Paleoclimates and paleoclimate change

Reconstructing climates from the Earth’s geologic past and human prehistory provides a context for evaluating the extent to which the present global warming trend is an anthropogenic phenomenon. Paleoclimatic records also allow projections of the impacts of global warming on natural ecosystems and human society.

Background

The Earth’s climate has not been uniform in geologic or even historic time. Climatic changes form the backdrop for much of human prehistory and are viewed by some as a driving force in the rise and fall of civilizations. Looking further back into the geologic record, the abrupt transitions between eras indicate periods of rapid climatic change. Despite decades of intensive research, there remain many uncertainties and unanswered questions about the causes and impact of climatic change, particularly in remote geologic time.

Paleoclimatology as a science owes its beginnings to the work of Louis Agassiz (1807-1873), who in 1840 tuned his attention from the study of fossilized fish to observations on glaciers in Switzerland and concluded that the presence of characteristic glacial geologic formations in widely separated localities indicated that an had once covered northern Europe. At about the same time, systematic study of fossilized plants in Britain, Germany, and Pennsylvania suggested that the climate at a more remote period had been tropical. as a scientific discipline remained largely a tool to study ancient ecosystems until the last quarter of the twentieth century, when the focus turned to understanding mass extinctions in the remote past and historical climatic extremes in order to predict and manage global warming in the present.

Research Techniques

Systematic instrumental records of temperature, precipitation, and wind speed in parts of Europe and North America exist for the last 150 years, with spotty records for the eighteenth century. Data such as timing of the grape harvest in Southern Europe and the maximum extent of the annual flooding of the Nile extend the range of human observations several centuries backward in time, and historic and even legendary sources chronicle catastrophic events throughout human history.

Most paleoclimatology relies on the study of proxies, of which tree rings (dendrochronology) are a good example. If a variation in ring width in historic times correlates with fluctuations in temperature or precipitation, a similar pattern in the more remote past indicates similar fluctuations. Within the roughly four-thousand-year lifespan of the oldest living trees, dendrochronology provides very high resolution of climatic fluctuations. It has helped establish that the El Niño-Southern Oscillation (ENSO) cycle of wet and dry years has occurred for many centuries in coastal Peru and the American Southwest. Some corals and mollusks also develop annual rings that are indicative of fluctuation in water temperature at a given locality.

The type of vegetation and species present in a region are both good climatic indicators. Leaves, woody material, and particularly pollen occur in abundance in bogs, lake sediments, and sedimentary rocks. Plant species and genera have changed very little over the last few million years, making it a safe assumption that the environment in which a fossil was deposited closely resembled the environment where the same species occurs today. The genus Metasequoia (dawn redwood) today thrives in wet temperate climates. Its occurrence in a fifty-million-year-old fossil assemblage of the Eocene Age on Ellesmere Island in the Canadian Arctic is one of many pieces of evidence for an unusually warm period in geologic time.

Pollen analysis is a powerful tool, because pollen is extraordinarily resistant to decay, many pollen grains can be identified to genus, and analysis of relative abundance provides a fairly complete picture of the vegetation that produced it. Pollen in bog cores provided the first evidence for the Younger Dryas, an episode of drastic cooling in Europe between 12,900 and 11,500 years ago. The prevalence of pollen of an arctic plant, Dryas octopetala, indicated an abrupt return to arctic-tundra conditions within a ten-year period.

Further back in the geologic record, general morphology of plant fossils can be used as a climate proxy. Forest vegetation indicates relatively high precipitation; broad-leaved evergreens, a tropical or subtropical climate; and slow-growing woody plants with small vessel elements, a semiarid ecosystem.

The abundance and species composition of oceanic plankton are highly sensitive to water temperatures. Foraminifera (protozoa) and diatoms (algae) produce distinctive resistant outer coatings. The ratio of stable isotopes of carbon and oxygen in carbonate-containing marine sediments reflects their concentration in seawater. Plants selectively use carbon 12 in photosynthesis, so oceanic sediments become enriched in carbon 13 during warm, moist periods. Evaporated water is enriched in oxygen 16 relative to oxygen 18, leaving seawater with a higher proportion of oxygen 18 during periods of glaciation.

There are many geologic indicators of past climatic conditions. Cool, arid conditions produce deposits of (windblown dust). Volcanic ash distributed over a wide area may signal the onset of a cooling period. Low sea levels indicate glaciation, and high sea levels indicate warming episodes. accumulates on land during cool wet periods. Continental glaciers produce characteristic scouring of rocks and glacial moraines. Ice-rafted debris found far from the continental margin, especially at low latitudes, indicates extensive glaciation.

The effects of geological and astronomic forcing mechanisms can be modeled based on a knowledge of plate tectonics and changes in the Earth’s orbit and solar luminosity, and the results can be compared with proxy measurements. The relative proportions of sea and land mass and the latitudinal distribution of continents have changed substantially over nearly four billion years of life on Earth, but continental drift occurs too slowly to account for dramatic climate changes in the Phanerozoic. do, however, spawn massive episodes of volcanism, such as those associated with the Permian-Triassic and Miocene-Pliocene transitions. Milanković cycles, which predict changes in the absolute amount of solar radiation reaching the Earth, show some positive correlation with Pleistocene pulses. Cores taken from glaciers in Greenland and Antarctica provide evidence of climate over the last 400,000 years in the form of rates of precipitation, amounts of atmospheric dust, and concentrations of in trapped air.

Climatic Change in the Geologic Record

Scientists who developed the geologic timescale used today based their division into eras and periods on marked discontinuities in both the inorganic constituents and fossils in sedimentary rocks. The discovery of based on decay of uranium at the end of the nineteenth century allowed an absolute time scale to be superimposed on the stratigraphic sequence. From a climatic perspective, the periods represent spans of millions of years, during which global climate was relatively uniform, divided by much briefer spans that were characterized by climatic extremes, elevated extinction rates, and the subsequent emergence of numerous new taxa.

The geologic record is punctuated by five recognized episodes of catastrophic extinction, when 60-80 percent of the then extant species disappeared in less than a million years. The best-known of these is the end-Cretaceous event marking the end of the dinosaurs 65 million years ago. Most biologists accept that the precipitating factor was an asteroid collision. The end-Ordovician extinctions, 440 million years ago, coincided with a drastic lowering of the sea level, possibly representing extensive glaciation. The causes of the late Devonian and Triassic-Jurassic extinctions are uncertain. From the point of view of present policy makers attempting to learn from the lessons of geologic history, the Permian-Triassic extinctions, 250 million years ago, are probably the most instructive, both because they were the most profound and because the postulated causes most closely mirror global anthropogenic changes that are rapidly creating what Richard Leakey and others have termed “the sixth extinction.”

A somewhat controversial interpretation of late Precambrian geologic formations postulates a “snowball Earth” between 790 and 630 million years ago. This period encompassed three or possibly four major glacial episodes, during which the Earth’s oceans may have completely frozen, halting the evolution of multicellular life. At the time, most of the Earth’s land mass centered over the South Pole. There is also evidence of major glaciation during the Huronian, 2 billion years ago. The postulated cause is depletion of methane due to release of oxygen by photosynthesis.

For most of the Phanerozoic, temperatures on Earth, as inferred from sea levels, isotope ratios, and vegetation at high latitudes have been significantly higher than they are at present. The late Cretaceous and the Eocene were particularly warm periods, characterized by temperate deciduous forests in Antarctica and the Canadian Arctic, and tropical jungles in Central Europe and the Pacific Northwest. The past 50 million years have witnessed gradual cooling and increasing aridity. Pronounced aridity and a drop in sea level in the Pliocene caused both the Mediterranean and Black Seas to become landlocked and shrink to a fraction of their present size.

Climatic Change in Human Prehistory

The tenure of modern humans on Earth encompasses the last Pleistocene glaciation and the ten thousand years of the Holocene, during which the Earth’s climate has fluctuated, with a temperature maximum roughly six thousand years ago and a temperature minimum, the Little Ice Age, from the fourteenth to early nineteenth centuries. There are many studies correlating prehistoric cultural changes with climatic changes. For agricultural societies, the droughts associated with colder periods are more devastating than lower temperatures themselves. The historic and geologic records contain no compelling evidence of rapid rises in temperature such as the Earth is currently experiencing—global warming in geologic time appears to be a gradual process to which life adapts itself. Cooling, on the other hand, can be extremely rapid and catastrophic. Massive volcanic eruptions cause by ejecting fine ash and sulfates into the atmosphere. Historic cold episodes beginning in 1470B.C.E., 535 C.E., 1315, and 1815 are dwarfed by the eruption of Mount Toba on the island of Sumatra seventy-four thousand years ago, which is believed by some to have nearly wiped out the human race.

Context

Probably the most important lesson to be learned from paleoclimatology in respect to global warming is that of the disruption of North Atlantic currents and resulting deep freeze in Europe in the 8.2ka event. Very rapid melting of the Greenland and release of freshwater into the Atlantic could well mimic the effects of the rapid draining of glacial Lake Agassiz through the St. Lawrence at the end of the Wisconsin Glaciation. The other lesson to be learned, although it does not so readily translate into international policy making, is that a massive volcanic eruption could, within a period of a few months, cause a substantial drop in global temperatures and global agricultural productivity, a much grimmer in today’s overpopulated and environmentally degraded world than in 1315 or 1815.

Key Concepts

• astronomical forcing: climatic change triggered by changes in solar luminosity, variation in the Earth’s orbit, and bolide impact
• Milanković cycles: variations in the eccentricity of the Earth’s orbit, the tilt of the Earth’s axis, and the precession of equinoxes, resulting in climatic variation on a scale of tens of thousands of years
• positive climate feedback loops: self-reinforcing climatic processes, such as increased snow cover increasing planetary albedo, promoting additional cooling and therefore more snow cover
• proxies: measurable parameters, correlated with climate, that are preserved in the geologic record

Bibliography

Alverson, Keith D., Raymond S. Bradley, and Thomas Pedersen, eds. Paleoclimate, Global Change, and the Future. Berlin: Springer Verlag, 2003.

"Climate Change in the Context of the Paleoclimate." National Centers for Environmental Information, 15 Dec. 2022, www.ncei.noaa.gov/news/climate-change-context-paleoclimate. Accessed 12 Dec. 2024.

Cronin, Thomas M. Principles of Paleoclimatology. New York: Columbia University Press, 1999.

Pap, Judit M., and Peter Fox, eds. Solar Variability and Its Effects on Climate. Washington, D.C.: American Geophysical Union, 2004.

Saltzman, Barry. Dynamical Paleoclimatology: Generalized Theory of Global Climate Change. New York: Academic Press, 2002.