Chapter 14: Climate Change

Transcription

1 Chapter 14: Climate Change Goals of Period 14 Section 14.1: To review the energy balance of the Earth and the enhanced greenhouse effect Section 14.2: To examine evidence for climate change Section 14.3: To describe the consequences of climate change Section 14.4: To address responses to climate change 14.1 Earth s Energy Balance and the Enhanced Greenhouse Effect As discussed in Period 13, the Earth receives a constant influx of solar energy. Some of this sunlight is immediately reflected by clouds, particles in the atmosphere, or snow cover back into space. Most of the rest is absorbed by the atmosphere or the surface of land and oceans and converted into thermal energy. The energy absorbed by the Earth s surface is reradiated into the atmosphere. If all of the solar energy incident on the Earth eventually escapes back into space, an energy balance exists between the influx of solar energy and the outflux of radiant energy back into space. Although the incoming solar radiation consists of all wavelengths and frequencies of the electromagnetic spectrum, the radiation peaks at the wavelengths of visible light. Once absorbed by the Earth, this energy is radiated away from the Earth as longer wavelength infrared radiation. The difference in radiation wavelength is due to the difference in temperature of the radiating bodies. The hotter the radiating body, the higher the frequency and the shorter the wavelength radiation is emitted. The Sun, with a surface temperature of about 6,000 K radiates shorter wavelength visible light, while the Earth, with an average temperature of about 300 K, radiates at longer wavelength infrared. These differences in wavelength result in the Earth s greenhouse effect. Incoming visible light can pass through the Earth s atmosphere with little absorption. However, outgoing longer wavelength infrared radiation is trapped by carbon dioxide, methane, and water vapor in the atmosphere. These gases, along with chlorofluorocarbons (CFC s) from Freon and aerosol propellants are known as green house gases. These greenhouse gases prevent infrared radiation from escaping from the Earth s atmosphere. Greenhouse gases in the atmosphere provide an essential protective layer above the Earth s surface that moderates surface temperatures. However, as the amount of carbon dioxide and other greenhouse gases in the atmosphere is significantly increased by combustion of fossil fuels, the greenhouse effect is enhanced and the temperature of the earth rises. Presently there is more carbon dioxide dissolved in the oceans than exists in the atmosphere. As the temperature of the earth increases, the amount of 119

2 dissolved carbon dioxide that the oceans can hold will decrease. The carbon dioxide released by the oceans would increase the greenhouse effect. This, in turn, would cause more carbon dioxide to be released from the oceans. This is an example of a positive feedback loop. There are other factors that influence the temperature of the Earth, such as an increase in the amount of particulate matter. Particulate matter in the atmosphere acts to decrease the temperature in two ways: by directly reflecting incoming sunlight back into space and by providing condensation centers that increase the formation of fog and clouds, both of which reflect the sunlight better than land or water. The interaction of the many factors influencing average global temperatures presents a complicated scenario for the future. However, as shown in Figure 14.1, there has been a definite increase in the departure from average global temperatures from 1980 to the present. Figure 14.1 Departure in Global Temperature from Average Values How do we know that that this temperature increase is due to the enhanced greenhouse effect and not a natural cycle of variations in climate? Next, we examine the factors that have determined past cyclical climate variations. 120

3 Historical global temperatures The Earth s climate has fluctuated between periods of warmer and colder average global temperatures as the result of variations in the Earth s motion: the eccentricity of the Earth s orbit and the tilt and precession of the Earth s axis. The eccentricity of the Earth's orbit causes its shape to vary from nearly circular to slight ellipse (a 5 O difference) with a cycle of about 413,000 years. The orbital shape affects the Earth-Sun distance. The tilt of the Earth s axis of rotation varies from 22.1 to 24.5 with a cycle of about 41,000 years. A smaller angle of tilt results in less difference between winter and summer solar radiation received. The precession of the Earth s axis results from variations in the direction the axis relative to the fixed stars with a cycle of about 26,000 years. Precession of the rotation axis causes changes in the time of year that the Earth is closest to the Sun. These cycles were calculated by the mathematician Milutin Milankovitch approximately 100 years ago. Milankovitch used these cycles to explain the occurrence of major ice ages every 100,000 and 41,000 years and minor cooling events every 19,000 to 23,000 years. At these times, cooler summer temperatures prevented the previous winter s snow from completely melting. Over time, snow and ice accumulated into miles thick glaciers covering large portions of the Northern Hemisphere. Is it possible that the current warming is due to variations in the Milankovitch cycles? The most recent temperature maximum based on the Milankovitch cycles occurred approximately 10,000 years ago. The Earth has been in a long-term cooling trend for the past 6,000 years. Natural factors in addition to the Milankovitch cycles influence climate. Cyclical changes in solar radiation impact the Earth. In the past, increases in the amount of particulate matter in the atmosphere following a volcanic eruption or very large meteor impact have reduced temperatures. As more particles in the atmosphere reflect incoming sunlight back into space, less radiant energy is absorbed by the earth and temperatures fall. A warm period, known as the Medieval Climate Optimum, occurred in the North Atlantic region between about AD 950 to Although warmer than the preceding and following centuries, temperatures were still slightly cooler than recent global temperatures. A cold period, known as the Little Ice Age, occurred between about 1550 and While far warmer than an actual glacial ice age, temperatures in the Northern Hemisphere cooled as shown in Figure These variations may be the result of regional impacts on temperature, such as volcanic activity, rather than global climate changes. 121

4 Figure 14.2: Temperature Variations over the Last 2,000 Years Source: //en.wikipedia.org/wiki/file:2000_year_temperature_comparison.png Next, we examine how past climates are determined from fossil evidence Evidence for Climate Change Tree Rings as Evidence of Past Climates In moderate climates, most trees produce annual growth rings which result from the new wood added to the tree s trunk each year. Growth rings of varying widths indicate how rapidly the tree grew and depend on environmental factors such as temperature and precipitation. Thus, the measuring the width of growth rings of trees in gives evidence of the environment conditions under which a tree grew. To account for individual differences among trees, an average of the growth ring widths of multiple trees for a particular year is used. Major environmental events, such as drought or volcanic eruptions, are reflected in narrow tree rings. These events serve as markers for patterns in tree growth. The pattern of rings in one tree sample can be matched to patterns in other trees growing in the same region. These patterns can be followed back in time with successively older trees and wood used in ancient structures. This method of cross-dating wood from the patterns of tree rings will be illustrated in class. Absolute dates of events can be estimated from the carbon dating of materials containing carbon, such as plant and animal remains. Two stable isotopes of carbon are 122

5 present in the environment: carbon-12 and carbon-14. These isotopes of carbon differ in their number of nucleons. The nuclei of carbon-12 atoms contain 6 protons and 6 neutrons, while the nuclei of carbon-14 atoms contain 6 protons and 8 neutrons. Carbon-14 atoms decay into carbon-12 atoms with a half life of 5,730 years; that is, after 5,730 years, one-half of the original sample of carbon-14 has become carbon-12. Living organisms absorb both isotopes of carbon. After an organism dies, it no longer absorbs any new carbon-14, and the carbon-14 within it decays. By measuring the ratio of carbon-12 to carbon-14 in a fossil, scientists can determine the number of half lives that have passed since the organism died. By multiplying the number of half lives by 5,730 years, the age of the fossil can be estimated. Radiocarbon dating will be discussed in more detail in Chapter 18. Lake and Ocean Sediments as Evidence of Past Climates Similar to the patterns in tree rings, patterns in the deposition of sediment in lake or sea beds provide evidence of past climate. Undisturbed sediments are deposited in annual layers called varves. The thickness of each varve is an indicator of the amount of silt and clay blown into the water and the amount and strength of the wind that blew these materials. Cores of lake sediment are removed and their content examined. Information on the aquatic life of an ancient lake bed is found from the calcium carbonate shells of tiny diatom organisms. Different species of diatoms flourish at different temperatures. Evidence of ancient plant life is determined from pollen in the sediment. Pollen grains provide the identity of the plants growing in the general area of the lake and give evidence of the surrounding temperature and precipitation. Examination of ocean sediments provides additional information on past climate: ocean currents, surface temperatures, salinity, and dissolved oxygen and carbon dioxide content. Glacial Ice Cores as Evidence of Past Climate Glacial ice is also deposited in annual layers. Snow precipitated during cold months accumulates over time, and pressure from the weight of this snow converts previous snow layers into ice. Annual rings, similar to the patterns in tree rings, delineate each year. Ice cores drilled from ice sheets provide invaluable information of past climates. The width of each ring indicates the amount of annual precipitation. Pollen and other particulate matter that blew onto the ice sheets is incorporated into the ice. But more important, within the ice are bubbles of air. This fossil air provides information on the content and concentration of gases in the atmosphere. As glaciers melt, obtaining and storing ice cores from them becomes more urgent. Teams, such as those from the OSU Byrd Polar Center have conducted a decades-long project of procuring ice cores and analyzing their contents. Distribution of Oxygen Isotopes as Evidence of Past Climate Two oxygen isotopes exist in the environment: oxygen-16 atoms, whose nuclei consist of 8 protons and 8 neutrons, and oxygen-18 atoms, whose nuclei consist of 8 123

6 protons and 10 neutrons. A small fraction of water molecules (about 1 in 500) contain oxygen-18 atoms. Because oxygen-18 atoms have two more nucleons than oxygen-16 atoms, water molecules made of the oxygen-18 isotope are heavier and evaporate less readily than water molecules made of oxygen-16. Those oxygen-18 water molecules that do evaporate are more likely to condense into liquid water. Not only do more of the lighter oxygen-16 molecules remain in the atmosphere, but they also diffuse more readily over land masses than the heavier oxygen-18 molecules. As warm air rises and moves in convection currents toward the Earth s poles, more of the heavier oxygen-18 water vapor molecules condense and precipitate first, allowing more of the lighter oxygen-16 water molecules to reach the higher latitudes as water vapor. When the oxygen-16 water molecules condense and precipitate, they add oxygen-16 to the ice and snow packs near the poles. During periods of glacial maxima, more water is locked into frozen ice sheets. That means a larger portion of the oxygen- 16 water molecules remain in solid ice, and liquid water in the oceans in enriched in oxygen-18 water molecules. Thus a change in the ratio of oxygen-16 to oxygen-18 in the oceans is an indicator of glacial advance or retreat. Next we examine how the past ratios of oxygen-16 to oxygen-18 in sea water can be determined. Sea Shells and Coral Reefs as Evidence of Past Climate The shells the aquatic microorganisms, such as foraminifera, contain calcium carbonate ( CaCO 3 ), which is formed from carbon dioxide dissolved in water. The oxygen in calcium carbonate consists of a mixture of oxygen-16 and oxygen-18 isotopes. The ratio of these isotopes in the shells of foraminifera is the same as the ratio of the isotopes in the water when the shells were formed. When these organisms die, their shells are deposited on the sea floor. Examination of cores of deep sea sediments provides evidence of past oxygen-16 to oxygen-18 ratios, and thus, the extent of glaciation at the time the shells formed. Periods of glacial maxima also correspond to lower sea levels, and more water is locked in glacial ice sheets. Evidence of changing sea levels is present in fossil coral reefs. Variations in sea level are evident in the growth of coral reefs, especially in shallow seas. During ice ages with lower sea level, coral reef growth was limited. During warmer periods of higher sea level, coral reefs expanded Consequences of a Warming Global Climate As discussed in Chapter 10, the consequences of a warming climate are difficult to quantify. However, consideration of the likely consequences of paints a concerning future. Next we discuss some possible scenarios and the feedback loops that are inherent in these changes. Recall that a positive feedback loop is a series of events that enhances the outcome and the events in a negative feedback loop diminish the outcome. An example of a process that provides both positive and negative feedback loops is cloud formation. 124

7 Cloud Formation As sea temperatures rise, more water will evaporate and enter the atmosphere. Water vapor in the atmosphere is a greenhouse gas that blocks the infrared energy emitted by the Earth from radiating into space, thus raising average global temperatures. Higher air temperatures result in higher water temperatures, and more water evaporates. This is an example of a positive feedback loop. However, some of the evaporated water condenses into water droplets or freezes into ice crystals, forming clouds. These liquid and solid water molecules are quite effective at reflecting incoming solar radiation back into space. Adding more particulate matter to the atmosphere in the form of water droplets and ice crystals reduces the amount of solar radiation that reaches the Earth s surface. Warmer water temperatures result in more water evaporation, which adds more particulate matter to the atmosphere, which reduces incoming solar radiation. This is an example of a negative feedback loop. Warming of Sea Water Large quantities of carbon dioxide are dissolved in sea water. The solubility of carbon dioxide is a function of water temperature: warmer water can hold less carbon dioxide. As air temperatures rise, sea water temperatures will also increase, causing previously dissolved carbon dioxide to be released into the atmosphere. The addition of carbon dioxide to the atmosphere will enhance the greenhouse effect further (a positive feedback loop). Increase in methane Methane (CH 4 ) is a powerful greenhouse gas that is produced when organic material decays. Methane trapped in previously frozen high latitude areas of permafrost will be released to the atmosphere when the permafrost thaws. Many areas of permafrost contain peat bogs with high concentrations of organic materials. Methane is also trapped under sea floor sediments in the form of methane hydrate, a water ice. Increasing water temperatures could result in the sudden release of large quantities of methane from these sea floor deposits. Melting of Glaciers The annual summer melt water from glaciers on mountains such as the Andes in South America, Mt. Kilimanjaro in Africa, and the Himalayas provide fresh water and hydroelectricity for billions of people living in these regions. These glaciers are expected to melt by the end of this century. Without this source of water, energy will be required pipe in water from other sources or to process waste water for household use. Greenhouse-free hydroelectric power will be replaced with other forms of generating electricity, which may involve fossil fuels. The melting of glaciers due to higher global temperatures will require replacements that likely add more greenhouse gases to the atmosphere. 125

8 Polar Ice Sheets The melting of polar ice sheets, such as the ice sheets covering Antarctica and Greenland, will add fresh water to the oceans. The result will be a reduction in the salt concentration (salinity) of sea water and a rise in sea level. As noted above, warmer ocean temperatures result in greater evaporation of water. More water vapor in the atmosphere will result in greater precipitation adding to the amount of ice contained in the ice sheets (a negative feedback loop). This added precipitation is expected to be outweighed by melting of the ice sheets. Melting polar ice also changes the reflectivity of the Earth s surface. Ice sheets are highly reflective (have a higher albedo) than land or water surfaces. More incoming solar radiation is reflected off of ice, and more radiation is absorbed by land or water. The more radiation that is absorbed, the warmer the Earth s surface, and the more ice or permafrost is melted (a positive feedback loop). Ocean Currents The circulation of the Gulf Stream current in the Atlantic Ocean brings water warmed near the equator to northern latitudes. Evaporation causes the water to become saltier as it moves northward. Because the saltier sea water is denser, it sinks below the ocean surface. Deep in the ocean, the current continues to flow southward and then turns eastward. As the current approaches the Pacific Ocean, the water is warmed by equatorial temperatures. In the Pacific, the warmed current rises and flows around the Pacific rim and back into the Atlantic, where the cycle begins again. This circulation of water acts as a conveyor belt of heat distribution for the Earth. Figure 14.3 Ocean Circulation 126

9 About 10,000 years ago, this ocean current stopped circulating. With less warm water reaching the northern latitudes, temperatures dropped and ice sheets expanded. Areas of Canada that had been moderate in temperature, cooled in a matter of only 70 years. The cause of this sudden change was a massive input of fresh water from prehistoric Lake Agassiz in Canada. Ice dams holding back the lake water melted, allowing very large quantities of fresh water to enter the northern Atlantic Ocean via the St. Lawrence Seaway. The added fresh water decreased the ocean water s saltiness to the point that the current no longer sank, thus temporarily ending the global circulation of water currents. The present concern is that climate change will again disrupt ocean circulation patterns. As melt water from the Greenland and Arctic ice sheets enters the northern Atlantic Ocean, this fresh water will dilute the saltiness of the Gulf Stream. As sea water becomes fresher and less dense, it may no longer sink and ocean circulation will stop Addressing Climate Change There are two major approaches to addressing climate change: mitigation of the factors causing that change and adapting to inevitable changes. The most important of the mitigation strategies is reducing the addition of greenhouse gases to the atmosphere. The adaptation strategies involve the development of new technologies to, for example, capture and sequester carbon dioxide underground. Both of these approaches require cooperation within and among world governments. They will require developed countries, which are responsible for the majority of the enhanced greenhouse gas emissions, to provide assistance and technology to developing countries. The issues are complex and the results may take more than one life time to become apparent. The solutions are costly and some may be difficult for citizens to accept. The remainder of Physics 1104 addresses these mitigation and adaptation strategies as we discuss each type of energy in detail and your role in using energy wisely. 127