Like many other issue areas that require specialist technical expertise, international climate cooperation relies on scientific evidence to inform policymaking. The reports of the Intergovernmental Panel on Climate Change (IPCC) as the United Nations’ main body on climate science structure climate negotiations by defining the scientific guardrails for governments’ climate policies, including on mitigation, adaptation, and technology choices. Since IPCC reports are produced jointly between scientist authors and governments, we study the importance of personal ties between report authors and political officials. Contributing to the literature on informal governance in international institutions, the observational part of our empirical analysis demonstrates that review comments are more likely to be accepted by report authors when authors and the comment-submitting government come from the same country. We provide explanatory evidence that shared education and shared employment histories are a plausible mechanism that underpins this relationship. In the experimental part of the study, we draw on an elite sample of former IPCC authors during Assessment Round 6 and randomize the nationality of the comment-submitting government to be either the same or different from each of the author’s own nationality for various substantively different sets of comments. We rely on the qualitative analysis of open-ended responses of IPCC authors from our elite survey to corroborate our central claim that personal ties shape international climate science. These findings challenge the standard assumption in much of the international cooperation literature that IO information is largely exogenous to national interests of member governments.