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
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.
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.
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.
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.
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.
Two important inferences were drawn from these results. The first was that introducing iron to iron-deficient regions in the Southern ocean does not increase the size of the phytoplankton bloom; grazers controlled the size. The second implication was that OIF’s ability to remove anthropogenic CO2 was lower than expected. The authors stated that more issues with OIF methods must be addressed. These issues encompassed the role of trace elements, the relationship between phytoplankton and zooplankton during time periods when zooplankton supply is lower, and the effect on the food chain with long-term OIF methods. The authors recognized the importance of OIF research for the discovery of important benefits. They believed commercialization and misplaced concerns would limit scientific research. The authors opposed OIF commercialization and stated that future research should be funded by carbon taxes instead of a carbon credit market. They concluded that scientific research should not be highly regulated.
Due to results of scientific studies that have indicated drastic climate changes, a scientific consensus has transpired that acknowledges the adverse consequences of global warming. As a result, methods to mitigate global warming have become widely deliberated. Resnik and Vallero (2011) addressed two basic policy options for responding to global warming and discussed the overall difficulties with climate change mitigation. The authors posed geoengineering as a useful method to combat global warming through carbon dioxide removal and solar radiation management. Two proposals for solar radiation management are assessed, and the advantages and disadvantages of each compared. Objections to geoengineering are then recognized through discussion of pragmatic and ideological critiques. Because other methods to combat global warming have been ineffective thus far, Resnik and Vallero conclude that geoenginnering is a viable proposal. Further research and discussion are needed before geoengineering tactics are implemented, but other methods of mitigation should be undertaken in the meantime. The authors conclude that mitigate both the symptoms and causes of global warming should ultimately be pursued simultaneously. —Michela Isono
The relationship between climate changes and global warming has been the subject of numerous studies that indicated record high surface temperatures and significant increases in sea levels. The emission of greenhouse gases, particularly carbon dioxide (CO2) and methane (CH4), into the atmosphere is correlated with global warming. Greenhouse gases trap heat in the earth’s atmosphere and prevent the earth from cooling. As a result, the earth’s surface temperature increases and polar ice sheets and glaciers melt causing sea levels to increase. Not only will the temperature and sea levels continue to rise, but numerous adverse consequences for human health and the environment will result if methods to lessen the production of human-produced (anthropogenic) greenhouse gases are not implemented.
Resnik and Vallero addressed two current policy options to counter global warming: mitigation and adaptation. The authors defined mitigation as a method to inhibit and repress climate change, and adaptation as a method to prepare for the effects of global warming. They cited efforts such as regulations to control emissions, carbon taxes, market-based incentives, changes in behavior, and control of population growth and development as mitigation techniques, and efforts such as building infrastructure to prepare for natural disasters, developing crops that can grow in a drought, increasing water supplies and improving responses to infectious diseases as adaptation techniques. These efforts proved difficult due to the demands and risks which included: major changes in human behavior and habits, potential adverse economic impacts, strict international cooperation, the responsibility of industrialized nations compared to developing nations, and the political debate surrounding the authenticity of climate change and global warming. Resnik and Vallero considered both options’ efforts ineffective as a result.
The authors then discussed the topic of geoengineering as an alternative method to combat global warming. They defined geoengineering as a method that would utilize technology to purposefully manipulate the planetary environment to mitigate the anthropogenic changes on a global scale. Two methods of geoengineering were cited: CO2 removal and solar radiation management. Carbon dioxide removal removes and stores excess CO2 from the atmosphere. Solar radiation management reflects sunlight by increasing the reflectivity of clouds. The main difference between the two types of geoengineering is that CO2 removal deals with a producer of climate change (the surplus of greenhouse gases in the atmosphere), whereas solar radiation management deals with the symptoms of climate changes that have already occurred.
Resnik and Vallero assessed and compared two proposals for solar radiation management. The first method was introduced by geochemist Paul Crutzen in 2006 and the second method was developed by David W. Keith in 2010. Crutzen’s proposal was developed based on findings from the eruption of Mount Pinatubo volcano in 1991. The eruption caused 10 million metric tons of sulfur dioxide (SO2) to be emitted into the stratosphere. The SO2 then transformed into sulfate particles, which increased cloud-reflecting power (cloud albedo) by reflecting more sunlight into space, and ultimately cooled the planet by 0.5˚C. Crutzen thus proposed that airplanes should spray sulfur dioxide into the stratosphere to increase cloud albedo to cool the planet. Crutzen advocated that SO2 should be sprayed into the stratosphere rather than the troposphere because the particles would have a greater impact on cloud albedo and would last for 1–2 years. He estimated the cost would be $25–$50 billion per year in order to combat climate change; however the cost would be dependent on the amount of emitted greenhouse gases.
Crutzen also acknowledged the potential risks of his proposal. First, SO2 is considered an air pollutant and can lead to respiratory problems that increase the number of emergency department visits and hospitalizations. The Environmental Protection Agency also regulates SO2 in the United States. Second, spraying SO2 could disrupt the ozone chemistry and cause the protective ozone layer to thin. The ozone layer is important because it acts as a protector from harmful ultraviolet light radiation. Third, SO2 transforms into sulfuric acid and becomes acid rain. Acid rain decreases the pH of soil, which is detrimental to ecosystems and plant species. Fourth, spraying sulfur dioxide would cause a higher level of CO2 to remain in the atmosphere. When CO2 dissolves in the ocean, it increases the acidity of the water, which endangers plankton and species with shells, which would impact other marine species and ecosystems. Fifth, increasing cloud albedo could alter rain patterns, wind, storms, and temperature distribution in addition to increasing the amount of diffuse light that hits the earth. This would case the sky to appear whiter and could affect plant photosynthesis and solar power. Lastly, if too much SO2 is sprayed, too much cooling could occur which could initiate an ice age.
The second proposal was developed by Keith as an alternative to Crutzen’s proposal. Keith recommended that circular nanoparticles made up of layers of aluminum oxide, metallic aluminum, and barium titanate, should be dispensed into the stratosphere. The advantage to the nanoparticles was that they would rise above the stratosphere, which would reduce their ability to interfere with the ozone chemistry. This would enable the particles to stay in the atmosphere longer than sulfur dioxide and decrease the need for continual replenishment of the SO2. Additionally, the nanoparticles could be specifically engineered to release the right amount of diffuse light and not produce acid rain.
Keith’s proposal also had risks. First, the proposal had not been tested previously and no natural occurrence like the volcano provided supporting evidence of its potential effects. Second, it could also affect ocean acidification, rain patterns, wind, storms, and temperature distribution. Third, the health and environmental risks of the nanoparticles are not well known. Lastly, the proposal could be extremely expensive, as the exact amount of materials needed to create and perform the task is not entirely known.
Resnik and Vallero next recognized the objections to geoengineering through the discussion of pragmatic and ideological critiques. The authors defined pragmatic critiques to address the practical issues to employing geoengineering solutions, and defined ideological critiques to represent the opinion that geoenginnerring proposals should not be utilized even if practical difficulties were irrelevant. The pragmatic critiques encompassed problems that pertained to environmental and public health issues, monetary costs, and effectiveness. The authors also recognized the difficultly in acquiring and maintaining international cooperation as well as the low level of scientific understanding regarding the impact of many gases. Resnik and Vallero concluded that geoengineering methods should not be implemented unless there is sufficient evidence that assures safety and effectiveness. The authors stated that smaller-scale proposals with lower risks should be tried before larger-scale proposals with greater risks are used.
The addressed ideological critiques stated that geoengineering proposals would take away monetary and intellectual resources from other non-geoengineering methods to mitigate global warming. The authors disagreed with this critique and stated that people in favor of stopping global warming should agree to any technique that is safe, feasible and effective. Another critique stated that geoenginnering methods should not be utilized at all and that environmental policies should revert nature to its natural state. Other opponents argued that geoengineering would favor the industry producers of greenhouse gases and would detract from efforts to reduce greenhouse gases. Lastly, critics opposed the use of technology as a solution to fix climate change. The authors did not agree with the ideological critiques due to the controversial suppositions. They addressed how major benefits could be overlooked if the consequences of not utilizing geoengineering were ignored.
Because other methods to combat global warming have been ineffective thus far, Resnik and Vallero conclude that geoenginnering is a viable proposal. Further research and discussion is needed before geoengineering tactics are implemented, but they believe that other methods of mitigation should be undertaken ardently; both the symptoms and causes of global warming should be addressed and methods to mitigate both the symptoms and causes can and should be pursued simultaneously.