In the study of past climates ("paleoclimatology"), scientists use climate proxies—physical clues from the past—to understand weather and climate conditions before direct measurements were recorded. Reliable global climate records began in the 1880s, but before that, proxies are the only way to learn about Earth's climate history.

Many types of climate proxies have been studied from different geological sources. Examples include measurements of stable isotopes in ice cores, the growth rates of tree rings, the types of pollen found in lake sediments, the species of foraminifera in ocean sediments, temperature data from boreholes, and the chemical composition and structure of corals and cave formations called carbonate speleothems. Each proxy is affected by a specific climate condition, such as summer temperature or the strength of monsoons, at the time it was formed or grew. To interpret these clues, scientists must conduct additional research, such as testing how sensitive each proxy is to climate changes and comparing different proxies to confirm their accuracy.

Climate proxies can be combined to create temperature records that extend far beyond the time when instruments began measuring temperatures. These records help scientists study global warming and Earth's climate history. Like the instrumental records, proxy records are not evenly spread across the globe, with more available in the northern hemisphere.

Proxies

In science, sometimes it is necessary to study something that cannot be measured directly. This can be done using "proxy methods," which involve measuring something else that is connected to the thing being studied. Proxy methods are especially helpful for studying past climates, especially when direct temperature measurements are not available.

Most proxy records must be compared to independent temperature measurements or other well-calibrated proxies during overlapping time periods. This helps scientists understand how the proxy relates to temperature. Once this relationship is known, the proxy can be used to estimate temperatures from earlier times.

Ice cores are long, cylindrical samples taken from ice sheets in Greenland, Antarctica, and North America. The first ice core drilling happened in 1956 as part of the International Geophysical Year. In 1968, the U.S. Army's Cold Regions Research and Engineering Laboratory used a special drill to extract ice cores in Greenland and Antarctica. Their equipment could drill through 15–20 feet of ice in 40–50 minutes. Ice cores taken from 1300 to 3,000 feet deep were about 4.25 inches in diameter and 10 to 20 feet long. Deeper samples of similar lengths were common. Each new drilling team has improved methods over time.

The ratio of oxygen isotopes in ice cores helps scientists determine past temperatures and snowfall. The heavier oxygen isotope condenses more easily when temperatures are lower and falls as precipitation. The lighter oxygen isotope requires colder conditions to form. The farther north scientists find higher levels of the heavier oxygen isotope, the warmer the period was.

In addition to oxygen isotopes, water also contains hydrogen isotopes (H and D, or deuterium) that are used as temperature proxies. Ice cores from Greenland are often analyzed for δO, while those from Antarctica are analyzed for δD. Some cores tested for both show differences. Air bubbles trapped in ice contain greenhouse gases like carbon dioxide and methane, which also help scientists study past climate changes.

From 1989 to 1992, the European Greenland Ice Core Drilling Project drilled in central Greenland at coordinates 72° 35' N, 37° 38' W. Ice in that core was 3,840 years old at 770 meters deep, 40,000 years old at 2,521 meters, and over 200,000 years old at 3,029 meters. Ice cores in Antarctica can show climate records from the past 650,000 years. Information about U.S. ice core drilling sites can be found on the National Ice Core Laboratory's website.

Dendroclimatology is the study of past climates using trees, mainly by examining tree rings. Wider rings form when growth conditions are favorable, and narrower rings form during difficult times. Temperature and water availability are key factors. Other tree ring properties, like maximum latewood density, are better proxies than simple ring width. Scientists have used tree rings to estimate local climates for hundreds to thousands of years. Combining multiple tree ring studies with other climate data helps estimate past regional and global climates.

Paleoclimatologists use leaf teeth to estimate past temperatures and leaf size to estimate past rainfall. Some researchers think natural processes may make smaller leaves more common in the fossil record, which could affect rainfall estimates. However, recent studies suggest this bias may not be significant. New methods use carbon-13 isotope ratios to estimate past atmospheric carbon dioxide levels. A 2014 study used these ratios to estimate carbon dioxide levels over the past 400 million years, suggesting higher climate sensitivity to carbon dioxide.

Borehole temperatures are used as temperature proxies. Heat moves slowly through the ground, so measuring temperatures at different depths and adjusting for heat from Earth’s interior can help estimate past surface temperatures. This process uses a mathematical formula, but it can produce multiple possible temperature records. Reconstructions become less precise further back in time. Boreholes can show warming trends over the past 150 years, but they do not provide detailed records for the 20th century. They are useful for confirming long-term temperature changes, such as showing that temperatures around 1000 AD were warmer than the late 20th century.

Boreholes have an advantage because they measure actual temperatures, not requiring calibration. However, they record surface temperatures, not the 1.5-meter temperature used for most weather observations. Differences may occur in extreme conditions or with surface snow. Groundwater contamination can also affect borehole temperatures, but this is rare in ice cores where the site remains frozen.

More than 600 boreholes on all continents have been used to study past temperatures. The highest number of boreholes are in North America and Europe. Depths typically range from 200 to over 1,000 meters. In Greenland, boreholes show a warming of about 1°C over the past 150 years, following a cooler period and a warm period around A.D. 1000. A borehole in Antarctica shows temperatures around A.D. 1 were about 1°C warmer than the late 20th century.

Borehole data corrected earlier assumptions about how temperature changes relate to location, showing that temperature changes over time are not always the same as changes across locations.

Ocean coral skeletal rings, like tree rings, provide climate information. In 2002, scientists studied oxygen isotopes in coral calcium carbonate to understand past ocean conditions. Cooler temperatures cause corals to use heavier oxygen isotopes, which helps scientists estimate past temperatures.