Paleoclimatology

Introduction

Paleoclimatology (or Palaeoclimatology) is the study of past climate changes throughout Earth history. Earth's climate has varied substantially over its long history, on a wide range of timescales. However, records of climate from satellites and meteorological stations are limited to the past 150 years or so. Everything we know about climate prior to about 1850 is, therefore, inferred via other methods, including analysis of historical data, direct measurements of past conditions (such as carbon dioxide [CO₂] in ice cores), and measurements of pale-oclimate “proxy” data. Historical or documentary evidence for past climatic conditions includes written temperature and precipitation records that can be found in ship logs, farm records, journals, and newspapers. These records, while useful, are limited to recent centuries.

The vast majority of paleoclimate research has therefore focused on direct and proxy-based climate reconstructions. Direct records of past change can be obtained by measuring the composition of gases trapped in bubbles in glacial ice, by measuring the temperature in bore-holes, by mapping the extent of past glaciers, and by looking at variations in ocean sediment pore-water chemistry. These different measurements provide direct records of atmospheric composition, surface temperature, and ocean chemistry at times in the past, all of which are closely related to climate. Paleoclimate proxy records are obtained by measuring some chemical, physical, or biological parameter preserved in a natural archive whose incorporation in that archive was controlled by environmental conditions at the time of formation. There are numerous types of paleoclimate archives, including ice cores, ocean sediments, tree rings, fossil pollen, corals, and speleothems (cave deposits such as stalagmites). Each of these archives has specific advantages and disadvantages and tells us about different aspects of the past climate. By studying multiple records from different time periods and different locations around the world, paleoclimatologists have built up a clear picture of past climatic conditions over millions of years. This paleoclimate record has helped us understand not only how climate has varied in the past, but why it has varied, and how it could possibly vary in the future.

We know that climate has been constantly changing throughout geologic time, though the farther back in time one goes, the more difficult it is to reconstruct past climates due to problems of dating and poor preservation of archives. Over million-year timescales, much climate variability has been driven by plate tectonics and the associated changes in the atmosphere, ocean, and biosphere. In contrast, climate change during the Quaternary period (about the past 2 million years), on 100,000-year timescales or less, has been largely caused by changes in Earth's orbit around the sun, and the associated changes in Earth's albedo (reflectivity) and greenhouse-gas composition of the atmosphere, both natural and anthropogenic (human-caused). Paleoclimatologists have largely focused on studying Quaternary climate change because there are abundant natural archives available during this period; there are adequate methods for dating these archives (geochemical and physical); it was a period of major environmental change; and, most importantly, understanding natural climate variability over this time period will help us to understand, predict, and prepare for future climate changes.

Historical Background and Scientific Foundations

One of the most important advances in paleoclimate science stemmed from the suggestion of Serbian mathematician Milutin Milankovitch (1879-1958) that past glaciations were principally controlled by changes in Earth's orbit, the so-called Milankovitch (or orbital) theory. Three major orbital parameters are known to vary periodically and produce measurable changes in

the amount and distribution of solar insolation (incoming solar radiation) that Earth receives: obliquity (or tilt) of Earth's axis; precession (or wobble) of Earth's axis; and eccentricity (or shape) of Earth's orbit around the sun. The first major test of this hypothesis was conducted in the mid-1970s. Scientists measured chemical and biologic proxies of global ice volume and sea surface temperature in a marine sediment core and found that most of the variability in these proxies corresponded closely to changes in Earth's orbit. Following numerous other studies, largely from marine sediment cores and ice cores, it is now well accepted that Earth's climate has fluctuated regularly, with approximately 100,000-year-long cycles between glacial and interglacial conditions (interglacials being the warmer intervals between ice ages) during the Quaternary and that these ice ages were controlled largely by orbital changes. Studying past ice ages allows scientists to better understand how the climate system responds to large changes in the radiative balance (the difference between incoming and outgoing solar radiation).

Although Quaternary glaciations were forced by orbital changes, numerous feedbacks amplified these temperature changes. Paleoclimate records from the EPICA ice core in Antarctica, for example, reveal a very close relationship between Antarctic temperatures, as recorded in the ice chemistry, and global atmospheric CO₂ concentrations over at least the past 800,000 years. Although the exact causes of glacial-interglacial CO₂ changes are still not well understood, it is likely that they were governed by changes in oceanic and biologic processes that affected the carbon cycling between ocean, atmosphere, and land. In any case, the lower CO₂ concentrations during glacial periods served to amplify orbitally induced glacial cooling, and higher concentrations during interglacials amplified warming due to the well known effect of CO₂ on Earth's radiative balance through the greenhouse effect. Based on the ice core records, we know that the current atmospheric CO₂ levels are higher than any period in the past 650,000 years.

Another important feedback is the ice albedo effect, in which greater amounts of ice result in increased reflection of incoming solar radiation back to space so that more ice amplifies cooling and less ice amplifies warming. Numerous other feedbacks are also important, including those related to land surface and vegetation changes. A major goal of paleoclimatologists, therefore, is reconstructing how and why CO₂ and other greenhouse gases, continental ice sheets, sea ice, and vegetation have varied in the past. This is accomplished through studying modern systems, paleoclimate records, and computer modeling.

Detailed paleoclimate studies of the last glacial period, which began about 116,000 years ago and culminated in the Last Glacial Maximum (LGM) about 21,000 years ago, have revealed numerous millennial-scale climate fluctuations that cannot be explained by orbital theory. There is good evidence from marine sediment cores that these abrupt climate changes, in which climate changed very quickly compared to the forcing, were linked with changes in ocean circulation. Abrupt cold events were sometimes caused by freshwater release to the North Atlantic through massive iceberg discharge (Heinrich events) or glacial meltwater. The cause of abrupt warmings, known as Dansgaard-Oeschger events, are less well understood, though also linked to changing ocean circulation. These abrupt climate changes had global effects that were most pronounced in the North Atlantic region, though global mean climate probably didn't change much.

Climate during the current interglacial, or Holocene, though generallymorestablethantheprecedingglacial period, has been shown to vary on decadal to centennial timescales, possibly due to changes in volcanic or solar forcing. Significant changes in the frequency of tropical cyclones, droughts, and El Niño events, as well as large changes in African-Asian summer monsoon rainfall, very likely occurred over this time period. Notable climatic events during the Holocene include the “8.2 event” (about 8,200 years ago), which was actually caused by a large pulse of glacial meltwater to the North Atlantic, the Medieval Warm Period (about AD 950-1100), and the Little Ice Age (about AD 1600-1850). The latter two events were probably due to changes in volcanic and/or solar activity. Much effort has been spent on reconstructing temperature over the last 2,000 years (based largely on tree rings) in particular, because this time period is highly relevant for assessing the human impact on climate. It is now considered likely that Northern Hemisphere temperatures in the second half of the twentieth century were warmer than any other period in the last 1,300 years and very likely that these temperatures were warmer than any period in the last 500 years.

Impacts and Issues

Paleoclimatologists have made significant advances in recent decades, with improved methodology yielding numerous high-resolution, well-dated paleoclimate records. In addition, integration of paleoclimate data with computer models of past climate has contributed to a better understanding of climate change mechanisms. Models are important for testing hypotheses about the physical mechanisms of climate change, like the Milankovitch theory and ocean circulation change. The ability to describe these processes mathematically is the best way to apply our knowledge about past climate change to predicting future change. Paleoclimate records are therefore widely used to test climate models. If a model can accurately reproduce past climates in which we know what the forcings were, we can be more confident in its ability to predict future changes.

Despite these advances, there is a great deal that we still don't know about how the climate system works. For example, mechanisms of abrupt changes in ocean circulation, sea level, ice sheets, drought frequency, El Niño events, and monsoon strength are still not well understood. More and better paleoclimate records and models are needed to understand the nature and cause of these issues in the past in order to understand the potential risk and prepare for future abrupt changes. In general, developing new climate records from more locations and over greater time periods, including earlier warm periods in Earth history, such as the Mid-Pliocene 3.3-3.0 million years ago) and the Paleocene Eocene Thermal Maximum (55 million years ago), will help reduce key uncertainties.

WORDS TO KNOW

ALBEDO: A numerical expression describing the ability of an object or planet to reflect light.

INTERGLACIAL: Geological time period between glacial periods, which are periods when ice masses grow in the polar regions and at high elevations. The world is warmer during interglacials. The world is presently experiencing an inter-glacial that began about 11,000 years ago.

PROXY DATA: Information (data) about past climate obtained indirectly from various long-lasting physical traces left by climate or weather, such as oxygen isotopes in ancient ice layers, tree-rings, coral reef layers, and more. “Proxy” means a thing that represents something else: in this case, directly measurable quantities stand for or represent ancient climate conditions.

RADIATIVE BALANCE: The balance between incoming solar radiation and outgoing infrared radiation.

SOLAR INSOLATION: The measure of electromagnetic radiation from the sun that reaches Earth. When measured over a specific surface area, solar insolation is often expressed watts per square meter (W/m²) or kilowatt-hours per square meter per day.

Another major challenge is in understanding exactly how changes in proxy data relate to changes in climate. Proxy interpretation is complicated by factors such as seasonal bias and multiple climatic influences. Proxies are therefore calibrated by measuring them in modern samples and comparing them with available instrumental climate data over the same time period to test exactly how they are recording climate. Some proxies are better understood than others and new proxies are still being tested and developed. Given these complications, paleoclimatologists prefer to use a multi-proxy approach in which several different climate proxies are utilized to more accurately reconstruct past change.

IN CONTEXT: PALEOCLIMATE INFORMATION

“Paleoclimate information supports the interpretation that the warmth of the last half century is unusual in at least the previous 1300 years. The last time the polar regions were significantly warmer than present for an extended period (about 125,000 years ago), reductions in polar ice volume led to 4 to 6 metres of sea level rise.”

• Average Northern Hemisphere temperatures during the second half of the 20th century were very likely higher than during any other 50-year period in the last 500 years and likely the highest in at least the past 1300 years. Some recent studies indicate greater variability in Northern Hemisphere temperatures than suggested in the TAR [the IPCC's Third Assessment Report in 2001], particularly finding that cooler periods existed in the 12 to 14th, 17th, and 19th centuries. Warmer periods prior to the 20th century are within the uncertainty range given in the TAR.”
• Global average sea level in the last interglacial period (about 125,000 years ago) was likely 4 to 6 m higher than during the 20th century, mainly due to the retreat of polar ice. Ice core data indicate that average polar temperatures at that time were 3 to 5°C higher than present, because of differences in the Earth's orbit. The Greenland ice sheet and other Arctic ice fields likely contributed no more than 4 m of the observed sea level rise. There may also have been a contribution from Antarctica.”

…As formally approved at the 10th Session of Working Group I of the Intergovernmental Panel on Climate Change (IPCC), Paris, France, February 2007.

SOURCE: Solomon, S., et al, eds. Climate Change 2007: The Physical Science Basis: Contribution of Working Group I to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change. New York: Cambridge University Press, 2007.

See Also Abrupt Climate Change; Heinrich Events; Ice Ages; Ice Core Research; Milankovitch Cycles; Temperature Record.

BIBLIOGRAPHY

Books

Solomon, S., et al, eds. Climate Change 2007: The hysical Science Basis: Contribution of Working Group I to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change. New York: Cambridge University Press, 2007.

Periodicals

Alley, R. B., and A. M.Ágústsdóttir. “The 8k Event: Cause and Consequences of a Major Holocene Abrupt Climate Change.” Quaternary Science Reviews 24 (2005): 1123-1149.

Alley, R. B., et al. “Abrupt Climate Change.” Science 99 (2003): 2005-2010.

Clark, P. U., et al. “The Role of the Thermohaline Circulation in Abrupt Climate Change.” Nature 415 (February 21, 2002): 863-869.

EPICA Community Members. “Eight Glacial Cycles from an Antarctic Ice Core.” Nature 429 (June 10, 2004): 623-628.

Hays, J. D., et al. “Variations in the Earth's Orbit: Pacemaker of the Ice Ages.” Science 194, 4270 (1976): 528-530.

Mann, M. E., et al. “Northern Hemisphere Temperatures During the Past Millennium: Inferences, Uncertainties, and Limitations. Geophysical Research Letters 26 (1999): 759-762.

Kathleen R. Johnson