Climate scientists use "proxy data" to study climates of the past, before humans with thermometers began keeping temperature records. These "proxies" include tree rings, layers within ice cores pulled from glaciers and ice sheets, growth layers in coral, and layers of sediments from the bottoms of lakes and oceans.
Debris that flows into a lake gradually settles to the bottom to add to the layers of sediments there. Climate scientists can study these layers of sediments for clues about past climates. The thickness of sediment layers tells us about the rate of flow of water into the lake, which helps us learn about precipitation rates in the region at various times in the past. The amount and types of organic debris in the sediments can help us determine the types of plants that grew nearby, which gives us another insight into the local climate of the past. Scientists use special tools to extract sediment cores from lakebeds, which they chemically analyze back in their labs.
The video clips in this video are from the "To What Degree? What Science is Telling Us about Climate Change" web site developed by the National Science Foundation.