A Persistent Warm West in the Sundarbans of Bangladesh: Spatial Contrasts Are Recoverable Where Multi-Decadal Trends Are Not

The Bangladesh Sundarbans is the largest continuous mangrove forest in the world, and the satellite thermal record over it is now long enough to invite decadal analysis. We ask a prior question: which parts of that thermal signal can the archive actually resolve? Using 25 years of MODIS April composites, we find the western third of the forest persistently warmer than the eastern third, by 0.654°C on average (95% confidence interval +0.428 to +0.880°C, paired p = 3.6 × 10 -6 ) and in 23 of the 25 individual years. The west is also warmer than the centre in 24 of 25 years (+0.463°C), while centre and east are not reliably separable (+0.191°C, p = 0.052): the pattern is a warm west against a uniform remainder, not a gradient across the forest. The same record cannot support a trend. Sen’s slope for the whole forest is +0.039°C per year, with a confidence interval of −0.037 to +0.101 that contains zero, as does every sector interval; a Landsat 1995 to 2025 difference of +1.36°C rests on three sensor generations and on 13 against 29 contributing scenes. The asymmetry is structural rather than statistical: a contrast measured inside one composite shares its instrument, acquisition time, quality filter and mask on both sides, so those errors cancel, whereas a comparison between composites shares none of them. Two things follow. The western Sundarbans is repeatedly and measurably the warmest part of this forest, which is a target conservation can act on; and a record that cannot support a trend may still support a spatial result, so the two should not be reported with equal confidence.