Paleoclimate archives consist of geologic (e.g., sediment cores) and biologic (e.g., tree rings) materials that preserve evidence of past changes in climate. They contain substances or features (climate proxies) that can be sampled and analyzed using a variety of physical and chemical methods. Using results from the analyses of proxies in archives, scientists reconstruct changes through time and use them to infer patterns of climate and environmental change at the sample site. When a number of individual reconstructions are combined they provide a picture of regional and global climate change through time.
Paleoclimate records are preserved in marine, aquatic, and terrestrial settings from around the world. The time period preserved in a given material varies, as does the resolution with which it can be analyzed. Some materials provide centennial to millennial-scale resolution climate records that collectively cover much of Earth's history, while others provide relatively short records that can resolve monthly to inter-annual climate variability.
Examples of paleoclimate archives include:
Sediment is deposited in layers in lakes, wetlands, estuaries, oceans, and on land. Sedimentary records collected through coring or geologic outcrops represent the most commonly studied paleoclimate archive. Because the oldest sedimentary rocks are about 3.9 billion years old, sediment records provide a means to study past climates throughout most of Earth's history. Combining paleoclimate records with climate models enables researchers to reconstruct and understand the mechanisms underlying climate change. Because climate models are based on what we know about modern climate, paleoclimate records also provide the opportunity to test and improve model capabilities to simulate climates that differ from today.
Sediment is transported by wind, water, ice, and, less commonly, biologic agents. Within the sediments, various proxies for past climate and environment are preserved, such as pollen and plant remains, molecular fossils, algae, charcoal, and planktic and benthic organisms.
Each year, snow falls on ice sheets in Greenland and Antarctica and on mountain glaciers throughout the world. Through time, these annual layers have accumulated to thicknesses of up to 4.8 kilometers (more than 2 miles) on the West Antarctic Ice Sheet and more than 3 kilometers (1.9 miles) on the Greenland Ice Sheet. Scientists collect ice cores from ice sheets and glaciers to study yearly changes in temperature, precipitation, atmospheric composition, volcanic activity, and wind patterns. Ice cores span up to the last 800,000 years and provide quantitative data on the long-term natural variability of atmospheric greenhouse gases that can be compared with post-Industrial changes.
Trees can grow for hundreds and, in some cases, even thousands of years. Each year, trees add a new layer of growth, known as a tree ring. These rings can be used to determine how long the tree lived, and they also record changes in temperature and precipitation, as well as more localized changes such as fire, insect attacks, and earthquakes. Changes in the thickness of tree rings result from seasonal and annual changes in water availability during the growing season. By analyzing patterns of tree-ring thickness, scientists have reconstructed annual variability in moisture and temperature over the last 14,000 years.
Speleothems are cave formations that result from deposition of minerals from groundwater. There are many forms of speleothems, including stalactites that hang from cave ceilings, stalagmites that grow upward from the cave floor, sheet-like flowstones, and other deposits. As the water flows into the cave, thin layers of minerals, typically calcium carbonate, are deposited. The thickness of these layers depends on the amount of water entering the cave. Very thin layers may indicate drought conditions, and thicker layers may indicate periods of high precipitation. Scientists analyze the thickness and chemical composition of speleothem layers to reconstruct climate changes over thousands to hundreds of thousands of years.
Corals are marine animals with a calcium carbonate skeleton that live in warm, shallow, well-lit ocean waters. Although some corals live as individuals, they typically live in colonies and are the main reef builders in modern oceans. Solitary corals are found in rocks more than 500 million years old, and the first extensive coral reef systems appeared around 410 million years ago.
Within the carbonate skeleton of corals, there are annual growth bands, with lighter-colored layers deposited during the summer and darker layers forming in the winter. Scientists analyze the chemical composition of each layer to reconstruct monthly, annual, and longer records past temperature and salinity. Although corals may preserve relatively short paleoclimate records, their ability to provide monthly and seasonal time resolution allows reconstruction of relatively short-term phenomena such as El Niño and La Niña events that occur on time scales of years and decades.
Packrats, which occur from the Northwest Territories in Canada southward to Nicaragua, have a unique habit of collecting items near their dens. These items can include plants, rocks, bones, insects, and other artifacts. There, the material is piled up into a refuse heap and the packrat urinates on it, binding and preserving the material in a packrat midden, The oldest known middens are 45,000 years old, preserving plants and animals that lived before the last Ice Age. Scientists identify and determine the age of fossilized material in packrat middens to reconstruct changes in local plant and animal communities and climate through time.