The Stratospheric Shield
A practical, low-cost way to reverse catastrophic warming of the Arctic—or the entire planet
Massive shifts are underway in the global energy system as humanity attempts to curtail the release of the gases responsible for greenhouse warming. Yet relatively little progress has been made so far in reducing global emissions of greenhouse gases; concentrations of carbon dioxide (CO2), methane, and other contributors to the warming continue to increase in earth’s atmosphere [Hoffmann 2009, Raupach et al. 2007]. At the same time, scientific projections of the possible future warming and associated changes to precipitation, heat waves, and major storms have generally grown more alarming[IPCC 2007].
A number of leading scientists have voiced concern that our best efforts to reduce emissions through conservation, improvements to energy efficiencies, and shifts to cleaner sources of energy may not be sufficiently fast to prevent intolerable climate change. They have proposed a variety of technologies— commonly called “geoengineering”—that could reverse global warming temporarily, buying time for the global economy to complete its move to a more sustainable energy system [Crutzen 2006, Cicerone 2006, Royal Society 2008].
These proposals have generated a lively and fruitful debate within the scientific community and the public at large about the cost and practicality of various geoengineering technologies, as well as possible unintended side effects of their use. Intellectual Ventures hopes to play a constructive role in this debate as a source of innovative technical ideas for solving some of these issues. We have funded research in this area— and are encouraging others to do so as well—because it would be irresponsible for the technical community to postpone such work until a climate emergency was actually underway. Intellectual Ventures does not advocate construction or deployment of geoengineering systems now, and we hope they will never be needed. But the prudent course is to begin studying options immediately. (See “Climate Science and Engineering at Intellectual Ventures” for further discussion about the role of geoengineering.)
A Global Cooling System
Scientists have proposed a wide variety of approaches for cooling part or all of the Earth [Blackstock et al. 2009]. One approach has received more attention than the others, however: the idea of increasing the amount of sulfur-bearing aerosols in the stratosphere and thereby decreasing slightly the amount of sunlight that reaches the earth [Kunzig 2009]. (The stratosphere is the weather-free portion of the atmosphere at altitudes between about 10 kilometers and 50 kilometers, or 33,000 to 165,000 feet.)
The attractiveness of this approach stems largely from the fact that it happens naturally during large volcanic eruptions, such as the eruption of Mount Pinatubo in the Philippines in 1991. Intensive scientific study of the Pinatubo eruption showed that sulfur dioxide aerosols injected high in the atmosphere cooled the planet by reflecting more incoming sunlight back into space [Robock 2002]. An even larger eruption in 1815 of Mount Tambora in Indonesia led to the second-coldest year in the northern hemisphere in four centuries, the “year without a summer” [Briffa et al. 1998].
Importantly, the cooling effect begins immediately, but is short-lived: unlike carbon dioxide emissions, which persist in the upper atmosphere, warming the earth for centuries [Matthews and Caldeira 2008], sulfur dioxide aerosols appear to remain in the stratosphere for only a year or two after injection before falling back to Earth [Caldeira and Wood 2008]. Any geoengineering system should ideally be not only quick-acting but also quickly reversible, so that the climate returns to its previous state soon after the system is turned off. This provides a measure of safety in case any damaging side effects appear when the system is deployed.
Also important is the fact that aerosols in the stratosphere tend to migrate toward the poles. Thus aerosols injected at the Arctic Circle would be expected to cool the Arctic but to have little or no effect on sunlight received by the temperate and tropical parts of the Earth. Aerosols injected into the atmosphere above Antarctica will similarly tend to disperse gradually toward the South Pole. To cover the entire planet, the spray would have to be released at a variety of latitudes, including sites near the equator.
The general poleward migration of high-altitude aerosols is useful for two reasons. First, it allows small-scale testing of a geoengineering system. A pilot project could be set up in northern Alaska or northern Europe, for example.
Second, the polar regions have so far experienced far greater warming than has the rest of the planet, and climate models project that this trend will continue [IPCC 2007]. If a climate emergency does occur that would warrant use of geoengineering, it seems probable that it will affect the Arctic or Antarctic ice caps first and more severely—indeed, an abrupt shift in climate may already be underway in the Arctic [Kerr 2007]. Systems that can concentrate their cooling effects to the northernmost or southernmost parts of the planet are thus more useful than those that only work uniformly on the entire Earth at once.
To estimate how much sunlight would need to be reflected to offset greenhouse warming of the Arctic or of the entire planet, scientists have turned to the same computer models that they use to project climate change scenarios [Caldeira and Wood 2008]. These models suggest that reducing incoming solar radiation by about 1.8% worldwide would offset the greenhouse warming caused by the doubling of CO2 concentration from its level in preindustrial times. (The CO2 concentration is currently about 1.4 times its preindustrial level and rising steadily. [Hoffman 2009])
Such a small change in solar radiation would almost certainly be imperceptible to our eyes. Because incoming sunlight would be more diffuse, scientists believe that stratospheric aerosols would increase plant growth, boosting agricultural productivity and increasing the rate at which carbon dioxide is absorbed out of the atmosphere [Robock et al. 2009]. More studies are needed to understand the magnitude of this effect and whether it could help to alleviate other consequences of high CO2 levels, such as changes to the pH of the oceans.
Preliminary modeling studies suggest that two million to five million metric tons of sulfur dioxide aerosols (carrying one million to 2.5 million tons of sulfur), injected into the stratosphere each year, would reverse global warming due to a doubling of CO2, if the aerosol particles are sufficiently small and well dispersed [Rasch et al. 2008]. Two million tons may sound like a lot, but it equates to roughly 2% of the SO2 that now rises into the atmosphere each year, about half of it from manmade
sources [Caldeira and Wood 2008], and far less than the 20 million tons of sulfur dioxide released over the course of a few days by the 1991 eruption of Mount Pinatubo [Robok et al. 2009]. Scientific studies published so far conclude that any increase in the acidity of rain and snow as several million additional tons a year of SO2 precipitate out of the atmosphere would be minuscule and would not disrupt ecosystems [Kravitz et al. 2009].
RESCUING ARCTIC SUMMER SEA ICE may be necessary—and possible—if CO2 levels continue to rise, according to computer models of the global climate. The extent of ice cover on the Arctic Ocean at the end of September is shown at top for a world with preindustrial CO2 levels (pink). The fraction covered by ice is much smaller if CO2 levels double (middle). Models indicate that if a stratospheric aerosol shield reduced sunlight over latitudes north of 60°N by 10%, the ice cap would be restored to its former extent each summer (bottom).
CREDIT: Maps by Wayt Gibbs; data courtesy of Ken Caldeira, Carnegie Institute of Washington
A more limited geoengineering system designed to rescue the Arctic ice cap and tundra from catastrophic warming (with much less cooling of the rest of the planet) would aim to attenuate the solar radiation hitting the Arctic and sub-Arctic latitudes of 60°N and higher by about 10%. Climate models indicate that this would lead to average temperatures in the region being about 2.8 °C (5 °F) lower than they would be without the system—enough to restore sea ice in the Arctic to its preindustrial extent. Snow depth might actually increase a bit over what it was before global warming began [Caldeira and Wood 2008].
Because about 10% of the planet lies north of 60°N— which is roughly the latitude of Anchorage, Alaska or Oslo, Norway—a rough first-order estimate is that injection of as little as 200,000 metric tons a year of sulfur dioxide aerosol into the stratosphere above this region could offset warming within the Arctic. A phenomena peculiar to the polar atmosphere, the polar stratospheric vortex, adds uncertainty to this estimate, however. The vortex causes mixing between stratospheric air and the lower part of the atmosphere to occur more rapidly in the Arctic than at lower latitudes. As a result, aerosol particles injected into the stratosphere at latitudes above 60°N will probably fall back to Earth in less than a year, on average. To compensate for this effect—and because the aerosols serve no purpose during the dark polar winter—it would thus make sense to concentrate the injection period to just the spring, so that the cooling effect is at maximum strength during the summer melting season.
Cutting the Cost: A Hose is Better than Bombs
Lifting large masses of aerosols—or of anything, for that matter— up to the stratosphere poses a substantial engineering challenge. One of the principal criticisms of geoengineering proposals so far has been cost: published estimates of the construction costs of delivery systems of various kinds have run from $784 million to $6.6 billion, with estimated operating costs ranging from $225 million to $30 billion a year, depending on whether aircraft, artillery, or sulfur-filled exploding balloons were envisioned as vehicles for the aerosols [Robock et al. 2009].
In a series of invention sessions over the past several years, scientists and engineers at Intellectual Ventures have fleshed out ideas for a geoengineering system that could be far less expensive and more practical than others proposed to date. Called a Stratospheric Shield, or StratoShield for short, the system would deliver sulfur dioxide to an altitude of 30 kilometers in liquid form, through a very long hose supported by large, longduration balloons. At the top of the hose, a series of atomizers would disperse the liquid into a fine mist of aerosol particles, each about 100 nanometers in diameter.
HIGH-FLYING BLIMPS, based on existing protoypes, could support a hose no thicker than a fire hose (above) to carry sulfur dioxide as a clear liquid up to the stratosphere, where one or more nozzles (below) would atomize it into a fine mist of nanometer-scale aerosol particles.
CREDIT: David Fierstein
In the calculations we performed to validate this approach (described below), we focused on an installation capable of pumping 100,000 metric tons a year (about 3.2 kilograms a second) of liquid sulfur dioxide up to the stratosphere, where it would be dispersed by atomizers into a fine mist. Several installations of this size—or one larger installation with several hoses— might be needed to save the Arctic from runaway warming, if they were operated only in the spring rather than year-round.
If at some point world leaders decided that a climate emergency warranted deployment of Stratospheric Shields on a global scale, a dozen or more installations of the size sketched out here could be set up around the world, with most of them at tropical and temperate latitudes, to erect an invisible reflective shield that could counteract greenhouse warming worldwide.
Our work so far, which represents substantial inventive activity but is still quite preliminary, suggests that the cost to construct a Stratospheric Shield with a pumping capacity of 100,000 tons a year would be roughly $24 million, including transportation and assembly. Annual operating costs would run approximately $10 million. The system would use only technologies and materials that already exist—although some improvements may be needed to existing atomizer technology in order to achieve wide sprays of nanometer-scale sulfur dioxide particles and to prevent the particles from coalescing into larger droplets.
Even if these cost estimates are off by a factor of 10 (and we think that is unlikely), this work appears to remove cost as an obstacle to cooling an overheated planet by technological means. The Stratospheric Shield, and geoengineering in general, must still clear many other obstacles, however, before such systems can reasonably be considered for deployment. A concerted, well-funded, long-term research effort is needed to answer the many questions that remain. What effect would cooling by stratospheric aerosols have on shifts in precipitation, increasing acidification of the oceans, and other environmental changes driven by rising levels of CO2? How would additional SO2 in the stratosphere interact with the ozone layer? Are there compounds that would perform better than sulfur dioxide as reflectors, that would be even less expensive, or that would be lighter and thus easier to lift?
Now is the time for the science and engineering community to engage fully in the research needed to answer such questions. There currently is no business model for geoengineering that would encourage creative firms such as Intellectual Ventures to ramp up and maintain a serious research effort. But it is too important a topic to leave for the indefinite future.
In the following sections, we present more details on the Stratospheric Shield in the hope that it will inform and inspire others to refine the idea and to generate other inventions for coping with the defining problem of the 21st century.
References and Further Reading
J. J. Blackstock, D. S. Battisti, K. Caldeira, D. M. Eardley, J. I. Katz, D. W. Keith, A. A. N. Patrinos, D. P. Schrag, R. H. Socolow and S. E. Koonin, Climate Engineering Responses to Climate Emergencies, Novim, 2009.
K. R. Briffa, P. D. Jones, F. H. Schweingruber, and T. J. Osborn. Influence of volcanic eruptions on Northern Hemisphere summer temperature over 600 years. Nature 393, pp. 450–455, 4 June 1998.
K. Caldeira and L. Wood. Global and Arctic climate engineering: numerical model studies. Philosophical Transactions of the Royal Society A, 366, pp. 4039–4056, 2008.
R. J. Cicerone. Geoengineering: Encouraging research and overseeing implementation. Climatic Change 77, pp. 221–226, 2006.
P. J. Crutzen. Albedo enhancement by stratospheric sulfur injections: a contribution to resolve a policy dilemma? Climatic Change 77, pp. 211– 220, 2006.
D. J. Hoffmann. The NOAA Annual Greenhouse Gas Index. NOAA, 2009. Intellectual Ventures. Climate Science and Engineering at Intellectual Ventures. White paper, 2009.
IPCC. Climate Change 2007: The Physical Science Basis. Cambridge University Press, 2007.
R. A. Kerr. Is Battered Arctic Sea Ice Down for the Count? Science 318, pp. 33–34, 5 October 2007.
B. Kravitz, A. Robock, L. Oman, G. Stenchikov, and A. B. Marquardt. Sulfuric acid deposition from stratospheric geoengineering with sulfate aerosols. Journal of Geophysical Research 114, D14109, 2009.
R. Kunzig. A Sunshade for Planet Earth. Scientific American, pp. 46–55, November 2008.
H.D. Matthews and K. Caldeira. Stabilizing climate requires near-zero emissions. Geophysical Research Letters 35, L04705, 2008.
P. J. Rasch, S. Tilmes, R. P. Turco, A. Robock, L. Oman, C. C. Chen, G. L. Stenchikov, and R. R. Garcia. An overview of geoengineering of climate using stratospheric sulphate aerosols. Philosophical Transactions of the Royal Society A 366, pp. 4007–4027, September 2008.
M. R. Raupach, G. Marland, P. Ciais, C. Le Quere, J. G. Canadell, G. Klepper, and C. B. Field. Global and regional drivers of accelerating CO2 emissions. Proceedings of the National Academy of Sciences of the U.S.A. 104: 24, 2007.
A. Robock. The Climatic Aftermath. Science 295, pp. 1242–1243, 15 February 2002.
A. Robock, A. Marquardt, B. Kravitz, and G. Stenchikov. Benefits, risks, and costs of stratospheric geoengineering. Geophysical Research Letters 36, L19703, 2009.
The Royal Society of London. Special issue on geoengineering. Philosophical Transactions of the Royal Society A 366, September 2008.
Nächste Seite >>> Hier weiterlesen: