Lake Superior summer surface water did not continue its 1979-2006 warming trend in 2007-2025; winter-to-summer air temperature predicted these years and ice cover added no skill (NDBC buoys)

Austin and Colman (2007) reported that Lake Superior July-September surface water warmed about 1.1 C per decade in 1979-2006, about two times faster than regional air, and gave an ice-albedo explanation. We did an out-of-sample test with the same three NDBC buoys, six buoys on Lakes Michigan, Huron, Erie and Ontario, GLERL daily ice cover and GHCN-CAMS basin air temperature. All models were fitted on 1979-2006 and tested on 2007-2025 (18 Superior seasons). We reproduced the 1979-2006 trend (+1.16 C per decade, 95% CI +0.24 to +2.07). In 2007-2025 the trend was +0.43 (-0.75 to +1.60), and the extrapolated trend line overpredicted the test mean by 1.58 C (RMSE 2.84 C, against 2.58 C for the training mean). A linear model with December-March, April-June and July-September air temperature (coefficient sum 2.27 C per C) predicted the test years with RMSE 1.20 C and bias +0.01 C, 1.63 C (95% CI 0.99 to 2.27) better than the trend line. With air temperature in the model, Jan-Apr ice cover (r = -0.86 with winter air) had no effect (-0.001 C per percentage point, 95% CI -0.033 to +0.032) and did not improve test or cross-validated error. The air model also explained the 1979-2025 trend (residual year term +0.06 C per decade). Michigan, Huron and Erie gave the same forecast result; Michigan and Huron showed a small in-sample ice effect (about -0.2 C per SD) that cross-validation did not clearly confirm. Limitations: short test record, point buoys near the surface, observed (not forecast) air in the test, and ice and winter air change together, so the test cannot separate an ice-albedo path from other winter heat-loss paths.

Lake Superior · Great Lakes · lake surface temperature · ice cover · ice-albedo feedback · out-of-sample test · NDBC buoys · GLERL · replication · climate warming · stratification onset · forecast verification