The global thermal unit (GTU) is a unit of temperature used when taking the mean temperature of a large body such as a planet. It is a refinement of the Celsius scale.

On 28 December 2143, in a report to the NAU Science Council, Doctor Kristeen Ballard gave an upbeat assessment of progress in the terraforming of Mars, reporting that the process was ahead of schedule. Volcanic activity had spread across the Olympus Mons region north and west of Habitat Mars. She also reported that the mean planet-wide temperature had climbed nearly 50 GTUs. As a result, the Martian icecaps had begun to melt, releasing water and carbon dioxide into the atmosphere.