'confronting the urgent climate crisis, caused by fossil fuels' and will continue to campaign for the cause despite her health battle. 'This diagnosis has only made me more determined than ever to continue to end the deadly effects of fossil fuels,' she continued. 'While most of us know that fossil fuels are the primary cause of the climate crisis, many may not know that fossil fuel emissions also cause cancer as well as other major health problems like birth defects, childhood leukemia, heart attacks, strokes, lung disease and preterm birth,' Fonda said.