• Long-term (millions of years) proxy records are represented by oxygen isotope ratios
• Examples of short-term (thousands of years) proxy climate indicators are pollen, oxygen isotopes, and tree ring data

limate fluctuations during the history of the earth can be determined from the analyses of a variety of biological and geological data sources. Some allow the reconstruction of climates stretching back millions of years, others provide precise annual records that cover the advance of human civilizations. These data represent proxy records of paleoclimates - data that can be interpreted to give indirect information on past climates.

Paleoclimatic data comes in a variety of forms, some give information on long-term climates (oceanic microfossils), while others provide precision in the recent, short-term climate record (tree rings, pollen).

Long-term Climate Changes
Changes in temperature over millions of years can be determined using oxygen isotopes (oxygen atoms with different numbers of neutrons). Two isotopes of oxygen, ¹⁶O (lighter, more abundant) and ¹⁸O (heavier, less abundant), are present in ocean water. These isotopes are preserved in the ice of Greenland and Antarctica and are incorporated into the skeletons of microscopic organisms that dwell in the oceans.

Water that evaporates from the oceans is relatively enriched in the lighter ¹⁶O isotope. During cold periods when the polar ice sheets expanded, much of this water vapor was converted to ice, leaving the oceans smaller and relatively enriched in ¹⁸O. In contrast, warm periods are characterized by melting glaciers that return more of the ¹⁶O isotope to the ocean, increasing ¹⁶O concentrations. During normal (equilibrium) conditions, ¹⁶O is lost by evaporation but is returned to the oceans as precipitation, resulting in no net change in the oxygen isotope ratio. As ice sheets expand, the relative volume of ¹⁶O in the ocean decreases, driving up the ¹⁸O/¹⁶O ratio. When the ice melts the ¹⁶O is returned to the oceans, causing the ration to decline.

The lighter 16O isotopes evaporate with sea water  and are incorporated into continental ice sheets causing the oceans to become enriched in the 18O isotope.

The ratio of ¹⁸O/¹⁶O in ancient ice or in organism’s skeletons can be compared with standard values. The difference can be used to estimate the temperature of the air in which the ice (snow) was precipitated or of the water in which the microscopic organisms grew. The ratio acts as a paleothermometer for ancient climates. The ¹⁸O/¹⁶O ratio is higher at lower temperatures (when oceans are enriched in ¹⁸O), and decreases as temperatures increase. This relationship has been used to interpret climate conditions during the Cenozoic Era, representing the last (most recent) 66 million years of geologic time.

The Earth was much warmer 52 million years ago.  Scientists have discovered that the temperature contrast between the equator and poles was much less than present. Much of the southern U.S. would have experienced a tropical climate. Islands in northern Canada contain fossils of alligator-like reptiles from this period. This warm interval was followed by a long cooling trend from 52 to 36 million years ago. Increasing ¹⁸O/¹⁶O ratios in marine microfossils reflect the development of permanent ice caps in East Antarctica and a drop in southern ocean surface temperatures to 5-8^oC.

Global temperatures fluctuated between 36-20 million years ago and had a short warming trend from 20-16 million years ago, before another dramatic drop in temperature between 16-10 million years. This mega-cold snap resulted in the development of glaciers on Greenland. There was a brief warming from 5-3 million years ago that was followed by a third cooling episode that continued to the present.

Short-term Climate Changes
Pollen - Plants produce pollen grains that collect in sediment on the bottom of ponds, lakes and oceans. Pollen can be analyzed from cores of sediments and can be used to obtain records of changing plant communities that reflect decade-scale climate changes stretching back several thousand years. For example, a dramatic cold period in Europe approximately 11,000 years ago (called the Younger Dryas) was marked by the appearance of the pollen of the polar wildflower Dryas octopetala in sediments.

Graph of temperature changes during last 18,000 years illustrates the rapid drop in temperature during Younger Dryas event (~11,000 years ago) and the smaller changes associated with the Little Ice Age (LIA) and Medieval Warm Period (MWP).

Vegetation distribution 12,000 years ago and today based upon pollen data. Dark green = evergreen forest; light green = mixed forest; orange = deciduous forest; black = prairie; purple = tundra. Red = no equivalent modern species. Large white area in left figure is extent of the continental ice sheet. Graphic modified from NOAA Paleoclimatology Program image.

Oxygen Isotopes - Oxygen isotope ratios in modern corals reveal temperature changes over periods up to hundreds of years on a year-decade scale that can not be distinguished in long-term climate change events. Oxygen isotopes in ice cores drilled from Antarctica and Greenland can provide a detailed climate history extending back over 200,000 years. Individual ice layers can be dated much like tree rings to determine their age and the air bubbles trapped within each layer are used to learn about climate variations. Dust and pollen particles trapped in the ice also yield clues to ancient climates.

Dendrochronology (Tree Rings Research) - Relatively short-term climate change can also be distinguished from tree ring records. Live trees add a new growth ring each year, therefore counting the rings in a dead tree reveals its age. The width of the rings can be used to decipher precipitation history during tree growth; wide rings occur during wet, warm years, narrow rings during cool or dry years. A single Huron Pine tree from Tasmania, Australia, was 2200 years old and revealed climate fluctuations from 270 B.C. to 1973. Records for several trees were used to graph temperature fluctuations over the last 1100 years. Scientists can use tree rings to determine climate conditions up to 8,000 years ago by matching tree ring patterns from trees of different ages.

Recently published tree-ring data reveals that the 20th century was the warmest of the last 600 years and that very short term climate fluctuations (1-5 years) were the result of large-volume volcanic eruptions that prompted periods of global cooling.