The Refugia habitability index is a composite raster built by classifying six environmental factors into three-class suitability masks (suitable, marginal, unsuitable) and combining them multiplicatively. A single failing factor drives the composite toward zero, ensuring that regions must satisfy all criteria simultaneously. Below, each factor is shown as its continuous input dataset alongside its reclassified index mask. The flooding factor combines projected sea-level rise with WRI Aqueduct 100-year riverine flood depth, and applies to all three epochs (current, 2100, 2300). Habitability is then crossed with two asset surfaces — gridded population and the Biodiversity Intactness Index — to produce a pair of risk rasters: Human Risk (people in declining habitability) and Biodiversity Risk (intact ecosystems becoming climate refugia).
Terrain steeper than 6° is classified as unsuitable for settlement and agriculture; slopes between 4° and 6° are marginal. Derived from NASA SRTM 30 m elevation data, resampled to the analysis grid. Topography is time-invariant across all epochs.
Soil quality classes from SoilGrids are reclassified into suitable (high fertility), marginal (moderate), and unsuitable (non-soil, water, permafrost). For 2100 and 2300 epochs, permafrost zones projected to thaw under CMIP6 SSP5-8.5 mean annual temperature are reclassified as marginal rather than unsuitable.
Annual runoff / water balance from the WRI Aqueduct dataset. Regions with runoff below 9 × 10⁶ m³ are unsuitable; above 5 × 10⁹ m³ are suitable. Water availability is held constant across time periods in the current model.
Wet-bulb temperature is computed from CMIP6 daily maximum temperature and relative humidity using the Stull (2011) approximation. Under SSP5-8.5, regions exceeding 35 °C wet-bulb are physiologically lethal; thresholds are 26 °C (current) and 31 °C (2100/2300) for the suitable boundary, with intermediate values classified as marginal.
The maximum number of consecutive dry days (CDD) per year from CMIP6 projections under SSP5-8.5. Regions with ≤ 63 CDD are suitable; 64–171 are marginal; above 171 are classified as unsuitable, representing extreme drought stress incompatible with rainfed agriculture.
Two flood hazards are unioned into a single mask: a cell is unsuitable if either it falls below a projected sea-level-rise threshold or the WRI Aqueduct 100-year riverine inundation depth exceeds 0.5 m. The 2100 epoch uses a 5 m SLR scenario; the 2300 epoch uses 65 m (total ice-sheet melt: Antarctica ~58 m + Greenland ~7 m + other glaciers). The current epoch has no SLR component but still applies the riverine layer. Elevation is the SRTM DEM used for topography. Riverine depth is the per-pixel ensemble maximum across four CMIP5 GCMs (NorESM1-M, GFDL-ESM2M, HadGEM2-ES, IPSL-CM5A-LR) for the RCP 8.5 / 2080 horizon, with the historical (WATCH) baseline used for the current epoch and 2080 reused as a proxy for 2300.
The Biodiversity Intactness Index (BII) from the PREDICTS database measures how much original biodiversity remains in each cell as a fraction of its pre-industrial baseline. As of the 2026-05 methodology refactor, BII no longer feeds the multiplicative habitability composite — instead it is treated as a static asset surface and combined with habitability downstream to produce the Biodiversity Risk raster, which surfaces intact ecosystems whose climate is becoming a refugium and that may therefore face human encroachment.
The six factor masks are multiplied element-wise. Because each mask uses values of 0 (unsuitable), 1 (suitable), or 2 (marginal), the product encodes which combination of factors constrain each cell. A product of zero (any single unsuitable factor) maps to a habitability score of 0.0; all-suitable maps to 1.0. Intermediate products are mapped to a continuous red–yellow–green color ramp. Finally, ocean and inland water bodies are masked out using slope-derived water detection.
The composite habitability raster describes where the planet remains liveable, but not what stands to lose by its decline. We cross habitability with two static 2020 asset surfaces — gridded population and the Biodiversity Intactness Index — to produce a pair of risk rasters. Human Risk = (1 − habitability) × population, surfacing where many people live in places whose climate is becoming uninhabitable. Biodiversity Risk = habitability × BII, surfacing intact ecosystems whose climate is becoming a refugium for displaced humans, and which may therefore face encroachment. Both asset surfaces are held at their 2020 baseline since no future projections are currently in scope; the change in risk through 2100 and 2300 is therefore driven entirely by climate-driven changes in habitability.