The Effects of Deep Ocean Carbon Sequestration on Different Oceanic Locations

Methods to mitigate global warming have been ineffective thus far. For this reason, geoengineering methods to combat climate change have become a topic of much interest. Because the ocean holds a significant amount of anthropogenic carbon, deep ocean carbon sequestration is proposed to be a long-term solution to reducing the amount of carbon accumulation in the atmosphere. However, this solution also enhances ocean acidification at the seafloor. The authors study the effectiveness and side effects of CO2injection at various locations using an Earth model system. They compare the effects at the injection sites to the effects that would occur without using this mitigation method at those sites. The authors conclude that sequestration of CO2 was more effective under climate change and with larger overall emission, but poorly chosen sites that are shallow and or less accessible to the ocean can exacerbate future climate change. There are also many obstacles to using this method: a lack of public acceptability, costly and under-developed technologies for ocean CO2 storage, and a lack of complete evaluation of the benefits and consequences. The authors conclude that more thorough research is needed before the method is employed. —Michela Isono
Ridgwell, A., Rodengen, T., Kohfeld, K. 2011. Geographical Variations in the Effectiveness and side Effects of Deep Ocean Carbon Sequestration. Geophysical Research Letters 38, doi:10.1029.

            The rising accumulation of carbon in the atmosphere has proven to affect the planet detrimentally . Methods to mitigate these effects have therefore been proposed and studied. Because the ocean holds a significant amount of carbon, deep ocean carbon sequestration is a specific technique of geoengineering. This method injects liquefied CO2thousands of meters deep into the ocean, where the carbon would sink and be stored. The geologic storage of CO2 in the ocean serves to prevent CO2 from entering the atmosphere and perpetuating the effects of global warming.  
            Methods: A low resolution Earth system model (GENIE) is used to represent ocean circulation and carbon cycling. Five models are used in total: a bathymetry of the Earth system model is used to track measurements of ocean depth; an observation model based on data-estimated observations from other studies; a control model based on a 10,000 year spin-up under pre-industrial boundary conditions which is continued to year 2010 where levels of atmospheric CO2are based on historical data; a model based on model-estimated distributions of water-column integrated anthropogenic CO2 inventory for year 1994; and an experimental model that incorporate SRES emission scenarios for years 2100 and 2000 where 10% of the emissions are directed towards the ocean and the other 90% enter the atmosphere. Seven location points for injection were used: Bay of Biscay, New York, Rio de Janerio, San Francisco, Tokyo, Jakarta, and Bombay. These points represented the Antlantic Ocean, the Pacific Ocean, and the Indian Ocean. The injected CO2, once dissolved, is referred to as Dissolved Inorganic Carbon (DIC).
            Results and Discussion: CO2 that is injected at the ocean floor instead of being released into the atmosphere man interact with CO2 taken up at the ocean surface. Sequestration efficiency is therefore considered in the context of how much CO2 would invade the ocean from the atmosphere. In the San Francisco location, DIC extended outward from the injection point but there was also a reduced DIC in the North Atlantic because less CO2was taken up from the atmosphere. Higher DIC concentrations were found in the Pacific, but there was also a reduction in the amount of calcium carbonate saturation and an increase in the amount of seafloor area that had unsaturated conditions. However, in the Atlantic, the reduction of atmospheric CO2increased the amount of seafloor area that had saturated conditions. 
            Data regarding the variation in effectiveness of CO2 depletion and relative mitigation of the surface ocean acidification as a function of time, injection depth, and ocean sector demonstrate that carbon sequestration can fail to work. The Pacific and Indian Ocean point of injection sites were more likely to fail and result in a negative sequestration at year 3000 compared to unmitigated atmospheric CO2 release. However, the Atlantic injection sites did not have negative sequestration even though there were more shallow and intermediate ocean depth levels within this location.
            Sequestration efficiency was mapped to visualize the retention of injected CO2 in the entire ocean for the release at each grid point. The relative efficiency of carbon sequestration in percent (RE) at the beginning of the time period and located away from shallow continental margins was more or less the same at over 70%. However, later in the millennium, many inter-basin gradients in CO2 retention developed and RE approached zero because the carbon equilibrium was reestablished between the ocean and atmosphere.
            In locations where climate change was strongly prevalent, RE was enhanced.  In this case, carbon mitigation led to lower CO2 in the atmosphere and decreased the temperature of ocean surfaces. This increased the solubility of CO2 and improved CO2 uptake at the ocean surface. RE was higher for greater emissions. This means that carbon buffering is reduced when more CO2 is released and absorbed by the ocean surface. Thus, the results indicated that CO2 injection improved the ability for uptake from the atmosphere. Locations that included shallow sites and sites that are not well connected to the entire ocean exhibited an RE < 0.0.
            The choice of CO2injection site was also analyzed based on levels of under saturated waters. At year 2100, there was little changed by injection in the NW Pacific compared to the unmitigated case. Injections in the SE Pacific and S Atlantic experienced a 10% additional increase in the seafloor area that was under saturated. By the end of the millennium, injection led to less than a 2% increase in additional under saturated seafloor area.
Conclusion: Sequestration of CO2was more effective under climate change and with larger overall emissions. For higher emissions, the naturally occurring CO2 buffer of ocean surface waters is depleted faster. Overall, RE is better than 70% by year 2100 and in certain places can stay above 50% by year 3000. Poorly chosen sites that are shallow and or less accessible to the ocean can exacerbate future climate change. Injection in the deep NW Pacific (a high efficiency site) minimizes the exacerbation of under saturated seafloor conditions. There are many practical constraints that would limit the use of this geoengineering technique such as negative environmental impacts, harmful effects on organisms, and the method’s effect on other associated biotic impacts still need further research and analysis. 

The Effects of Sulfate Aerosol Injection Across Various Stratospheric Locations

The detrimental effects of global warming have become increasingly apparent, and the current mitigation methods have proven insufficient. Therefore, geoengineering techniques have been proposed as the necessary means to combat global warming. Specifically, solar radiation management using the sulfate aerosol has been named the most effective solution. The authors analyze temperature and precipitation responses to stratospheric sulfate aerosol as a function of both latitude and altitude of release. They model the injection of the sulfate aerosol (comprised of H2S rather than the commonly analyzed SO2) in the Northern and Southern Hemispheres. The authors conclude that sulfate aerosol injections at higher latitudes reduce global mean temperature, precipitation, and total ozone. They also conclude that warming in the stratosphere follows cooling in the troposphere. —Michela Isono
Volodin, E., Sergey, V., Ryaboshoapko, A. 2011. Climate Response to Aerosol Injection at Different Stratospheric Locations. Atmospheric Science Letters 12, 381–385.

            Model: The climate model used is Earth system model INMCM. This model includes the general circulation of the atmosphere and oceans. It is used to determine the most effective injection circumstances in order to reduce changes in temperature and precipitation in the Arctic and on a global scale. The observed climate changes in the 20th century will also be reproduced in the model. The model also has a sulfate aerosol component that describes the formation of the sulfate aerosol particles and their removal by gravitational settling. H2S was chosen instead of SO2 because it is a gas with the highest sulfur content by mass, and it has a prolonged lifetime that enables greater dispersion before the aerosol particles are formed and the reduction in particle concentrations by coagulation. Lastly, the gravitational settling velocity is calculated using the Stokes-Cunningham formula:
vg= 109Ccunngd2p / (18 µ), where Vg is gravitational velocity (m/s), g is acceleration due to gravity (9.8 m/s2), d is particle diameter (m), p is particle density (1.63 g/cm3), µ is dynamic viscosity of air (µPa s) and Ccunn is a correction factor.
            Geoengineering scenarios: Six scenarios in the Northern and Southern hemispheres are used in this study. They are at different altitudes above the tropopause and within different latitudinal belts. Each hemisphere experiences a continuous injection of 2 Mt per year. The sulfur injection is continued for 30 year in each model where the level of atmospheric CO2 is constant at 288 ppb. The climate response was determined by the average temperature and precipitation over the latter 20 years. Each model was then compared to a control model with no inclusion of geoengineering. The climate response was calculated as the difference between the temperature or precipitation in the models with geoengineering compared to the control model without geoengineering. The annual mean values were calculated separately for the globe and Arctic (north of 65˚N).
            Results: Scenario 1, 2, and 3 with injections near the equator at 26–28, 22–24, and 20–22 km respectively, were the most effective. Scenarios with lower injections towards the poles were less effective for global cooling. It was determined that injections at higher heights ranging from 20–28 km lead to a higher concentration of gaseous H2SO4 rather than the aerosol. The level of global precipitation reduction and cooling were proportional. The total ozone loss was also proportional to aerosol mass and global cooling. The Arctic temperature change, however, was two times more than the global temperature change. The models also demonstrated significant aerosol concentration located outside of the injection region. The equilibrium aerosol mass in the atmosphere is reached after about two to three years, where the lifetime of the aerosol was 0.9 years. The equilibrium temperature and precipitation response was achieved after 10–15 years.
            The annual mean cooling over land was stronger than over the ocean. The strongest cooling was over high latitude land and sea ice, above 2˚. There was a decrease in precipitation by 5–10% over parts of Eurasia, and North and South America. In contrast, there was an increase in precipitation by 10–20% over Mediterranean, tropical, and subtropical regions. Overall, as the aerosol cooled the troposphere, the stratosphere warmed by 2–8˚. This was due to the aerosols’ absorption of small amounts of visible radiation in the stratosphere.
            Conclusion: The authors concluded that the best scenarios for geoengineering with sulfate aerosol was when the aerosol was injected at 26–28, 22–24, and 20–22 km in the latitudinal band 0–10˚. Additional conclusions were that cooling is greater over land and high altitudes, the injection of aerosols decreases global mean precipitation and total ozone, the cooling in the troposphere is followed by warming in the stratosphere, and an increase of diffuse radiation increases vegetation primary production. 

Reducing the Solar Constant as a Mitigation Technique to the Increasing CO2 Levels

Due to the detrimental effects of global warming, in particular the drastically increasing levels of CO2, the Danish Climate Centre (DKC) focused on methods of mitigation. In particular, DKC focused on solar radiation management experiments. The DKC uses the EC-Earth climate model in their experiments because it includes a good representation of the stratosphere. Their experiment includes three simulations: a pre-industrial control simulation (Control), a quadrupled CO2 simulation (4CO2), and a quadrupled CO2 simulation balanced by a reduction of the solar constant (Balanced). The authors also considered the situation in the Northern Hemisphere extra-tropical winter. The mean temperature and precipitation responses in each simulation were analyzed. The authors conclude that reducing the solar constant can significantly mitigate the detrimental affects of increasing CO2 levels. However, they acknowledge that a better understanding about the effects of cooling on the stratospheric ozone is needed. —Michela Isono
Christiansen, B., Yang, S., 2011. Mitigating a Quadrupling of CO2 by a Reduction of the Solar Constant: A Geoengineering Experiment with the EC-Earth Climate Model. Danish Meteorological Institute, 1399–1973.

Background
The solar radiation constant is a measure of the amount of incoming solar electromagnetic radiation per unit area. The experiment lasted for 50 years and the second 25 years are used for analysis purposes. A reduction of the solar constant of 56 W/m2 was used. This value was derived from the equation ∆F = (1-a) / 4∆S, where ∆F is the radiative forcing from the quadrupled CO2(~8.5 W/m2) and where a is the planetary albedo (0.33). The authors investigated the annual mean temperature responses and the annual mean precipitation responses, and acknowledged that their experiments have been performed in previous scientific studies. The authors determine the statistical significance of the differences among the simulations by using a t-test.
The authors chose to study the Northern Hemisphere extra-tropical winter region. In this area, the impact of the reduced solar radiation is minimal; however the indirect effects associated with dynamical changes were expected. The authors stated that changes in the stratospheric temperature cause changes in the stratospheric vortex, which causes changes in the North Atlantic Oscillation (NAO) and then affects the troposphere.
Annual Mean Temperature Responses
            The annual mean surface temperature responses were calculated over a 25-year time period. The 4CO2simulation showed significant warming across all regions (values ranged from 2 to over 16 K). The largest response was in the Arctic, and the smallest response was in the tropics and over the Southern Ocean. The Balance simulation showed a much lower response across all regions (values infrequently went above 1 K). The warming that occurred was located in the polar areas contrasted by the slight cooling in the tropics. The responses in the tropics and Arctic were statistically significant, however responses in many parts of the extra-tropics proved statistically insignificant.
            The zonal and annual mean temperature responses as a function of latitude and altitude were also performed. The 4CO2 simulation showed that the troposphere warmed ubiquitously. The most warming was observed in the mid-troposphere over the tropics. In contrast, the stratosphere cooled ubiquitously (values reached –15 K). The Balance simulation showed much smaller responses in the troposphere, although cooling was seen overall in the tropical and extra-tropical troposphere (values were statistically significant decreasing by –1 or –2 K). Comparatively, there was more cooling in the stratosphere in the Balance simulation than in the 4CO2 simulation.
Annual Precipitation Responses
            The monthly global mean precipitation as a function of time for the three simulations over 25 years was analyzed. The Control simulation showed a consistent value of about 2.85 mm/day/m2. The value in the 4CO2 simulation increased about 0.2 mm/day. The authors noted that this result could be caused by the fact that saturation water vapor pressure increases with temperature. The value in the Balance simulation decreased about 0.1 mm/day. The authors stated that this finding has been found in previous studies and is the result of the reduced solar radiation at the Earth’s surface, which ultimately slows the continuous movement of water on, above, and below the surface of the Earth (hydrologic cycle). 
            The geographical distributions of the annual mean precipitation responses were also shown. In the 4CO2simulation, the polar-regions and tropics showed the greatest increase in precipitation and were statistically significant. The extra-tropics showed a weaker change, in which there was a negative response in some locations. In the Balance simulation, a small decrease in precipitation response was generally shown. The response of only a small number of regions was statistically significant— over seas or the Western part of tropical Africa. 
NH Winter Responses
            The annual zonal mean temperature response showed that the troposphere warmed and the stratosphere cooled in the 4CO2 simulation. This occurrence is statistically significant in all regions except in the polar stratosphere. However, a greater response in the stratosphere in the winter towards the poles was found as well. This is the result of the induced temperature response that reduces the negative temperature gradient in the stratosphere, which reduces the stratospheric vortex. The results of the zonal mean wind responses support this idea.
Additionally, the Balance simulation for winter (December, January, and February) zonal mean temperature response over the last 25 years showed that the gradient in the stratosphere temperature response was almost gone. This is due to cooling in the polar stratosphere and causes smaller changes in the stratospheric vortex and no changes in the troposphere. The mean surface temperature and precipitation responses during the winter were determined as well. The results showed a significant response in the 4CO2 simulationbut not in the Balance simulation.
Conclusion
            These experiments showed that reducing the solar constant can mitigate the increase of the annual mean surface temperature that is caused by the drastically increasing levels of CO2. This mitigation method proved successful near the surface, and cooled both the troposphere and the stratosphere. However, the effects of significant cooling in the upper stratosphere could affect the stratospheric ozone.
The authors also took into account the NH winter response. In this case, the effect of reducing the solar constant was small. However, the surface temperature and precipitation was combated well.

The Effects of Different Aerosol Types and Sizes on Stratospheric Heating

There have been major changes in the Earth’s climate and attempts to mitigate the greenhouse effect have been ineffective thus far. For this reason, geoengineering has been proposed as another method to combat global warming. One common method of geoengineering is through solar radiation management (SRM). One such technique involves injecting aerosol particles into the stratosphere to increase planetary albedo. Studies have show that using SRM in the stratosphere using sulfate particles ultimately cools the planet. Ferraro et al. use a fixed dynamical heating model to examine the affects of various types and sizes of aerosols on stratospheric temperature change. The authors recognize that aerosols heat the tropical lower stratosphere, but can either heat, cool, or not affect the polar regions. They therefore employ various aerosol types and sizes during different seasons in order to further investigate the aerosols’ effect on atmospheric temperature change. The authors conclude that additional research in modeling the impacts of geoengineered aerosols is needed to better understand the effect of this method on stratospheric circulation. —Michela Isono

Ferraro, A., Highwood, E., Charlton-Perez, A. 2011. Stratospheric Heating by Potential Geoengineering Aerosols. Geophysical Research Letters 38, L24706, doi:10.1029/2011GL049761.

Background

The majority of SRM research has taken into account the eruption of the Mt. Pinatubo volcano. This eruption increased the sulfate layer produced by the sulfur dioxide emission from the volcano, which ultimately decreased the stratospheric temperature. A change in the stratospheric temperature is also thought to affect the dynamics of the stratosphere; studies have showed irregular weather patterns in various parts of the world such as a warm winter in Northern Europe and a positive phase of the Arctic Oscillation (an index of opposing atmospheric pressure patterns in the Northern middle and high latitude). It is thought that these occurrences were the result of an intensified meridional (in the north-south direction) temperature gradient in the lower stratosphere, which was enhanced by ozone depletion and reduced planetary wave activity.

Ferraro et al. believe that it is necessary to quantify the radiative impact of SRM in the stratosphere before the potential dynamical changes are examined. The size distribution of aerosols is a key factor of which little is known and understood. Previous studies have indicated that the size distribution affects the surface cooling and the stratospheric radiative heating. The authors state that the compositions of aerosols are also significant and propose that soot, limestone dust, and titanium dioxide are viable alternatives. For this reason, the authors investigate the temperature change based on the type and size of aerosols used in order to better understand the impact of SMR in the stratosphere.

Methods

Model: When an aerosol layer is introduced into the stratosphere, the authors use a two-stream radiative transfer code to calculate radiative fluxes (measure of the flow of radiation from a given radioactive source) and heating rates. The temperature change in the stratosphere is calculated using the fixed dynamical heating approximation (FDH). The temperatures are then changed until the stratosphere meets the radiative equilibrium. This method differentiates the radiative impacts from dynamical changes. However, their model does not incorporate the radiative effects on the stratosphere of changing surface temperature. Their model instead shows the activity within the stratosphere before the surface temperature has changed.

Aerosol Layer Properties: When injecting at high altitudes, the amount of time the aerosol resides there is maximized. The authors indicated the existence of technological limits to the input altitude of geoengineering aerosols: plastic balloons burst at about 25 km, and the size of aerosols decrease due to decreasing amalgamation which enhances shortwave (SW) scattering. As a result, the authors introduced the aerosol between the tropopause and 22 km in a standardized layer. It is important to note that the standardized layer is an idealized model because in reality the layer would slope downwards towards the poles.

The authors used aerosols composed of sulfuric acid (sulfate), titanium dioxide (titania), limestone dust, and soot. They also used six size distributions for each type of aerosol, which they characterized as small, medium, and large, and narrow and wide. As a result of the parameters of the aerosols, the authors used Mie calculations (shows scattering of electromagnetic radiation by a sphere) of absorption and scattering. Because radiative properties are a function of aerosol size, the different aerosol distributions will show different radiative forcing (change in net power of electromagnetic radiation per unit area on a surface where a positive forcing warms the system and a negative forcing cools the system). Specifically for this reason, the authors chose the aerosol mass so that the small/wide cases all had instantaneous radiative forcing at the tropopause of –3.5 ± 0.1 Wm–2. Instantaneous radiative forcing measures radiative impact instead of changes in stratospheric temperatures. The authors stated that adjustments in the stratosphere do not significantly impact radiative forcing for most aerosol forms.

Results

The result of the FDH stratospheric temperature adjustment for the small/wide case in the December-January-February (DJF) season and in the June-July-August (JJA) season are compared; the September-October-November and March-April-May seasons are not reported because these seasons include transitions between a warm and cold polar stratosphere. The DJF season and the JJA season showed a very similar patter except with the poles reversed. The mean temperature change in DJF for the small/wide aerosol size distribution for sulfate and soot aerosols showed the same instantaneous radiative forcing at the tropopause. The strong heating in the stratosphere increases downward longwave (LW) emission to the surface, which is why the differences between the instantaneous and stratosphere-adjusted radiative forcings fall under 10% of the instantaneous values, while soot demonstrated a forcing that is 48% less than the instantaneous farcing.

The authors note the importance of understanding the energy balance of a layer in the lower stratosphere when analyzing the data. The stratosphere cools be emitting LW radiation in proportion to the layer’s temperature. They also stated that the main input to the layer is from solar SW radiation. Although some LW radiation from the troposphere goes into the stratosphere, the amount of LW is usually smaller than the incoming SW. As a result, the authors stated that the temperature changes from the aerosols are ruled by LW emission and SW absorption.

The sulfate aerosol showed that the temperature change in the stratosphere correlated strongly with the result of the volcanic natural analogy. There was heating in the tropical lower stratosphere and cooling over the summer pole. The stratosphere is mildly warm over the summer pole, and sulfate emits strongly in the LW and radiative cooling occurs. It was also shown that sulfate absorbs moderately in the LW part of the spectrum. The authors stated the tropical heating was due to the absorption of LW radiation from the warm troposphere and the negligible emission from the cold tropical lower stratosphere.

The titania aerosol showed a temperature change of about 30% of sulfate’s. Heating occurred at all latitudes expect for at the winter pole. This occurrence was due to the fact that titania mainly absorbs the shortest wavelengths, which causes heating in latitudes with solar radiation. The LW cooling takes over, as there is no solar heating because the North Pole is under polar night conditions in DJF.

The limestone aerosol showed patterns similar to titania although they were enhanced; however, limestone showed cooling at lower levels over the South (summer) Pole. The authors noted that the absorption and scattering of incoming solar radiation decreases the radiation accessible for heating at lower latitudes. Therefore, cooling only occurs at the pole because this region is below the aerosol layer. The temperature of the troposphere could not be calculated because the FDH model only applies to the stratosphere.

The soot aerosol showed the most intense heating over the summer pole. This is because soot mainly absorbs SW so its heating pattern in limited by the latitudinal variation of solar radiation. The results showed that soot’s heating is significantly greater than the other aerosol types.

As previously stated, heating always occurs at the tropical lower stratosphere with aerosols, but either heating, cooling or a neutral effect can take place at the poles. Thus, the temperature difference between the Pole and the Equator is affected. The authors use the equation TTR-NP = TTR – TNP to define the difference in temperature between the Tropics (20N–20S) and the North Pole (90N–50N) per unit negative forcing. This is calculated by dividing the temperature change from each model performed by its instantaneous radiative forcing. A positive number indicates cooling at the Pole and warming at the Equator. The authors do not consider the Southern Hemisphere due to redundant characteristics and do not show the large/wide case for titania because the results are irrelevant to SRM.

The results showed that each aerosol type and size distribution increased TTR–NP. IN DJF, titania showed the smallest change in temperature difference (~0.3 K) whereas soot showed the largest change in temperature difference (~2.8 K). Sulfate and limestone both showed an increase in temperature change by approximately 1 K. In JJA, SW absorption heats the North Polar stratosphere. Therefore, sulfate showed a similar temperature change in both seasons, titania and limestone did not show a significant change in temperature, and soot showed a strong negative change (meaning warming at the Pole and cooling at the Equator) of about –5 K.

Results of change in regards to aerosol size distribution were also found. The majority of DJF cases showed that increasing the radius and size distribution increased the temperature change between the Tropics and the North Pole. The large/wide sulfate and limestone aerosol showed a significant change where their mass was ruled by large particles; large particles absorb LW. However, the narrow sulfate case showed negligible affects to changing size, the wide soot case showed negligible affect to radius changes, and the large radius showed the least impact.

In JJA, the temperature change depended on the size distribution of the aerosol. The titania aerosol showed an increased difference in temperature expect for in the small/narrow case (the poles does not cool because LW absorption is low). For this reason, the pole is warmer with this particular titian aerosol. The limestone aerosol also showed a change in the sign: the sign was positive for the small case and negative in the medium and large cases. This is also the result of polar heating, as the larger the limestone aerosol, the greater the absorption of SW radiation. In the JJA season, the pole receives consistent sunlight, which allows more SW radiation to be absorbed, heats the pole, and reduces the temperature difference between the Tropics and the North Pole. In all cases for the soot aerosol, significant SW heating over the poles were shown which decreased the temperature difference.

Discussion

The study showed that different types of aerosols cause different patterns of stratospheric temperature change. The DJF small/wide aerosol types showed the same instantaneous radiative forcing at the tropopause. This means that they all have the same global cooling, however, the atmospheric cooling response may differ from case to case. Stratospheric temperature changes from aerosols result from their rates of SW absorption and LW emission. Aerosols also decrease the temperature of the surface and troposphere, however, because the authors’ model does not calculate temperatures for these regions, the result was not included. The authors focus on the stratospheric temperature changes because surface temperature changes occur over hundreds of years, whereas dynamical changes occur over months to years.

Because the soot aerosol heats the stratosphere intensely, a larger mass of soot is needed per unit radiative forcing despite the results of the calculations without stratospheric adjustment to aerosol heating. The titania aerosol was very responsive to size distributions in regards to its instantaneous radiative forcing- the large/wide aerosol produced a small positive forcing. Therefore, the authors stated that aerosol geoengineering could be ineffective if their thoughts about aerosol size distributions were wrong, which would ultimately alter the nature of regional and planetary responses to SRM.

The authors also compared their results with the yearly variability in the pole-Equator temperature differences. The calculation was based on the standard deviation of the difference in DJF and JJA over the same 20-year period. The yearly standard deviation was for DJF and JJA was 2.09 K and 0.64 K, respectively. To compare those standard deviations with their results, the authors multiplied TTR–NP by the radiative forcing, where the small/wide case was multiplied by 3.5 Wm–2. The calculations showed that in DJF sulfate, limestone, and soot increased the temperature difference more than the standard deviation. In JJA, sulfate increased the temperature difference, and soot decreased the temperature difference past the standard deviation. For the titanita aerosol, no changes in temperature difference were significant in either season.

Conclusion

The results showed strong heating disturbances in the stratosphere. Therefore, it is not adequate to model SRM in the stratosphere by only decreasing the amount of solar irradiance as the stratospheric aerosol heating will not be shown. The authors also conclude that using the same radiative forcing with various aerosol types will not have the same ultimate effect, and that different changes in the stratospheric temperature will lead to various dynamical responses and thus cause the climate to react subjectively. Additionally, the lower stratospheric temperature changes will affect the power of the polar vortex (a big cyclone near both of the earth’s poles in the middle and upper troposphere and the stratosphere). This affect could alter the Arctic Oscillation and various weather patterns. The authors conclude that more dynamical modeling is needed to examine the effect of aerosol radiative absorption on the circulation of the stratosphere and troposphere.

Removal or Failure of Climate Engineering by Sulfate Aerosol Injection Poses Dangerously Abrupt Temperature Increases

Sulfate aerosol injection as a method of geoengineering and cooling the planet has been showing great promise and growing in popularity in regards to fixing the climate crisis. It has long been suggested that geoengineering could function independently and provide researchers with more time to improve and create methods of removing CO2 from the atmosphere, which is the real fix. Ross et al. (2009), however, claim that if we rely on geoengineering and do not implement it alongside the removal of CO2, there will be drastic temperature changes if the geoengineering strategy is removed or fails. If CO2 emissions do not decrease, the failure or removal of climate engineering methods would result in a large temperature spike, increasing global temperatures by a maximum of 4.5 °C, which would have catastrophic impacts on the planet’s ecosystems and possibly result in mass species extinctions.— Ellie Pickrell
Ross, A., Matthews, H., 2009. Climate engineering and the risk of rapid climate change. Environmental Resolution Letters 4, 045103.

 In this study, Ross et al. used a climate model to predict the effects of the implementation and subsequent removal of climate engineering by injection of sulfate aerosols with the A1B emissions scenario. The control group consisted of a business as usual emissions scenario. The second simulation consisted of a model exposed to climate engineering that started in the year 2020 and was removed in 2060. These two simulations were repeated 40 times each, varying with climate sensitivity of the model from 0.5 to 10 ° C. Climate sensitivity is the response of global mean surface air temperature to a doubling of atmospheric CO2 concentrations. An estimated climate sensitivity probability density function was used from another paper (Hegerl et al. 2006) to identify the likelihood of each set of model situations.
In the control group where climate engineering was not applied, temperatures increased consistently from 1990 to 2100, ranging from 0.6 to 5.1 °C for climate sensitivities ranging from 0.5 to 10 °C.  Atmospheric CO2 concentrations at the year 2100 ranged from 690 to 739 ppmv, with higher climate sensitivities containing the higher concentrations. In the climate engineered simulations, temperatures dropped to values very similar to the temperatures in 1990 between 2020 and 2059, with respect to the control scenario. As soon as the engineering was removed, however, temperatures increased rapidly, ranging from 0.15 to 4.5 °C between 2060 and 2100. The temperature change after the removal of the engineering was higher with higher values of climate sensitivity. The final CO2 concentrations in the geoengineering runs were similar to those in the control simulations (between 689 and 722 ppmv).
Next, Ross et al. looked at the annual rate of temperature change between 1990 and 2100 for each simulation. In the control scenario, the annual rate of temperature change increased until 2060, whens greenhouse gas emissions decline with the A1B emissions scenario. This resulted in a decreased rate of temperature change. In the climate engineering scenarios, the rate of temperature change was relatively small up until 2020, when geoeningeering was implemented and temperatures dropped. From 2020 to 2060 the rate of temperature change was insignificant, until temperatures abruptly increased after the removal of geoengineering. The maximum rate of warming ranged from 0.13 to 0.76 °C/year. These high rates of warming, however, only lasted for a few years and within a decade, the rates decreased to less than 0.1 °C/year. The maximum rate of sea level rise was also higher in the geoengineering simulations than in the controls.
Finally, Ross et al. looked at the probability density functions between 1990 and 2100, which measures the likelihood that these temperatures will change. For the control group, the most likely maximum annual temperate change was 0.031°/year. The geoengineering simulation showed a likely maximum rate of temperature change just under 0.5 °C/year, which occurred in 2060 at the high temperature spike. 

Positive Effects of Geoengineering on Ocean Acidification and Aragonite Saturation Levels

In the past geoengineering has been considered as a promising strategy for global cooling, although it has had some drawbacks. One of these drawbacks was the common belief that engineering the climate would not have any beneficial effects on ocean acidification, which is a negative component of climate change. The writers of this paper, however, proposed that geoengineering could have a beneficial impact on ocean acidification and offset some of the impacts that greenhouse gas emissions have had on our planet’s oceans, specifically pH levels. Aquatic organisms that rely on shells for survival can only build these shells in waters with higher aragonite saturation values, and as the ocean becomes more acidic, the aragonite saturation levels go down, and these organisms cannot survive. Climate engineering could potentially slow the ocean’s current pH decreases, which would ideally slow the rapid reduction of aragonite saturation in the oceans (Matthews et al, 2009). But, although simulations from this experiment do show an increase in oceanic pH values, but not a significant enough increase to stop the rapid decline in aragonite saturation levels. — Ellie Pickrell
Matthews, H., Cao, L., Caldeira, K., 2009. Sensitivity of ocean acidification to geoengineered climate stabilization. Geophysical Research Letters, 36.

 Matthews et al. conducted a series of experiments on an earth system model that resembled a world exposed to climate engineering. They performed five simulations which all began at a preindustrial climate equilibrium, and compared the model’s results at what represented conditions in the year 2100. The control group, A2, lacked climate engineering and consisted of prescribed SRES CO2 emissions. The next simulation, A2+eng consisted of prescribed CO2 emissions and climate engineering, which began after 2010. Next was the A2A+ eng, which consisted of the same CO2 emissions as simulation A2, but again was exposed to climate engineering after 2010. The first three simulations were representing a world with an active biosphere, which means that the land biosphere was exchanging carbon with the atmosphere, i.e. carbon sinks. The fourth simulation, A2nb, consisted of a neutral biosphere (the land biosphere does not exchange carbon with the atmosphere after 2010), prescribed CO2 emissions, and no geoengineering. The final simulation, A2nb+eng was the same as the previous simulation, but was exposed to climate engineering.
     After all of the simulations were tested, Matthews et al. compared the results of the pH and the aragonite tests between the different scenarios. In both the A2 and A2+ eng simulations, pH values were reduced (7.6 and 7.85), compared to the control group with a pH value of 8.05, and aragonite concentrations of 1.85. A model with climate engineering showed a slightly higher pH value at the 2100 mark in comparison to the A2 non-engineered simulation, but a lower aragonite saturation value (1.80 to 1.90). Climate engineering was also effective at reducing the average atmospheric temperatures, as well as lower atmospheric CO2 concentrations, due to an increase in carbon uptake by natural carbon sinks as a result of the cooler temperatures.
     In the A2A+eng simulation, the change in ocean pH was smaller and was extremely close to the control simulation’s pH, but the aragonite saturation decreased more rapidly as a result of climate engineering. An increase in dissolved inorganic carbon and colder temperatures lead to aragonite saturation values that were 9% lower than the values in the A2 simulation (from 1.72 in A2 to 1.58 in A2A+eng), because colder temperatures lead to slightly higher pH values, but result in lower aragonite saturation values.
     Next, A2nb+eng and A2nb were compared. In A2nb+eng, surface temperatures were colder, and ocean dissolved inorganic carbon values were higher than in A2nb, which created unaffected pH values and a further decrease in aragonite saturation relative to the non-engineered simulation. The A2nb+eng simulation had a pH value of 7.75, and aragonite saturation values of 1.7, while the A2nb simulation had a pH value of 7.25 and aragonite saturation values of 1.85.
     These effects and results are dependent on the enhanced accumulation of carbon in the land biosphere. Without this accumulation of carbon, climate engineering will have little effect on ocean pH levels, which would then lead to accelerated declines in aragonite saturation. 

Although Sulfate Aerosol Injections May Cool the Planet They Still Reduce the Amount of Ozone in the Earth’s Atmosphere

Geoengineering is a climate manipulating strategy that could potentially cool the planet and give researchers more time to find an efficient way to remove CO2 from the atmosphere. Injecting sulfate aerosol particles into the Earth’s atmosphere is one of the most popular strategies of geoengineering, as it would increase the albedo of the planet and reflect more light back into space. These sulfate aerosols, however, have to be a certain size to be effective in increasing the planet’s albedo, and particles with a radii of roughly 0.1 mm are the most efficient in cooling the planet (Heckendorn, et al, 2009). This paper shows which injection strategies will produce the smallest and most efficient aerosol particles, and how long these particles will stay in the atmosphere. This is of large concern since there is great speculation regarding aerosol injection, and if it ends up having negative impacts it would be even worse if the particles remained in the atmosphere for long periods of time. This paper also explains how risky injecting sulfate aerosols into the atmosphere can be, as increase aerosols reduce the amount of ozone in the atmosphere. Ellie Pickrell
Heckendorn, P., Weisenstein, D., Fueglistaler, S., Luo, B., Rozanov, E., Schraner, M., Thomason, L., Peter, T., 2009. The impact of geoengineering aerosols on stratospheric temperature and ozone. Environmental Resolution Letters 4.

     Heckendorn et al ran a series of experiments on a global model. These calculations were carried out with two types of sulfur injection methods. The first was a continuous pumping of sulfur into the clouds with fluxes of 1,2,5 and 10 Mt/a, while the other simulations were pulsed injections with periods of one month and six months with fluxes of 5 Mt/a. The control group consisted of zero sulfur injections. All simulations were run for twenty-years with present day concentrations of ozone depleting substances, green house gas, carbon dioxide emissions, sea ice and sea surface temperatures.
     The results of the simulations show that the surface area density of the aerosol particles increases as concentration increases.  In simulation GEO5 (continuous injection with fluxes of 5 MT/a) the surface area density is larger than 40 mm2 cm-3. In the simulation where the injection takes place twice a year (GEO5p2), the surface area density is larger than 100 mm2 cm-3.
     Next, Heckendorn et al looked at how the injection strategies affected the stratospheric residence time of the aerosols. They found that smaller particles have a longer residence time. For the GEO1 simulation (continuous injection with fluxes of 1 Mt/a), the residence time, referred to as aerosol burden, is 1.4 Mt S. For all the other simulations, the aerosol burden is less than one year with 3.7 Mt S for the GEO5 simulation and 6.0 Mt S for the GEO10 simulation (continuous injections with fluxes of 10 Mt/a). They also found that if the sulfur injection occurs at a higher altitude, the residence time increases.
     Finally, Heckendorn et al looked at how the injection strategies would affect the mean O3 column. For the GEO5 simulation, the O3 column is predicted to decrease by 4.5%, and the GEO10 model predicted a decrease by 5.3%. These values are greater than the O3 loss due to the emission of greenhouse gases from 2002 to 2005, a decrease by 3.5%.
     Although geoengineering by injection of sulfur aerosols into the Earth’s clouds has shown promise in cooling the planet and decrease levels of atmospheric CO2 concentrations, it also has the negative impact of reducing the Earth’s ozone layer. 

Solar Radiation Management Geoengineering: Possible Solution for the Shrinking Greenland Ice Sheet

The effects that climate change has on polar ice sheets, particularly Greenland in this study, are important for many reasons. The two most important reasons that are discussed in this article involve rising sea levels and decreased planetary albedo as the globe’s ice sheets melt. Solar radiation management has been suggested to reduce the warming of the globe and buy some time while engineers and scientists address the larger problem of removing CO2 from the atmosphere. The installation of a solar “sunshade” or the injection of sulfate aerosols into the clouds are the two most promising methods of geoengineering. Previous studies have shown that a world exposed to climate engineering would experience warming at the poles, cooling in the tropics, and a decreased precipitation rate, which may have certain effects on the Greenland ice sheet (Irvine et al.). In this study, the melting of the Greenland ice sheet was prevented at levels of partial climate manipulation, which suggests that the geoengineering required to cool the planet and reduce the impacts of greenhouse warming may not be as thorough as geoengineers originally believed. Ellie Pickrell
Irvine, Peter J., Lunt, Daniel J., Stone, Emma J., Ridgwell, Andy, 2009. The Fate of the Greenland Ice Sheet in a Geoengineered, High CO2 World. Environmental Research Letters, 4.

Irvine et al. conducted twelve 400-year simulations on a climate model. The first model was control simulation that modeled a climate similar to that of a pre-industrial world, and wasn’t exposed to climate engineering. The second has atmospheric CO2 concentrations of 1120 ppmv, which is four times the pre-industrial amount, and 0% climate manipulation. The last ten simulations have the same CO2 concentrations and range from 10% to 100% climate engineering by intervals of 10%. At each simulation, Irvine et al. measured the temperature and precipitation anomalies in comparison to the control simulation. The results were then combined with an observed climatology to create an ice-sheet model—Glimmer. Glimmer is a three-dimensional ice sheet model representing the Greenland region, and provided results showing the impact of solar radiation management on the ice sheet.
In the simulation with 0% climate engineering, the center of the Greenland ice sheet had an annual temperature increase of 8°C, and an average summer temperature that increased by 6°C when compared to the pre-industrial simulation. This 0% geoengineering simulation also showed an increase in annual precipitation of over 6 meters a year, which would increase the amount of annual snowfall, which could potentially cause the ice sheet to grow.
For simulations experiencing 100% engineering, the annual average surface air temperature was significantly lower than simulations with lower climate manipulation, although Greenland remained warmer than it was in the pre-industrial period. The island showed an increase of at least 0.5°C, with its northern and southern coasts undergoing an increase of 0.75°C, and a 1°C increase at the southern tip. For simulations experiencing 50% engineering, the model predicted a warming of 3°C across the majority of Greenland. Both 100% and 50% simulations showed an increase in precipitation rates, although it was lower than the 0% engineering simulation. A 100% simulation resulted in a precipitation rate of 21 mm per year.
The results from the Glimmer test were then used to predict the change in sea level of the Greenland region. In the pre-industrial control simulation, the sea levels were at 8.6 m. In the 0% simulation, only 12.8% of the original ice sheet remained, which could result in a sea level rise of 6.4 m.  The remaining 12.8% of the ice sheet is located at the high altitude regions on the southern tip and on the eastern coastline. In the 100% simulation, there was a sea level increase of 0.1 cm. These results show that as the climate engineering percentage came closer to 100%, the volume of the ice sheet increased.
The Glimmer test also showed that there was no linear relationship, rather a step-like behavior, between an increase in climate engineering and an increase in height and coverage of the ice sheet. The 20% simulation showed an ice sheet that was slightly larger than the 0% simulation, but the remaining ice sheet was more inter-connected. The 30% and 40% simulations show slight increases from the previous simulation, with a partial ice sheet in the north that wasn’t present in the 20% simulation. The ice sheet at the 60% simulation was at full height and coverage, and the pre-industrial ice sheet was maintained.
For all of the simulations that include geoengineering, Greenland experiences a warmer and wetter climate in comparison to the pre-industrial period. On average, the temperature and precipitation rates of Greenland decrease relatively linearly with increases in the level of climate manipulation.

Cloud Seeding: A Promising Strategy for Cooling the Planet and Rebuilding the Polar Ice Caps

Cloud seeding has been seen as a possible method of decreasing the overall surface temperature of the globe. Seeding our planets maritime boundary layer clouds would increase the number of raindrops released from these clouds and reduce the average droplet size, thus increasing their albedo (Rasch et al. 2009). This could result in the cooling of the planet and compensation for some of the negative effects of climate change. The effects of cloud seeding were looked at on a model that represented a globe whose atmospheric CO2 concentrations were twice as high as they are today. Global surface temperature, polar sea ice cover, and the global precipitation rate would experience drastic changes if this cloud seeding strategy were put into action. We would see an overall cooling of the planet, a halt in the rapid shrinking of the polar ice caps, and an overall decrease in the global rate of precipitation.—Ellie Pickrell
 Rasch, Philip J., Latham, John, Chen, Jack, 2009. Geoengineering by Cloud Seeding: Influence on Sea Ice and Climate System. Environmental Research Letters, 4.
     Philip J. Rasch and the Pacific Northwest National Laboratory conducted an experiment where they examined the effects of cloud seeding on an “Earth” with atmospheric CO2 concentrations that were twice as high as present day values. They used a Community Climate System Model and set up four different geoengineering situations, with a control system that consisted of zero climate engineering. The four cases were 20%, 30%, 40%, and 70% cloud seeding of the areal extent of the ocean surface. They then examined the effects that these four situations had on global surface temperature, polar sea ice, and global precipitation.
     The test showing effects of cloud seeding on the Earth’s surface temperature produced promising results. The control group showed an increased surface temperature by 1.8 K compared to the Earth’s current conditions, but the models that included cloud seeding show much more positive results. In the 20% case, the warming is reduced to 0.8 K more than the current day temperatures, which is almost half as much heating if we were to dismiss the idea of cloud seeding. The 70% case actually produced a cooling of 0.4 K less than the current day temperatures, which would actually result in an overcooling of the planet. Based on the results, it is clear that the maximum amount of cloud seeding isn’t necessary, and even the minimum amount of 20% would make a fifty percent difference in the surface temperature.
     Next, they compared the results regarding the polar sea ice covers and their reaction to cloud seeding. In this experiment they looked at the effects that cloud seeding would have on the Northern Hemisphere and the Southern Hemisphere separately, as the clouds in the Southern Hemisphere require less seeding than the clouds in the Northern Hemisphere (they are more susceptible to brightening). The control group shows a 20% decrease in the Northern Hemisphere and a 36% decrease in the Southern Hemisphere from the current sea ice levels. In the 40% case the sea ice is 9% smaller than the control group in the Northern Hemisphere, and 8% smaller in the Southern Hemisphere. To really make a difference in the polar ice caps, the Earth requires a 70% cloud seeding strategy, which is almost impossible as it may overcool the Earth. Regardless, in the 70% case the sea ice is restored to within 2% of the present day level.
     Finally, they looked at the effect that cloud seeding could have on the global precipitation rate. As the percentage of cloud seeding increases, the global precipitation rate decreases. The control group shows an increase by 0.1 mm of rain, compared to the current day precipitation rate, per day. The 20% case shows an increase by 0.01 mm of rain, while the 70% case shows a decrease by 0.08 mm of rain. These reductions in precipitation occur along the equator between the eastern Pacific and the maritime subcontinent, especially across South America. For all the cases there is, however, an increase in precipitation in the South Pacific convergence zone.
     It is important to realize that this study shows how difficult it is to address multiple changes resulting from climate change. If the atmospheric CO2 concentrations were to double, it would be impossible to simultaneously cool the planet, or return sea ice and global precipitation to the present day amounts. —Ellie Pickrell