Lake mud builds a year-by-year diary of ancient weather
Deep beneath the surface of Swedish and North American lakes, alternating bands of light and dark sediment act as a high-resolution climate record stretching back thousands of years.
Deep inside the quiet basins of glacial lakes, time is recorded in a rhythmic pulse of mud. These annual layers, known as varves, function like the pages of a diary written by gravity and the seasons. During the energetic spring and summer, rushing meltwater carries coarse, light-colored grains into the lake. When winter arrives and ice seals the surface, the water turns still, allowing the finest, darkest clays to slowly drift to the bottom. This predictable cycle creates a distinct pair of stripes for every single year, allowing researchers to count back through time with the precision of a stopwatch.