For the purposes of this talk, Bodansky defines geoengineering as large-scale, intentional efforts to change the climate system. Climate engineering proposals include those aimed at removing GHGs from the atmosphere, for example, through afforestation or iron fertilization of plankton in the oceans and those aimed at screening out sunlight by injecting aerosols into the atmosphere to create cloud condensation nuclei and hence more clouds, by injecting dust into the stratosphere to screen out sunlight, by launching reflective balloons into the stratosphere, or by space mirrors or screens to act as a constant shield from the sun.

Proponents of geoengineering claim that its benefits include technical feasibility (injecting dust in the stratosphere with airplanes or cannons), relatively low costs ($30 billion to inject dust), and administrative feasibility (wouldn't require complex regulatory regime; could be done unilaterally without collective action problems).

Problematic features include the fact that it is intentional (and thus attracts greater scrutiny), has global effects, involves high uncertainties (with an indeterminate risk of some thing going wrong), and non-uniform effects (winners and losers result). These features of geoengineering raise several governance issues. The fact that geoengineering is an intentional activity with global effects raises the issue of who should decide whether to proceed. Should all countries be able to participate in decision making since all will be affected and there will be both positive and negative impacts? Also, how should liability and compensation for damages be addressed?

There are not many precedents for how to address geoengineering in international law. Weather modification activities that took place in the 1970s are one precedent. In 1980, the United Nation Environment Programme (UNEP) produced a set of Weather Modification Guidelines. Weather modification was defined as "Any action performed with the intention of producing artificial changes in the properties of the atmosphere for purposes such as increasing, decreasing or redistributing precipitation or cloud coverage, moderating severe storms and tropical cyclones, decreasing or suppressing hail or lightning or dissipating fog." Recommendations of the guidelines included:

• States should undertake prior environmental assessments of prospective activities likely to affect other states or the global commons.
• States should exchange technical and scientific information on weather modification activities through the World Meteorological Organization.
• States should provide timely notice to and consult with potentially affected states.
• Weather modification activities should be conducted in a manner designed to ensure that they do not cause damage to the environment of other states or of areas beyond the limits of national jurisdiction.

The second precedent was prompted by the use of weather modification by the U. S. in Vietnam for military purposes. The Convention on the Prohibition of Military or Any Other Hostile Use of Environmental Modification Techniques (ENMOD) was adopted in 1977. It prohibits any hostile use of environmental modification techniques, but was neutral regarding the use of such techniques for peaceful purposes. Relevant provisions

• recognize the potential of environmental modification techniques to preserve and improve the environment for the benefit of present and future generations.
• state that ENMOD does not hinder the use of environmental modification techniques for peaceful purposes, and is without prejudice to the generally recognized principles and applicable rules of international law concerning the use of environmental modification for peaceful purposes.

A number of general norms in international law provide background and set the terms of the debate on whether climate geoengineering or similar activities should be allowed to proceed. These include:

• Duty to prevent transboundary harm
• Precautionary principle, which states that if there is a potential for irreversible or catastrophic harm, the burden of proof should be on those proposing the action.
• Principle of intergenerational equity
• Duty to undertake prior assessments
• Duty to provide notice to and consult with potentially affected states

The United Nation (UN) Framework Convention on Climate Change (FCCC) is silent on the subject of geoengineering. In general, it promotes scientific cooperation and mentions a duty to minimize adverse effects from projects to mitigate climate change. Most importantly, it creates a governance structure to address climate issues, including a Conference of the Parties (CoP) and a Subsidiary Body for Scientific and Technological Advice (SBSTA).

In addition to general international norms and the FCCC, there are a number of specific norms that relate to climate engineering proposals. With respect to afforestation, if a country is planting trees in its own territory, then as a matter of international law this is a permissible exercise of national sovereignty because it takes place entirely within the country's borders. But when one country wants to offset emissions by planting trees in another country, sovereignty and internal political issues may arise. For example, a national government may approve a tree planting scheme as part of an international agreement, but the local people may not want the trees planted in their area since it may have the effect of hindering development.

On the subject of ocean fertilization schemes, the 1982 UN Convention on the Law of the Sea generally provides for freedom of the high seas for research and other peaceful uses. There is also the Antarctic Treaty System which contains no specific prohibition on ocean fertilization but establishes an effective governance system through the Antarctic Treaty Consultative Meetings.

Regarding the space mirrors proposal, the Outer Space Treaty of 1967 applies. It says that outer space is the "province of all mankind" and any state is free to engage in peaceful uses. There is a duty to avoid adverse environmental changes and to consult with other potentially affected states if there is any question about adverse effects.

Schemes to inject dust or release balloons into the atmosphere are the most problematic of the geoengineering proposals in terms of existing international law because the atmosphere above a country, including the stratosphere, is part of its air space. Nations have claimed this area and acted on their claims (e.g., by shooting down aircraft). Geoengineering proposals involving the atmosphere thus could be viewed as an infringement and incursion on national territory.

Although existing international legal norms are generally permissive, they are unlikely to be a reliable guide to how the international community will react if geoengineering schemes are seriously proposed. Instead, there is likely to be a great deal of resistance. Absent some crisis, there will probably be a drive for the regulation of these activities, and perhaps for their prohibition, because it is very difficult to discern what the inadvertent consequences of such proposals might be. The precautionary principle says, in essence, "when in doubt, don't," and this would probably be the general response to geoengineering. Some international precedents that might be relevant to geoengineering include:

• Ban on Antarctic mineral activities
• Ban on commercial whaling and driftnet fishing
• Ban on ocean incineration and ocean dumping of low-level radioactive waste

Thus, the ultimate obstacles to geoengineering may not be technical or economic, but may instead be political.