The prospect of climate engineering (CE) – also known as geoengineering, referring to modification of the global environment to partly offset climate change and impacts from elevated atmospheric greenhouse gases – poses major, disruptive challenges to international policy and governance. If full global cooperation to manage climate change is not initially achievable, adding CE to the agenda has major effects on the challenges and risks associated with alternative configurations of participation – for example, variants of partial cooperation, unilateral action, and exclusion. Although the risks of unilateral CE by small states or non-state actors have been over-stated, some powerful states may be able to pursue CE unilaterally, risking international destabilization and conflict. These risks are not limited to future CE deployment, but may also be triggered by unilateral research and development (R&D), secrecy about intentions and capabilities, or assertion of legal rights of unilateral action. They may be reduced by early cooperative steps, such as international collaboration in R&D and open sharing of information. CE presents novel opportunities for explicit bargaining linkages within a complete climate response. Four CE-mitigation linkage scenarios suggest how CE may enhance mitigation incentives, and not weaken them as commonly assumed. Such synergy appears to be challenging if CE is treated only as a contingent response to a future climate crisis, but may be more achievable if CE is used earlier and at lower intensity, either to reduce peak near-term climate disruption in parallel with a programme of deep emission cuts or to target regional climate processes linked to acute global risks.