with abrupt removal in 2035