This paper presents the concept of greening as a function of mitigating climate change and how greening affects the increase in tourism competitiveness. In the first part, the concepts of tourism and climate crisis are explained, how the impact of the climate crisis affects the environment, how the climate crisis affects tourism, and how the crisis is connected to urban environments. In the following, the concept of green cities is explained and analyzed, how the greening of the urban environment can affect the reduction of climate change, what are the methods of greening urban environments, the paper presents examples of green cities in Europe, what new forms of tourism are emerging and how they affect tourism competitiveness. After the clarification of green cities, the previously mentioned topics were drawn through the aspect of the City of Zagreb. The paper describes the environmental sustainability of tourism in the City of Zagreb, describes the bio- and geo-diversity of Zagreb, presents the greening methods used in the City of Zagreb, and explains how the aforementioned concepts affect the quality of life of the people in Zagreb. At the end of the paper, the results of the research and interviews were presented, and the results were recommendations for greening the City of Zagreb with the aim of reducing climate change and developing tourism competitiveness.