Human activities have induced unprecedented global shifts in natural systems including the climate, the oceans, cryosphere and biosphere. The impacts of these changes on physical health are clear and are accelerating at an alarming rate. Climate change and its consequences, especially disruptive events like floods, droughts and heat waves also impact the mental health of affected populations, increasing risk for post-traumatic stress, depression and anxiety disorders. However, the impact of climate change on mental health is not well examined and has received less attention than climate's impacts on physical health. The paper examines the planetary health-mental health nexus. It assesses the existing state of knowledge on the association between climate events, natural disasters, pollution, access to green space and mental health. It also presents a global analysis of the economic costs of climate-related mental health disorders by developing scenarios estimating the costs of mental illness at the country level predicted to be attributable to changes in environmental factors during the period 2020-2050. Globally, the additional societal costs of mental disorders due to changes in climate-related hazards, air pollution and inadequate access to green space are estimated to be almost US$47 billion annually in 2030. These estimated costs will continue to grow exponentially to US$537 billion in 2050, relative to a baseline scenario in which these environmental factors remain at 2020 levels. Our scenario analysis shows that the costs associated with climate-related mental health morbidity and mortality are high already and continue to will increase sharply in coming decades. There is need therefore to strengthen the evidence linking climate change to mental health and to prioritize the development of evidence-based and impactful interventions to address the global burden of environment-related mental disorders.