Quaternary Period

The Quaternary period, the most recent geologic interval, represents the last 1.8 million years of time. Its most striking feature is that the Earth had cold polar regions, which led to periodic development of continental glaciers. The prolonged ice ages, comprising the main part of the Quaternary interval, ended 10,000 years ago, when the continental glaciers had melted. Compared to the many cold periods (glacial intervals) of the Quaternary, the last 10,000 years of the present interglacial has been comparatively warm.

The changes in climate over the past 150 years have shown a varied history. The world endured a historic cool period during the late 1800s (the Little Ice Age), followed by a warm period of the 1930s (the Dust Bowl years), and since about 2000, the climate has been more variable, reaching extremes in warmth at high latitudes. Reports from Vikings show that 1,000 years ago, the climate of Greenland and Iceland was warmer than today (the Medieval warming). While these historic shifts in temperature are relatively moderate, much larger changes in temperature on Earth have occurred in the geologic past.

Proxy Data

Using the changing oxygen isotope ratios from ice cores and marine sediment cores, scientists have discovered global changes, recorded synchronously over a wide range of latitudes. One long climate proxy record was taken in the Antarctic, the Vostok ice core. From it, the inferred temperature over a long interval is based on the temperature-sensitive ratio of oxygen isotopes, 18O to 16O. In the Vostok ice core, isotopes of oxygen have been used to develop Earth temperature histories extending over 400,000 years.

Trapped gas bubbles record the history of atmospheric CO2 concentrations for this period (data from the National Climatic Data Center, Asheville, North Carolina). Because the isotope 18O is heavier than 16O, the proportions of each vary depending on the climate region. In alpine areas such as in the Alps of Switzerland and in the polar regions, 18O is more abundant, while in the lowlands of the middle and low latitudes, 16O prevails in the atmosphere. By calibrating these, scientists can use the proportions of oxygen isotopes taken from ancient sediments or ice cores as an index of average annual temperature.

The changing proportions of isotopes can be matched and dated with a geomagnetic signal of polar reversals (the Earth’s poles changed their magnetic signals), a known time scale based on isotopically dated magnetic signals that are worldwide. Another source of long climate records are the deep-sea sediment cores from which oxygen isotopes can be extracted from calcareous plankton (foraminifera). These data carry the paleoclimatic records back through more than 60 million years through the Tertiary period and the time of the last dinosaurs.

The ice ages, which comprised the main part the last million years, was a globally cold period with increasingly variable swings of climate. The glacial pattern continued over long intervals, with only a few comparatively short warm or interglacial periods. The last major glaciation came in two parts; in the United States, these are called the early Wisconsin (80,000–28,000 years before present, oxygen isotope stage 4) and the Full Glacial or late Wisconsin (23,000–15,000 years ago, oxygen isotope stage 2); between these a somewhat warmer middle Wisconsin period occurred 28,000–23,000 years ago. The maximum of the last major glaciation occurred about 18,000– 15,000 years ago (called the Full Glacial). After 15,000 years ago, a global warming began, and continental ice sheets melted by about 10,000 years ago. The period after 10,000 years ago, or postglacial, is called the Holocene or Recent period representing the present interglacial.

The period of the last glaciation (Full Glacial) brought continental ice down to the middle latitudes in both hemispheres. In Europe, ice from the Scandinavian highlands spread southward over the Netherlands and mountain glaciers covered the Alps. Ice stood over parts of northern Siberia, Greenland, and parts of Alaska. Permafrost ice developed underground in Siberia, northern Canada, and Alaska as much as 300 ft. (91 m) thick. Equivalent ice expansions occurred in the Southern Hemisphere.

As temperatures warmed between 15,000 and 9,000 years ago, the average annual temperatures at mid-latitudes increased by about 11 degrees F (6 degrees C). At Lamont National Observatory, scientists estimated that the difference in solar insolation between the Full Glacial and the midpostglacial was about 8 percent.

An overall trend within the Quaternary is apparent. In the first half, the amount of variance from year to year was fairly low, while in the last half of the Quaternary, variance became more and more extreme. Looking back over records of the past million years, geologists estimate that there were at least 40 cold or glacial intervals, and these were of varying length and were not regular in occurrence. In the last 400,000 years of the ice ages, proxy data indicate that there were four extreme cool periods (glacials) interspersed with five variable warm periods (interglacials). The last major interglacial (spanning the interval of about 80,000–122,000 years before present) was similar in warmth to the present interglacial climate, but the previous interglacials were definitely cooler than the present one.

Causes

The main climatic changes of the Quaternary are linked to the orbital position of Earth in relation to the sun. Astronomer Milutin Milankovitch proposed the orbital theory that is established by a variety of evidence, starting with tree ring variation. The precession of the equinox, obliquity of the Earth’s orbit around the sun, and eccentricity of the Earth’s orbit all contribute to the level of insolation received by Earth and are therefore the cause of geologic shifts in climate.

Before the Quaternary was the warm Tertiary period that brought tropical conditions to the mid-latitudes, and extensive temperate forests grew in the Northern Hemisphere. The north and south polar areas were at least 36 degrees F (20 degrees C) warmer than today, perhaps around 41 degrees F (5 degrees C) average annual temperature. Orbital causes for this part of Earth’s history have not been specified, but probably were important factors.

Impacts

Global changes in climate forced major changes in plant and animal distributions, especially in the high and mid-latitudes. During the Full Glacial, remains of trees that now grow in the northern boreal forest were found in Tennessee where their fossils were associated with deciduous hardwoods and even cypress-swamp types, creating strange mixtures of genera. Plants and animals that had dispersed to lower latitudes and in areas of southerly environments during glacials dispersed northward again during the postglacial warming. Thus, the Quaternary period records some of the most extreme climate changes known in Earth’s history.