Paleoclimatology

Paleoclimatology is the study of past climate. The word is derived from the Greek root "paleo-," which means "ancient," and the term "climate." Paleoclimate is climate that existed before humans began collecting instrumental measurements of weather (e.g., temperature from a thermometer, precipitation from a rain gauge, sea level pressure from a barometer, wind speed and direction from an anemometer). Instead of instrumental measurements of weather and climate, paleoclimatologists use natural environmental (or "proxy") records to infer past climate conditions. Paleoclimatology not only includes the collection of evidence of past climate conditions, but the investigation of the climate processes underlying these conditions.

Proxy data is data that paleoclimatologists gather from natural recorders of climate variability, e.g., tree rings, ice cores, fossil pollen, ocean sediments, coral and historical data. By analyzing records taken from these and other proxy sources, scientists can extend our understanding of climate far beyond the 140 year instrumental record.

Listed below are some widely used proxy climate data types:

Corals:

Corals build their hard skeletons from calcium carbonate, a mineral extracted from sea water. The carbonate contains oxygen and the isotopes of oxygen, as well as trace metals, that can be used to determine the temperature of the water in which the coral grew. These temperature recordings can then be used to reconstruct climate during that period of time that the coral lived.

Fossil Pollen:

Each species and genus of plants produces pollen grains which have a distinct shape. These shapes can be used to identify the type of plant from which they came. Since pollen grains are well preserved in the sediment layers that form in the bottom of a pond, lake or ocean, an analysis of the pollen grains in each layer tell us what kinds of plants were growing at the time the sediment was deposited. Inferences can then be made about the climate based on the types of plants found in each layer.

Tree Rings:

Since tree growth is influenced by climatic conditions, patterns in tree-ring widths, density, and isotopic composition reflect variations in climate. In temperate regions where there is a distinct growing season, trees generally produce one ring a year, and thus record the climatic conditions of each year. Trees can grow to be hundreds to thousands of years old and can contain annually-resolved records of climate for centuries to millennia.

Ocean & Lake Sediments:

Between 6 and 11 billion metric tons of sediment accumulate in the ocean and lake basins each year. Scientist drill cores of sediment from the basin floors. Ocean and lake sediments consist of materials that were produced in the lake/ocean or that washed in from nearby land. These materials (preserved tiny fossils and chemicals in the sediments) can be used to interpret past climate.

Ice Cores:

Located high in mountains and deep in polar ice caps, ice has accumulated from snowfall over many centuries. Scientists drill through the deep ice to collect ice cores. These cores contain dust, air bubbles or isotopes of oxygen. Since the heavy isotope content in precipitation decreases with the condensation temperature, the isotopic composition of ice can be used to reconstruct past climate.

Compared to marine sediments, deep ice cores have several advantages. These studies provide high resolution and they contain valious information on the load and composition of aerosols as well as trace gases content of the atmosphere, parameters which may be involved in past climate changes.

Ice cores contain an abundance of climate information --more so than any other natural recorder of climate such as tree rings or sediment layers. Although their record is short (in geologic terms), it can be highly detailed. An ice core from the right site can contain an uninterrupted, detailed climate record extending back hundreds of thousands of years. This record can include temperature, precipitation , chemistry and gas composition of the lower atmosphere, volcanic eruptions, solar variability, sea-surface productivity and a variety of other climate indicators. It is the simultaneity of these properties recorded in the ice that makes ice cores such a powerful tool in paleoclimate research.

Ice cores from Vostok, Antarctica, were the first to cover a full glacial-interglacial cycle. And, despite recent drillings in central Greenland, they still carry the distinction of being the only ice cores that scientists are certain have remained undisturbed for the last interglacial and the penultimate glacial periods.

Interpreting Paleoclimate From Ice Cores

Two elements - deuterium and oxygen 18 - are important because they can be used to reconstruct past temperature changes in polar regions. In Antarctica, a cooling of 1°C results in a decrease of 9 per mil deuterium. An accurate chronology is essential for interpreting ice core paleoclimate data.

Because the accumulation rate is governed by saturation water vapor pressure, past accumulation may be estimated from the temperature record. Accumulation rates inferred in this way are supported by measurements of beryllium 10 (¹⁰Be), an isotope produced by the interaction of cosmic rays and the upper atmosphere, can be used to determine past snow accumulation in Vostok ice. Deposition of this cosmogenic isotope is assumed to be constant.

Beyond their use as dating tools, ice cores convey specific geochemical information.

Changes in terrestrial aerosols hold the key to past climate. More dust was present in glacial periods than during interglacials; this suggests that glacial periods were characterized by extensive deserts, intense surface winds in the desert source regions, and more efficient transport along the imaginary circular path that runs perpendicular to the equator through the poles. This idea of stronger circulation during glacial periods is reinforced by the fact that glacial values of marine aerosols are much higher than interglacial levels.

Another important aspect of change in the past atmosphere's aerosol load is a secondary aerosol composed of nonseasalt sulfate and methanesulfonic acid (MSA), an oxidation product emitted by marine organisms. Although studies based on MSA measurements show that the link between climate and biogenic marine activity could be more complex than initially thought, both nonseasalt sulfate and MSA records indicate that the ocean-atmosphere sulfur cycle is extremely sensitive to climate change. Sulfate aerosols affect climate by "thickening" the atmosphere.

From a climatic viewpoint, CO₂ and CH₄ have played an important role. Together with the growth and decay of the Northern Hemisphere ice sheets, these greenhouse gases have amplified the initial orbital forcing, and they account for about half of the glacial-interglacial climate changes. This supports the idea that significant greenhouse warming will occur in the next century.

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Text: Liliana del Blanco - Bahia Blanca - Argentina