Climate Citizen 3.5 is a large-scale, campaign-based public art project that urges all-out climate actions. The project is inspired by the work of sociologist Erica Chenoweth, who, based on the analysis of data on various social movements that have occurred throughout the globe for the past 100 years, showed that social change is possible when 3.5% of the population act. This is a collaborative campaign bringing together artists and theorists in the fields of art, architecture, film, and design; environmental organizations and civic groups; research institutes and citizens. It is divided into eight sub-themes: Climate City, Climate Classroom, Climate Kitchen, Climate Camp, Climate Cinema, Climate Museum, Climate Jeju, and Climate Global. Each sub-theme consists of production, curating, research, and practical application projects that explore diverse models of climate action, ranging from small individual practices to policy proposals for countries, corporations, and local communities. Each individual project provides a spatial vision of how cities should be reorganized in the age of climate crisis, seeks ways in which citizens can contribute to the survival of other forms of life which have become threatened due to human activities, or archives examples of alternative responses to the climate crisis currently in action in the areas of food production, distribution, and consumption. In addition, the projects take up a campaign strategy that urges social interest in the effects of global warming, which is becoming visible all over the Korean peninsula, such as rising sea levels, destruction of rivers and marine ecosystems, and mass mortality of alpine conifers. In the face of the current situation where the number of people in the streets and plazas have drastically decreased due to Covid-19, and the use of cultural spaces such as theaters and art museums have been restricted, Climate Citizen 3.5 adopts a spatial strategy that is not limited to specific places in order to reach as many people as possible. In addition to spatial interventions using videos projected from around thirty large-scale electronic billboards scattered throughout Seoul, advertisements inside subway cars, and handmade banners that can be hung on apartment balconies like laundry, it also makes use label-type posters that can be attached to clothing and backpacks as well as campaign hats, gloves, scarves, handbags and other fashion items hand-knit by elderly women, worn on the moving bodies of citizens as part of the exhibition space. Moreover, participation programs urging climate actions are also conducted using various online media such as online games, exhibitions, film festivals, workshops, and online challenges. While primarily a campaign that aims at mobilizing 3.5% of the Korean population to take climate action, it is hoped that Climate Citizen 3.5 will contribute to the spread of climate action globally through solidarity actions with over 600 civil societies that are active throughout the globe operating under the umbrella of the UN Convention to Combat Desertification (UNCCD).