The impact amplifies with every degree, and beyond 4 C could be disastrous, said the report.
Climate change could drive turbulence and conflict, prompted by migration from newly uninhabitable areas and jockeying for water and food, it said.
"There are many things that make people vulnerable, and when you combine a climate shock with these factors, you can have bad outcomes," said Chris Field, co-chair of the conference.
"With high levels of warming that result from continued growth in greenhouse gas emissions, risks will be challenging to manage, and even serious, sustained investments in adaptation will face limits."
Rainfall patterns will be disrupted, resulting in a significantly higher flood risk, especially for Europe and Asia -- and magnified drought risks will add to water stress in arid, heavily populated areas, the report said.
This, in turn, will have consequences for agriculture. Yields of staples such as wheat, rice and corn will be squeezed, just as demand will soar because of population growth, it predicts.