Metta Spencer was invited to speak to the fellows of McLaughlin College, York University about climate change. Her title, “How to Subtract" reviews five of the possible methods of either shielding the planet from the sun's heat with whiter clouds or removing carbon from the atmosphere to store it in the soil or concrete. Spencer chose these as the most promising partial solutions, though of course the essential solution will simply be to cease emitting greenhouse gases. Wadhams and Petersen were participants in the zoom meeting and explained their approaches during the Q and A period. For the video, audio podcast, transcript, or comments: https://tosavetheworld.ca/how-to-subtract/.