This lake is fed by glaciers in the Ahklun Mountains. Glacier meltwater enters the lake on the left in this picture. The amount of sediment delivered to the lake can vary with the intensity of the summer melt season and is registered in sediment cores through changes in depositional rates and other indicators. Sediment cores were taken as part of a study, supported primarily by the National Science Foundation, that indicates arctic temperatures suddenly increased during the last 50 years of the period from A.D. 1 to the year 2000. Because this warming occurred abruptly during the 20th century while atmospheric greenhouse gases were accumulating, these findings provide additional evidence that humans are influencing climate.