From Wikipedia
Six Degrees: Our Future on a Hotter Planet is a non-fiction book by author Mark Lynas about global warming. The book looks and attempts to summarize results from scientific papers on climate change.
The first chapter describes the expected effects of climate change with one degree (°C) increase in average global temperature since pre-industrial times.
The second chapter describes the effects of two degrees average temperature and so forth until Chapter 6 which shows the expected effects of six degrees (°) average global temperature. The effects are also compare to paleoclimatic studies, with six degrees of warming compared back to the Cretaceous.[citation needed]
Special coverage is given to the positive feedback mechanisms that could dramatically accelerate climate change. The book explains how the release of methane hydrate and the release of methane from melting permafrost could unleash a major extinction event. Carbon cycle feedbacks, the demise of coral, the destruction of the Amazon rainforest, and extreme desertification are also described, with five or six degrees of warming potentially leading to the complete uninhabitability of the tropics and subtropics, as well as extreme water and food shortages, possibly leading to mass migration of billions of people.