February 13, 2002
Global Climate Change and Plate Tectonics
Solar Radiation, Earth's Albedo, and the Greenhouse effect
Change in Earth's climate has had a profound effect upon life on Earth, causing both terrestrial and marine animals to migrate and in some cases leading to the extinction of numerous plant and animal species. Earth's weather patterns and climate are driven by electromagnetic radiation emanating from our Sun. Our Sun produces radiation across a very broad range of wavelengths, but its peak output is within the part of the spectrum that is visible to us (between the wavelengths termed infrared and ultraviolet, which we cannot see, but which can be detected by equipment that can produce an interpretation in visible light). The peak wavelength of radiational output for a body (be it human or stellar) is determined by its temperature (the Sun's spectral profile is shown in this figure, along with that of one hotter and two cooler stars). The relatively cool Earth also radiates energy (primarily energy from absorbed sunlight), largely in the infrared wavelengths.
The balance of energy influx and outflow determines how warm Earth's surface will be (on average). Importantly, not all of the incident solar radiation is absorbed by the Earth; a sizable proportion is reflected directly off the Earth before it can be absorbed. The proportion of incident sunlight that is reflected in this manner is termed Earth's albedo. Earth's albedo is not uniform over the surface but varies depending upon whether the surficial cover is locally light-colored or dark-colored. Deep blue oceans and deep green forests have a low albedo and absorb a lot of sunlight. Grasslands, deserts, clouds, and ice (especially the latter two) have a higher albedo, reflecting a greater amount of sunlight (and thus tending to cool the Earth).
Additionally, not all of Earth's infrared radiation escapes into space. Some of it is trapped by greenhouse gases (water vapor, methane, and carbon dioxide, which traps the greatest amount of radiation because it is much more common in the atmosphere than are the other two gases). Greenhouse gases are transparent with respect to visible light (they are colorless and clear), so they do not block incident sunlight, but they keep the Earth warmer than it would otherwise be (this is the greenhouse effect) by blocking the escape of infrared rays.
Important sources of atmospheric carbon dioxide are volcanism, animal respiration, bacterial decomposition of dead matter, and burning of fossil fuels such as coal, oil, and natural gas (methane). Important sinks for carbon dioxide are the production of organic carbon through photosynthesis, the burial of organic carbon and limestone (calcium carbonate), and the chemical weathering of rocks. Rainfall strips carbon dioxide (CO2) from the atmosphere and brings it in contact with rock, where chemical reactions break down minerals within the rock, yielding CO2-bearing ions that remain in aqueous solution. Rates of chemcial weathering are greatly influenced by the presence of mountains (large areas of exposed rock) and vegetation (which increases weathering rates by chemically altering its surroundings).
Changes in Earth's albedo and the magnitude of the Greenhouse effect are the immediate causes of global temperature change. Given the complexity of the Earth-atmosphere-ocean system, there are a number of ways that climate can change; this change may be reinforced by positive feedback or opposed by negative feedback. As an example of positive feedback, a cooling Earth might lead to the expansion of glaciers in the Northern Hemisphere. As these glaciers overrun boreal forests, they increase the albedo of the Earth, making conditions even colder. As an example of negative feedback, a warming Earth leads to greater rates of evaporation. Greater amounts of water vapor in the air will lead to condensation in the form of clouds, which increase Earth's albedo, cooling the Earth.
Paleothermometers and Indicators of Ancient Atmospheric Chemistry
A valuable method available for determing how warm the Earth was during the geologic past is the examination of ratios of the oxygen isotopes O-16 and O-18 in marine sediments. Warm periods of Earth history are characterized by high rates of sea water evaporation. Overwhelmingly, the molecules of water which evaporate to the atmosphere contain the light isotope, O-16, leaving the remaining sea water enriched in the heavy isotope, O-18. The warmer the weather, the more sea water is evaporated, leaving the oceans more and more depleted in O-16. Most open ocean sediments consist of the skeletons of tiny planktonic organisms that incorporate oxygen from the sea water into their shells, preserving an ancient record of the proportions of the two oxygen isotopes in the sea.
Additionally, local climates can be assessed on the basis of plant and animal fossils of species with temperature-restricted ranges. Especially helpful in this regard is the proportion of leaf fossils bearing serrated edges. In the past, as today, the proportion of leaves with serrated margins as compared to those with smoothe margins increases from the equator toward the poles.
To look back beyond the few years for which direct measurements of atmospheric carbon dioxide have been made, important indicators of past atmospheric chemistry include fluid inclusions, such as air bubbles trapped in amber or ice. Massive amounts of buried carbon in the rock record (as in rocks deposited during the Pennsylvanian period, a time of extensive coal swamps) indicate carbon dioxide was being pulled from the atmosphere by plants, but was not returned by decomposing bacteria. Thus the atmosphere at the time of extensive carbon burial was likely rich in oxygen and depleted in carbon dioxide.
Earth's Climatic History
For most of Earth history, the planet has been considerably warmer than the present. Long stretches of time were dominated by greenhouse climate, characterized by high levels of atmospheric carbon dioxide fostering a strong greenhouse effect, high rates of evaporation and rainfall, a lack of continental glaciation, and as a result, continents that were more greatly submerged by a swollen global ocean. (During the Cretaceous, for example, polar Antarctica was covered with lush temperate forest while the ocean cut across North America.) These warmer times have been interrupted by spans of icehouse climate, characterized by cold temperatures, ice covering a portion of the continents, relatively low sea level (because water is bound up in glaciers), and low levels of atmospheric carbon dioxide.
Long term factors that determine whether greenhouse or icehouse conditions prevail are 1) the proportion of atmospheric carbon dioxide, and 2) the positions of the continents. Low levels of carbon dioxide and the presence of a large continental mass over one of the poles favor the formation of glacial ice and albedo-fed icehouse climate. Plate tectonic activity plays an important role in both of these factors, because the continents are pushed around the globe by mid-ocean ridge volcanoes that release carbon dioxide.
The coldest span of icehouse climate in Earth history occurred during the late Proterozoic. Jumbled glacial deposits termed tillites produced by late Proterozoic glaciers have been found on all continents, suggesting that glacial ice approached the equator (a unique event in Earth history). Greenhouse warmth prevailed from the end of the Proterozoic through the Cambrian and most of the Ordovician. There was a short-lived episode of late Ordovician glaciation, but the greenhouse quickly recovered.
The next major span of icehouse climate occurred in the late Paleozoic (Mississippian, Pennsylvanian, and part of the Permian). A massive supercontinent (Gondwana) covered the south pole at the same time that carbon dioxide was being pulled from the atmosphere by the highly successful plants of the coal swamps. Their swampy surroundings buried these plants before they could be decomposed, so their carbon content was never recycled to the atmosphere. Carbon dioxide levels rebounded at the end of the Permian, and conditions were relatively warm through the Mesozoic and the first half of the Cenozoic.
Late Cenozoic Climatic Deterioration (from Greenhouse to Icehouse to the Ice Age)
Since the Oligocene epoch (30-35 million years ago), icehouse conditions have prevailed. Glaciers did not advance towards Illinois overnight, but rather climatic deterioration took place over a span of millions of years in a number of steps (see figure 7.14 in your text, p. 107). The onset of icehouse conditions (marked by glacial ice beginning to cover Antarctica) was influenced by a number of factors, including the collision of India with Asia to produce the Himalayas (a C02 sink), the development of encircling currents around Antarctica (so that the cold waters surrounding it are not mixed with warmer waters from the north), and the spread of grasses (grasses reflect more sunlight than do forests and they require less rainfall, so they are well suited to relatively dry habitats that are common during times of icehouse climate). At the end of the Miocene, Earth became yet cooler because North America and Eurasia began to surround and constrict the Arctic ocean, and the north polar ice cap (consisting of pack ice as well as continental glaciation) built up over the course of the Pliocene epoch.
The last 2.7 million years are somewhat informally termed the "Ice Age," during which glaciers have extended at times into the interior of Eurasia and North America. The critical factor that allowed this great advance appears to be the collision of North and South America (which were unconnected prior to this time). This deflected warm, equatorial Atlantic ocean water northward, which increased the amount of precipitation (winter snowfall) that fell over northern Europe and (what is now) Canada. The ice over these snow centers built up so high that it began to spread out to the south under the pressure of its own weight.
The Ice Age is climatically divided into glacial episodes (times of maximum glacial advance over the continents) and interglacial episodes, such as the Holocene in which we live (the last 10,000 years of Earth history), during which the glaciers have retreated back toward the poles. (Despite the warm weather of the past few years, we are still in the Ice Age; unless carbon dioxide levels rise greatly enough to melt polar ice over the long term, glaciers are expected to return to North America in a few tens of thousands of years.)
The advance and retreat of ice sheets during times of icehouse climate appears to be strongly influenced by three periodic fluctuations in Earth's orbit about the sun. The eccentricity (deviation from a circle) of Earth's elliptical orbital path around the Sun varies in a cycle of approximately 100,000 years. A greater amount of eccentricity means that Earth-Sun distance varies substantially over the course of a year from perihelion (closest approach to the sun) to aphelion (orbital point farthest from the Sun).
Earth is currently tilted at 23.5 degrees from a perpendicular axis to Earth's orbital plane about the Sun. As Earth progresses in its orbit, this axial tilt creates the familiar seasonal variation (northern winter occuring when the north pole is tilted away from the Sun, as on the right side of this figure). The current amount of tilt is approximately in the middle of the range of historical tilt values, which vary by a few degrees over a cyclical span of about 40,000 years. Greater amounts of tilt increase the magnitude of seasonality (leading to hotter summers and colder winters).
The third and final cyclical change in Earth's orbit (with a period of about 20,000 years) is precession (sometimes termed the precession of the equinoxes). The Earth wobbles in its orbit like a spinning top, so the direction (with respect to the fixed background of stars) in which the north pole points varies over time. This change in direction also brings a change in the timing of the onset of the seasons with respect to Earth's position in its orbital path. Currently (part A of this figure) the northern summer soltice occurs near aphelion, and the north pole points toward a star in Ursa Minor named Polaris (the famous north star). About ten thousand years ago, the north pole pointed toward the bright blue star Vega in the constellation Lyra, and the onset of northern summer occurred near perihelion (part B).
During icehouse conditions, glacial advance is favored by relative warm, wet (snowy) winters (at very cold temperatures, little snow falls because the air cannot hold much moisture) and cool summers that minimize glacial melting. Thus conditions are optimal for glaciation in the Northern Hemisphere when the orbit is highly eccentric (which is opposite the state of the present orbit, which is relatively circular), when tilt is at a minimum, and when the northern summer occurs at aphelion (as in the present) so that the northern summer is moderated by a great distance between the Earth and Sun. That we are currently in an interglacial episode suggests that orbital eccentricity is a more important factor than precession.