Temperature Sentinels Global Project: Global patterns in lake thermal structure show diverging trends in deepwater temperature and thermocline depth

In the first phase of the Temperature Sentinels Project of the Climate Sentinels working group, two small lakes of differing transparency were examined to understand which aspects of full temperature profiles (including stratification) were most responsive to a range of climate variables (temperature, precipitation, acid recovery, solar radiation, wind, and others; Pilla et al. 2018 Journal of Geophysical Research: Biogeosciences, 123: 1651–1665). This global project extends this work by analyzing long-term temperature profile data from 100 lakes around the world, spanning 18 countries in 5 continents. These data are being used to assess trends in lake thermal structure, with specific focus on deepwater temperatures and thermal stratification. We focus on lake thermal trends during two time periods, 1970-2009 and 1990-2009, where lake temperature data are most available. The specific questions that we are addressing are:  (1) How has thermal structure, particularly hypolimnion temperature and thermal stratification, changed in lakes across the world? (2) What geographic, morphometric, and water quality variables best explain the trends in each metric across the suite of lakes? As expected, most lakes show epilimnion warming and increases in strength of stratification, with the most consistent trends in northeastern North America and northern Europe. However, trends in hypolimnion temperature and thermocline depth show high variation in both the direction and magnitude even in lakes that are very near to one another. We are using random forest analysis to understand the importance of geographic, morphological, and water quality variables in predicting the observed trends in thermal structure. We envision several projects stemming from this first set of research questions, including understanding how lakes have responded to regional climate patterns and changes in seasonal lake temperature patterns and phenology.