Six Decades of Thermal Change in a Pristine Lake Situated North of the Arctic Circle

AbstractThe majority of lake temperature studies have investigated climate‐induced changes occurring at the lake surface, primarily by analyzing detailed satellite images of surface water temperature. Whilst essential to observe long‐term change, satellite images do not provide information on the thermal environment at depth, thus limiting our understanding of lake thermal responses to a warming world. Long‐term in situ observational data can fill some of the information gap, with depth‐resolved field measurements providing a detailed view of thermal change throughout the water column. However, many previous studies that have investigated multi‐decadal changes in lake temperature, both at the surface and at depth, have typically focused on north temperate lakes. Relatively few studies have investigated temperature variations in lakes situated north of the Arctic Circle, which is one of the most rapidly warming regions on Earth. Here, using a 60‐year (1961–2020) observational data set of summer water temperature (July–September) from Lake Inari (Finland), we investigate changes in the thermal environment of this pristine lake. Our analysis suggests a statistically significant summer warming trend at the lake surface (+0.25°C decade−1, p‐value <0.1), whilst deepwater temperatures remain largely unchanged. This contrasting thermal response of surface and bottom water temperature to climatic warming has likewise resulted in a strengthening of summer stratification in this high latitude lake. Implications of the observed change in both temperature and stratification on the lake ecosystem will likely be extensive, including impacts on aquatic organisms which this lake supports. Our work builds on the ever‐growing literature regarding lake thermal responses to climate change.