Climate change is different things to different people. Understanding what climate change is and according to whom is especially important in the context of the Great Barrier Reef ("the Reef") as its health is diminishing and at the same time, authority over the Reef is being renegotiated. Governance institutions have committed to co-management with the Reef's Traditional Owners ("TOs"). This study asks how TOs, the Australian Department of Climate Change, Energy, the Environment and Water ("DCCEEW") and the Great Barrier Reef Marine Park Authority ("GBRMPA") discursively construct climate change, and what ontological imaginaries, or configurations of what exists in the world, are involved in those constructions. Through a critical discourse analysis of documents published by DCCEEW and GBRMPA and 20 in-depth interviews with TOs, I conclude that where DCCEEW and GBRMPA express the same ontological imaginary, many TOs hold multiple, sometimes conflicting ontological imaginaries simultaneously. To TOs, climate change is a collection of harmful, local impacts on the Reef, produced by interactions between, amongst others, people, Country, and God, but also by fossil fuels, ocean acidification and mining. Traditional Owners also point at the challenges that continued colonization poses to caring for the Reef. DCCEEW and GBRMPA, on the other hand, construct climate change as an autonomous entity that jeopardizes human interests, including interests in the Reef. Responding to climate change, for these institutions, requires governments to invest in scientific and technological innovation. These results indicate that differences exist in how authorities on the Reef understand their role in the causes of and responses to climate change. They also show that where TOs navigate multiple ontological imaginaries and realities of climate change, DCCEEW and GBRMPA do not recognize other ways of knowing and engaging with climate change, which may impede co-management.