"It is unlikely that a single small actor could implement and sustain global-scale geoengineering that harms much of the world without intervention from harmed world powers," wrote the scientists in Environmental Research Letters (ERL). "However, a sufficiently powerful international coalition might be able to deploy solar geoengineering."

Researchers from the Carnegie Institution for Science and the Georgia Institute of Technology applied game theory to the international politics of solar geoengineering, basing the damages from climate change on climate-model simulations and allowing different regions to form solar geoengineering coalitions.

"Research on this type of geoengineering shows that different countries would likely want the planet to be at different temperatures," explained Katharine Ricke of the Carnegie Institution for Science. "In our game, the coalition holding the majority of power in the world gets to set the knob on the thermostat of the planet and this could lead to conflict."

Ricke and her colleagues found that, because of this dynamic, the incentives for countries to participate in coalitions to reduce greenhouse gases are fundamentally different to the incentives to participate in solar geoengineering coalitions.

"It is well known that attempts to form coalitions to reduce greenhouse-gas emissions have so far failed because it is difficult to get everybody to participate in a substantive and meaningful way," Ricke told environmentalresearchweb. "Members of coalitions to reduce emissions have incentives to include more countries, but countries have incentives not to participate, to avoid costs associated with emission reduction while benefiting from reductions made by other countries."

Coalitions to geoengineer the climate have the opposite problem, the researchers found. "Coalitions that are powerful enough to geoengineer climate have an incentive to exclude other countries, because other countries might want them to set the thermostat at a temperature less to their liking," said Ricke. "Excluded countries want to participate so that they could have the thermometer set closer to where they might like it."

The researchers compared two different types of models – an exclusive membership game versus an open membership game. They examined the results of the game over a number of decades of geoengineering and over different international power metrics, for example power based on economic output and power based on population. They found that the members of the winning coalition change with these different power metrics and over different decades as well as with different climate-damage functions.

"The strategic effects of solar geoengineering are appropriately characterized by an exclusive, rather than open membership, coalition game," wrote the team.

The dynamics of solar geoengineering coalitions are different to those formed for greenhouse-gas mitigation because solar geoengineering has much lower direct costs and heterogeneous physical climate effects that mean potentially different preferences among members. Mitigation, on the other hand, is expensive but everyone wants the same amount – as much as possible.

Ricke and colleagues believe that, from a policy perspective, a central challenge to achieve equity in geoengineering coalitions is to maintain openness and inclusiveness, so that all people who want a voice in the decision-making process are able to have that voice. "We believe that the framework we provide in this paper will be a starting point for bridging the gap between scientific results and political debate on the governance of climate engineering," said Ricke.

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