Wildfire science tracks dead-fuel moisture content — the percentage of a dead branch, leaf or grass blade's weight that is water — because it is the single strongest predictor of whether a spark catches and how fast it spreads. Fuel exchanges moisture with the air constantly; live vegetation and litter dry out during hot, low-humidity, high-vapor-pressure-deficit (VPD) stretches and rewet during cool, humid ones.
Historically, wetter and shorter dry seasons kept fine dead fuel above roughly 20–30% moisture for most of the year. A well-documented climate trend — longer, hotter dry seasons and rising VPD across regions like the western US, Australia and the Mediterranean — now pushes fuel moisture down toward 6–15% for much longer stretches, and that is exactly the range where flammability rises steeply.
flammability = clamp((M_ext − moisture) / M_ext, 0, 1)²
P(ignite neighbor, per tick) = flammability × baseSpreadRate
burnDuration = lerp(maxBurn, minBurn, flammability)
- M_ext — the moisture of extinction (~30% here): above it, fuel resists ignition almost entirely, mirroring the real extinction-moisture concept used in fire-behavior models like Rothermel's.
- Climate scenario — sets the average fuel moisture and how patchy it is across the stand; switching it re-draws the forest's moisture field.
- Squared response — flammability rises non-linearly as moisture drops below the threshold, which is why a small climate-driven drop in moisture can produce a disproportionate jump in spread rate.
- Trees are colored from green (moist, resistant) through amber to grey-brown (dry, flammable); once alight they flare orange-red, then burn out to black.
Ignite the same spot under both scenarios: in the historical run the fire typically stalls or crawls; in the current-climate run the identical spark can race across the stand — the spark never changed, only the fuel's moisture did.