To understand the potential range and effect of future climate and how its changes may impact marine and terrestrial systems and society, scientists use instrumental records that are at most a few hundred years old and longer geologic records that extend back over thousands and millions of years. Reconstructed records of paleoclimate provide crucial insights into potential rates and magnitudes of climate change, warm and cold extremes that lasted for thousands of years, and large sea level changes.

What is paleoclimatology?

Paleoclimatology is the study of Earth's climate during Earth's entire history. Paleoclimate research uses geologic and biologic evidence (climate proxies) preserved in sediments, rocks, tree rings, corals, ice sheets and other climate records to reconstruct past climate in land and water environments globally. Paleoclimate reconstructions provide evidence for baseline levels of climate and environmental variability prior to humans using instruments to measure different aspects of climate and weather.

How far back in Earth's history can paleoclimate be reconstructed?

Paleoclimate research covers the history of Earth. Studies that focus on the last few centuries to millennia provide high-resolution temporal reconstructions of temperature and precipitation that provide a basis for measuring and understanding natural climate variability. Studies that examine the past tens-of-thousands to millions of years show climate change and variability associated with the rotation and alignment of the Earth in relation to the Sun and variations in greenhouse gases that controlled the timing of ice ages, abrupt changes related to changes in ocean circulation, and geologic processes such as mountain uplift. "Deep-time" paleoclimate studies (prior to ~2.6 million years ago) provide a way to understand extreme climate states and long-term patterns of atmospheric CO2 and climate.

How is past climate reconstructed?

Past climates are reconstructed from a variety of geologic and biologic archives that preserve climate proxies, or evidence of past climate and environment. Examples of archives include terrestrial or aquatic sediments, ice cores from glaciers and ice sheets, tree rings or corals. These archives contain climate proxies, i.e. physical, chemical, or biological features that provide information on past climate and environment (such as sea level, air and ocean temperature, atmospheric composition, and precipitation).

How do we know the time period represented by a paleoclimate record?

Multiple techniques are used to determine the ages of archives and proxies. Usually, dating is used to establish the time of onset, termination, and rate of change of climate events. Many dating techniques employed are based on analyzing the nature of radioactive isotopes (e.g. radiocarbon, uranium-thorium) present in sample material. These dating techniques are used in addition to other methods such as biostratigraphy (which uses fossil assemblages contained within a sample to estimate its age) and counting tree rings or annual sediment layers deposited in ice and lakes. Other methods such as surface exposure dating techniques are used to estimate the amount of time a sample material such as a boulder deposited by an ice sheet or shoreline has been exposed on the Earth's surface to cosmic rays. Whenever possible, scientists employ more than one dating method in order to maximize accuracy and precision of their findings.

How can paleoclimate studies help us better understand potential consequences of future climate change?

All components of the Earth system affects or is affected by climate. Ecosystems, water availability, carbon cycling, sea level rise, ocean circulation, and ocean acidification all interact with the climate system and react to changes in climate. Paleoclimate studies provide an essential perspective for assessing the potential impacts of future climate on natural systems (U.S. Geological Survey, 2019).

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