Key Climate Intervals in the Earths Past

This chapter looks at the Earth's climate over geologic time and demonstrates when key long- and short-term changes took place. Cli-matologists have been able to reconstruct a representative climatic time line by tapping into natural storehouses of data, such as corals, ocean and lake sediments, ice cores, fossil pollen, tree rings, and other physical clues left behind from natural processes. These types of data, known as proxy data, are natural records of climate variability. Through the analysis of proxy records, scientists can extend their understanding of past climate backward in geologic time. This chapter first looks at the Earth's most ancient climates (2 million to 4.6 billion years ago), then progresses to the Pleistocene climates (10,000 years to 2 million years ago), and finally looks at the Holocene/recent climates. It also addresses what climatologists have identified as the key uncertainties.

THE EARTH'S GEOLOGIC PAST

Being able to look to the past has helped climatologists better understand long-term processes relating to the Earth's climate. Although lack of usable data means that much of the most ancient time intervals is not understood as well as are more recent time intervals, as new discoveries are made and analysis techniques are improved, the Earth's past climate history becomes clearer.

In order to put the Earth's past in a historical context, it must be looked at on a timescale that spans 4.6 billion years—what geologists refer to as the geologic timescale. The Earth's history has been divided into major time intervals (see page 166 for Geological Timescale), the intervals determined by significant events in the history of the Earth, such as extinctions, glaciations, and changes in dominant life-forms; which is why the intervals all represent different lengths of time instead of preset consistent intervals. For example, the division between the Permian and Triassic periods (also the boundary between the Paleozoic and Mesozoic eras) corresponds to the Earth's largest mass extinction. At this point, 100 percent of the trilobites (a marine arthropod) were rendered extinct, 50 percent of animal families were lost, 95 percent of all marine species died, and many species of trees were lost. Geologists have determined this could have been caused by glaciation or volcanism.

The geological timescale consists of four division types: eons, eras, periods, and epochs. Eons are the largest intervals of geologic time— they can be hundreds of millions of years long. Eons are subdivided into eras. Eras are then subdivided into periods or even smaller epochs. Epochs are used for relatively young geologic deposits. This hierarchical classification scheme divides time periods based on significant events.

When looking at paleoclimate change throughout geologic time, scientists look at factors such as changes in solar output, changes in the Earth's orbit, precession, and tilt, changes in the positions of the Earth's continents, and changes in the atmospheric concentration of greenhouse gases. The following table illustrates what general temperature regimes and life-forms existed during different geologic periods.

Characteristics of Geologic Periods

GEOLOGiC PERIOD

AGE (MYA)*

GENERAL TEMPERATURE

DOMINANT LIFE-FORMS

Cambrian

550-480

Mild

Trilobites

Ordovician

480-415

Cold through hot

Plants, corals, fish

Silurian

415-390

Warm

Insects, vascular plants

Devonian

390-345

Hot

Land plants, amphibians

Mississippian

345-300

Warm

Winged insects

Pennsylvanian

300-270

Mild

Reptiles, ferns

Permian

270-230

Cooler

Amphibians, trees

Triassic

230-200

Hot

Dinosaurs, mammals

Jurassic

200-135

Hot

Dinosaurs, birds, conifers

Cretaceous

135-60

Warm

Dinosaurs, snakes, butterflies, marsupials

Tertiary

60-2.6

Cool

Mammals, grasses, birds

Quaternary

2.6-present

Warm

Humans

*million years ago

Concerning long-term change, scientists at the United States Geological Survey (USGS) believe it has been controlled primarily by plate tectonics and its influence on the atmospheric greenhouse effect. In plate tectonics, the Earth's continents "drift" on top of a fluidlike layer of the crust over geologic time. When the plates collide, some of the plate material is subducted (pushed under) the Earth's crust. The subducted crust

C02, H20

Plate tectonics provides one mechanism for carbon dioxide to be released to the hydrosphere, biosphere, and atmosphere.

can melt, forming a volcano on the Earth's crust directly above. Plate tectonics is responsible for releasing CO2 into the atmosphere. Having an understanding of plate tectonics has allowed climatologists to study and date the rocks and landforms on the Earth's surface and recreate models of where the continents were at different periods in time.

Some climatologists have suggested ancient periods of glaciation may have even been correlated with plate tectonics. The idea has been discussed that there may have been a drop in the production rate of CO2 by volcanoes, which would have lowered the CO2 content in the atmosphere, thereby lowering the Earth's temperature and making it prone to glaciation. Some scientists have suggested that global warming may result from rapid plate spreading and that global cooling may result from slowed plate spreading during the tectonic process.

There are also mid-term changes. These changes focus on what scientists refer to as the "astronomical theory of climate change" and concern the Milankovitch cycles. Named after Serbian astronomer Milutin M. Milankovitch, the Milankovitch cycles explain the changes in the Earth's seasons that result from changes in the Earth's orbit around the Sun. There are three types of orbital changes: (1) changes in the Earth's tilt, (2) changes in its eccentricity, and (3) changes in its precession. As the Earth revolves in its orbit around the Sun, these three separate cyclic movements—whose lengths of cycle all differ in duration—combine to produce variations in the amount of solar energy that reaches the Earth and help determine its climate.

The first cycle—change in the Earth's tilt—operates on a 41,000-year cycle. The Earth's tilt can vary from 22° to 24.5°. The smaller the tilt, the less seasonal variation there is between summer and winter at the middle and high (polar) latitudes. Today, the Earth's tilt is 23.5 degrees. When the tilt is smaller, winters are milder and summers are cooler, creating optimal conditions for the formation of glaciers and ice sheets.

Once glaciers and ice sheets build up, then positive feedbacks in the climate system come into play. A positive feedback is an interaction that amplifies the response of the system to what it is subjected to—it increases the condition (such as makes it colder, makes it hotter, etc). In this case, when the Earth is covered with more snow and ice, it reflects more of the Sun's energy into space, which causes additional cooling to occur. Scientists have also determined that the amount of CO2 in the atmosphere falls as ice sheets grow, which cools the climate further (another positive feedback).

The second cycle—eccentricity—deals with the shape of the Earth's orbital path around the Sun. The orbital path is not a perfect circle. It shifts through time and becomes more or less oval. This means that the Earth is slightly closer to the Sun at some times of the year than others. In a cycle of 100,000 years, the orbital path varies from nearly circular to very elliptical (oval). When the orbit is more circular (as it is today), the seasonal change in solar energy is not great (approximately 7 percent), but when the orbit is highly eccentric, the seasons are much more exaggerated (approximately 20 percent) and the lengths of the seasons are changed. This cycle affects the relative severity of summer and winter and helps control the growth and retreat of ice sheets.

Cool summers in the Northern Hemisphere, where most of the Earth's landmasses are located, allow snow and ice to exist until the next winter, which enables large ice sheets to develop over hundreds to thousands of years. Warm summers shrink the ice sheets by melting more ice than can accumulate during the winter.

The third cycle—change in precession—deals with the spin of the Earth on its axis. Like a spinning top that begins to wind down and wobble as it traces a small circular path, the Earth's axis does the same thing. This wobble is called precession, and it operates on a cycle of around 23,000 years. Because of this, the northern spin axis traces a circle through the nighttime sky. Currently, the Earth's North Pole points at Polaris, the North Star, but as the spin axis wobbles, it will not always point toward Polaris. At this time, the Earth is closest to the Sun in January and farthest away in July, but because of precession, in approximately 11,000 years, the opposite condition will exist, which will give the Northern Hemisphere more severe winters.

Paleoclimatologists can use this information to look at the climatic record over the past million years to identify any cause-and-effect relationships. They have been able to determine that there is a correlation between periods of low eccentricity (circular orbit) and glacial periods. In addition, the interglacial periods (the intervals between glacial periods) over the last 160,000 years show the cyclical patterns of the 41,000-year tilt cycle and the 23,000-year precession cycle.

According to Dr. Perry Samson, a meteorologist at the University of Michigan's Department of Atmospheric, Oceanic, and Space Sciences, other paleoclimate factors exist that also correlate with the Milankov-itch cycles and assist in the reconstruction and understanding of past climates. They include factors such as:

• the amount of dust in the atmosphere

• the reflectivity of the ice sheets

• the concentration of greenhouse gases

• characteristics of clouds, and

• the rebounding (uplifting) of land that has been previously depressed by the enormous weight of glaciers and ice caps.

Once the weight is removed, the ground slowly rises to the level it was before the weight of the ice pressed on it—a phenomenon called iso-static rebound.

A good understanding of the Milankovitch cycles has greatly helped scientists put together pieces of the puzzle to explain the advance and retreat of ice over the last 10,000 to 100,000 years.

Milankovitch And Glacials

O Infobase Publishing

The Milankovitch cycles are one method used to correlate cycles of the Earth's warming and cooling periods. The orbital patterns combine to vary the amount of incoming solar radiation (insolation) reaching the Earth at a given time. One theory is that ice ages are caused by changes in insolation.

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