Proxy (climate)
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.
Proxy methods are of particular use in the study of the past climate, beyond times when direct measurements of temperatures are available.
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.
[6] The ratio between the 16O and 18O water molecule isotopologues in an ice core helps determine past temperatures and snow accumulations.
Air bubbles in the ice, which contain trapped greenhouse gases such as carbon dioxide and methane, are also helpful in determining past climate changes.
Using tree rings, scientists have estimated many local climates for hundreds to thousands of years previous.
[10] However, recent research suggests that the leaf fossil record may not be significantly biased toward small leaves.
[11] 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.
Since heat transfer through the ground is slow, temperature measurements at a series of different depths down the borehole, adjusted for the effect of rising heat from inside the Earth, can be "inverted" (a mathematical formula to solve matrix equations) to produce a non-unique series of surface temperature values.
Central Greenland borehole temperatures show "a warming over the last 150 years of approximately 1°C ± 0.2°C preceded by a few centuries of cool conditions.
Lisa Greer and Peter Swart, associates of University of Miami at the time, in regard to stable oxygen isotopes in the calcium carbonate of coral.
The identified plant community of the area at the relative time from that sediment layer, will provide information about the climatic condition.
Dinoflagellates occur in most aquatic environments and during their life cycle, some species produce highly resistant organic-walled cysts for a dormancy period when environmental conditions are not appropriate for growth.
Their living depth is relatively shallow (dependent upon light penetration), and closely coupled to diatoms on which they feed.
Several studies, including [20] and [21] have compiled box and gravity cores in the North Pacific analyzing them for palynological content to determine the distribution of dinocysts and their relationships with sea surface temperature, salinity, productivity and upwelling.
Similarly,[22] and [23] use a box core at 576.5 m of water depth from 1992 in the central Santa Barbara Basin to determine oceanographic and climatic changes during the past 40 kyr in the area.
Similar to their study on other proxies, paleoclimatologists examine oxygen isotopes in the contents of ocean sediments.
Likewise, they measure the layers of varve (deposited fine and coarse silt or clay)[24] laminating lake sediments.
A study published in 2017 called the previous methodology to reconstruct paleo ocean temperatures 100 million years ago into question, suggesting it has been relatively stable during that time, much colder.
[31] A novel climate proxy obtained from peat (lignites, ancient peat) and soils, membrane lipids known as glycerol dialkyl glycerol tetraether (GDGT) is helping to study paleo environmental factors, which control relative distribution of differently branched GDGT isomers.
The study authors note, "These branched membrane lipids are produced by an as yet unknown group of anaerobic soil bacteria.
"[32] As of 2018[update], there is a decade of research demonstrating that in mineral soils the degree of methylation of bacteria (brGDGTs), helps to calculate mean annual air temperatures.
This proxy method was used to study the climate of the early Palaeogene, at the Cretaceous–Paleogene boundary, and researchers found that annual air temperatures, over land and at mid-latitude, averaged about 23–29 °C (± 4.7 °C), which is 5–10 °C higher than most previous findings.
[33][34] The skill of algorithms used to combine proxy records into an overall hemispheric temperature reconstruction may be tested using a technique known as "pseudoproxies".
