The role of the IPCC as a boundary, or hybrid organization, between science and policy has been well-documented. Much of this research focuses on the role that the IPCC has had in shaping policy outcomes, or how its limitations have potential impacts in uptake and political acceptability of its knowledge claims. Less has been said on its role as an organizer of scientific research. This paper takes the IPCC 1.5 degree Special Report (SR15), requested by the UNFCCC in conjunction with the Paris Agreement in 2015, as a case of how the politics of climate change framed specific work of the IPCC. We argue that this led to 1.5 becoming a boundary object shared by the policy and scientific communities. The paper draws on interviews with SR15 authors, alongside document analysis to trace the origins of the report, and explores what role science had in the transformation of 1.5 degrees from an unrealistic political target to a symbol of climate action. We highlight how 1.5 degrees’ substantive scientific meaning has arisen through political negotiation, demonstrating the complexities of the relationship between science and policy, and the role of the IPCC in these processes. The writing of SR15 may herald the ways that the IPCC evolves its function and work for the post-Paris era.