Scientists agree that some human activities, like burning fossil fuels, cause the Earth’s atmosphere to trap more heat, leading to climate change. We wanted to find out how climate change affected large freshwater lakes, so we took a series of sediment cores in Lake Superior (the biggest of the Great Lakes in North America) to study samples of the mud at the lake bottom. Back in the lab, we analyzed tiny bits of dead plants and algae trapped within the sediments. From these samples, we could see that the amount of algae in Lake Superior has slowly increased over time. However, in the past hundred years, the amount of algae and plants in the Lake has increased dramatically. We think this recent drastic change happened because human-caused climate change led to warmer temperatures and longer ice-free periods in the lake. This made it easier for algae and plants to grow in the water.