The Refugia habitability index is a composite raster built by classifying seven 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). A downstream risk index then crosses habitability with gridded population to surface where exposure to declining habitability is greatest.
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. The classification is intentionally inverted: regions with BII ≥ 70% (high ecological integrity) are marked unsuitable for development to preserve biodiversity refugia; BII < 40% is suitable; 40–70% is marginal.
The seven 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 who is exposed to its decline. The risk index crosses unsuitability (1 − habitability) with a normalized population layer, so a cell scores high only when both many people live there and habitability is poor. Population is loaded from WorldPop's 1 km global mosaic (unconstrained 2020) for the current epoch, with optional WorldPop FuturePop SSP projections for 2100; absent a future projection, today's population distribution is reused under future climate as a “static-population” exposure baseline. Both inputs are min-max normalized to [0, 1] before multiplication, then the product is min-max normalized again so the output ramp spans the full red-yellow scale regardless of epoch.