Past Climate
Some natural processes record the passage of time: for instance, layers of sediments accumulate over many years, and some of them preserve information about weather conditions during the time they were deposited. Throughout Earth's history, global and regional climate has changed on very long time scales, and many of the past conditions can be discovered by studying natural records.
How Do We Study Past Climates?
Paleoclimatology is the study of climate records from hundreds to millions of years ago. Information for paleoclimate studies come from natural records rather than instruments: these indirect records of climatic conditions are called proxy records.
Coring a tree to gain access to tree ring data.
Image source: Penn State Department of Meterology
One of the most easily recognized type of paleoclimate records is tree ring data. Trees that grow a single annual ring can preserve a record of the conditions they experienced each year. Rocks deposited by glaciers are another example of proxy records. Scientists can recognize the distinctive pattern of sediments in rocks formed by glaciers. Wherever they find glacial rocks then, they know that glaciers were present in that location at some point in the past. Once they examine fossils and other clues about when the rocks were deposited, they use the information to reconstruct the climate story for that location. Other sources of proxy data for climate include lake and ocean sediments, layers of ice (cored from ice sheets), corals, fossils, and historical records from ship logs and early weather observers.
How Do Scientists Interpret Natural Records?
Paleoclimate records come from all around the world – from the tops of mountains, to the bottom of the ocean, from the tropics to the poles.
Scientists use a variety of methods to access and analyze climate proxies. The most familiar methods involve taking a core sample of tree rings, corals, sediments, and ice. By measuring the width, chemical composition, and physical structure of each layer, scientists can deduce the conditions present when each layer formed. Examples of paleoclimate data »
Another way to learn about past climate is to take the temperature of rocks at different depths by mounting thermometers at different depths within boreholes drilled directly into the Earth’s crust. Rocks respond very slowly to changes in temperature conditions, and deeper rocks change temperatures more slowly than shallower rocks. Precise measurements of the rate of temperature change of rocks at various depths can be correlated to past temperatures at the surface.
More about boreholes, including links to borehole data:
NOAA Borehole data »
University of Michigan Borehole Information »
What Can I Do with Paleoclimate Data? What is it Good For?
Records of past climate provide context for present climate. For instance, one might ask, “How does the drought in the western United States this year compare with droughts of the past?” By analyzing tree rings and sediments from lakes and dunes, scientists can reconstruct the history of drought in the region, and compare the severity and extent of present conditions to droughts of the past. These sorts of comparisons are very valuable for water and other resource managers when they plan for the future.
Descriptions of past climates deduced by scientists can be found at NOAA Paleoclimate »
Climates of the Past
Paleoclimate records, collected by independent science teams worldwide, become even more valuable when plotted and compared with one another. As the following graphs of past temperatures indicated by various methods show, common patterns of temperature trends are broadly consistent across a variety of data collection methods. By comparing multiple sets of proxy records, scientists have been able to reconstruct a fairly consistent story of Earth’s climate for the past few thousand years. While the methods of calculating global average temperature from different proxies vary from team to team, the outcomes are broadly consistent over time, and they converge with the instrumental record. The broad agreement of several datasets increases our confidence that proxies reveal valid temperature records. The set of charts below shows the general agreement among results from multiple science teams using different methods to understand how temperatures have changed over time.
Northern Hemisphere Climate Reconstruction. Image source: IPCC AR4 WG 1 figure 6.10.
References and Further Information
Learn more about methods of climate reconstruction:
Climate Change: Past, Present, and Future »
More information and access to datasets
NOAA Paleoclimatology »
Learn more about the tools and methods that are used in past climate research
Paleoclimate Education and Outreach »