The Reduction in Precipitation due to the Effects of Volcanic Aerosols

Major volcanic eruptions inject stratospheric aerosols into the atmosphere, which causes a cooling effect. This phenomenon has been greatly studied, however its impact on precipitation lacks substantial analysis. The authors employ a coupled atmosphere-ocean-land-vegetation model paired with observations to study the effects of volcanic aerosol on precipitation in tropical and subtropical regions. They focused particularly on three large volcanic eruptions that occurred in the late twentieth century: Pinatubo, El Chichon, and Agung. The authors conclude that average precipitation over land decreases faster than the average precipitation over ocean. They state that volcano-induced droughts could greatly impact the ecosystem, agriculture, and the carbon cycle, especially in the monsoon regions. –Michela Isono

Joseph, R., Zeng, N., 2011. Seasonally Modulated Tropical Drought Induced by Volcanic Aerosol. American Meteorological Society, 2045–2060.

            Volcanic eruptions inject sulfer dioxie gas into the stratosphere, which then combines with water and oxygen to form aerosol particles that block incoming solar radiation to the earth. This process has a cooling effect. For this reason, aerosols have been proposed as a geoengineering technique to mitigate global warming. However, aerosols can also affect the hydrologic cycle, particularly precipitation in the tropics. Precipitation in the tropics has strong seasonal movement, where the response to volcanic forcing should differ between land and ocean.
The paper discusses the effect of volcanic aerosols on hydrologic processes. The tropical and subtropical land regions are the locations of focus because agriculture depends on the seasonal migration of monsoon rainfall.
            Methods:A coupled atmosphere-ocean-land-vegetation model is used to simulate a realistic seasonal climate compared to observations in the tropics and midlatitudes. The data used as reference for sea surface temperature (SST) is the Hadley Center’s sea ice and SST analysis. The analyses spans years 1870–2005, but the study focuses on years 1960–2000. The volcanic aerosols are injected uniformly across all latitudes. The three volcanic events used all occurred during El Niños. All model results are averages.
            Results: The time series of precipitation and temperature in the model and observations in relation to the volcanic events averaged between 40 degrees N and 40 degrees S, showed that the Pinatubo event was the largest and the model responded with a greater reduction in SAT and precipitation. The magnitude of reduction was approximately 0.15 mm day–1 in the model is similar to the GPCP observational data. The SAT and SST observational data showed a significant trend of 0.5 K per year, but a decrease in temperature of 0.35 K was seen for all three volcanic events in the SAT for both model and observations.
            The composites of precipitation anomalies of the three volcanic events for the model and CRU observations for one year during the peak response period to volcanic aerosol forcing showed a decrease in precipitation over equatorial Africa, the northern part of South America, northern Australia, and the Indian subcontinent. Differences between observations and model in the tropics in the African and the South American continent were also shown. Overall, the precipitation in the observations is less standardized compared to the model; however, when the average of the entire region was calculated they had a comparable decrease in precipitation of about 0.15 mm day–1.
            A seasonal shift in precipitation was observed when the spatial plots of the austral and boreal summer plots were analyzed. In the austral summer (January-March), there was a decrease in precipitation over South Africa, northern Australia, and South America in both observations and model. In the Africa model, the decrease in precipitation spread further south. In the boreal summer (July-September), there was a decrease in precipitation over the Indian and Asian subcontinent in both observations and model.
            The 1-year composite of SAT over land and SST for model and observations with CRU SAT over land and HADISSTs over the ocean was examined. There was a decrease in temperature over land and ocean in the tropics in both observations and model. Significant cooling was shown in the subtropics over the dry arid desert over land in both observation and model. A distinct land-sea thermal contrast was observed in both observations and model outside of regions with decreased precipitation: North Africa-Mediterranean and East Asia.
            The Hovmoller plots of optical depth and the model precipitation and temperature anomalies over land and ocean for the Pinatubo event showed more cooling over land than in the nearby oceans in the summer hemisphere. Once the zonal mean optical depth reached 0.02 in the tropics, the seasonal migration of the precipitation anomalies was observed. There was about a 10% decrease in precipitation over land that lasted for approximately three years. Slightly larger cooling occurred over land than oceans by about 0.1 K. Cooling for SST took longer, lasted longer, and was weaker than the land SAT at the same latitude. After initial inconsistent responses, the precipitation anomalies followed the same seasonal cycle of climatological precipitation. The optical depth of the EL Chichon and Agung volcanoes peaked in July but the Pinatubo volcano peaked in November. The most significant decrease in precipitation was in the Southern Hemisphere in the austral summer.
            The total moisture convergence overlaid with vertically integrated moisture transport for the austral and boreal summer time periods showed a distinct reversal in the expected wind directions during monsoon seasons. For the Australian and the South African monsoon, the winds were reversed in the austral summer. For the East Asian, North American, and East African monsoon, the winds were reversed in the boreal summer. In both observations and model, there was a seasonal decrease in anomalous precipitation over land in regions where the total precipitation was at maximum.
            The time evolution of the volcanic events was also compared. The oceanic SST response to Pinatubo was slow and reached a peak value at about –0.3 K after one year of the time marker (October 1991). Evaporation decreased peaks at one month after SST and precipitation followed after another month. In contrast, land had a different result in response to Pinatubo: the land precipitation response was fast and reached a peak value of –0.15 mm day–1 after about 5 months of the time marker, whereas evaporation lagged and surface temperature followed precipitation by four months. Overall, over the ocean SST response to volcanic which was followed by evaporation and then precipitation, whereas land precipitation responded fast to the forcing which was followed by evaporation, as surface temperature responded the slowest.
            The response of the land-atmosphere carbon flux, net primary production, heterotrophic respiration for the one-year period for the three volcanic effects was a reduction in the net flux, indicating that the cooling effect on respiration was stronger than the precipitation effect on productivity. However, in northern Australia and western South America, the net primary production response was different from most of the other regions. This showed that the competing effects of cooling and drying on the net primary production and heterotrophic respiration (the release of CO2 during the decomposition process of organic matter in soil) are not always dictated by heterotrophic respiration.  
Conclusion: The model used alongside observational data was employed to better understand the mechanisms that dictate precipitation decrease in response to volcanic aerosols. The study showed that precipitation responses over land to volcanic aerosol followed seasonal patterns of monsoons. The reduction in precipitation and cooling of the planet has also been shown to have significant implications for the ecosystem and carbon cycle over the tropics. In conclusion, the results demonstrates the affects that stratospheric aerosol geoengineering could have on the hydrological cycle. 

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. 

The Effects of Solar Radiation Management Geoengineering on Global Crop Yields

Climate change studies have demonstrated an increase in global mean temperature. This increase in temperature has detrimental affects for both the environment and human health. It has also been predicted that climate change may have adverse effects on crop yields. For this reason, solar radiation management (SRM) has been discussed as a method to mitigate climate change and global warming. While the effects of SRM could combat climate change, it could also threaten food and water supply for billions of people. Pongratz et al. (2012) use climate models to simulate the effects of a geoengineered climate on global crop yields. The authors found that crop yields increase in the model with SRM in a high-CO2 climate. They reason that the reduction of temperature stresses while maintaining the benefits of CO2 fertilization account for this result. However, they conclude that even so, with the many potential adverse consequences of geoengineering, the best way to protect crop yields is to reduce greenhouse gas emission. —Michela Isono
Pongratz, J., Lobell, D., Cao, L., Caldeira, K., 2012. Crop Yields in a Geoengineered Climate. Natural Climate Change, 10.1038.

            Global warming has become a main topic of focus within the scientific community. To counteract this effect, geoengineering techniques have been discussed as a mitigation method. Specifically, SRM has been proposed because of its ability to deflect sunlight away from the planet and reduce the amount of solar insolation absorbed. However, SRM also affects precipitation rates. For this reason, crop yields could be adversely affected. Pongratz et al. studied the impacts of changes in temperature, precipitation and the atmospheric CO2 concentration on crop yields.
            Methods: Pongratz et al. combined climate-model simulations with models of crop-yield responses to climate changes. Three global climate simulations were used: a climate to represent today’s environment with an atmospheric concentration of CO2 of ~400ppm (control); a climate with twice the amount of CO2 (2 x CO2); and a climate with twice the amount of CO2 and sulphate aerosols to stabilize global mean temperatures at control levels (SRM). The models are used to isolate the effects of a high-CO2 environment with and without SRM compared with the present-day climate. The crops used are: wheat, maize, and rice. These crops were chosen because they provide about half of the calories consumed by humans and a large fraction of calories consumed by livestock.
            Results and Discussion: When comparing results from 2 x CO2 and SRM with control for all three crops, 2 x CO2 showed much lower percent yield and production than SRM, where 2 x CO2 also showed a negative percentage yield and production for maize when compared to control (meaning the control model produced more yield and had a greater production). This result was due to the detrimental influences of climate change and the beneficial influences of CO2 fertilization. Warming climate changes at most latitudes have negatively effect maize and wheat yields. In contrast, the increased temperatures may benefit rice at high latitudes by enabling a longer growing season. The yield decreases at low latitudes are a result of heat and drought. CO2 fertilization also greatly compensated for yield losses of maize, and yield increases of wheat and rice.
            In the SRM compared to the control, the yields and production increased for all three crops across all latitudes. This is due to the effects of CO2 fertilization. However, there were few small negative impacts on yields across some latitudes in the SRM, but these impacts were smaller than in the 2 x CO2 case.
In the SRM compared to 2 x CO2 case, a significant loss in yield was only shown for rice growing at high altitudes. The production of maize, wheat and rice was higher under SRM than 2 x CO2overall. Therefore, the simulations demonstrated that SRM would create an increase in global yields compared to the global yields in a 2 x CO2climate.
Even with a stabilized temperature, specific regions may experience different changes in their yield and crop productivity due to climate changes. The authors concluded that their simulations counter concerns about SRM threatening food security in large regions, but small regions may also experience greater changes in yields. This may endanger local food security and shift market shares and producer ranking.
In conclusion, the authors did not final substantial reductions in yields by SRM compared to the control. Warming, rather than precipitation change, caused most of the climate-induced yield reductions. When SRM was applied in the high-CO2 climate, the yields and production of maize, wheat and rice increased at the global mean temperature and across most latitudes. This phenomenon represents stabilization in temperature, reduced CO2 levels, and beneficial effects of CO2 fertilization on plant productivity.
Recommendations: Because the effects of climate change on a global scale is better understood than on a regional scale, the authors stated that more research is needed to better understand the effect of SRM on a smaller geographical scale. The authors do not believe that SRM can maintain the economic status quo because market shares of agricultural output will change with climate changes. Therefore,an analysis of the environmental and socioeconomic consequences of SRM is needed as well. Lastly, the authors stated that further research is needed to understand the anticipated and unanticipated effects of SRM. 

The Effects of Solar Radiation Management Geoengineering on Global Crop Yields

Climate change studies have demonstrated an increase in global mean temperature. This increase in temperature has detrimental affects for both the environment and human health. It has also been predicted that climate change may have adverse effects on crop yields. For this reason, solar radiation management (SRM) has been discussed as a method to mitigate climate change and global warming. While the effects of SRM could combat climate change, it could also threaten food and water supply for billions of people. Pongratz et al. (2012) use climate models to simulate the effects of a geoengineered climate on global crop yields. The authors found that crop yields increase in the model with SRM in a high-CO2 climate. They reason that the reduction of temperature stresses while maintaining the benefits of CO2 fertilization account for this result. However, they conclude that even so, with the many potential adverse consequences of geoengineering, the best way to protect crop yields is to reduce greenhouse gas emission. —Michela Isono
Pongratz, J., Lobell, D., Cao, L., Caldeira, K., 2012. Crop Yields in a Geoengineered Climate. Natural Climate Change, 10.1038.

            Global warming has become a main topic of focus within the scientific community. To counteract this effect, geoengineering techniques have been discussed as a mitigation method. Specifically, SRM has been proposed because of its ability to deflect sunlight away from the planet and reduce the amount of solar insolation absorbed. However, SRM also affects precipitation rates. For this reason, crop yields could be adversely affected. Pongratz et al. studied the impacts of changes in temperature, precipitation and the atmospheric CO2 concentration on crop yields.
            Methods: Pongratz et al. combined climate-model simulations with models of crop-yield responses to climate changes. Three global climate simulations were used: a climate to represent today’s environment with an atmospheric concentration of CO2 of ~400ppm (control); a climate with twice the amount of CO2 (2 x CO2); and a climate with twice the amount of CO2 and sulphate aerosols to stabilize global mean temperatures at control levels (SRM). The models are used to isolate the effects of a high-CO2 environment with and without SRM compared with the present-day climate. The crops used are: wheat, maize, and rice. These crops were chosen because they provide about half of the calories consumed by humans and a large fraction of calories consumed by livestock.
            Results and Discussion: When comparing results from 2 x CO2 and SRM with control for all three crops, 2 x CO2 showed much lower percent yield and production than SRM, where 2 x CO2 also showed a negative percentage yield and production for maize when compared to control (meaning the control model produced more yield and had a greater production). This result was due to the detrimental influences of climate change and the beneficial influences of CO2 fertilization. Warming climate changes at most latitudes have negatively effect maize and wheat yields. In contrast, the increased temperatures may benefit rice at high latitudes by enabling a longer growing season. The yield decreases at low latitudes are a result of heat and drought. CO2 fertilization also greatly compensated for yield losses of maize, and yield increases of wheat and rice.
            In the SRM compared to the control, the yields and production increased for all three crops across all latitudes. This is due to the effects of CO2 fertilization. However, there were few small negative impacts on yields across some latitudes in the SRM, but these impacts were smaller than in the 2 x CO2 case.
In the SRM compared to 2 x CO2 case, a significant loss in yield was only shown for rice growing at high altitudes. The production of maize, wheat and rice was higher under SRM than 2 x CO2overall. Therefore, the simulations demonstrated that SRM would create an increase in global yields compared to the global yields in a 2 x CO2climate.
Even with a stabilized temperature, specific regions may experience different changes in their yield and crop productivity due to climate changes. The authors concluded that their simulations counter concerns about SRM threatening food security in large regions, but small regions may also experience greater changes in yields. This may endanger local food security and shift market shares and producer ranking.
In conclusion, the authors did not final substantial reductions in yields by SRM compared to the control. Warming, rather than precipitation change, caused most of the climate-induced yield reductions. When SRM was applied in the high-CO2 climate, the yields and production of maize, wheat and rice increased at the global mean temperature and across most latitudes. This phenomenon represents stabilization in temperature, reduced CO2 levels, and beneficial effects of CO2 fertilization on plant productivity.
Recommendations: Because the effects of climate change on a global scale is better understood than on a regional scale, the authors stated that more research is needed to better understand the effect of SRM on a smaller geographical scale. The authors do not believe that SRM can maintain the economic status quo because market shares of agricultural output will change with climate changes. Therefore,an analysis of the environmental and socioeconomic consequences of SRM is needed as well. Lastly, the authors stated that further research is needed to understand the anticipated and unanticipated effects of SRM. 

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.