Professionals have an ethical obligation to bear witness to climate change. They should report, warn, criticize, and lobby to bring attention to the existential threat that climate change poses. But they also have an obligation to respect the knowledge that is the basis of their authority to witness. Witnessing carries risks to this professional authority. Witnessing professionals should avoid letting bias distort their advocacy, simplifying their statements excessively, overplaying the consensus in the field, neglecting their own conflicts of interest, and claiming authority beyond their areas of expertise. To witness ethically, the professional should advocate responsibly.