Proxy (climate)

• 1. Introduction

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  Climate proxies fill a silence that stretches back before the 1880s, when reliable global temperature records begin. They are the indirect physical evidence preserved in materials scattered across the Earth's surface and buried beneath its floors.

  The gap they bridge is immense. Systematic global temperature records span roughly 140 years. Every climate event before that window must be read from materials that were never designed to be measured.

  Scientists have learned to read them anyway. The questions driving that work are about reliability: how accurately can these materials record temperature, and how far back can those readings extend? At the deepest Antarctic drilling sites, ice has preserved a climate record stretching 650,000 years into the past.

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• 2. The Ice Archives

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  In 1956, the first attempts to drill into polar ice sheets took place as part of the International Geophysical Year. The goal was to extract cylindrical core samples from ice sheets in Greenland, Antarctica, and North America.

  The U.S. Army's Cold Regions Research and Engineering Laboratory advanced the method in 1968. Using an 80-foot modified electrodrill, teams worked at Camp Century in Greenland and Byrd Station in Antarctica. That machinery could penetrate ice in 40 to 50 minutes. Core samples from depths of 1,300 to 3,000 feet were typically 10 to 20 feet long; deeper cores sometimes reached 15 to 20 feet.

  From 1989 to 1992, the European Greenland Ice Core Drilling Project worked at coordinates 72 degrees 35 minutes North, 37 degrees 38 minutes West in central Greenland. Ice at 770 meters depth was 3,840 years old. At 2,521 meters, the same core held ice that was 40,000 years old. At the bedrock depth of 3,029 meters, the ice was 200,000 years old or more.

  The ratio between oxygen-16 and oxygen-18 in the water molecules of an ice core shifts with temperature. Oxygen-18, the heavier isotope, condenses more readily as temperatures drop and falls more easily as precipitation. The farther north a scientist must travel to find elevated levels of oxygen-18, the warmer the period being studied. Air bubbles trapped in the ice add a separate dimension. They preserve ancient greenhouse gases, including carbon dioxide and methane, at the moment of burial.

  Standard practice analyzes Greenland cores for a value called delta-18O and Antarctic cores for delta-deuterium, a related measure built from hydrogen isotopes. Calibration work has established the relationship between isotope ratios and actual temperature. In Greenland, a shift of one per mille in delta-18O corresponds to 0.67 degrees Celsius. In East Antarctica, the same shift corresponds to 0.76 degrees Celsius. Borehole thermometry later revised that relationship for glacial-interglacial variations to 0.33 per mille per degree Celsius. That implies ancient temperature swings were twice as large as previously believed.

  Cores that analyze for both delta-18O and delta-deuterium show a lack of agreement, a finding that researchers have yet to fully explain.

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• 3. What Trees Record

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  Temperature and humidity control ring width in annual tree rings, a relationship that scientists have built into a field called dendroclimatology. Tree rings are wider when conditions favor growth and narrower when times are difficult.

  Maximum latewood density, known as MXD, has been shown to be a better climate proxy than ring width alone. Using tree rings, scientists have estimated local climates covering hundreds to thousands of years. By combining multiple tree-ring studies, sometimes alongside other proxy records, researchers have extended those estimates across the past 1,000 years at regional and global scales.

  Paleoclimatologists also use leaf teeth to reconstruct mean annual temperature in past climates. Leaf size serves as a proxy for mean annual precipitation. One complication is that taphonomic processes may cause smaller leaves to be overrepresented in the fossil record, which could bias reconstructions. Recent research suggests that bias may not be significant.

  A 2014 study used carbon-13 isotope ratios from fossil leaf stomata to estimate atmospheric carbon dioxide levels across the past 400 million years. Those findings hint at higher climate sensitivity to carbon dioxide concentrations than most previous models had estimated.

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• 4. Boreholes and Buried Heat

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  More than 600 boreholes on every continent have been used to reconstruct past surface temperatures. Their highest concentration is in North America and Europe, and depths typically range from 200 to over 1,000 meters.

  At roughly 492 feet of depth, a borehole captures temperatures from about 100 years ago. At 1,640 feet, the signal reaches back about 1,000 years. Temperature measurements taken at different depths are adjusted for the Earth's internal heat and then mathematically inverted to produce a series of past surface temperatures. The solution from that inversion is not unique: multiple possible temperature histories can produce the same borehole profile. Reconstructions also become less precise further back in time, a phenomenon researchers describe as smearing.

  Unlike standard weather records, which measure air temperature at 1.5 meters above the ground, boreholes record actual surface temperature. No calibration is required to convert that reading. The two measurements can diverge under extreme conditions or when snow covers the ground, though the effect is generally considered small. Groundwater seeping into a borehole can carry more modern temperatures into the measurement. This effect is believed to be small and more relevant at humid sites.

  Temporal resolution is a limitation. Reconstructing temperatures around 1500 AD, boreholes offer a resolution of only a few centuries. By the start of the 20th century, that resolution sharpens to a few decades.

  Data from central Greenland boreholes shows warming of approximately 1 degree Celsius over the past 150 years, with a margin of 0.2 degrees Celsius. Before that warming came a few centuries of cool conditions. Earlier still, centered around A.D. 1000, a warm period ran roughly 1 degree Celsius above late-20th-century levels. A borehole in the Antarctic ice cap places temperatures at A.D. 1 at approximately 1 degree Celsius above late-20th-century levels. That single reading predates the instrumental record by nearly two millennia.

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• 5. Coral Skeletons

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  In 2002, a report by Drs. Lisa Greer and Peter Swart, then associates of the University of Miami, examined stable oxygen isotopes in the calcium carbonate of coral. Their work offered a detailed picture of how coral skeletal chemistry encodes past ocean conditions.

  Coral skeletal rings, called bands, share the same climate-recording principle as tree rings. The oxygen isotopes in those bands shift with temperature: cooler water causes coral to use heavier isotopes in its structure, while warmer water results in lighter, more typical oxygen composition. Denser, saltier water also tends to incorporate the heavier isotope.

  Greer's coral sample from the Atlantic Ocean was collected in 1994 and dated back to 1935. Analyzing averaged annual data across those six decades, she found a striking pattern. Greer recalled: "When we look at the averaged annual data from 1935 to about 1994, we see it has the shape of a sine wave." She continued: "It is periodic and has a significant pattern of oxygen isotope composition that has a peak at about every twelve to fifteen years." Surface water temperatures, measured independently, had been peaking on a cycle of roughly twelve and a half years.

  Direct ocean temperature recording has been practiced for roughly fifty years. That limits how far back the correlation between coral chemistry and measured water temperature can be confirmed. The structural record in coral extends much further than any thermometer reading, leaving scientists to read the older portion without an independent check.

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• 6. Life in the Sediment

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  Plants produce pollen in large quantities, and the grains are extremely resistant to decay. Individual pollen grains can be matched to the plant species that produced them, so sediment samples reveal which plants grew in an area at the time each layer formed.

  The abundance of pollen from a given period reflects weather conditions from the preceding months. That relationship gives researchers a record of short-term climate variation within each sediment layer. The study of prehistoric pollen is called palynology.

  Lake sediment layers, known as varves, consist of alternating deposits of fine and coarse silt or clay. Three climate factors primarily shape them: summer temperature, which determines how much seasonal snow and ice melts; winter snowfall, which controls the level of sediment disturbance during spring thaw; and rainfall.

  Diatoms, foraminifera, radiolarians, ostracods, and coccolithophores are among the microscopic organisms whose shells accumulate in ocean sediments. The optimal living conditions for each species leave a recognizable signature. Where their remains appear in sediment, and in what proportions, tells scientists what temperatures prevailed when those creatures died. Oxygen isotope ratios in their shells provide a quantitative temperature signal alongside that distributional evidence.

  Rodent associations have become a recognized biological proxy for past local climate. Small home ranges, habitat specificity, and high reproductive rates make small mammals sensitive recorders of environmental change. Advanced methodologies combining the Bioclimatic Analysis approach with a spatial generalized linear mixed model have reconstructed continental-scale temperatures. Those methods have produced estimates of mean annual temperature, warmest-month temperature, and coldest-month temperature for regions including the Western Palearctic.

  Dinoflagellates occur in most aquatic environments. Certain species produce highly resistant organic-walled cysts during dormancy, and those cysts accumulate in sediment over time. Their distribution in surface waters closely tracks temperature, salinity, productivity, and upwelling patterns. Studies of box and gravity cores in the North Pacific have documented how dinoflagellate cyst assemblages reflect sea-surface conditions. A 1992 box core from the central Santa Barbara Basin, drawn from 576.5 meters of water, traced climate changes across the past 40,000 years.

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• 7. Novel and Computational Tools

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  Glycerol dialkyl glycerol tetraether, known as GDGT, is a membrane lipid found in peat and soils. More than a decade of research has established a link between the degree of methylation in branched GDGTs and mean annual air temperature. The organisms responsible remain unidentified; researchers describe them as "an as yet unknown group of anaerobic soil bacteria."

  At the Cretaceous-Paleogene boundary, annual air temperatures over land at mid-latitude averaged about 23 to 29 degrees Celsius, with an uncertainty margin of 4.7 degrees. That is 5 to 10 degrees higher than most previous findings for the same period. Researchers applied GDGT analysis to reach those figures, studying the climate of the early Palaeogene.

  Pseudoproxies offer a way to test the statistical methods used to combine proxy records into hemispheric temperature reconstructions. Climate model output is sampled at locations matching the known proxy network. The resulting temperature record is then compared against the model's known overall temperature. That comparison reveals how accurately the reconstruction algorithms perform without relying on incomplete real-world data.

  A study published in 2017 questioned the previous methodology for reconstructing ocean temperatures 100 million years ago. Its evidence suggested the ancient ocean was relatively stable and much colder than earlier reconstructions had proposed.

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