Paleoenvironmental Reconstruction - What Was Climate Like in the Past?

How Do Scientists Know That Past Climates Were Different Than Today?

Greenland: A Laboratory For The Symptoms Of Global Warming
Professor David Noone from the University of Colorado uses a snow pit to study the layers of ice in the glacier at Summit Station on July 11, 2013 on the Glacial Ice Sheet, Greenland. Joe Raedle / Getty Images

Paleoenvironmental reconstruction (also known as paleoclimate reconstruction) refers to the results and the investigations undertaken to determine what the climate and vegetation were like at a particular time and place in the past. Climate, including vegetation, temperature, and relative humidity, has varied considerably during the time since the earliest human habitation of planet earth, from both natural and cultural (human-made) causes.

Climatologists primarily use paleoenvironmental data to understand how the environment of our world has changed and how modern societies need to prepare for the changes to come. Archaeologists use paleoenvironmental data to help understand the living conditions for the people who lived at an archaeological site. Climatologists benefit from the archaeological studies because they show how humans in the past learned how to adapt or failed to adapt to environmental change, and how they caused environmental changes or made them worse or better by their actions.

Using Proxies

The data that are collected and interpreted by paleoclimatologists are known as proxies, stand-ins for what can't be directly measured. We can't travel back in time to measure the temperature or humidity of a given day or year or century, and there are no written records of climatic changes that would give us those details older than a couple of hundred years.

Instead, paleoclimate researchers rely on biological, chemical, and geological traces of past events that were influenced by the climate.

The primary proxies used by climate researchers are plant and animal remains because the type of flora and fauna in a region indicates the climate: think of polar bears and palm trees as indicators of local climates.

Identifiable traces of plants and animals range in size from whole trees to microscopic diatoms and chemical signatures. The most useful remains are those that are large enough to be identifiable to species; modern science has been able to identify objects as tiny as pollen grains and spores to plant species.

Keys to Past Climates

Proxy evidence can be biotic, geomorphic, geochemical, or geophysical; they can record environmental data that range in time from yearly, every ten years, every century, every millennium or even multi-millennia. Events such as tree growth and regional vegetation changes leave traces in soils and peat deposits, glacial ice and moraines, cave formations, and in the bottoms of lakes and oceans.

Researchers rely on modern analogs; that is to say, they compare the findings from the past to those found in current climates around the world. However, there are periods in the very ancient past when the climate was completely different from what is currently being experienced on our planet. In general, those situations appear to be the result of climate conditions that had more extreme seasonal differences than any we've experienced today. It is particularly important to recognize that atmospheric carbon dioxide levels were lower in the past than those present today, so ecosystems with less greenhouse gas in the atmosphere likely behaved differently than they do today.

Paleoenvironmental Data Sources

There are several types of sources where paleoclimate researchers can find preserved records of past climates.

  • Glaciers and Ice Sheets: Long-term bodies of ice, such as the Greenland and Antarctic ice sheets, have annual cycles which build new layers of ice each year like tree rings. Layers in the ice vary in texture and color during warmer and cooler parts of the year. Also, glaciers expand with increased precipitation and cooler weather and retract when warmer conditions prevail. Trapped in those layers laid down over thousands of years are dust particles and gases which were created by climatic disturbances such as volcanic eruptions, data which can be retrieved using ice cores.
  • Ocean Bottoms: Sediments are deposited in the bottom of the oceans each year, and lifeforms such as foraminifera, ostracods, and diatoms die and are deposited with them. Those forms respond to ocean temperatures: for example, some are more prevalent during warmer periods.
  • Estuaries and Coastlines: Estuaries preserve information about the height of former sea levels in long sequences of alternating layers of organic peat when the sea level was low, and inorganic silts when the sea level rose.
  • Lakes: Like oceans and estuaries, lakes also have annual basal deposits called varves. Varves hold a wide variety of organic remains, from entire archaeological sites to pollen grains and insects. They can hold information about environmental pollution such as acid rain, local iron mongering, or run-offs from eroded hills nearby.
  • Caves: Caves are closed systems, where average annual temperatures are maintained year-round and with a high relative humidity. Mineral deposits within caves such as stalactites, stalagmites, and flowstones gradually form in thin layers of calcite, which trap chemical compositions from outside the cave. Caves can thus contain continuous, high-resolution records which can be dated using uranium series dating.
  • Terrestrial Soils: Soil deposits on land can also be a source of information, trapping animal and plant remains in colluvial deposits at the base of hills or alluvial deposits in valley terraces.

Archaeological Studies of Climate Change

Archaeologists have been interested in climate research since at least Grahame Clark's 1954 work at Star Carr. Many have worked with climate scientists to figure out the local conditions at the time of occupation. A trend identified by Sandweiss and Kelley (2012) suggests that climate researchers are beginning to turn to the archaeological record to assist with the reconstruction of paleoenvironments.

Recent studies described in detail in Sandweiss and Kelley include:

  • The interaction between humans and climatic data to determine the rate and extent of El Niño and the human reaction to it over the last 12,000 years of people living in coastal Peru.
  • Tell Leilan in northern Mesopotamia (Syria) deposits matched to ocean drilling cores in the Arabian Sea identified a previously-unknown volcanic eruption that took place between 2075-1675 BC, which in turn may have led to an abrupt aridification with abandonment of the tell and may have led to the disintegration of the Akkadian empire.
  • In the Penobscot valley of Maine in the northeastern United States, studies on sites dated to the early-middle Archaic (~9000-5000 years ago), helped establish a chronology of flood events in the region associated with falling or low lake levels.
  • Shetland Island, Scotland, where Neolithic-aged sites are sand-inundated, a situation believed to be an indication of a period of storminess in the North Atlantic.