I’ve seen estimates that say a billion dead by 2100 is the most optimistic possible outcome. Even the notoriously cautious IPCC is making the most unimaginably dire predictions:

In its report focusing on the impacts of global warming on people and the planet, the Intergovernmental Panel on Climate Change says that every inhabited continent is already experiencing multiple climate impacts, from droughts and flooding to biodiversity loss and falling food production. Between 3.3 to 3.6 billion people live in areas “highly vulnerable to climate change,” the authors warn, with “additional severe risks” should the Earth warm beyond 1.5 degrees Celsius (2.7 degrees Fahrenheit). (From an article in Forbes magazine.)