Over the past few decades, climate shifted from a local to a global issue with rising concern over global climate change. Now, studies on urban climate and heat island mitigation are re-localizing climate science, often adapting global models to the city scale. This paper explores how environmental planners engaged with climate scientists to model heat island mitigation strategies, including urban forestry, green roofs, and light surfaces. Drawing on original data documenting the New York Regional Heat Island Initiative, we present three case studies highlighting how global science was localized as researchers struggled to present scientifically valid and politically acceptable results. Through the cases, we demonstrate how science-policy expertise was co-produced. The frame of co-production suggests that legitimate science policy must draw from both technical and social insights. In the first case, we highlight the process of co-production by exploring how the local knowledge of planners was incorporated into regional-scale climate models, altering simulated reductions in urban air temperature. We characterize local knowledge as contextually valid information that is not derived solely through disciplinary techniques but relies on the experiences and 'expertise' of local actors. The second case explores how municipal agency involvement shaped the mitigation scenarios by providing context on available space for tree planting and high-albedo paving materials, altering the study's policy implications. A third case explores how the collaborative research altered the implementation of urban forestry programs in New York City, and particularly in the South Bronx. Since heat island mitigation requires intervention in the face of high technical and political uncertainty, the process of co-production – where planners and researchers collaboratively review policy-relevant science – is necessary for both professional and local knowledge to use the best available science to design locally appropriate policies. We conclude that local knowledge, too often overlooked or dismissed by climate scientists, is crucial for making locally relevant urban heat island science policy.