Page 356 - Geosystems An Introduction to Physical Geography 4th Canadian Edition

320 part II The Water, Weather, and Climate Systems
• Earth’s elliptical orbit about the Sun, known as eccen- tricity, is not constant and changes in cycles over sev- eral time scales. The most prominent is a 100000-year cycle in which the shape of the ellipse varies by more than 17.7 million km, from a shape that is nearly circu- lar to one that is more elliptical (Figure 11.16a).
• Earth’s axis “wobbles” through a 26000-year cycle, in a movement much like that of a spinning top winding down (Figure 11.16b). Earth’s wobble, known as pre- cession, changes the orientation of hemispheres and landmasses to the Sun.
• Earth’s axial tilt, at present about 23.5°, varies from 21.5° to 24.5° during a 41000-year period (Figure 11.16c).
Although many of Milankovitch’s ideas were rejected by the scientific community at the time, these consistent orbital cycles, now called Milankovitch cycles, are today accepted as a causal factor for long-term climatic fluctua- tions, though their role is still being investigated. Scientific evidence in ice cores from Greenland and Antarctica and in the accumulated sediments of Lake Baikal in Russia have confirmed a roughly 100000-year climatic cycle; other evi- dence supports the effect of shorter-term cycles of roughly 40000 and 20000 years on climate. Milankovitch cycles ap- pear to be an important cause of glacial–interglacial cycles, although other factors probably amplify the effects (such as changes in the North Atlantic Ocean, albedo effects from gains or losses of Arctic sea ice, and variations in green- house gas concentrations, discussed just ahead).
Continental Position and Topography
In Chapters 12 and 13, we discuss plate tectonics and the movement of the continents over the past 400 million years. Because Earth’s lithosphere is composed of moving plates, continental rearrangement has occurred through- out geologic history (look ahead to Figure 12.13). This movement affects climate, since landmasses have strong effects on the general circulation of the atmosphere. As discussed in previous chapters, the relative proportions of land and ocean area affect surface albedo, as does the position of landmasses relative to the poles or equator. The position of the continents also impacts ocean currents, which are critical for redistributing heat throughout the world’s oceans. Finally, the movement of continental plates causes episodes of mountain building and spreading of the seafloor, discussed in Chapter 13. These processes affect Earth’s climate system as high-elevation mountain ranges accumulate snow and ice during glacial periods, and as CO2 from volcanic outgassing enters the atmosphere (from volcanoes) and the ocean (from spreading of the seafloor).
Atmospheric Gases and Aerosols
Natural processes can release gases and aerosols into Earth’s atmosphere with varying impacts on climate. Natural outgassing from Earth’s interior through volca- noes and vents in the ocean floor is the primary natural source of CO2 emissions to the atmosphere. Water vapour is a natural greenhouse gas present in Earth’s atmosphere
and is discussed ahead with regard to climate feedbacks. Over long time scales, higher levels of greenhouse gases generally correlate with warmer interglacials, and lower levels correlate with colder glacials. As greenhouse gas concentrations change, Earth’s surface heats or cools in response. Further, these gases can amplify the climatic trends (discussed ahead with climate feedbacks). In cases where huge amounts of greenhouse gases are released into the atmosphere, such as may have happened during the PETM (56 million years ago), these gases can potentially drive climatic change.
In addition to outgassing, volcanic eruptions also produce aerosols that scientists definitively have linked to climatic cooling. Accumulations of aerosols ejected into the stratosphere can create a layer of particulates that increases albedo so that more insolation is reflected and less solar energy reaches Earth’s surface. Studies of 20th-century eruptions have shown that sulfur aerosol ac- cumulations affect temperatures on timescales of months to years. As examples, the aerosol cloud from the 1982 El Chichón eruption in Mexico lowered temperatures world- wide for several months, and the 1991 Mount Pinatubo eruption lowered temperatures for 2 years (discussed in Chapter 6, Figure 6.1; also see Figure 11.17). Scientific evidence also suggests that a series of large volcanic erup- tions may have initiated the colder temperatures of the Little Ice Age in the second half of the 13th century.
Climate Feedbacks
and the Carbon Budget
Earth’s climate system is subject to a number of feed- back mechanisms. As discussed in Chapter 1, systems can produce outputs that sometimes influence their own operations via positive or negative feedback loops. Positive feedback amplifies system changes and tends to destabilize the system; negative feedback inhibits sys- tem changes and tends to stabilize the system. Climate feedbacks are processes that either amplify or reduce cli- matic trends, toward either warming or cooling.
A good example of a positive climate feedback is the ice–albedo feedback introduced in Chapter 1 (see Figure 1.8) and discussed in the Chapter 4 Geosystems Now. This feedback is accelerating the current climatic trend toward global warming. However, ice–albedo feedback can also amplify global cooling, because lower temperatures lead to more snow and ice cover, which in- creases albedo, or reflectivity, and causes less sunlight to be absorbed by Earth’s surface. Scientists think that the ice–albedo feedback may have amplified global cooling following the volcanic eruptions at the start of the Little Ice Age. As atmospheric aerosols increased and temper- atures decreased, more ice formed, further increasing albedo, leading to further cooling and more ice forma- tion. These conditions persisted until the recent influx of anthropogenic greenhouse gases into the atmosphere at the onset of the Industrial Revolution in the 1800s.