With each passing year the reality that the Earth’s climate is significantly changing becomes more and more prevalent: plants bloom earlier than in the past, arctic and Antarctic ice sheets continue to lose thickness and/or melt resulting in higher sea levels, animals migrate further north, tropical diseases are starting to take root in more temperate climates, etc. The progression of these changes only increases the odds for permanent climate change due to passage of tipping points. To argue that these changes are principally driven by anything but the continued consumption of fossil fuels and alteration of land for agricultural purposes by humans is foolish.
A significant portion of the Earth’s climate is driven by global temperature, which in its simplest form is the result of a balance between solar energy that strikes the Earth and the heat that is radiated back into space. The reflectivity of the planet is referred to as albedo. Humans have altered this energy balance through two different means. First, they have negatively affected Earth albedo largely through the release of large quantities of black carbon (soot) and various changes to land through cultivation1,2 Second and more importantly they have negatively affected the path of radiation reflection by dramatically increasing the concentration of greenhouse gases in the atmosphere relative to its natural balance; the most notable increase has been carbon dioxide concentrations. Increasing atmospheric concentrations of certain gases increases the probability that radiated heat is returned to Earth instead of released into space, thus increasing surface temperatures.
Knowing the cause of these imbalances the best solution is clear: rapidly reduce carbon emissions. Unfortunately knowing the solution to a problem and executing it are two entirely different things and the rate at which humans are applying the solution versus the scale and speed at which it needs to be applied is lacking. This lack of urgency increases the need to delay the onset of the more permanent climate changes. Delaying the onset of these changes in effort to procure more time to reduce emissions demands that society address the other imbalance of albedo. However, due to the problems with greenhouse gases it is impractical to alter the albedo at surface level because the impact of any change will be negatively affected by the ‘reflective’ action of the gases. Instead the best means to change albedo in order to reduce the effects of climate change appears to be to limit the amount of solar energy that actually reaches Earth.
Techniques that accomplish this reduction in solar energy have largely been referred to as ‘geo-engineering’. Technically geo-engineering encompasses two different types of methodologies: the removal of carbon from the atmosphere (carbon remediation) and the aforementioned reduction of solar energy entry (solar radiation management).3,4 However, most individuals, especially in public discourse, define geo-engineering solely as solar radiation management techniques. For the purpose of clarity and due to the limited controversy surrounding carbon remediation techniques (other than iron fertilization) the rest of this blog post will associate the term ‘geo-engineering’ with solar radiation management techniques.
There are only three solar radiation management techniques that have demonstrated a sufficient level of credibility to be taken seriously theoretically: 1) injection of sulfur or other reflective aerosols into the stratosphere;5,6 2) deployment of a space-based solar mirror (with models usually placing the point of congregation at Lagrange Point 1);7 3) cloud seeding through water mist or other vapor injection to increase the number of marine stratocumulus clouds.8 While these methods are theoretically valid, most individuals do not believe that the deployment of a space-based solar mirror is economically viable or could be carried out quickly enough to avoid permanent detrimental climate changes. Cloud seeding is handicapped by the uncertainty surrounding whether or not clouds formed in this fashion would increase or decrease overall heat retention and potential issues regarding changes in precipitation patterns. Therefore, most of the attention given to solar radiation management techniques revolves around stratospheric sulfur injection.
Numerous model simulations have demonstrated that on theoretical level stratospheric sulfur injection can eliminate any existing warming (based on pre-industrial levels i.e. 1700s) and depending on the amount of sulfur injected can neutralize any warming gains.9-11 However, despite a theoretical success at the principle goal, opponents of geo-engineering have compiled a long list of objections to aerosol injection. Unfortunately most opponents have not taken the time to identify which of these objections are legitimate and which are driven not by logic, but simple bias against geo-engineering.
First, the insistence of geo-engineering opponents to incessantly cite the continuation of increasing ocean acidity even under solar radiation management techniques demonstrates the previously mentioned bias against geo-engineering that some individuals bring to the discussion.12 No rational person would expect ocean acidification to be corrected by solar radiation management techniques because that is not the rationality behind these strategies. Using increasing ocean acidity as a negative point of argument against any geo-engineering technique is akin to complaining that Prozac does nothing to lower cholesterol.
The irrationality of this complaint notwithstanding it may also be inaccurate on a more discrete level. While solar radiation management techniques cannot directly influence ocean acidity either in a positive or negative manner, it is possible that they could decrease ocean acidity in an indirect way. Discussed later will be the issue of plants increasing photosynthetic efficiency when exposed to more diffuse light. Greater photosynthetic efficiency typically results in greater levels of carbon dioxide absorption. Thus solar radiation management techniques that create more diffuse sunlight over direct sunlight could increase carbon dioxide absorption by plants, which would decrease atmospheric concentration of carbon dioxide increasing the probability for oceanic out-gassing of carbon dioxide. This out-gassing would reduce ocean acidity.
However, it must be noted that while current theory suggests ocean acidity would decrease under a number of solar radiation management techniques there is no accurate means to determine the total increase in out-gassing potential provided by these strategies and it would be reasonable to assume only a small amount of out-gassing, probably insignificant overall, as a result of increasing diffuse light. In the end though it is silly to reject solar radiation management techniques on the basis that they will not solve the problem of ocean acidity.
One rather benign, but noteworthy side-effect of aerosol based solar radiation management techniques is that when aerosols are at sizes similar to photons the interaction between photons and these aerosols create a white cloudy appearance to the sky.13 In addition these aerosols can increase the probability of red and yellow skies during sunrises and sunsets.14 Some wonder if these visual changes, which would be ongoing and permanent during the application of these types of solar radiation management techniques, would have a negative psychological impact on the populous.
While it is possible that the loss of the normalcy of a blue sky could create psychological problems in some individuals, it stands to reason that the most severe problem that could be expected is a slight increase in depression. It seems improbable to anticipate any greater problems arising out of a non-blue sky. Also it must be stated that the application of geo-engineering techniques will not be carried out on a whim, but instead will be utilized to salvage a livable climate, thus to allow merely the possible increase of unknown magnitude in depression of certain types of individuals to prevent the application of such techniques seems incredibly foolish.
Another concern opponents have is that the delivery system designed to facilitate the solar radiation management technique will have a negative environmental impact. The extent of this concern should be tied to the type of system. For example the use of jets to release aerosols into the stratosphere would draw more concern than using balloons to release the aerosols. Thus, this concern is really only relevant pertaining to the selection process of the applied system not to whether or not any system at all should be applied. The major concern with delivery methodology is not the methodology itself, but the numerous times that it needs to be utilized over the lifetime of application. For example eating one apple a day is healthy, but eating 10 apples a day is not healthy.
This ‘damage through repetition’ has been discussed in the methodology of releasing aerosols from high flying jets as in one study it was suggested that one million flights per year would be required to release the recommended amount of aerosols under a given proposal.15 Initially it would be difficult to approve of such a delivery system based on the amount of carbon emissions released by those all of those flights because the chief goal of saving the climate requires a reduction in climate emissions, thus such a delivery system would be counter-productive, especially because there are other options. Artillery delivery does not appear to be without negative consequence as well namely the production and recovery of the spent shells as well as the energy required to fire the magnitude necessary. Thus the best delivery system appears to be the most benign in hoisting and tethering a balloon system to release aerosols. In fact it stands to reason that all energy requirements for such a system could be provided by a small module nuclear reactor or geothermal plant.
Some are concerned that if the application of the solar radiation management technique is stopped at sometime during the process due to some unforeseen circumstance (political or global strife, etc.) that global warming will then proceed at an accelerated pace.11,12 The reasoning for this conclusion comes from the understanding that solar radiation management techniques mask surface and ocean temperature increases brought on by global warming, but until the carbon dioxide and other greenhouse gases are reduced in the atmosphere the underlying warming still remains. Opponents believe that masking this increase is detrimental because of the ‘sudden’ increase in temperature and greater difficulty the global environment will have at adjusting to such an increase if stopped.
The problem with this rationality is that it seem counter to the general trend in population-based genetic adaptation. Genetic adaptation, the only genuinely effective response, occurs over hundreds to thousands of years for a species. The current temperature changes have occurred only over the past 150-200 years and very slowly at that, thus plants and animals are not genetically adapting to the changing temperatures at anything near population levels. Current adaptation strategies largely involve animals scurrying north to colder climates.
Also the overall level of change between a scenario utilizing a solar radiation management technique and a scenario that does not is minimized because the failure of a solar radiation management technique does not significantly change the amount of greenhouse gases in the atmosphere. Only the rate of change in temperature will change due to the stoppage of the technique. While one could argue that due to the overall impact of temperature that an increase of 0.1 degrees over 10 years accelerating into an increase of 1 degree over 10 years would provide more environmental detriment than an increase of 1.1 degrees over 20 years, the overall impact is negligible because the overall increase in temperature will exceed the habitat thermal maxima for most creatures anyways. Basically while a sudden stoppage of an applied solar radiation management technique after a significantly long time (probably at least a decade, but overall the minimum time period required to see any real change in temperature pattern is unknown) would result in a more rapid change, the total change will be insignificant because the overall level of change already outpaces genetic adaptation.
Interestingly enough some are concerned not about premature stoppage, but the ability to stop at all. The concern that once a solar radiation management technique begins it cannot or will not be stopped demonstrates a significant misunderstanding of the purpose of geo-engineering. The point of geo-engineering is to provide a sufficient amount of time to execute carbon mitigation and remediation strategies. To this point no solar radiation management technique is permanent. Aerosols injections can be stopped and what has already been injected will be consumed in natural atmospheric chemical reactions, space mirrors can be repositioned to change the amount of incoming sunlight and cloud formation/water vapor techniques can be ceased with eventual cloud dissipation. Both aerosol and water vapor residence times have been estimated at 3-12 months depending on the injection concentrations.10,16 Therefore, there is no rational reason to conclude that once a solar radiation management technique is started that it creates some unstoppable chain reaction forcing humans to adapt permanently.
The criticism of geo-engineering using the idea that it will replace mitigation instead of aid in its application by extending the amount of time society has to evolve its energy infrastructure to one using trace carbon is foolish. As mentioned above only a fool would support a solar radiation management technique without corresponding carbon mitigation strategies. In short this criticism, unfortunately a rather popular one, is entirely driven by bias and those who carry it care nothing about solving the problem of global warming.
Similar to the trepidation surrounding the concern regarding undermining emission mitigation are concerns regarding human error in the application of geo-engineering techniques. Not surprisingly people do not like uncertainty; however, the uncertainty that is associated with geo-engineering cannot be utilized as an excuse to refrain from its application. Using human error as an excuse improperly characterizes the role of geo-engineering techniques in the fight against global warming as a luxury. While the exact consequences resulting from continued climate change brought on by global warming are unclear, enough information can be surmised that they will be severely detrimental and significantly challenging to societal arrangement and long-term survival.
With a general understanding about the eventual global warming derived detriments it is rational to take action against the realization of those outcomes, hence the application of geo-engineering techniques. The application of these techniques are not on a whim, but designed to lessen the known future consequences of global warming. Think of geo-engineering like an experimental cancer treatment. Due to incomplete understanding regarding biochemistry and biology there are certain elements, usually mechanistic, to a given treatment that may not be understood which could result in detrimental side effects. However, experimental cancer treatments are not given to individuals with the flu, they are given to individuals with significant unresponsive cancer. Suppose there is a negative outcome and the cancer treatment does not work and even hastens the individual’s death; in the overall picture very little has changed because the patient was going to die soon anyways, thus the negative effects of the cancer treatment were of little consequence.
Also the probability of permanent detrimental results from the application of geo-engineering techniques is reduced due to the non-permanent nature of their application. The climate has certainly demonstrated a robustness that if a significant detrimental unforeseen outcome emerges from the application of a given geo-engineering technique the application can be ceased with assumed limited probability of long-term damage. However, although society can stop the experiments does not mean that caution should be excluded. For all applications of techniques that can influence the climate careful measurements need to be taken routinely and studied to identify any anomalies and decipher their causes. If necessary the applied techniques can be ceased to deduce their role, if any, in any developed anomaly.
Economics are always an issue with any strategy due to money being a finite resource. The general sparring over the cost of geo-engineering projects, especially solar radiation management techniques, is frequent between proponents and opponents. Proponents claim that most techniques will have limited costs relative to associated mitigation techniques.9,17 This thought process is incorrect because it develops a comparison mindset in that society only needs to execute either the geo-engineering technique or the mitigation technique. If society wishes to properly address global warming carbon mitigation is not negotiable or replaceable, mitigation must occur; thus geo-engineering will be complementary not competitive.
However, opponents to geo-engineering are also in error in the viewpoint that application of geo-engineering techniques is a sunk cost. This mindset appears to stem from overconfidence that society will embark on appropriate mitigation techniques with enough haste to prevent significant detrimental consequences to the environment. Based on past and current evidence of human inaction with regards to global warming and the scale of the problem, it is difficult to have faith in this viewpoint. Thus, functioning under the realistic conclusion that there will be significant detrimental consequences to the environment brought on by global warming before proper mitigation techniques can be completed, the issue of geo-engineering cost is encapsulated in the analogy ‘an ounce of prevention is worth a pound of cure’. Basically the ability of geo-engineering to delay the onset of these detrimental effects more than justifies their costs.
The element of time is generally ignored in the cost estimates of geo-engineering techniques, which is unfortunate because it is so important. The longer society takes to execute mitigation strategies the more cost-effective geo-engineering techniques become. The only justification for arguing cost as a negative factor for the application of geo-engineering techniques is if mitigation techniques can be applied very quickly. As mentioned above based on current patterns of evidence and behavior relative to the scale of the global warming problem it stands to reason that the costs associated with geo-engineering will be beneficial relative to the costs its application abates in any situation.
The final issue of cost relates to division of economic resources. Some argue that if mitigation is required then it would be rational to forgo the monetary and infrastructure development required for any geo-engineering technique and instead invest those funds in trace emission energy, energy efficiency and/or carbon remediation strategies. While on its face such an argument makes sense, there is a significant problem. One must never forget the scope of time that embodies the issue of global warming. Devoting financial resources from geo-engineering to mitigation projects would hasten their development, but based on the total scale of carbon that needs to be mitigated in the associated time period the application of those additional funds lose significance.
For example most estimates of aerosol based solar radiation management techniques have costs at hundreds of millions to tens of billions of dollars, yet even the most conservative estimates regarding carbon mitigation (transformation to a trace emission energy infrastructure) are in the trillions of dollars.18,19 Thus, a couple of hundred million to a couple of billion more is rather irrelevant, especially in the unfocused hodge-podge time-insensitive mitigation methodology currently practiced by the global community.
Theoretically the issue of technological control and global application of geo-engineering techniques should not be an issue. If, although sadly ‘when’ is probably the more appropriate word, geo-engineering techniques need to be applied the United Nations would be the group with sole control over their application. All governments would agree that any private application of these techniques would be immediately stopped with a seizure of assets by the government controlling the region where the unsanctioned technique is being applied. Funding for the U.N. program would come from the largest, by absolute tonnage not per capita because per capita is a bad measurement system due to it being a ratio, carbon emitters at a proportional scale. All governments would also disavow any patent protection to any methodology or chemical that was applied to solar radiation management techniques.
In fact a controversy surrounding a patent is what has temporarily stopped one of the first major controlled test of geo-engineering methodology that would be used in releasing aerosols. A European collaboration called “Implications and Risks of Novel Options to Limit Climate Change” was planning to release water from a tethered balloon over a kilometer in the air after pumping the water up a hose attached to the balloon in order to assess how the water behaved when released and extrapolate that behavior to other substances like aerosols. Unfortunately the experiment never got off the ground because certain private parties within the initiative had filed patents in the initial stages of the proposal and this action was viewed by other parties as controversial enough to delay the experiment.20
Note also that stripping patent protection only applies to solar radiation management techniques not carbon mitigation/remediation techniques because the mitigation/remediation techniques are not altering the climate directly in the short-term, but instead are either removing unnecessary greenhouse gases or are limiting the addition to unnecessary greenhouse gases to the atmosphere. Therefore, due to the fewer negative effects and controversies associated with mitigation techniques over solar management techniques innovation in that particular field should not be restrained through patent blocks.
Excluding patent protection would heavily discourage private application of solar radiation management techniques because it would eliminate all short-term profitability and direct long-term profitability. Also due to the uncertainty of how climate changes in the future would influence various business practices most private companies could only view solar radiation management techniques as a sunk cost, not an investment. With no expectation of profit in the short-term and unclear profit projections in the long-term the probability that any private corporation would apply solar radiation management techniques is extremely low partially because they could not justify the associated short-term present costs to shareholders.
Some are concerned with interference by private companies in that those companies may try to utilize geo-engineering, not to help save the climate but as a form of terraforming, changing the localized temperature or climate of a particular region to improve agricultural production or some other activity/commodity that could generate a profit. Fortunately these fears are unfounded because localization is difficult to isolate in existing solar radiation management techniques. For additional protections all global governments could simply ban import of any product produced by an organization utilizing geo-engineering techniques; it would not be difficult to identify those utilizing them.
Any concerns over the military applications of geo-engineering are overblown largely because of the uncertainty factor. For example the ability to inject large amounts of aerosols into the atmosphere has been technologically available for decades, but no country has ever done it because of uncertainty regarding their overall benefits pertaining to a conflict against a potential enemy. In addition what is known also limits military applicability largely due to the free mixing of the atmosphere basically eliminating the ability to localize the influence of the aerosols over the long-term. The aerosols will simply mix with the atmosphere and spread all over the world instead of remaining above the given localized target area. Finally every country that has the reasonable ability to initiate and maintain such a methodology for military purposes has signed the U.N. Convention on the Prohibition of Military or Any Other Hostile Use of Environmental Modification Techniques (ENMOD), thus violation of this treaty would result in severe global backlash.
Some raise the question of whether or not humans have the moral authority to further the alteration of the environment beyond natural processes through the application of geo-engineering techniques? One of the biggest concerns of climate scientists and some environmentalists is that there are certain environmental ‘tipping points’ that once passed will permanently alter the environment. These tipping points largely relate to surface and ocean temperatures, thus controlling those temperatures will either hasten or slow the passage of the tipping points. Both carbon mitigation and geo-engineering techniques have the ability to influence temperature with geo-engineering having a faster execution rate relative to impact and mitigation having an advantage in long-term stability.
The issue of moral authority for the application of geo-engineering techniques comes from the mindset of restoring natural processes. The application of geo-engineering techniques is in effort to restore the environment to a more natural state. As long as this is the goal of the application, not to change the environment of a given region as a means of punishment, control, economics, etc. then moral authority is not an issue. The non-permanent nature of geo-engineering techniques further the viability of this moral authority to restore nature. Arguing a lack of moral authority in this situation is akin to arguing that one cannot put out a fire burning down a house because it will further change the environment of the house.
The fear of the unknown and the potential detriment to the environment is a common tactic used by individuals who are opposed to a given strategy, in a similar vein to raising concerns about human error, and geo-engineering opponents are no exception. While concerns about the complexity and limited overall knowledge about the climate and how various geo-engineering strategies would operate is understandable the point of argument is mistaken. Once again it must be stated that geo-engineering strategies would not be applied on a whim, but instead are applied out of necessity. The necessary application is due to the fact that the environmental damage developed through a continued lack of scale-appropriate action will be devastating. In the scenario where civilization is seriously handicapped by environmental changes, there is little additional damage that can be catalyzed by the application of geo-engineering techniques. Also it must be noted that geo-engineering opponents never consider that unknowns could be beneficial instead simply assuming that all unknowns that come into play when applying geo-engineering techniques will be detrimental.
The most common and justifiable concern about geo-engineering is how increasing the concentrations of aerosols will impact the South Asian Summer Monsoon (SASM) with a number of individuals believing the impact will be negative. This influence is important because the SASM provides up to 80% of the annual mean precipitation for India.21 Overall the SASM is largely affected by both aerosols and greenhouse gases. Aerosols are thought to apply influence through a change in surface cooling creating a reduction in the meridional thermal contrast between the northern and southern Indian Ocean.22,23 Greenhouse gases are though to apply influence through their ability to increase sea surface temperatures, which weakens tropical circulation. This reduction also occurs because global precipitation levels cannot increase fast enough to compensate for the lower tropospheric water vapor concentration increase due to increased evaporation from higher atmospheric and sea surface temperatures.24,25 However, interestingly enough despite the weakening of the monsoon circulation, models project an increase in monsoon based rainfall if global warming continues as is.26
Unfortunately there appears to be some contradiction between what the models predict for the future and what has been happening empirically. Looking at the last 50 years there has been a significant reduction in precipitation (drying) over central-northern India and other parts of Southeast Asia with a slight increase in precipitation over southern India and northwestern India with Pakistan with an overall precipitation decrease in India of 4-5%.27,28 This decrease in rainfall despite increasing temperatures over the same time frame indicates that either the models are missing a significant component in their monsoon predictions or aerosols have increased faster than temperatures.
Based on empirical changes to society in Asia, an increase in aerosols appears to be a better explanation. This local increase in aerosols most likely stemmed from increased burning of charcoal and carbon black by economically poor Indians driven by an increase in population. Also natural forces have been calculated as too weak to produce the level of drying. Most models actually capture the drying trend adding support to their accuracy.29 Overall greenhouse gases and ozone are thought to cause a slow down in the circulation in the meridional equatorial zone due to an eastward shift in the convergence zone.29
The ability of aerosols to reduce the local land-ocean surface thermal contrast as well as the large-scale meridional atmospheric temperature and sea level pressure gradients results in the slowdown of the tropics-wide meridional overturning circulation which is another element to why the South Asian monsoon has weakened.29 Interestingly changes in the meridional SST gradient over the Indian Ocean could possibly be explained by an uneven distribution of aerosol forcing.23,27 If geo-engineering is executed there should be a more even distribution of aerosol coverage with sufficient consistency after a certain period of application. Could a more even distribution reduce some of the negative influence on the SASM? Regardless of the balance of distribution there is evidence to suggest that the increases in atmospheric aerosol concentration around India and other parts of South Asia have counter-acted the predicted increase in precipitation due to increased rates of evaporation brought on by global warming.
In fact monsoons have decreased in frequency, but increased in force and intensity,30 so much so that areas that typically do not receive precipitation have seen significant increases. Overall if there is counter-play between global warming increasing monsoon activity and aerosols reducing it, the real question is how will this interplay progress after applying geo-engineering?
At the moment temperatures in South Asia are increasing, yet the monsoon, partially due to increases in aerosol concentration, is weakening; more aerosols will need to be injected to control temperatures, which should lead to greater reduction of the monsoon. However, there is the issue of uneven vs. even aerosol distribution and how it may influence the monsoon. Finally it needs to be acknowledged that the monsoon is being influenced away from historical patterns by global warming, even without the influence of aerosols, thus doing nothing pertaining to geo-engineering will not salvage the monsoon if emissions are not rapidly reduced. Overall if geo-engineering is necessary, which it will be in the very near future, then temporarily adding another interference to the monsoon is a justifiable consequence.
Ozone depletion is one of the more interesting potential side effects of geo-engineering because of the positive and negative feedbacks associated with the interaction between the sulfur aerosols and ozone. Ozone depletion largely occurs through two methods one transient and one more permanent.31,32 The transient method occurs when solar energy from sunlight strikes an ozone molecule breaking it down into a free radical of oxygen and an oxygen molecule. However, due to the generally large concentration of other oxygen molecules in the upper atmosphere the free radical of oxygen typically reacts with another oxygen molecule reforming another ozone molecule to replace the photolyzed ozone molecule.
The more permanent method involves ozone breakdown due to interaction with a free radical catalysts like nitric oxide (NO), nitrous oxide (N2O), hydroxyl (OH) chlorine (Cl), chlorine monoxide (ClO) or bromine (Br). In these reactions there is no free radical of oxygen that can later bind to another oxygen molecule to reform the lost ozone molecule. Of the free radicals that drive this process chlorine is viewed as the most potent. In fact large (relatively speaking) concentrations of chlorine still exist in the stratosphere due to excessive chlorofluorocarbon use in the 1950s and 1960s before the passage of the Montreal Protocol and the lack of a natural removal process. Overall reactions that destroy ozone depend on UV flux, temperature or existing surfaces for heterogeneous actions.31,33
While large concentrations of chlorofluorocarbons were released into the atmosphere in the past, chlorofluorocarbons are not reactive to ozone they need to be broken down into more reactive species like free chlorine or chlorine monoxide. Two ways this breakdown occurs is through photolytic dissolution (not that efficient) or the formation of polar stratospheric clouds (PSCs) (very efficient). PSCs form at extremely low temperatures (at least –80 degrees C), which typically only occur in the lower stratosphere in winter around the Antarctic.34 The formation of PSCs hasten ozone destruction largely through increasing the probability of breaking down chlorine containing molecules releasing free chlorine by providing a specific surface to hasten chlorine-ozone reactions as well as reacting with nitric acid (one of the agents responsible for PSC formation) removing it from the stratosphere.32,35 PSCs are a significant reason why the first ozone hole was detected over Antarctica, but because they are seasonal due to the low temperature requirements ozone destruction hastens during the winter and is reduced during the summer hence why the biggest holes are seen in the spring.
PSCs are not thought to be significantly influenced by sulfur aerosols due to the heavy dependence on temperature and water vapor.31 However, similar to PSCs in colder stratospheric regions, sulfur aerosol particles provide the heterogeneous surfaces which aid chemical reactions that prevent nitric acid from reducing the probability that chlorine is liberated.31 Thus the larger the concentration of sulfur aerosols in the stratosphere the higher the probability that chlorine atoms are available to react with ozone. Such an understanding is important because unlike PSCs sulfur aerosols do not require such extreme temperatures to facilitate chlorine liberation, thus theory dictates that higher sulfur aerosols should deplete ozone at higher rates throughout the ozone layer in a more uniform manner. Fortunately because PSCs still exist Antarctica can still act as a ‘canary’ of sorts relative to the rate of how the addition of sulfur aerosols are affecting the overall ozone layer.
Empirical evidence exists for sulfur aerosols hastening ozone breakdown from both the El Chich´on eruption (3-5 Tg Sulfur)36 in 1982 and Mount Pinatubo eruption (10 Tg Sulfur) in 1991.37,38 Local ozone destruction for El Chich´on was approximately 16% at 20 km altitude at mid-latitudes36 and Mount Pinatubo generated a global column loss of 5%-7% for mid-latitudes39,40 and 2% for the tropics.40 Most of the hastened ozone destruction due to sulfur aerosols occurred in the lower latitudes with increasing ozone concentrations with respect to increasing latitudes. In fact at some high latitudes ozone concentrations actually increased.41
There are multiple additional points of note on the relationship between sulfur aerosols and ozone. First, almost no model estimates single release concentrations equal to those released in the El Chich’on and Mount Pinatubo eruptions, so to assume initial ozone destruction similar to those seen from those eruptions would be questionable over a short-time frame. However, the eruptions were single occurrence events and geo-engineering sulfur injection will require multiple injection events, so the rate of accumulation needs to be furthered studied to determine to influence.
Second, recall that sulfur aerosols do not actually destroy ozone molecules. The aforementioned free radicals are the actual reactants that lead to ozone destruction; sulfur aerosols simply increase the probability of reaction by providing a surface for reactions. Therefore, if the free radicals are removed from the stratosphere, ozone should not be destroyed regardless of how much sulfur is injected into the atmosphere. Unfortunately humans released so many CFCs into the stratosphere that it is estimated to take until 2050 until concentrations abate to levels where sulfur injections will do insignificant harm to the ozone layer.42 Some estimate that under certain sulfur injection regiments ozone damage will take an additional 20-30 years to recover versus if sulfur was not injected.42
Third, due to the sulfur solar radiation shielding effect there will be some compensation for the loss of ozone. The total level of compensation is unknown although a previous study attempted to estimate the compensation from the Mount Pinatubo eruption and concluded that there was some compensation, but greater UVB radiation did reach Earth due to ozone destruction.43 While that estimate is a good starting point, it may not be applicable to the compensation seen from geo-engineering based injection due to changing sulfur concentrations. Finally ground level ozone is becoming a greater problem in the environment; one question that has not been addressed in sulfur injection models of whether or not this ground level ozone will be influenced by sulfur injection.
One positive benefit of sulfur injection that most opponents of geo-engineering avoid is the effect of diffuse light on plants. The understanding that plant growth is enhanced during cloudy days over clear cloudless days is widely acknowledged.44-47 The different growth rates stem from the different transfer regimes between plant canopies for more dense regions and the non-linearity of photosynthesis.48 As the level of irradiant light strikes the leaf, electron transport photosynthesis increases in efficiency increasing growth. However, if irradiant light continues to increase the mechanism of photosynthesis eventually shifts from electron transport with RuBP regeneration limitation to Rubisco control,49 which reduces efficiency of growth due to saturation. Efficiency is also affected by the bimodal distribution of light striking a plant shifting between low intensity diffuse light and high intensity direct light. Under direct light saturation is attained quickly and at the highest levels can actually reduce photosynthetic efficiency due to elevated temperature and increased respiration.48
Basically due to the higher probability of wasted efficiency from saturation under direct light, diffuse light results in higher average light use efficiencies, thus greater plant growth and carbon sequestration over a large sample size (a forest or large above ground farm crop). However, the difference in efficiency between direct and diffuse light is also influenced by atmospheric temperature and vapor pressure, so there is no linear or static change in efficiency between light sources.48
The reduction in direct temperature change from contact with direct light in favor of diffuse light may also be important in the context of respiration. Unfortunately the dramatic increase in carbon dioxide in the atmosphere has lead to the trend of less dense (up to 34%) stomata in various plants.50 This reduction in density reduces the amount of water vapor plants can release through transpiration, which affects the ability of the plants to cool themselves and the air around them. Less dense stomata could also reduce the amount of carbon dioxide absorbed by plants. Finally this change in the water cycle of a given plant may also change the rate of photosynthesis with water becoming the limiting factor over carbon dioxide.
Another concern is that the continuous injection of sulfur compounds into the stratosphere will increase acid deposition in some context to the Earth either in liquid form (acid-based precipitation) or gas form and will harm the environment. Such a concern does not appear to be significant because various studies have demonstrated that any potential increase from reasonable sulfur injection methodologies will be negligible relative to the total sulfur cycle in the atmosphere.15,51 The perceived magnitude of this concern largely stems from the understanding that sulfur injection methodologies would result in an increase of 15-30 times the current non-volcanic sulfur concentrations in the stratosphere.15 Such an increase is certainly significant, but most of the sulfur that exists in the atmosphere resides in the troposphere and the increase in the stratosphere is small relative to the tropospheric concentration and its interaction with the lower environment.
However, there is an important issue that should be acknowledge in that while any increases in acid deposition from human-derived sulfur injection will be negligible on an absolute scale, there exists a small probability that small isolated environments will see a significant increase in acid deposition. Due to the overall rate of mixing that is desired in a sulfur injection methodology this probability is small, but early in the application of such a strategy society should be aware of any dramatic proportional increases of acid deposition in isolated regions and respond accordingly.
The biggest mystery element in global warming is the role of clouds. Incorrect interpretation of cloud influence in global warming could lead to bad climate decisions in the future. Uncertainty regarding clouds pertains to their dual nature in influencing temperatures. Clouds are able to trap outgoing long-wave radiation that is reflected from the Earth reradiating it to the surface increasing temperatures and are also able to reflect incoming solar radiation due to their inhabited ice/water particles, similar to aerosols, decreasing temperatures. Clouds are thought to scatter, depending on type, anywhere from 20 to 90 percent of contacted sunlight. The overall influence of clouds on temperatures is thought to be negative (decreasing), but how or even if that current influence will change during global warming is unclear.
One crucial aspect of the relationship with clouds and temperature is their altitude. Higher clouds are contacted by lower amounts of sunlight. Their cold temperatures also reduce their ability to return outgoing radiation as well, but because the net effects of clouds are negative and influence is generally proportionally handicapped higher clouds have a net warming effect. Not surprisingly lower clouds have a net cooling effect due to a higher albedo characterized by greater sunlight contact as well as equalized temperature profiles relative to the surface, thus lower clouds tends to emit similar amounts of outgoing radiation back to space as they return to Earth.
With respect to sulfur aerosol injection there are some who believe that the injected particles that fail to react in the upper atmosphere will fall to Earth and increase the probability for the formation of cirrus type clouds in the troposphere.52 This formation typically involves the formation of haze particles which homogeneously freeze at temperatures lower 235 K aided by a supersaturation condition.53
However, there are some concerns with this theory as some believe that if homogenous freezing is the chief formation method for cirrus clouds the chief influencing factors are local updraft velocity and temperature with little sensitivity to the number of aerosol particles in the local environment.54 The reason for a suspected little sensitivity regarding number of aerosol particles is that soluble particles are not the limiting factor in creating the requisite cirrus ice crystals.55
Heterogeneous freezing is different because of the multiple nuclei involved; an increase in ice nuclei can increase the number of ice crystals formed. In addition some believe that the addition of sulfur aerosols to a cloud where homogeneous freezing is largely dominant reduces the relative humidity relative to the amount of ice reducing ice crystal number density.56-58 Not surprisingly this effect is influenced by updraft velocity, temperature and number and properties of the ice nuclei in the region.56
Based on information comparing pre-industrial atmosphere to current atmosphere alterations it has been theorized that homogeneous freezing influences clouds that form between 100 and 250 hPa and in polar regions and heterogeneous freezing influences clouds that form between 250 and 500 hPa mostly in the northern mid-latitudes.59 So sulfur aerosols decrease ice crystal concentrations in higher cirrus clouds and increase ice crystal concentrations in lower cirrus clouds. Basically it appears that both types of clouds (those with a warming effect and those with a cooling effect) have their influences reduced by sulfur aerosols, the magnitude of that reduction is still unclear, but it is assumed that due to the dependency of long-wave forcings on cirrus type clouds the net overall influence of sulfur aerosol addition will be to cool the surface.59
Finally another viable concern regarding geo-engineering has been created by poor circumstances. The principle advantage of geo-engineering, blocking sunlight to reduce atmospheric and oceanic temperatures, becomes a disadvantage with regards to solar energy. Numerous individuals, despite a lack of future foresight regarding intermittence, storage and construction materials, believe that massive deployment of solar energy is critical to hastening emission reduction in the short-term. Unfortunately this mass deployment will be negatively affected by geo-engineering due to the increase in light diffusivity. This reduction in energy generation is a very serious issue because as previously stated it appears that based on current emission reduction patterns some form of geo-engineering will be required.
Thus, there is conflict for solar supporters who would argue that solar deployment should proceed as fast as possible to replace coal and gas in order to reduce emissions. If geo-engineering techniques are applied then these new solar instillations will more than likely suffer significant inefficiencies due to their inability to harness the energy of the more diffuse light. In this scenario replacing fossil fuels with solar will result in reduced energy generation more than likely leading to massive brown and blackouts or heavy power rationing. With a reduced energy payoff under geo-engineering, heavy deployment of a solar infrastructure may not be a wise strategy due to limited available resources; however, continuing fossil fuel energy generation is also a non-starter because emission reduction is the principle goal.
If geo-engineering is planned to be utilized society must explore compensation options with two immediately coming to mind. First, solar power deployment and resources can be redistributed to other trace emission technologies like nuclear and enhanced geothermal without question. One might suggest redistribution for wind, but with current wind expansion having taken the best producing land wind costs per MW generated will increase reducing their energy economics. Such reasoning is cautionary due to the sheer scale of energy that these alternatives, that would have included solar in most eyes, need to replace to fully displace fossil fuel use.
Second, an important experiment would need to be conducted measuring exactly how diffused light would interact with various solar cell designs. The necessity of this experiment is to quantify what type of an efficiency drop-off will be developed under an aerosol-based geo-engineering strategy. It stands to reason that based on the scale of energy that solar needs to supply, taking from deployment demands created by solar proponents, that any significant drop-off in efficiency under these diffuse light conditions (10% or greater) it would be difficult to consider an unmodified solar infrastructure in the near-future.
Therefore, either solar deployment needs to be delayed in favor of other options as outlined or one needs to develop a removable adaptor unit that can be placed on solar panels, which can enhance efficiency. The ‘adaptor’ needs to be removable because the geo-engineering technique will be temporary, thus the adaptor will be removed after the geo-engineering technique is stopped lest all of the solar materials constructed under the aerosol condition be wasted. This waste is presumed because it stands to reason that any ‘adaptor’ will not function as efficiency under more direct non-diffuse sunlight after aerosols dissipate. At least one company, Green Sun, is attempting to produce diffuse solar panels. Unfortunately as mentioned the direct manufacture of diffuse solar panels may be a mistake, but depending on the success level of Green Sun what is learned from their research can be taken to develop the attachment module.
Most of the concerns that are raised against geo-engineering are driven more by uncertainty and fear than actual potential for detrimental consequences. The three major potential negative issues surrounding the application of geo-engineering techniques are negative changes in the SASM, hastened destruction of the ozone layer and reduction in solar energy output. However, even among these three legitimate concerns the monsoon reduction is complicated by the influence of global warming and how it impacts the monsoon in a non-effective manner. The destruction of the ozone layer is the most difficult to reconcile even with some of the uncertainty. Society will simply have to accept a greater rate of destruction of the ozone layer as a side effect of geo-engineering with the understanding of a greater benefit coming from geo-engineering versus this particular detriment. The issue with solar energy output is more easily managed by foregoing the deployment of solar power in favor of other trace emission energy sources or, if possible, developing a means to temporarily harness the diffuse light. Overall the simple reality at this moment is that geo-engineering will be required to maintain a preferential climate for humans on Earth so people better start getting used to the idea and developing the appropriate methodologies for their application.
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