Martine Weitzman writes on PBS.org:
The key links in the chain connecting increased atmospheric CO2 concentrations to global well-being are the following. Increased CO2 concentrations lead to increased global average temperatures. Increased global average temperatures lead to increased climate changes (and planetary changes, like higher sea levels). Increased climate (and planetary) changes eventually result in increased damages to humans and the planet.
It is critical to recognize that every link of this chain is full of deep uncertainty that makes it very difficult to answer the question: by how much?