Climate engineering—the deliberate large-scale manipulation of the Earth’s climate system—is a set of technologies for reducing climate-change impacts and risks. It is controversial and raises novel governance challenges [T. C. Schelling, Climatic Change, 33, 303–307 (1996); J. Virgoe, Climatic Change, 95, 103–119 (2008)]. We focus on the strategic implications of solar geoengineering. When countries engineer the climate, conflict can arise because different countries might prefer different temperatures. This would result in too much geoengineering: the country with the highest preference for geoengineering cools the planet beyond what is socially optimal at the expense of the others—a theoretical possibility termed “free-driving” [M. L. Weitzman, Scand. J. Econ., 117, 1049–1068 (2015)]. This study is an empirical test of this hypothesis. We carry out an economic laboratory experiment based on a public “good or bad” game. We find compelling evidence of free-driving: global geoengineering exceeds the socially efficient level and leads to welfare losses. We also evaluate the possibility of counteracting the geoengineering efforts of others. Results show that countergeoengineering generates high payoff inequality as well as heavy welfare losses, resulting from both strategic and behavioral factors. Finally, we compare strategic behavior in bilateral and multilateral settings. We find that welfare deteriorates even more under multilateralism when countergeoengineering is a possibility. These results have general implications for governing global good or bad commons. Significance Governing climate change and other global commons is challenging. Climate engineering has recently gained attention as a way to manage climate risks. Solar geoengineering is a technology that allows countries to unilaterally influence the world temperature. Solar geoengineering could trigger conflicting interventions by countries who prefer different temperatures; economic theory suggests that countries wanting a cooler climate impose it on others. Other countries may react through countergeoengineering interventions. We study the governance of solar geoengineering using a laboratory experiment in which participants engage in a public good-or-bad game. Results confirm that too much geoengineering can occur, leading to considerable economic losses and increased inequality. The experiment also highlights unforeseen governance risks associated with such technologies.