Over the past five to ten years, policy debate on climate change has seen a surge of attention, and controversy, over technologies of climate engineering. These are engineered interventions that intentionally manipulate global-scale processes with the aim of controlling the climate, which are mainly being proposed as ways to offset some of the climate disruptions caused by elevated atmospheric concentrations of carbon dioxide (C[O.sub.2]) and other greenhouse gases. These are not new ideas, having first been discussed in the 1950s and 1960s. But after being only marginally present in debates on climate-change policy and law since these began in the 1980s, they are now receiving renewed prominence, for two reasons: nations' continued failure to make serious efforts at lowering greenhouse gas emissions; and increasing recognition of the magnitude of climate-related uncertainties, which represent a significant source of continuing risk whether or not we shift to serious emissions reductions. The world's major governments and economies have been sinners on emissions--but they could turn from sinners to saints and still face severe climate-change impacts, if uncertainties about the response of the global climate system and resulting impacts turn out to be near the worst end of the current distribution of possibilities.
There are two broad types of climate-engineering interventions, which are distinguished by the basic mechanisms by which they operate. The first type works by intervening in the global carbon cycle, increasing the rate of removal of C[O.sub.2] from the atmosphere. I do not consider this type of intervention today. This is not because these approaches are not important or valuable: they are potentially both. Rather, it is because, in contrast to the second type of intervention, the problems and opportunities these pose to international law and governance are not significantly novel. In their potential benefits, costs, and risks, these resemble a new, alternative form of emissions reduction, so they can be understood within the current framework of options of climate response, now being handled--albeit quite ineffectually-by the existing international process under the UN Framework Convention on Climate Change.
The second type of approach works by changing the Earth's absorption of incoming solar radiation, reflecting a little sunlight so it does not reach the Earth's surface. Various specific approaches have been proposed within this broad type, including making the land or ocean surface brighter, increasing the number or whiteness of clouds, or placing mirrors or sunshields in space. The most promising approach, and the one now attracting the most attention, would spread a fine mist of light-colored or reflective particles in the upper atmosphere (the stratosphere). These would scatter and reflect a little sunlight, making the sun a little dimmer and the sky a little whiter when viewed from the Earth's surface (and also making the Earth a little brighter when viewed from space). This approach is characterized by extremely high leverage, in that a small input can achieve a large perturbation of the climate: using this approach, the total global-average heating effect of one ton of increased C[O.sub.2] in the atmosphere can be offset by roughly five to ten grams of sulfur, suitably distributed in extremely fine particles, in the stratosphere. High leverage is the origin of the challenges and opportunities these technologies pose for international law and governance. For this reason, I will take spraying sulfur aerosols in the stratosphere as the exemplar of climate engineering technologies--even if better approaches are developed, as is likely, they will retain the high leverage that makes this approach so attractive and challenging, and the resultant effects for law and governance that flow from this high leverage.
These high-leverage technologies have three prominent characteristics that broadly shape the challenges they pose to international law and governance. …