A note on stratospheric aerosols, the cryosphere and future climate change....

Stratospheric sulfate aerosols, the cryosphere and climate change …

Recently in my search for literature on stratospheric sulfate aerosols, I came across a few pieces on stratospheric aerosols and the cryosphere and as a recent lecture highlighted the importance of the cryosphere and the seriousness of the changes that are taking place within it I thought I would include a post about the viability of stratospheric aerosol injections in alleviating the loss of ice and sea level rise.

Changes in the crysophere such as the loss of Arctic sea ice and the Greenland and Antarctic ice sheets in recent years have been more rapid than expected. Global warming effects are projected to be the largest in the polar regions, however, it is these regions where climatic responses to warming are most uncertain. The potential impacts of further changes have consequently raised the question of whether geo-engineering, or more specifically, stratospheric sulfate aerosols, can be used to alleviate the loss of ice and sea level rise.

Using the Community Climate System Model version 3, Bitz et al., (2010) imposed stratospheric sulfate aerosol loadings in a series of integrations with the model and found that aerosols can counter ‘many effects of greenhouse warming’ however perfect cancellation is not possible. Rather, when aerosols roughly cancel global average warming, temperature changes in the polar regions are still 20-50% of the changes in the warmed world (McCusker et al., 2011). The aerosols cause a southerly migration of the southern hemisphere jet (McCusker et al., 2011) and these atmospheric circulation changes cause the over-cooling in Arctic summers and residual warming during Arctic winters (McCusker et al., 2010).

The conclusion that arises is that aerosol geo-engineering can serve to counter some of the climate impacts of stratospheric ozone recovery and rising CO2 but not all. Having said that, projected responses to geo-engineering in polar regions have high levels of uncertainty due to the high levels of uncertainty that are present in projections for the polar regions in general. Consequently, the question of whether we can avoid polar climate emergencies is not certain. More robust depictions of a climate that has been geo-engineered are thus needed – a conclusion of more research being needed is yet again the case.

A Closer Look at the Environmental Impacts of Stratospheric Aerosol Injections

Stratospheric Geoengineering with Sulfate Aerosols

One of the possible impacts of stratospheric aerosol injections is sulfuric acid deposition. The acidic deposition would occur as injections of sulfuric acid would combine with water in the stratosphere forming sulfuric acid which would in turn fall to earth. The negative effects of sulfur deposition include damage to freshwater fish populations, reductions in lake bacteria, changes in plant parasite interactions and changes to forest bird populations.

In 2009, Kravitz et al. investigated the impacts of sulfuric acid deposition in association with the effects it could produce on the earth’s surface. The authors assumed all sulfur deposition to be wet in order to examine the worst possible scenario. The extent to which sulfate deposition is harmful depends on three things; the amount of sulfur introduced, the amount of hydrated sulfate and ecosystem sensitivity.

To examine impacts they used a general circulation model to look at the impacts in different regions and a chemical model which calculates the sulfur cycle in the stratosphere where the conversion of SO2 is based upon respective concentrations of SO2 and humidity values. Over the same time period of 20years, two scenarios were modelled:

•  Daily injections of SO2 into the lower stratosphere in the tropics - 5Tg per/year in total
• Daily injections of SO2 into the lower Arctic stratosphere – 3Tg per/year in total

Their results (see figure 1 below) showed that under the 5Tg scenario, global temperatures could be reduced to levels similar to 1980 and 0.3 degrees of cooling would occur by 2026. Not nearly as impressive, but still an marked result, changes under the 3Tg scenario would see global average temperatures immediately reduced to 2000 levels and only 0.3 degrees of warming would occur by 2026. Sulfur injected over the tropics increased sulfate deposition across the all of the globe except the tropic due to poleward transportation. Deposition in the polar regions would also be particularly apparent. Sulfur injected in the Arctic would lead to deposition in the Northern Hemisphere.

Figure 1: Annually averaged total sulphate deposition

The study concluded that sulfur injections of both 3Tg and 5Tg magnitudes were not significant enough to cause any damage to the both the regions of the sea and land. This initial study was however flawed in two ways and consequently in 2010, Kravitz et al. published a follow uppaper to their study correcting these errors.

The first error of the paper was the calculation for average global surface sulfur emission. When corrected, the calculation strengthens the conclusion that the additional amount of sulfur from geo-engineering would be much smaller that current sources. The second error was their incorrect application of a formula that converts the model output of sulfate deposition to a form used in critical loading studies.

The incorrect application of the conversion formula means that figure 2 (see below) their original study needs to be replaced with the figure below. As you can see the magnitude of the values change significantly. Despite the changes, the authors still conclude that acid deposition from geo-engineering would be smaller than the amount already being experienced in industrialised areas. Furthermore, they hold their conclusion that everywhere in the world has a significant buffering capacity to the additional sulfuric acid that would result from geo-engineering except the most sensitive and pristine areas of the world.

Figure 2: Original model for Tropical SO2 Injections 5 Tg

Figure 3 - Corrected model for Tropical SO2 Injection 5 Tg 

Even after correcting their errors, the conclusions of Kravitz et al., (2009; 2010) are still the same. Sulfur injections from geo-engineering are not enough to cause any damage except in sensitive, poorly buffered areas; they will not have negative ecosystem impacts. The studies model are accurate however local differences may occur and there may be a need for more significant sulfur injections in the future if GHGs alter atmospheric circulations as aerosol lifetime would be shorter – in which case the model would not serve as accurately. Despite these possibilities, stratospheric aerosol injections are nonetheless a promising geo-engineering technique. The climate cools following sulfur injections and ecosystems are not impacted negatively. As is the case with most geo-engineering techniques their reputation as a taboo topic means more work is often needed to further examine and develop the robustness of techniques. More work on stratospheric aerosols would in my opinion be a good move and a step in the right direction towards fixing climate change!

Thanks for reading! 

Kravitz, B., A. Robock, L. Oman, G. Stenchikov, and A. B. Marquardt (2009), 'Sulfuric acid deposition from stratospheric geoengineering with sulfate aerosols', Journal of Geophysical Research, 114, D14109.

Kravitz., BA. Robock, L. Oman, G. Stenchikov, and A. B. Marquardt (2010), Correction to “Sulfuric acid deposition from stratospheric geo-engineering with sulfate aerosols,” J. Geophys. Res., 115, D16119

A Human Volcano

A second Solar Radiation Management (SRM) option is to increase albedo through anthropogenically enhancing sulfate particle concentrations in the Earth’s stratosphere and troposphere.

Fossil fuel burning releases about 25PG of CO2 per year into the atmosphere leaving us with the global warming we are faced with today (Prentice et al., 2001).  Fossil fuel burning also emits 55Tg S as SO2 per year and research has shown that warming is counteracted by sulfate particles that scatter solar radiation back to space through increasing cloud albedo (Ramanathan et al., 2001).

We are faced with two main climate problems today – global warming and rising CO2 emissions. A stabilization of CO2 emissions would require an emissions reduction of around 60-80% which seems unlikely when current reductions are considered. This leaves us therefore with the option of anthropogenically enhancing the earth’s albedo through adding aerosols to cool the climate.

The conversion of SO2 into sub-micrometer sulfate particles by chemical and micro-physical processes has been observed in volcanic eruptions (Crutzen, 2006), however, it was Paul J. Crutzen who recognised the potential of these observations to alleviate global warming.

Whether it was due to him winning a nobelprize or the robustness of his work Paul Crutzen changed opinions towards SRM and geo-engineering in 2006 when he asserted that further research on ‘the feasibility and environmental consequences of climate engineering …which might need to be deployed in future, should not be tabooed’ (Crutzen, 2006) as the world may be coming closer to being characterised by conditions that would have ‘catastrophic implications for ecosystems’ (Schneider, 1996).

A loading of 1Tg S in the stratosphere yields a global average vertical optical depth of about 0.007 in the visible and corresponds to a global average sulphur mixing ratio of 1nmol/mole – 6 times more than the natural background (Albritton et al., 2001). Through looking at previous volcanic eruptions the radiative forcing caused by 1Tg S is estimated to have a cooling efficiency of 0.75 W/m² (Crutzen, 2006). The estimated cost to put 1Tg S into the stratosphere is around US $25Billion (NAS, 1992). To address climate warming therefore 1.9Tg S would be required producing an optical depth of 1.3% at a cost of US $25 -50 billion per year for residence times of 1-2 years.

At a first glance this figure may seem large, but when the benefits it will bring about are considered and expenditures such as the US $1000 billion or so that has been spent on the military in the U.S.A. is used as a comparable figure then this cost to fix climate change does not seem so high.

A doubling of CO2 would cause a greenhouse warming of 4 W/m2 meaning a sulfate loading of 5.3Tg S would be needed leaving a sizeable amount of whitening on the sky and a much bigger dent in the pockets of those funding SRM schemes.

There are also environmental risks and negative side effect involved with aerosol injections which cannot be ignored. They include

-       Effects of the stratospheric ozone – local ozone depletion has been observed as a by-product of previous volcanic eruptions.

-       Possible increases in drought severity

-       Constant injections of sulfate are required

-       The appearance of the sky is altered as it becomes much whiter

-       Can lead to acid precipitation and deposition of SO2 and sulfates which cause ecological damage

-       Pollution particles have been said to affect health (Nel, 2005)

Consequently the best way to conduct a stratospheric modification scheme has been debated.

Proposed methods include releasing an S-containing gas at the earth’s surface, using ships in remote locations, launching reflective balloons or adding other highly reflective nano particles. To achieve maximum cooling, however, the location of the particles should also be considered as residence times of sulfate particles in the stratosphere are around 1-2 years while in the troposphere they are can be as little as 1 week.

There is, however, still a lot of research that needs to be done before an albedo enhancement scheme can be deployed. In fact, Crutzen (2006) asserts that one should only be deployed when ‘there areproven net advantages’ and ‘its possibility should not be used to justifyinadequate climate policies but merely create a possibility to combatpotentially drastic climate heating’.  Despite this, I think there is something to be said about stratospheric albedo enhancement as not only are its costs more realistic that surface albedo enhancement, the climatic response of aerosol injections can be as little as six months (Hansen et al. 1992)- this is much faster than (and therefore could counteract) the rate of warming caused by CO2. Moreover, if undesired changes are observed then the sulfate injections could be stopped quite readily and the atmosphere could be left to return to its prior state. I never thought I would be in favour of humans artificially replicating anything never mind volcanos however when climate change (and the doom that entails) is the cost I feel there isn’t much to loose in developing these methods further as they can as provide something as little (or not so little if it's needed) as an escape plan that might never be used but could be, and at short notice I might add, should we ever reach dangerous levels of warming! Perhaps then, we are to become human volcanos! 

Summary of Stratospheric Aerosols (Royal Society, 2009)

Thanks for reading! 

Solar Radiation Management - Surface

Solar Radiation Management (SRM)

The main aim of SRM methods is to drastically, if not entirely, reduce net radiative forcing. They way in which the methods propose to do this is through making the Earth more reflective in order to balance the positive forcing of greenhouse gases with the negative forcing of the absorption of solar radiation (Royal Geographical Society, 2009). Achieving this balance would produce a reduction in global mean temperatures which in turn, could lessen the impacts and risks of global warming (Lane and Bickel, 2013).

Discussions about SRM have been going on since the 1960s however it was a largely ignored area of climate science, a topic considered taboo due to concerns that public discussions of SRM would lessen the incentives for political action (Kiehl, 2006).

Attitudes towards SRM only recently changed in response to the editorial essay of Paul Crutzen (2006) which urged a more systematic consideration of SRM. Crutzen’s (2006) paper gave ground to SRM research and debates within the wider climate change debate.

Two trends within the SRM debates are evident. The first is the fact that climate change has lost political salience due to failed GHG control efforts while the second is that it has become hard to deny the lack of control over GHG emissions.

The place of SRM has risen and subsequent debates over its proper governance are growing. Recent debates have been marked by hearings in UK Parliament and US Congress, various expert panels and national research programs have been created and there has been the formal resolution by the Convention on Biological Diversity (Mercer et al, 2011). Having said that, dispite the evolving dialog there range of experts who participate in debates is narrow (Lane and Bickel, 2013) and little is to be known about the public’s awareness and feelings towards geo-engineering due to  lack of data (Mercer et al., 2011).

Impact of different SRM methods on Solar Radiation Fluxes

There are three broad categories, or more specifically, heights at which SRM methods can operate (see above). Firstly, solar radiation can be reflected at the surface, it can be reflected in the air and lastly space based techniques can be used to reduce solar radiation. In the following posts I will look at each these SRM areas.

First up … surface albedo approaches of SRM…

Surface based approaches of SRM aim to reflect solar radiation by making the surface brighter. Surface albedo, therefore, is a measure of the reflectivity, or brightness, of the earths surface.

There are a number of methods that have been proposed such as white roof methods and the brightening of human settlements, more reflective crop varieties and grasslands, desert reflectors, reforestation and ocean albedo (Royal Geographical Society, 2009). Methods could that could be developed in the future include plant morphology modifications to increase albedo such as altering leaf characteristics to increase leaf pubescence, surface waxes, or canopy architecture to maximize albedo.

The overlying issue with surface approaches of SRM is that relative to their cost they are inefficient. The implementation costs of these methods range from billions (roof whitening) to trillions (desert reflectors) while albedo adjustments are minute.

From reading about SRM an important factor brought to attention in the effectiveness of SRM methods is height. The closer to earth an SRM method is, the less efficient it is at increasing surface albedo and ultimately reducing global temperatures.

Further to this, looking at the energy balance of the climate, the planets surface albedo is currently about 0.15 but to cool the planet 0.17 would be needed (Royal Geographical Society 2009). A 0.02 increase might seem rather modest however when you factor in the proportion of the Earth’s surface that is covered by oceans – which have a very low albedo – the figure does not seem quite so modest as we are only left with land to make up that increase and within that cohort there are further restrictions of what land can be used as not all land surface is available for brightening.

Conclusion:

The literature surrounding surface SRM methods is not expansive but perhaps this is due to a lack of support, particularly in comparison to other more popular SRM methods such as stratospheric aerosol injections that seemingly offer more promise (Lane and Bickel, 2013). Nonetheless one can conclude that the discounting the common argument of environmental impacts that is put forward by those opposed to geo-engineering, SRM methods simply do not provide the global cooling that would be desired based upon their costs. As they do not reduce GHG the level of cooling they cause is key and as it as of yet cannot be achieved it seems it is just being left behind.

Thanks for reading.