The methods behind the development of the Global One-Eighth Degree Population Base Year and Projection Grids Based on the SSPs, v1.01 (2000 – 2100) are in the peer-reviewed open access journal article:
Jones, B., and B. C. O’Neill. 2016. Spatially explicit global population scenarios consistent with the Shared Socioeconomic Pathways. Environmental Research Letters, 11: 084003. https://doi.org/10.1088/1748-9326/11/8/084003
The spatial population projections were conducted on a country-by-country basis using future scenarios of population growth and urbanization that were developed as part of the Shared Socioeconomic Pathways (SSPs). A short summary of the scenarios behind the SSPs is available through a Population-Environment Research Network cyberseminar expert statement by Leiwen Jiang entitled "Internal consistency of assumption and projection of demographic factors in the SSPs scenarios". Additional information is available via the International Institute for Applied Systems Analysis (IIASA) SSP Database.