In the study of past climates ("paleoclimatology"), climate proxies are preserved physical, chemical, and even biological characteristics of the past that stand in for direct meteorological, climatological, and environmental measurements and enable scientists to reconstruct the climatic conditions over a longer fraction of the Earth's history. Reliable global records of climate only began in the 1880s, and proxies provide the only means for scientists to determine climatic patterns before record-keeping began. A large number of climate proxies have been studied from a variety of geologic contexts. Examples of proxies include stable isotope measurements from ice cores, growth rates in tree rings, species composition of sub-fossil pollen in lake sediment or foraminifera in ocean sediments, temperature profiles of boreholes, and stable isotopes and mineralogy of corals and carbonate speleothems. In each case, the proxy indicator has been influenced by a particular seasonal climate parameter (e.g., summer temperature or monsoon intensity) at the time in which they were laid down or grew. Interpretation of climate proxies requires a range of ancillary studies, including calibration of the sensitivity of the proxy to climate and cross-verification among proxy indicators. Proxies can be combined to produce temperature reconstructions longer than the instrumental temperature record and can inform discussions of global warming and climate history. The geographic distribution of proxy records, just like the instrumental record, is not at all uniform, with more records in the northern hemisphere.
Proxies
In science, it is sometimes necessary to study a variable which cannot be measured directly. This can be done by "proxy methods," in which a variable which correlates with the variable of interest is measured, and then used to infer the value of the variable of interest. Proxy methods are of particular use in the study of the past climate, beyond times when direct measurements of temperatures are available. Most proxy records have to be calibrated against independent temperature measurements, or against a more directly calibrated proxy, during their period of overlap to estimate the relationship between temperature and the proxy. The longer history of the proxy is then used to reconstruct temperature from earlier periods.
Ice cores
Drilling Ice cores are cylindrical samples from within ice sheets in the Greenland, Antarctic, and North American regions. First attempts of extraction occurred in 1956 as part of the International Geophysical Year. As original means of extraction, the U.S. Army's Cold Regions Research and Engineering Laboratory used an 80-foot (24 m)-long modified electrodrill in 1968 at Camp Century, Greenland, and Byrd Station, Antarctica. Their machinery could drill through 15–20 feet (4.6–6.1 m) of ice in 40–50 minutes. From 1300 to 3,000 feet (910 m) in depth, core samples were 4+1⁄4 inches (110 mm) in diameter and 10 to 20 feet (6.1 m) long. Deeper samples of 15 to 20 feet (6.1 m) long were not uncommon. Every subsequent drilling team improves their method with each new effort.
Proxy
The ratio between the 16O and 18O water molecule isotopologues in an ice core helps determine past temperatures and snow accumulations. The heavier isotope (18O) condenses more readily as temperatures decrease and falls more easily as precipitation, while the lighter isotope (16O) needs colder conditions to precipitate. The farther north one needs to go to find elevated levels of the 18O isotopologue, the warmer the period. In addition to oxygen isotopes, water contains hydrogen isotopes – 1H and 2H, usually referred to as H and D (for deuterium) – that are also used for temperature proxies. Normally, ice cores from Greenland are analyzed for δ18O and those from Antarctica for δ-deuterium. Those cores that analyze for both show a lack of agreement. (In the figure, δ18O is for the trapped air, not the ice. δD is for the ice.) Air bubbles in the ice, which contain trapped greenhouse gases such as carbon dioxide and methane, are also helpful in determining 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. The ices in that core were 3840 years old at a depth of 770 m, 40,000 years old at 2521 m, and 200,000 years old or more at 3029 m bedrock. Ice cores in Antarctica can reveal the climate records for the past 650,000 years. Location maps and a complete list of U.S. ice core drilling sites can be found on the website for the National Ice Core Laboratory.
Tree rings
Dendroclimatology is the science of determining past climates from trees, primarily from properties of the annual tree rings. Tree rings are wider when conditions favor growth, narrower when times are difficult. Two primary factors are temperature and humidity / water availability. Other properties of the annual rings, such as maximum latewood density (MXD) have been shown to be better proxies than simple ring width. Using tree rings, scientists have estimated many local climates for hundreds to thousands of years previous. By combining multiple tree-ring studies (sometimes with other climate proxy records), scientists have estimated past regional and global climates (see Temperature record of the past 1000 years).
Fossil leaves Paleoclimatologists often use leaf teeth to reconstruct mean annual temperature in past climates, and they use leaf size as a proxy for mean annual precipitation. In the case of mean annual precipitation reconstructions, some researchers believe taphonomic processes cause smaller leaves to be overrepresented in the fossil record, which can bias reconstructions. However, recent research suggests that the leaf fossil record may not be significantly biased toward small leaves. New approaches retrieve data such as CO2 content of past atmospheres from fossil leaf stomata and isotope composition, measuring cellular CO2 concentrations. A 2014 study was able to use the carbon-13 isotope ratios to estimate the CO2 amounts of the past 400 million years, the findings hint at a higher climate sensitivity to CO2 concentrations.
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