Here I present a participatory system approach based on cognitive mapping that can be used to identify and relatively quantify climate adaptation tradeoffs. Fuzzy cognitive mapping (FCM) allows the integration of different participant views in the form of semi-quantitative cause-effect maps, each expressing particular knowledge or experience of a similar or distinctive part of a system. This particular nature of FCM allows to co-produce knowledge around a phenomenon and can be also used in the context of climate adaptation planning. To show its potential to identify and assess climate adaptation tradeoffs, I use a case study that explores heatwave impacts and adaptation options in the city of Madrid (Spain). Based on 22 individual maps elicited from agents with different backgrounds (urban planning, energy efficiency, water, green infrastructures, health, climate change…), an integrated multisectorial representation of urban heatwaves phenomena in Madrid has been obtained. In FCM, each participant interprets phenomena and connects cause and effects in very diverse manners. This information, far from being contradictory, explains the complexity of the system and provides complementary knowledge to assess impacts of different policy options. Alongside its potential as a knowledge co-production method and as a communication and learning tool, FCM allows analysis of what-if scenarios and assessment of cascading effects that would be difficult to identify otherwise. This application enables the identification and assessment of tradeoffs of policy options (so-called what-if scenarios). In the case study presented, 4 heatwave adaptation policy options are simulated: use of air conditioning, use of recycled water, deployment of green infrastructures and heat warning systems. Tradeoffs of each policy option are identified, quantified and compared. I discuss the benefits of this approach and the limitations of using this information for decision-making.