Researchers with the University of California analyzed climate and water-temperature data from 19 lakes scattered throughout the Sierra Nevada, including Emerald Lake in Sequoia-Kings Canyon National Park, where UC runs a long-term study site. They found that summer air temperatures at Emerald Lake are warming 1.5 degrees Fahrenheit per decade — a rate lead author Steven Sadro says is as high as nearly anywhere on the planet. Yet to the researchers’ surprise, says Sadro, an assistant professor of Environmental Science and Policy, that alone wasn’t enough to drive a similarly steady increase in water temperature on par with what has been observed in other lakes around the world. Instead, the most important factor in water temperature was snowpack. That’s because the level of snow determines when the lake becomes free of ice and can absorb radiation from the sun, which heats the water. “We would’ve expected to see a constant trend of rising lake temperature through time,” Sadro says. “What really popped out in our study was that we saw this linear trend of lake temperature with time that’s only visible in dry years. So we have this pattern where the Sierra is warming, but that pattern is being obscured by the high degree of variability that we see in snow.” This discovery, along with additional research into how non-climactic factors like depth and size influence lake warming, could help point scientists toward the most sensitive lakes, and thus the best targets for future conservation efforts.