In the report, titled Rings of Fire, it is argued that summer heatwaves in Tokyo during the past three years indicate that conditions will be tough.
Concerns over heat have caused the marathon to be moved from Tokyo.
Professor Mike Tipton said he expects Tokyo to be the most "thermally stressful Olympics" of recent times.
The University of Portsmouth professor helped to produce the report, which was backed by athletes, the British Association for Sustainability in Sport (Basis) and scientists from the Priestley International Centre for Climate at Leeds University.