Excess joint heat exposure rose between distant reef regions from 1986–2005 to 2006–2025

We tested joint annual heat exposure in 194 fixed reef regions with NOAA Coral Reef Watch data. The sample had 53,605 complete-record pixels and 18,276 region pairs with centers at least 1,000 km apart. Exposure meant annual maximum Degree Heating Weeks of at least 4 degrees C-weeks. We compared observed joint exposure with an independent-year reference that kept each region's local exposure history. Joint exposure was 0.534% in 1986–2005 and 9.480% in 2006–2025. The reference gave 0.366% and 6.414%. The excess thus rose by 2.897 percentage points, with a nominal 95% space and shared-year-block resampling interval of 0.743 to 5.217. Separate local linear-trend removal reduced the change to 1.278 points, with an interval of 0.230 to 2.307. Planned controls for distance, region size, threshold, weights, January maxima and recent years gave the same sign. The probability ratios were similar: 1.46 and 1.48. Thus, the main finding is a larger absolute gap above independence. Annual surface heat exposure does not measure coral loss or exposure on the same day. The package has compact source data, full provenance, a source rebuild, code, all results and three figures. A clean run reproduced 22 outputs.

coral reefs · joint heat exposure · spatial covariance · Degree Heating Weeks · conservation risk · reproducible data