The Earth’s climate has a history stretching back billions of years, but humanity’s direct measurements of temperature and precipitation only cover the last century and a half. This short instrumental record is insufficient for understanding the full range of natural climate variability or for testing complex climate models. To look back thousands or even millions of years, scientists rely on natural environmental archives that have passively recorded climate conditions. These natural recorders, known as climate proxies, are the foundation of paleoclimatology, the study of past climates.
Why Scientists Rely on Indirect Evidence
A climate proxy is a preserved physical, chemical, or biological characteristic of the past environment that can be substituted for a direct weather measurement. Since a thermometer could not be placed in a prehistoric ocean, scientists must analyze a measurable feature that consistently tracks with a climate variable, such as temperature or rainfall. By studying proxies, researchers can extend climate records far beyond the 1850s, allowing for a perspective on climate change over vast timescales. Analyzing these archives permits scientists to reconstruct past conditions, including atmospheric composition, ocean temperatures, and precipitation patterns, across millennia.
Climate Records from Land and Life
Some of the most accessible climate proxies are found in terrestrial environments and living organisms, providing high-resolution records over hundreds to thousands of years. Dendroclimatology uses the annual growth rings of trees to reconstruct past climate conditions. A tree’s growth rate is sensitive to local environmental conditions. Wider rings indicate a warm year with ample moisture, while a narrow ring often signals a period of drought or a cold growing season.
In addition to ring width, scientists also analyze the wood’s maximum latewood density, which is often a more reliable indicator of summer temperature in cold regions. Another terrestrial archive comes from palynology, the study of preserved pollen grains. Pollen has a tough, chemically stable outer wall that allows it to survive in lake and bog sediments for thousands of years.
Each plant species produces uniquely shaped pollen, allowing scientists to identify the types of vegetation that dominated an area in the past. Since specific plant communities are adapted to particular temperature and moisture regimes, a change in the dominant pollen assemblage found in a sediment layer reflects a corresponding shift in climate. For instance, an abundance of spruce pollen in a region now dominated by oak suggests a much colder past climate.
Deep Time Archives in Ice and Water
For the longest and most continuous climate records, scientists turn to geological archives found in ice sheets and deep-sea sediments. Ice cores drilled from Greenland and Antarctica contain layers of compressed snow that hold two types of information. Trapped air bubbles within the ice provide direct samples of the ancient atmosphere, allowing scientists to measure past concentrations of greenhouse gases like carbon dioxide and methane.
The ice itself, formed from ancient snowfall, is analyzed using the ratio of oxygen isotopes, specifically oxygen-18 (O-18) to oxygen-16 (O-16). Water molecules containing the lighter O-16 isotope evaporate more easily but condense less readily than those with the heavier O-18 isotope. During colder periods, more O-16 is locked up in ice sheets, leaving the ocean enriched in O-18, which results in a lower O-18/O-16 ratio in the ice core, a measurable proxy for past temperature.
Ocean and lake sediments also contain layers of accumulated material that record deep-time climate shifts. Microscopic marine organisms, such as foraminifera, build their shells from calcium carbonate, which incorporates the O-18/O-16 ratio of the surrounding seawater. By analyzing the chemistry of these preserved shells, scientists can reconstruct ancient ocean temperatures and global ice volume. The ratio of magnesium to calcium (Mg/Ca) in the shell calcite also serves as a precise paleothermometer, as warmer water causes foraminifera to incorporate more magnesium.
How Scientists Read the Past
Translating a proxy measurement into a reliable climate record requires two main methodological steps: dating and calibration. High-resolution archives like tree rings and ice cores are dated by simply counting annual layers. For older archives like marine sediments, scientists use radiometric dating techniques, such as Carbon-14, to assign an absolute age to a specific layer.
Once the proxy is dated, calibration begins, based on the principle that the physical and biological processes linking the proxy to the climate variable have remained constant. Scientists compare the proxy data with modern instrumental records during a period of overlap to establish a reliable relationship. This comparison generates a statistical rule, known as a transfer function, which converts the proxy measurement (e.g., a specific Mg/Ca ratio) into a quantitative estimate of a past climate variable.