Temperature Record

Introduction

The temperature record consists of all the millions of atmospheric, soil, and ocean temperature measurements that have been made over the last several centuries. It provides information on how the temperatures of Earth's atmosphere and oceans have changed over time. Widespread and reliable instrument records, such as thermometer-based data, are available only since the 1850s.

In order to study how climatic conditions varied prior to the invention of thermometers and the keeping of reliable science records, paleoclimatologists—scientists who study changes in the climate over the history of Earth—rely on proxy evidence. Proxies are phenomena that record some other phenomenon, such as temperature, indirectly. Temperature proxies include tree rings, corals reefs, ocean and lake sediments, cave deposits, fossils, isotopic ratios in ice cores, boreholes, glacier length records, and other documentary evidence. Instrumental and proxy data show that Earth warmed by roughly 1°F(0.6°C) during the twentieth century, and that the Northern Hemisphere was warmer during this period than at any other time in the past thousand years.

Historical Background and Scientific Foundations

Most instrument-based temperature data have been taken with mercury-in-glass thermometers which can be used down to –38.2°F (–39°C), the freezing point of mercury. For lower temperatures, alcohol is usually substituted for mercury. Recently, digital instruments are favored, both for ease of data collection and because mercury is highly toxic. Using instrumental records to determine large-scale surface temperatures requires a sufficient number of recording sites with a wide geographic distribution. Instrumental temperature records extend back more than 250 years in some locations, such as Europe, but only since the middle of the nineteenth century has there been a sufficient number of observing stations to estimate the average temperature over the Northern Hemisphere or all of Earth.

One important proxy, used for temperature reconstruction before the instrument record, is tree-ring formation. Tree-ring patterns are influenced by climatic conditions, especially where a given tree species is on the edge of its natural habitat. At high latitudes or elevations, tree-ring growth is related to temperature fluctuations with annual and sometimes even seasonal resolution.

Similar to tree rings, annual bands in coral skeletons provide information about climatic conditions at the time that each band was formed. Changes in the chemical composition of the coral reflect the temperature of the water in which it grew. Corals generally live in tropical or subtropical waters, and provide a reliable complement to data gathered from tree rings.

Two more examples of temperature proxies are ice cores and sediment cores. Oxygen isotopes (different forms of atoms of the same element) measured in ice cores cut from glaciers and ice caps can be used to infer the atmospheric temperature at the time when the snow originally fell. Sediment cores taken from the bottom of lakes and oceans can be analyzed to determine the temperature of the water from which various components of the sediment were deposited, which is in turn used to infer the temperature at the surface.

WORDS TO KNOW

ANTHROPOGENIC: Made by people or resulting from human activities. Usually used in the context of emissions that are produced as a result of human activities.

BOREHOLES: Hole or shaft drilled into solid ground or a glacier for the purpose of extracting information rather than some resource (such as petroleum). In climate research, temperature measurements are often made along the height of a borehole: this information can be mathematically processed (deconvolved) to give information about past changes of surface temperature up to several centuries past.

CLIMATE CHANGE: Sometimes used to refer to all forms of climatic inconsistency, but because Earth's climate is never static, the term is more properly used to imply a significant change from one climatic condition to another. In some cases, climate change has been used synonymously with the term, global warming; scientists, however, tend to use the term in the wider sense to also include natural changes in climate.

ICE CORE: A cylindrical section of ice removed from a glacier or an ice sheet in order to study climate patterns of the past. By performing chemical analyses on the air trapped in the ice, scientists can estimate the percentage of carbon dioxide and other trace gases in the atmosphere at that time.

MEDIEVAL WARM PERIOD: Interval from AD 1000 to 1300 during which some parts of the Northern Hemisphere (e.g., Europe) were warmer than during the Little Ice Age, which followed. Global climate, contrary to claims made by some climate skeptics, was not as warm during the Medieval Warm Period as it has been since about 1950.

PALEOCLIMATOLOGISTS: Scientists who study paleoclimate, that is, climate throughout Earth's history up to the beginning of instrumental record-keeping.

SEDIMENT CORE: Cylindrical, solid sample of a layered deposit of sediment, cut out of the deposit at right angles to its bedding planes. Sediment cores have been obtained from the bottoms of the oceans and many large lakes, such as Titicaca in South America and Tanganyika in Africa. Information about past climate changes can be derived from sediment cores, as the types and abundances of dead marine organisms (e.g., algae and foraminifera) in each sediment layer reflect climate conditions at the time the layer was laid down.

TREE RINGS: Marks left in the trunks of woody plants by the annual growth of a new coat or sheath of material. Tree rings provide a straightforward way of dating organic material stored in a tree trunk. Tree-ring thickness provides proxy data about climate conditions: most trees put on thicker rings in warm, wet conditions than in cool, dry conditions.

Data from proxy records in recent history are calibrated against instrument-based temperature records to estimate the relationship between temperature and proxy data. That is, modern proxy data are checked against thermometer readings to increase confidence in more ancient proxy data. The longer history of the proxy is then used to reconstruct temperature from earlier periods.

Impacts and Issues

Starting in the late 1990s, scientists began combining proxy evidence from many different locations in an effort to create large-scale surface temperature reconstructions for the last few hundred to few thousand years. These reconstructions have enabled researchers to estimate past temperature variations over the entire globe, often with enough resolution to examine particular decades or even individual years. This research, especially that of scientists Michael Mann and his colleagues Raymond Bradley and Malcolm Hughes (1998 and 1999), attracted considerable attention because of their conclusion that the Northern Hemisphere was warmer during the late twentieth century than at any other time during the past millennium (thousand years). The graph of their data, often referred to as the “hockey stick graph,” shows the temperature curve over the last 1,000 years as relatively flat (with a hump during the Medieval Warm Period around 1000 AD) until the end of the twentieth century, at which point the curve shoots up, causing the graph to resemble a hockey stick.

Most climate scientists have interpreted the results of Mann and colleagues as definitive evidence of anthropogenic (human-caused) climate change because there is a strong correspondence between recent temperature rise and increased concentration of CO₂ in the atmosphere. Others have criticized the methods and data that were used and argued that the conclusion of anthropogenic climate change is not supported.

IN CONTEXT: EXTREME TEMPERATURES

“Widespread changes in extreme temperatures have been observed over the last 50 years. Cold days, cold nights and frost have become less frequent, while hot days, hot nights, and heat waves have become more frequent.”

Statement of the Intergovernmental Panel on Climate Change (IPCC) as formally approved at the 10th Session of Working Group I of the IPCC in Paris, France, during February 2007.

SOURCE: Solomon, S., et al, eds. Climate Change 2007: The Physical Science Basis: Contribution of Working Group I to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change. New York: Cambridge University Press, 2007.

The Intergovernmental Panel on Climate Change (IPCC) has assembled the work of many paleoclimatologists who have also produced “hockey stick” graphs from the temperature record. Although researchers continue to iron out the problems with the use of temperature proxies, especially prior to about AD 900, most of the scientific community accepts the validity of the findings of Mann and colleagues, which were affirmed by a special committee of the National Academy of Sciences in 2005. The great majority of climatologists, meteorologists, and geophysicists agree that global warming is occurring and is anthropogenic, as shown not only by the temperature record but by numerous other lines of evidence.

See Also Hockey Stick Controversy; Ice Core Research; Paleoclimatology; Temperature and Temperature Scales.

BIBLIOGRAPHY

Books

Committee on Surface Temperature Reconstructions for the Last 2,000 Years, National Research Council. Surface Temperature Reconstructions for the Last 2,000 Years. Washington, DC: National Academies Press, 2006.

Parry, M. L., et al, eds. Climate Change 2007: Impacts, Adaptation and Vulnerability: Contribution of Working Group II to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change. New York: Cambridge University Press, 2007.

Periodicals

Mann, Michael, et al. “Global-Scale Temperature Patterns and Climate Forcing Over the Past Six Centuries.” Nature 392 (1998): 779–787.

Web Sites

“NOAA Satellite and Information Service.” National Climatic Data Center, November 16, 2006. <http://www.ncdc.noaa.gov/paleo/globalwarming/instrumental.html> (accessed November 12, 2007).

Michele Chapman