Proxy (climate)

• 1. Defining Climate Proxies

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  Reliable global records of climate only began in the 1880s. Before that decade, scientists had no direct meteorological measurements to study Earth's history. They needed a way to reconstruct climatic conditions over longer fractions of time. This necessity gave rise to the concept of climate proxies. These are preserved physical characteristics from the past that stand in for missing data. A proxy acts as a substitute variable when direct measurement is impossible. Scientists measure a correlated property and use it to infer the value of interest. For example, they might analyze stable isotopes or growth rates rather than reading a thermometer from 500 years ago. Each proxy indicator has been influenced by a particular seasonal climate parameter at the time it was laid down. Interpretation requires ancillary studies like calibration of sensitivity. Cross-verification among different indicators helps ensure accuracy. The geographic distribution of these records mirrors the instrumental record. More records exist in the northern hemisphere than elsewhere.

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• 2. Ice Core Analysis Methods

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  First attempts at extraction occurred in 1956 as part of the International Geophysical Year. The U.S. Army's Cold Regions Research and Engineering Laboratory used an electrodrill in 1968 at Camp Century, Greenland. Their machinery could drill through ice in 40 to 50 minutes. From 1300 meters to deeper levels, core samples were 10 centimeters in diameter and 10 to 20 meters long. Deeper samples of 15 to 20 meters were not uncommon. Every subsequent drilling team improved their method with each new effort. The ratio between the 16O and 18O water molecule isotopologues in an ice core helps determine past temperatures. The heavier isotope 18O condenses more readily as temperatures decrease. It falls more easily as precipitation while the lighter isotope needs colder conditions. Air bubbles trapped in the ice contain greenhouse gases such as carbon dioxide and methane. These bubbles help determine past climate changes. From 1989 to 1992, the European Greenland Ice Core Drilling Project drilled in central Greenland. The ices in that core were 3840 years old at a depth of 770 meters. They were 40,000 years old at 2521 meters and 200,000 years or more at 3029 meters bedrock. Ice cores in Antarctica can reveal climate records for the past 650,000 years.

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• 3. Dendroclimatology And Tree Rings

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  Tree rings are wider when conditions favor growth and narrower when times are difficult. Two primary factors influencing this width are temperature and humidity. Other properties like maximum latewood density have been shown to be better proxies than simple ring width. Scientists use these annual rings to estimate local climates for hundreds to thousands of years previous. By combining multiple tree-ring studies with other proxy records, researchers estimated past regional and global climates. Paleoclimatologists often use leaf teeth to reconstruct mean annual temperature in past climates. They use leaf size as a proxy for mean annual precipitation. Some researchers believe taphonomic processes cause smaller leaves to be overrepresented in the fossil record. This bias could skew reconstructions if not accounted for. However, recent research suggests the leaf fossil record may not be significantly biased toward small leaves. New approaches retrieve data such as carbon-13 isotope ratios from fossil leaf stomata. A 2014 study used these ratios to estimate CO2 amounts of the past 400 million years. The findings hint at higher climate sensitivity to CO2 concentrations than previously thought.

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• 4. Sediment And Fossil Records

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  Pollen can be found in sediments where plants produce it in large quantities. It is extremely resistant to decay so identification remains possible. The identified plant community provides information about climatic conditions at that relative time. Pollen density provides information on short-term climatic conditions based on weather patterns from previous months. Lake varves are layers of fine and coarse silt or clay deposited annually. Summer temperature shows energy available to melt seasonal snow and ice. Winter snowfall determines the level of disturbance to sediments when melting occurs. Rainfall also influences these sedimentary layers. Diatoms, foraminifera, radiolarians, ostracods, and coccolithophores serve as biotic proxies for lake and ocean conditions. Their distribution in preserved sediments acts as clues to reveal what the environment was like when creatures died. Oxygen isotope ratios in their shells function as additional proxies for temperature. Dinoflagellates occur in most aquatic environments during their life cycle. Some species produce highly resistant organic-walled cysts for dormancy periods. Their living depth depends upon light penetration and they feed closely coupled to diatoms. Several studies compiled box and gravity cores in the North Pacific analyzing palynological content. A 1992 study used a box core at 576.5 meters water depth from the central Santa Barbara Basin.

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• 5. Oceanographic Proxy Indicators

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  Ocean coral skeletal rings share paleoclimatological information similarly to tree rings. In 2002, a report published findings by Drs. Lisa Greer and Peter Swart regarding stable oxygen isotopes. Cooler temperatures tend to cause coral to use heavier isotopes in its structure. Warmer temperatures result in more normal oxygen isotopes being built into the coral structure. Denser water salinity also tends to contain the heavier isotope. Greer's coral sample from the Atlantic Ocean was taken in 1994 and dated back to 1935. When looking at averaged annual data from 1935 to about 1994, the pattern showed a sine wave shape. It had a significant pattern of oxygen isotope composition with peaks every twelve to fifteen years. Surface water temperatures have coincided by peaking every twelve and a half years. Since recording this temperature has only been practiced for the last fifty years, correlation can be drawn so far back. Dinoflagellate cysts contribute to understanding average sea-surface conditions that determine distribution patterns. Their abundance and presence help researchers reconstruct historical precipitation and vegetation patterns alongside other marine indicators.

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• 6. Borehole Temperature Inversion

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  Since heat transfer through the ground is slow, measurements at different depths allow inversion. A mathematical formula solves matrix equations to produce surface temperature values. The solution is non-unique because multiple possible reconstructions can produce the same profile. Reconstructions become smeared further back in time due to physical limitations. When reconstructing temperatures around 1500 AD, boreholes have a temporal resolution of a few centuries. At the start of the 20th century, their resolution reaches a few decades. They do not provide a useful check on the instrumental temperature record but remain broadly comparable. More than 600 boreholes on all continents have been used as proxies for reconstructing surface temperatures. The highest concentration exists in North America and Europe. Drilling depths typically range from 200 meters to greater than 1,000 meters into the crust. Central Greenland borehole temperatures show warming over the last 150 years of approximately 1 degree Celsius. This was preceded by a few centuries of cool conditions. Preceding this was a warm period centered around A.D. 1000 which was warmer than the late 20th century by approximately 1 degree Celsius.

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