Solar radiation management
Solar radiation management (SRM) projects (proposed and theoretical) are a type of climate engineering which seek to reflect sunlight and thus reduce global warming. Proposed examples include the creation of stratospheric sulfate aerosols. They would not reduce greenhouse gas concentrations in the atmosphere, and thus do not address problems such as ocean acidification caused by these gases. Their principal advantages as an approach to climate engineering is the speed with which they can be deployed and become fully active, as well as their low financial cost. By comparison, other climate engineering techniques based on greenhouse gas remediation, such as ocean iron fertilization, need to sequester the anthropogenic carbon excess before they can arrest global warming. Solar radiation management projects can therefore be used as a climate engineering 'quick fix' while levels of greenhouse gases can be brought under control by greenhouse gas remediation techniques.
A study by Lenton and Vaughan suggest that marine cloud brightening and stratospheric sulfate aerosols are each capable of reversing the warming effect of a doubling of the level of CO
2 in the atmosphere (when compared to pre-industrial levels).
- 1 Background
- 2 Limitations
- 3 Atmospheric projects
- 4 Terrestrial albedo modification
- 5 Farming, forestry, and land management
- 6 Space projects
- 7 Public attitudes
- 8 See also
- 9 References
- 10 Further reading
The phenomenon of global dimming is widely known, and is not necessarily a climate engineering technique. It occurs in normal conditions, due to aerosols caused by pollution, or caused naturally as a result of volcanoes and major forest fires. However, its deliberate manipulation is a tool of the geoengineer. The majority of recent global dimming has been in the troposphere, except that resulting from volcanos, which affect mainly the stratosphere.
By intentionally changing the Earth's albedo, or reflectivity, scientists propose that we could reflect more heat back out into space, or intercept sunlight before it reaches the Earth through a literal shade built in space. A 0.5% albedo increase would roughly halve the effect of CO2 doubling.
These climate engineering projects have been proposed in order to reduce global warming. The effect of rising greenhouse gas concentrations in the atmosphere on global climate is a warming effect on the planet. By modifying the albedo of the Earth's surface, or by preventing sunlight reaching the Earth by using a solar shade, this warming effect can be cancelled out — although the cancellation is imperfect, with regional discrepancies remaining.
The applicability of many techniques listed here has not been comprehensively tested. Even if the effects in computer simulation models or of small-scale interventions are known, there may be cumulative problems such as ozone depletion, which only become apparent from large scale experiments.
As early as 1974, Russian expert Mikhail Budyko suggested that if global warming became a problem, we could cool down the planet by burning sulfur in the stratosphere, which would create a haze. Paul Crutzen suggests that this would cost 25 to 50 billion dollars per year. It would, however, increase the environmental problem of acid rain. However, this is now believed to be a minor side effect.
A preliminary study by Edward Teller and others in 1997 presented the pros and cons of various relatively "low-tech" proposals to mitigate global warming through scattering/reflecting sunlight away from the Earth via insertion of various materials in the upper stratosphere, low earth orbit, and L1 locations.
As well as the imperfect cancellation of the effect of greenhouse gases on global warming, there are other significant problems with solar radiation management as a form of climate engineering not least of these are effects on the global hydrological cycle and the inability of such techniques to reduce ocean acidification.
Particular to solar radiation management, a risk of abrupt cessation exists. If SRM were to abruptly stop, the climate would rapidly warm. This would cause a sudden rise in global temperatures towards levels which would have existed without the use of the climate engineering technique. The rapid rise in temperature may lead to more severe consequences than a gradual rise of the same magnitude.
SRM has been suggested to control regional climate, but precise control over the geographical boundaries of the effect is not possible.
Methods based on increasing the aerosol content in the lower stratosphere for climate modification were proposed by a Russian scientist, Budyko.
Stratospheric sulfate aerosols as proposed by Paul Crutzen, with the purpose to modify the Earth's albedo with reflective or absorptive materials spread over portions of its surface. This would typically be achieved using hydrogen sulfide or sulfur dioxide, delivered using artillery, aircraft (such as the high-flying F15-C) or balloons.  (Alternative approaches using photophoretic particles have been proposed.) Ozone depletion is a risk of such techniques, but only if high enough quantities of aerosols drift to, or are deposited in, polar stratospheric clouds before the levels of CFCs and other ozone destroying gases fall naturally to safe levels because CFCs can settle on larger sulfate particles, increasing their ozone destroying potential. This proposal, not unlike the others, carries with it considerable risks, including increased drought or acid rain.
Broadly speaking, this technique is seen as a credible climate engineering scheme, although not one without major risks, and challenges for its implementation. This technique can give >3.7 W/m2 of globally averaged negative forcing, which is sufficient to entirely offset the warming caused by a doubling of CO2.
Sulfate is the most commonly proposed aerosol for climate engineering, since there is a good natural analogue with volcanic eruptions. Explosive eruptions inject large amounts of sulfur dioxide gas into the stratosphere, which form sulfate aerosol. Studies of the Earth's climate have shown that such aerosol can achieve significant cooling. Sulfate aerosols have been shown to enhance ozone depletion. However, other aerosol types may be more efficient at cooling the climate or less damaging to the ozone layer. Such aerosols include the highly reflective titanium dioxide.
United States Patent 5003186 suggested that tiny metal flakes could be "added to the fuel of jet airliners, so that the particles would be emitted from the jet engine exhaust while the airliner was at its cruising altitude." Alternative proposals, not known to have been published in peer-reviewed journals, include the addition of silicon compounds to jet fuel to make silicon dioxide particles in the exhaust.
A more sophisticated approach, using multi-layered nanoparticles (consisting of aluminium and barium titanate), was published by David Keith in 2010. He suggests utilizing the effects of photophoresis to increase the amount of time the aerosols stay airborne.
David Keith has also developed an experiment involving the release of sun-reflecting sulfate from a balloon more than 80,000 feet above Fort Sumner in New Mexico to replicate the cooling effects of volcanoes.
In 1992, a report by the US National Academy of Sciences (NAS) on geoengineering noted that dust is a better choice compared to sulphur, because dust is from natural soil and so should have no noticeable effect on the ground as it gradually falls into the troposphere and rains out. It estimated that about 1010 kg dust would be required to mitigate the warming from a doubling of atmospheric CO
2 or about 1 kg dust per 100 t of carbon emissions.
An example of the effects of the imposition of aerosol particles in the atmosphere can be found in history. Comets have been blamed for the dramatic but brief cooling period which commenced in 1159 BCE, and resulted in widespread disruption to civilisations at the time. However, this mechanism, and even the involvement of a comet, is not universally accepted. If a comet was indeed to blame, the action of its aerosols could also have been by the mechanism of cloud condensation nuclei. Other examples of climate change events linked to comets include the famines around 536 CE.
Cloud reflectivity enhancement
Various schemes have been suggested, such as that proposed by John Latham and Stephen Salter, which works by spraying seawater in the atmosphere to increase the reflectiveness of clouds. The extra condensation nuclei created by the spray will change the size distribution of the drops in existing clouds to make them whiter. The sprayers would use fleets of unmanned rotor ships known as Flettner vessels to spray mist created from seawater into the air to thicken clouds and thus reflect more radiation from the Earth. The whitening effect is created by using very small cloud condensation nuclei, which whiten the clouds due to the Twomey effect.
Ocean sulfur cycle enhancement
Enhancing the natural sulfur cycle in the Southern Ocean ocean by fertilizing a small portion with iron in order to enhance dimethyl sulfide production and cloud reflectivity. The goal is to slow Antarctic ice from melting and raising sea level. Such techniques also tend to sequester carbon, but in this specific project the enhancement of cloud albedo was both the desired outcome and measured result. An alternative technique proposes the vertical mixing of ocean water, to bring deep-water nutrients to surface plankton. This technique can give only 0.016 W/m2 of globally averaged negative forcing, which is essentially insignificant for climate engineering purposes.
These reflectors would be placed at a high enough altitude so that they do not interfere with air traffic. The cost estimate is about 20 times as much as the distribution of dust in the stratosphere, making these schemes economically nonviable. The large number of reflectors and the trash problem posed by their fall make the system unattractive.
Cloud seeding has been proposed using various methods to distribute the cloud-seeding materials, including airliners and ships or power plants. Reck (1978) studied the effect of increases in cloud cover and, using a radiative-convective atmospheric model, found that a 4 to 5 percent increase in low-level cloud cover would be sufficient to offset the warming predicted from a doubling of preindustrial CO2. This value is in reasonable agreement with Randall et al. (1984), who estimated that a 4 percent increase was required in the amount of marine stratocumulus, which comprises the bulk of the low clouds on a global basis."
Terrestrial albedo modification
Painting roof materials in white or pale colours to reflect solar radiation, known as 'cool roof' technology, and encouraged by legislation in some areas (notably California). This is a benign technique, although limited in its ultimate effectiveness by the costrained surface area available for treatment. This technique can give between 0.01-0.19 W/m2 of globally averaged negative forcing, depending on whether cities or all settlements are so treated. This is generally insignificant when compared to the 3.7 W/m2 of positive forcing from a doubling of CO2. However, in many cases it can be achieved at little or no cost by simply selecting different materials. Further, it can reduce the need for air conditioning, which causes CO2 emissions which worsen global warming.
Adding reflective plastic sheets covering 67,000 square miles (170,000 km2) of desert every year between 2010 and 2070 to reflect the Sun’s energy. This technique can give globally averaged 1.74 W/m2 of negative forcing, which is insufficient to offset the 3.7 W/m2 of positive forcing from a doubling of CO2, but is still a very significant contribution and is sufficient to offset the current level of warming (approx. 1.7 W/m2). However, the effect would be strongly regional, and would not be ideal for controlling Arctic shrinkage, which is one of the most significant problems resulting from global warming. Furthermore, the total area required during 2010-70 is larger than all non-polar deserts combined.
An early geoengineering idea was to use pale coloured floating litter within certain stable oceanic gyres. This litter would tend to group into large and stable areas, such as the Great Pacific Garbage Patch.
Farming, forestry, and land management
Changes to grassland have been proposed to increase albedo. This technique can give 0.64 W/m2 of globally averaged negative forcing, which is insufficient to offset the 3.7 W/m2 of positive forcing from a doubling of CO2, but could make a minor contribution.
High-albedo crop varieties
Selecting or genetically modifying commercial crops with high albedo has been suggested. This has the advantage of being relatively simple to implement, with farmers simply switching from one variety to another. Temperate areas may experience a 1 °C cooling as a result of this technique. This technique is an example of bio-geoengineering. This technique can give 0.44 W/m2 of globally averaged negative forcing, which is insufficient to offset the 3.7 W/m2 of positive forcing from a doubling of CO2, but could make a minor contribution.
Space-based climate engineering projects are seen by many commentators and scientists as being far-fetched at present.
Detonation of a nuclear bomb on the lunar surface
It has been proposed that a nuclear missile could be fired into the lunar surface, thus creating a dust cloud between the Earth and the sun. This would deflect solar radiation and reduce warming, until the cloud eventually dissipated. Benefits of this strategy include that it is relatively cheap compared to other climate engineering projects, and that it does not involve specific ecosystem changes, and the resultant unknowns in effects on specific species on the planet Earth. Disadvantages include its uncertain level of effectiveness, i.e., that it might be 'too effective', and actually result in global cooling. The disadvantage quoted by many, that it would destabilise the moon's effect on tides and other earthly phenomena, is of course false, since the effect on the moon as a whole would be entirely negligible.
Several authors have proposed dispersing light before it reaches the Earth by putting a very large diffraction grating (thin wire mesh) or lens in space, perhaps at the L1 point between the Earth and the Sun. Using a Fresnel lens in this manner was proposed in 1989 by J. T. Early. Using a diffraction grating was proposed in 1997 by Edward Teller, Lowell Wood, and Roderick Hyde. In 2004, physicist and science fiction author Gregory Benford calculated that a concave rotating Fresnel lens 1000 kilometres across, yet only a few millimeters thick, floating in space at the L1 point, would reduce the solar energy reaching the Earth by approximately 0.5% to 1%. He estimated that this would cost around US$10 billion up front, and another $10 billion in supportive cost during its lifespan. One issue with implementing such a solution is the need to counteract the effects of the solar wind moving such megastructures out of position. Side-effects include that, if this lens were built and global warming were avoided, there would be less incentive to reduce greenhouse gases, and humans might continue to produce too much carbon dioxide until it caused some other environmental catastrophe, such as a chemical change in ocean water that could be disastrous to ocean life.
There have been a limited number of studies into attitudes to solar radiation management. Notable examples include Mercer et al.
- [dead link]
- "A Framework to Prevent the Catastrophic Effects of Global Warming using Solar Radiation Management (Geo-Engineering)". Thehardlook.typepad.com. Retrieved 2013-10-16.
- "Metapress Hosting Services for Academic Publishers". Journals.royalsociety.org. Retrieved 2013-10-16.
- Lenton, T. M., Vaughan, N. E. (2009). "The radiative forcing potential of different climate geoengineering options". Atmos. Chem. Phys. Discuss. 9 (1): 2559–2608. doi:10.5194/acpd-9-2559-2009.
- Committee on Science, Engineering, and Public Policy (1992). Policy Implications of Greenhouse Warming: Mitigation, Adaptation, and the Science Base. National Academy Press. p. 447. ISBN 0-309-04386-7.
- Geoengineering the Climate: Science, Governance, and Uncertainty. London: The Royal Society. September 2009.
- Mark, Jason (2009). "Hacking the Sky: Geo-Engineering Could Save the Planet.. And in the Process Sacrifice the World". Earth Island Journal (San Francisco, CA: Earth Island Institute) 24 (3): 40–46. ISSN 1041-0406. 472240324.
- Spencer Weart (July 2006). "Aerosols: Effects of Haze and Cloud".
- Crutzen, P. J. (2006). "Albedo Enhancement by Stratospheric Sulfur Injections: A Contribution to Resolve a Policy Dilemma?". Climatic Change 77 (3–4): 211–220. doi:10.1007/s10584-006-9101-y.
- Harshvardhan (June 1978). "Albedo enhancement and perturbation of radiation balance due to stratospheric aerosols". Unknown. Bibcode:1978aepr.rept.....H. 1978aepr.rept.....H.
- "Acid Deposition from Stratospheric Geoengineering with Sulfate Aerosols". Climate.envsci.rutgers.edu. Retrieved 2013-10-16.
- Edward Teller; Roderick Hyde; Lowell Wood (1997). Global Warming and Ice Ages: Prospects for Physics-Based Modulation of Global Change (PDF). Lawrence Livermore National Laboratory. Retrieved 30 October 2018. See pages 10-14 in particular.
- Tilmes, S.; Fasullo, J.; Lamarque, J. F.; Marsh, D. R.; Mills, M.; Alterskjaer, K.; Muri, H.; Kristjánsson, J. N. E.; Boucher, O.; Schulz, M.; Cole, J. N. S.; Curry, C. L.; Jones, A.; Haywood, J.; Irvine, P. J.; Ji, D.; Moore, J. C.; Karam, D. B.; Kravitz, B.; Rasch, P. J.; Singh, B.; Yoon, J. H.; Niemeier, U.; Schmidt, H.; Robock, A.; Yang, S.; Watanabe, S. (2013). "The hydrological impact of geoengineering in the Geoengineering Model Intercomparison Project (GeoMIP)". Journal of Geophysical Research: Atmospheres 118 (19): 11,036. doi:10.1002/jgrd.50868.
- "Geoengineering Could Slow Down Global Water Cycle". Sciencedaily.com. 2008-05-28. Retrieved 2013-10-16.
- Ross, A.; Damon Matthews, H. (2009). "Climate engineering and the risk of rapid climate change". Environmental Research Letters 4 (4): 045103. Bibcode:2009ERL.....4d5103R. doi:10.1088/1748-9326/4/4/045103.
- Bernstein, D. N.; Neelin, J. D.; Li, Q. B.; Chen, D. (2013). "Could aerosol emissions be used for regional heat wave mitigation?". Atmospheric Chemistry and Physics 13 (13): 6373. doi:10.5194/acp-13-6373-2013.
- Budyko, M. I., Climate Changes, American Geophysical Union, Washington, DC, English translation of 1974 Russian volume, 1977, p. 244
- Davidson, P.; Burgoyne, C.; Hunt, H.; Causier, M. (2012). "Lifting options for stratospheric aerosol geoengineering: Advantages of tethered balloon systems". Philosophical Transactions of the Royal Society A: Mathematical, Physical and Engineering Sciences 370 (1974): 4263. Bibcode:2012RSPTA.370.4263D. doi:10.1098/rsta.2011.0639.
- "Programmes | Five Ways To Save The World". BBC News. 2007-02-20. Retrieved 2013-10-16.
- "Can a Million Tons of Sulfur Dioxide Combat Climate Change?". Wired.com. 23 June 2008. Retrieved 2013-10-16.
- "NOAA Status Alert". Pmel.noaa.gov. Retrieved 2013-10-16.
- Keith, D. W. (2010). "Photophoretic levitation of engineered aerosols for geoengineering". Proceedings of the National Academy of Sciences 107 (38): 16428–16431. doi:10.1073/pnas.1009519107. PMC 2944714. PMID 20823254.
- "Geoengineering side effects : Heating the tropical tropopause by sedimenting sulphur aerosol?". Cosis.net. Retrieved 2013-10-16.
- Hargreves, Ben (2010). "Protecting the Planet". Professional Engineering (Professional Engineering Publishing) 23 (19): 18–22. ISSN 0953-6639.
- Catherine Brahic (2 August 2007). "'Sunshade' for global warming could cause drought". Retrieved 29 April 2009.
- David Over. "Sulphur screens - 21st Century Challenges". Royal Geographic Society with IBG. Retrieved 29 April 2009.
- [dead link]
- Keith, David W.: Photophoretic levitation of engineered aerosols for geoengineering, PNAS, September 07. 2010
- "Sun Reflecting Chemicals Could Tackle Climate Change". Enviro-News.com. July 17, 2012.
- National Academy of Sciences, Policy implications of greenhouse warming: Mitigation, adaptation and the science base. National Academy Press, Washington DC, 1992, pp. 433–464.
- G. Bala (10 January 2009). "Problems with geoengineering schemes to combat climate change". Current Science 96 (1).
- "Cambridge Conference Correspondence". Abob.libs.uga.edu. Retrieved 2013-10-16.
- "Comet smashes triggered "dry fog" that caused famine 1,500 years ago | TopNews". Topnews.in. Retrieved 2013-10-16.
- Latham, J. (1990). "Control of global warming" (PDF). Nature 347 (6291): 339–340. Bibcode:1990Natur.347..339L. doi:10.1038/347339b0.
- Latham, J.; Salter, S. Preventing global warming by increasing cloud albedo (PDF). Retrieved 20 April 2008. (A brief handout, with artist's renderings.)
- Keith Bower, et al. (2006). "Assessment of a Proposed Technique for Global Warming Mitigation via Albedo-Enhancement of Marine Stratocumulus Clouds". Atmospheric Research 82 (1–2): 328–336. Bibcode:2006AtmRe..82..328B. doi:10.1016/j.atmosres.2005.11.013.
- Latham, J. (2002). "Amelioration of global warming by controlled enhancement of the albedo and longevity of low-level maritime clouds" (PDF). Atmos. Sci. Lett. 3 (2–4): 52–58. Bibcode:2002AtScL...3...52L. doi:10.1006/asle.2002.0099.
- Salter, S, G. Sortino & J. Latham (2008). "Sea-going hardware for the cloud albedo method of reversing global warming". Phil. Trans. R. Soc. A 366 (1882): 3989–4006. Bibcode:2008RSPTA.366.3989S. doi:10.1098/rsta.2008.0136. PMID 18757273.
- Panel on Policy Implications of Greenhouse Warming, National Academy of Sciences, National Academy of Engineering, Institute of Medicine (1992). Policy Implications of Greenhouse Warming: Mitigation, Adaptation, and the Science Base. The National Academies Press. ISBN 0-585-03095-2.
- Latham, J., P.J. Rasch, C.C.Chen, L. Kettles, A. Gadian, A. Gettelman, H. Morrison, K. Bower, T.W.Choularton (2008). "Global Temperature Stabilization via Controlled Albedo Enhancement of Low-level Maritime Clouds". Phil. Trans. Roy. Soc. A 366 (1882): 3969–87. Bibcode:2008RSPTA.366.3969L. doi:10.1098/rsta.2008.0137. PMID 18757272.
- Wingenter, Oliver W.; Elliot, Scott M.; Blake, Donald R. (November 2007). "New Directions: Enhancing the natural sulfur cycle to slow global warming". Atmospheric Environment 41 (34): 7373–5. doi:10.1016/j.atmosenv.2007.07.021.
- [dead link]
- Coale, K. H.; Johnson, K. S.; Buesseler, K.; Sofex Group; Johnson; Buesseler; Sofex Group (December 2002). "SOFeX: Southern Ocean Iron Experiments. Overview and Experimental Design". American Geophysical Union. Fall. Meeting 2002: 01. Bibcode:2002AGUFMOS22D..01C. 2002AGUFMOS22D..01C.
- "A proposal to force vertical mixing of the Pacific Equatorial Undercurrent to create a system of equatorially trapped coupled convection that counteracts global warming". Johnduke.com. Retrieved 2013-10-16.
- "A proposal to force vertical mixing of the Pacific Equatorial Undercurrent...". Cosis.net. Retrieved 2013-10-16.
- Edward Teller; Roderick Hyde; Lowell Wood (2002). Active Climate Stabilization: Practical Physics-Based Approaches to Prevention of Climate Change (PDF). Lawrence Livermore National Laboratory. Retrieved 30 October 2018.
- "IPCC Third Assessment Report - Climate Change 2001 - Complete online versions | GRID-Arendal - Publications - Other". Grida.no. Retrieved 2013-10-16.
- Mitchell, D. L.; Finnegan, W. (2009). "Modification of cirrus clouds to reduce global warming". Environmental Research Letters 4 (4): 045102. Bibcode:2009ERL.....4d5102M. doi:10.1088/1748-9326/4/4/045102.
- "Climate Controls". Reason.com. Retrieved 2013-10-16.
- Hashem Akbari, et al. (2008). "Global Cooling: Increasing World-wide Urban Albedos to Offset CO2".
- [dead link]
- Alvia Gaskill. "Desert Area Coverage". Global Albedo Enhancement Project.
- "Global Albedo Enhancement Project: Surface Albedo Enhancement Principle". Global-warming-geo-engineering.org. Retrieved 2013-10-16.
- "Solar Radiation Management (GeoEngineering)". Retrieved 23 April 2011.
- "What is the Great Pacific Garbage Patch?". 21st Century Challenges. Retrieved 23 April 2011.
- Evans, J.; Stride, E.; Edirisinghe, M.; Andrews, D.; Simons, R. (2010). "Can oceanic foams limit global warming?". Climate Research 42 (2): 155–160. doi:10.3354/cr00885.
- Hamwey, Robert M. (2005). "Active Amplification of the Terrestrial Albedo to Mitigate Climate Change: An Exploratory Study". Mitigation and Adaptation Strategies for Global Change 12 (4): 419. arXiv:physics/0512170. doi:10.1007/s11027-005-9024-3.
- "A high-albedo diet will chill the planet - environment - 15 January 2009". New Scientist. Retrieved 2013-10-16.
- Ridgwell, A; Singarayer, J; Hetherington, A; Valdes, P (2009). "Tackling Regional Climate Change By Leaf Albedo Bio-geoengineering". Current Biology 19 (2): 146–50. doi:10.1016/j.cub.2008.12.025. PMID 19147356.
- David W. Keith (2000). "Geoengineering the climate: History and Prospect". Annual Review of Energy and the Environment 25 (1): 245–284. doi:10.1146/annurev.energy.25.1.245.
- Journal of the British Interplanetary Society, vol 60, p 1
- Roger Angel, S. Pete Worden (Summer 2006). "Making Sun-Shades from Moon Dust". National Space Society, Ad Astra 18 (1).
- "Space Ring Could Shade Earth and Stop Global Warming". LiveScience. 2005-06-27. Retrieved 2013-10-16.
- J. T. Early (1989). "Space-Based Solar Shield To Offset Greenhouse Effect". Journal of the British Interplanetary Society 42. pp. 567–569. This proposal is also discussed in footnote 23 of Edward Teller; Roderick Hyde; Lowell Wood (1997). Global Warming and Ice Ages: Prospects for Physics-Based Modulation of Global Change (PDF). Lawrence Livermore National Laboratory. Retrieved 30 October 2018.
- See Russell Dovey, "Supervillainy: Astroengineering Global Warming and Bill Christensen, "Reduce Global Warming by Blocking Sunlight".
- Gregory Benford (Comments at the 64th World Science Fiction Convention, August 2006.)
- Mercer, A. M.; Keith, D. W.; Sharp, J. D. (2011). "Public understanding of solar radiation management". Environmental Research Letters 6 (4): 044006. doi:10.1088/1748-9326/6/4/044006.