Deeper waters are changing less consistently than surface waters in a global analysis of 102 lakes

Globally, lake surface water temperatures have warmed rapidly relative to air temperatures, but changes in deepwater temperatures and vertical thermal structure are still largely unknown. We compiled a long-term (1970–2009) summertime dataset of vertical temperature profiles from 102 lakes worldwide to examine trends and drivers of whole-lake vertical thermal structure. Surface water temperatures increased significantly at an average rate of +0.37 °C decade⁻¹, and water column stability increased (+0.08 kg m⁻³ decade⁻¹). In contrast, deepwater temperatures showed little change on average (+0.06 °C decade⁻¹) but exhibited high variability across lakes (−0.68 to +0.65 °C decade⁻¹). Variability in deepwater trends was not explained by surface temperature or stability trends within lakes, and only 8.4% was explained by lake thermal region or local characteristics in a random forest analysis. These results suggest external drivers beyond the tested lake characteristics—such as local to regional climate patterns or other anthropogenic influences—are important in explaining long-term trends in lake thermal structure.