I read this book in my teens (90s) so it's no newer than say 1995. The plot revolved around an older woman on the west coast of America (possibly San Franciso or San Deigo) during a time when climate change/global warming has made life difficult for humanity. Climate change progressed at an "exponential rate" (I remember that specific phrase from the book) and due to a lack of resources, many people live together in quasi-communes and grow as much as they can in gardens.
Here's the most distinctive thing I remember: It is discovered that humans have the genetic potential to care about the environment (by increasing their sensitivity to it or something like that), and the protagonist somehow became aware of or involved in a conspiracy or plot to create and release a virus that will switch this gene on in every human. The result will be a population that will better steward the planet and not greedily consume it. The term "The Green Man" was relevant in some way; I think as the inspiration for the eco-terrorists to create the virus, or maybe as a code for recognizing those in on the project. I also believe this book was penned by a female author, although I may be mistaken.