Increasing concentrations of greenhouse gases in the atmosphere are causing global warming. On this, there is, outside of the denialist fraternity (members of whom will doubtless be posting in the comments section of this article), near-unanimity. It is a scientific consensus.

Discussion of the best way or ways to address this, however, reveals a more fractured policy analysis. The great majority of proposed solutions focus on reducing the amount of carbon dioxide being emitted into the atmosphere via different emphases in energy policy, such as promoting nuclear power, alternative energy sources or energy efficiency (or any combination of the above). 

Some, not by any means necessarily to the mutual exclusivity of the above, focus on attempting to reduce carbon dioxide already in the atmosphere: by planting more trees, for example, which is generally lauded as a worthy goal in and of itself, or by fertilizing the ocean with iron to promote the growth of phytoplankton, which is not.

Then there are those who skip past the issue of carbon dioxide emissions altogether and focus on mitigating the warming that results. One such example will be presented in a webinar on Tuesday, by Peter Davidson, a fellow of the Institution of Chemical Engineers and the Royal Academy of Engineering, and a former senior innovation adviser to the British government.

Davidson's proposal, which he outlined last week in the journal tcetoday, takes as its starting point the eruption of Mount Pinatubo in the Philippines in 1991. Pinatubo's eruption caused temperatures to drop by around 0.5 degrees Celsius around the globe for two years, a consequence of the 20 million tons of sulfur dioxide particles it threw into the stratosphere. The size of volcanic aerosol particles (about 1 micrometer) and its placement in the stratosphere were key in allowing for a large and long-lasting (about two years) dispersal that also acted to scatter a small proportion of light (~1 percent), and hence its heat, back into space.

Davidson is not proposing that humanity spread sulfur dioxide around the globe; among other things, adding sulfuric acid to the stratosphere degrades the ozone layer. But he does propose an alternative particle of smaller size: the "benign" titanium oxide, "mankind’s most commonly-used pigment. It is stable in air, non-toxic and seven-times more effective at scattering light than sulfuric acid." At about half the size of the sulphuric acid aerosols from Pinatubo, Davidson's proposed titanium oxide aerosols would reflect sunlight in the wavelength of green light.

As a means of delivering these particles, Davidson proposes a series of very large balloons, tethered to ships, releasing streams of particles — a process that he acknowledges would need to continue for many years to be effective. He estimates the operating costs at roughly a billion dollars per year.

While Davidson and other proponents of such geoengineering, or "climate remediation," projects argue that their development is essential given what might charitably be described as a slow march toward reducing greenhouse gas emissions, they have many critics.

Climatologist Ken Caldeira of the Stanford School of Earth Sciences, who has studied geoengineering projects extensively, has argued that, inevitably, deployment of such schemes would lead to a reduction in pressure to decrease greenhouse emissions: "Thinking of geoengineering as a substitute for emissions reduction is analogous to saying, “Now that I’ve got the seatbelts on, I can just take my hands off the wheel and turn around and talk to people in the backseat. It’s crazy … If I had to wager, I would wager that we would never deploy any geoengineering system.”

He later expanded his argument to observe that:

A similar case was made by the authors of a paper in the journal Climatic Change, who, echoing Caldeira, posited that such geoengineering schemes pose risks through potential unintended consequences, are unlikely to be cost-effective, and are ethically suspect, in that they push the hard decisions concerning greenhouse gas reduction to future generations. They conclude that:

Furthermore, of course, adding aerosols to the air does nothing to address the other impacts of elevated carbon dioxide emissions, such as ocean acidification. Davidson anticipates and acknowledges some of these concerns, but offers his scheme as an interim 'Plan B': "It would be short-sighted to put-off research of such a safety-device –- like trying to develop a life-jacket when you’re swept out to sea and struggling in the water." He, too, however, recognizes that any such efforts could at best only function as stop-gaps, and that there is ultimately only one real solution, however it manifests: "It’s essential that we work to reduce carbon dioxide emissions now," he writes.

Top image: MODIS (Moderate Resolution Imaging Spectroradiometer) image from NASA's Terra satellite showing a thick blue haze stretched over the South Pacific archipelago of Vanuatu on the morning of April 12, 2010. The haze was volcanic fog emitted by the Gaua and Ambrym volcanoes. Both are known for producing volcanic plumes rich in sulfur dioxide. The sulfur dioxide gas emitted by the volcanoes reacts with moisture in the air to create small droplets (called aerosols) of sulfuric acid, which scatters blue light, coloring the plume. Credit: Corbis.

Bottom image: Davidson's proposed dispersal method. Courtesy of the Institution of Chemical Engineers.