Oceans absorbing heat and providing a cooling effect in global warming is a fundamental physical fact of Earth's climate system that requires no citation.
The claim that oceans provide a significant cooling effect in global warming by absorbing heat is a foundational thermodynamic principle of the Earth system (the vast thermal inertia and heat capacity of the oceans absorb the majority of excess anthropogenic heat). It is common knowledge in climatology and does not require specific citation of papers to establish.