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Atmosphere Resources and Climate Change Chapter 14 © 2011 Pearson Education, Inc. You will learn • The nature of atmospheric resources, including the atmosphere’s composition, physical character, and role in Earth systems interactions • The nature and sources of atmosphere pollutants, including NOx, SO2, VOCs, low-level ozone, CO, and greenhouse gases, especially CO2 • The effects of atmospheric pollution, including smog, acid rain, and changes in the natural stratospheric ozone layer • How variations in Earth’s orbit influence climate by changing the amount of solar radiation Earth receives © 2011 Pearson Education, Inc. You will learn (cont.) • How variations in the atmosphere’s composition influence climate, what greenhouse gases are, where they come from, and how they work • How scientists study climate change • How human activities have changed the composition of the atmosphere, how these changes have altered climate and contributed to global warming, and what future challenges people will face as a result • How science is helping people understand global warming and develop ways of dealing with it, including the capture and storage of CO2 © 2011 Pearson Education, Inc. Composition of the Atmosphere FIGURE 14-2 Composition of the Atmosphere, Including Variable Components (by Volume) © 2011 Pearson Education, Inc. Composition of Atmosphere (cont.) • Three elements (N, O, Ar)—make up over 99.9% of the atmosphere • Pure nitrogen (N2) is used in its very cold, liquid state • Instantly freezes biological specimens • Nonreactive; used to purge vessels of reactive gases like oxygen • Prevents food spoilage, and refrigerate perishables during shipping • Prevents corrosion of metal surfaces • Pure oxygen (O2)—is used in iron and steel smelting • Cutting and welding torches to achieve higher temperatures • Oxidizing agent in processes that produce a variety of chemicals • Increases the efficiency of waste incinerators • Medical applications © 2011 Pearson Education, Inc. Composition of Atmosphere (cont.) • Argon (Ar)—is a colorless, odorless, nontoxic, and nonreactive gas • Creates inert environments for growing crystals—in semiconductors • Protects materials against corrosion • Fills the air space in double-pane insulating windows • The gas in incandescent and fluorescent lightbulbs. • Water vapor, methane, carbon dioxide— greenhouse gases • Keep Earth’s climate from being unbearably cold © 2011 Pearson Education, Inc. The Structure of the Atmosphere FIGURE 14-3 The Structure of the Atmosphere Temperature variations define the atmosphere’s four principal layers: the troposphere, stratosphere, mesosphere, and exosphere. The ozone layer is within the stratosphere. © 2011 Pearson Education, Inc. Ozone • Ozone layer—within the stratosphere, some 15 kilometers (9 mi) up at higher latitudes and 35 kilometers (22 mi) up at low latitudes • Ozone molecules = (O3) rather than the usual two (O2). • forms when UV radiation breaks apart some O2 • makes O—combines with other O2 to form O3 • Ozone layer (few ppm) screens out • harmful, shortest-wavelength UV radiation • longer-wavelength UV radiation • (causes sunburn and skin cancer) © 2011 Pearson Education, Inc. The Atmosphere Screens Earth from Harmful Solar Radiation FIGURE 14-4 The Atmosphere Screens Earth from Harmful Solar Radiation Shorter wavelength gamma and X-ray radiation and large amounts of infrared radiation are completely absorbed by the atmosphere. The ozone layer absorbs the most harmful ultraviolet wavelengths. Only radio waves, visible light, and some ultraviolet radiation reach Earth’s surface relatively unimpeded. © 2011 Pearson Education, Inc. Air Pollution • The EPA considers common air pollutants to be: • Volatile organic compounds (vocs) • Nitrogen oxides (NO and NO2) • Sulfur dioxide (SO2) • Carbon monoxide (CO) © 2011 Pearson Education, Inc. Air Pollution (cont.) • Common greenhouse gases are: • Water vapor (H2O), methane (CH4), CO, nitrous oxide (N2O) • Volatile organic compounds—(VOCs) • Chemicals that contain carbon and hydrogen and vaporize easily • Benzene, toluene, butane, and propane are examples • Paint thinners, solvents, plastics, and gasoline • VOCs evaporate from crude oil when exposed to atmosphere • In the presence of sunlight—chemical reactions that lead to smog © 2011 Pearson Education, Inc. NOx and SO2 • Nitrogen oxides—NOx • Family of nitrogen-oxygen compounds where N:O ratio varies • Nitrogen oxide (NO) and nitrogen dioxide (NO2) • Nitrous oxide (N2O) is not toxic like NO and NO2— effective greenhouse gas • Produced during combustion of fossil fuels • Key ingredient in the formation of smog, contributes to developing acid rain, and causes respiratory problems for people © 2011 Pearson Education, Inc. NOx and SO2 (cont.) • Sulfur dioxide • Oil and coal combust and S combines with O = sulfur dioxide (SO2) • In the atmosphere, SO2 + H2O + O = sulfuric acid (H2SO4) • Source of acid rain • Causes respiratory problems for people • Reacts with nitrogen in the atmosphere to form ammonium sulfate • Creates atmosphere haze • Volcanic eruptions may release SO2 into the atmosphere • Special effect—global cooling © 2011 Pearson Education, Inc. Emissions FIGURE 14-5 Emission Trends Regulations developed by the EPA have had a major influence on pollutant emissions in the United States. A a result, air pollution levels are near or significantly below national standards and the trends continue downward. FIGURE 14-6 Sources of SO2 Pollution in the Atmosphere Although SO2 emissions in the United States have significantly decreased, over 10 million tonnes (11 million tons) are released into the atmosphere each year. © 2011 Pearson Education, Inc. CO and CO2 • Carbon monoxide—CO • Colorless, odorless, tasteless, and dangerous gas • 75% of CO—produced by incomplete combustion of fossil fuels • CO is a good general indicator of air pollution • Can be detected and mapped by satelliteborne instruments • EPA vehicle regulations—effective at lowering levels of CO © 2011 Pearson Education, Inc. CO and CO2 (cont.) • Carbon dioxide—CO2 • Source of CO2 emissions—burning fossil fuel with CO2 as product • Metabolism of food—source of CO2 that we exhale • Volcanic eruptions may release large amounts of CO2 • The Supreme Court—greenhouse gases are atmospheric pollutants • EPA can regulate under the clean air act • This ruling signals changes for CO2 emitters • Example—EPA permits for new coal-fired power plants must now consider a “best available control technology” for CO2 emissions—as for other air pollutants © 2011 Pearson Education, Inc. Smog • Combination of smoke and fog • Originally referred to the haze from extensive burning of coal • CO, NOx, and VOC emissions cause most of today’s smog problems FIGURE 14-9 Two of the Reactions Involved in Forming Ozone (O3) and Smog There are several other reactions involving NOx, VOCs, CO, and O (in the presence of sunlight) that also contribute to ozone and smog formation. © 2011 Pearson Education, Inc. Photochemical Smog and Acid Rain • Photochemical smog—brownish haze over cities on warm, sunny days • Brownish color comes from Nox, smell comes from ozone • Most dangerous component is low-level ozone • Irritates lungs—damages vegetation • Cause—photochemical reactions involving VOCs, NOx, and CO • Solution—more efficient combustion of fossil fuels • Reformulating gasoline • Catalytic converters, electronic engine controls, exhaust gas recirculators © 2011 Pearson Education, Inc. Photochemical Smog and Acid Rain (cont.) • Acid rain • NOx and SO2 react with water (atmosphere) to form droplets of acid • SO2 emissions = principal cause of acid rain • Coal-burning power plants—release 70% of SO2 emitted each year • Acid rain has a ph less than 5.6; NE U.S.—rain can have ph = 3.6 !! • Acid rain in NE has affected vegetation, aquatic systems, buildings—damaging plant leaves, degrading soil, dissolving nutrients © 2011 Pearson Education, Inc. Formation of Acid Rain FIGURE 14-11 Both NOx and SO2 can react with water in the atmosphere to form droplets of acid, but SO2 emissions have been the principal cause of acid rain © 2011 Pearson Education, Inc. Acid Rain in the NE U.S. Where acid rain accumulates in surface waters, acidity can reach levels that are unhealthful for fish and other aquatic life. Some lakes in the northeast U.S. have pH levels <5. Acid rain falls on limestone and marble (buildings, tombstones, etc.) and dissolves bits of the building stone and pits, etches, and discolors the building surfaces. In places, structural components are weakened and need to be replaced. FIGURE 14-13 Acid Rain and Power Plant Emissions (a) The location of coal-burning power plants. (b) The acidity of rainwater in different parts of the United States. The distribution of plants combined with prevailing winds (from west to east) make acid rain common in the east and northeast parts of the country. © 2011 Pearson Education, Inc. The Ozone Layer • The ozone layer in the stratosphere absorbs shorterwavelength UV radiation in sunlight that would otherwise be lethal to surface life • Photosynthesis releases oxygen • Migrates to high altitudes—reacts with UV radiation to form ozone • Ozone layer effectively shields surface life • Is carried to higher latitudes by stratospheric winds • Chlorofluorocarbons (CFCs)—destroy ozone • UV radiation breaks cfcs down in stratosphere • Releasing Cl and Br atoms that react with ozone • Cl and Br atoms—very effective at destroying ozone • Seasonal depletion over Antarctica is known as the ozone hole • The ozone hole—well developed between August and late November © 2011 Pearson Education, Inc. How Chlorine Destroys Ozone FIGURE 14-15 How Chlorine Destroys Ozone CFCs have such a devastating effect on ozone because the reactions involved form a cycle. Once a single chlorine atom is dislodged from a CFC molecule, it can just keep reacting with one ozone molecule after another. © 2011 Pearson Education, Inc. Atmosphere and Climate Change • Earth’s climate is strongly influenced (forced) by: 1) atmosphere composition; 2) solar radiation Earth receives • Composition—greenhouse gases • Let shorter-wavelength incoming sunlight through to Earth’s surface • Warm the ground, the ocean, and lower regions of the atmosphere • Reradiate some of their heat energy back at longer wavelengths • Infrared wavelengths are absorbed by greenhouse gases • Trapping energy in the atmosphere • Part of this trapped energy is, in turn, reradiated back to Earth © 2011 Pearson Education, Inc. Atmosphere and Climate Change (cont.) • This trapping of heat in the atmosphere is often likened to the effect of glass in a greenhouse—hence the term greenhouse effect • This greenhouse effect keeps Earth’s surface habitable • CO2—low warming potential—very abundant, hence important • Methane—56x warming potential of CO2—not abundant • Nitrous oxide—280 warming potential of CO2—not abundant • Water vapor too is a very potent greenhouse gas— temperature-dependent © 2011 Pearson Education, Inc. The Greenhouse Effect FIGURE 14-18 The Greenhouse Effect Solar radiation reflected or reradiated by Earth’s surface is absorbed by greenhouse gases and warms the atmosphere. The warmed atmosphere reemits radiation back to Earth, causing surface warming. © 2011 Pearson Education, Inc. How Axis Tilt Affects the Intensity of Solar Radiation FIGURE 14-19 It is because the axis is tilted that we have seasons. (a) The tilt of Earth’s axis of rotation changes from 22.1 to 24.5 degrees and back in a 41,000-year cycle. At higher latitudes, even the shift of a few degrees can cause a significant change in the intensity of solar radiation. (b) Solar radiation reaching Earth’s surface is strongest when the Sun is directly overhead and becomes weaker when Sun's rays strike at a more oblique angle. © 2011 Pearson Education, Inc. How Axis Tilt Affects the Intensity of Solar Radiation (cont.) FIGURE 14-19 How Axis Tilt Affects the Intensity of Solar Radiation In (c), the Northern Hemisphere is tilted toward the Sun and experiences summer while the Southern Hemisphere experiences winter. Six months later, when Earth is on the opposite side of its orbit (d), the Northern Hemisphere is tilted away from the Sun and experiences winter while the Southern Hemisphere experiences summer. © 2011 Pearson Education, Inc. Influence of Orbital Eccentricity FIGURE 14-20 How Orbital Eccentricity Changes the Amount of Solar Radiation Earth Receives Earth’s orbit around the Sun changes from more elliptical (a) to less elliptical (b) and back about every 100,000 years. (The extent of the elongation in part (a) has been exaggerated.) When the orbit is more elliptical, the Earth moves both closer to and farther from the Sun during a year than it does when the orbit is less elliptical. © 2011 Pearson Education, Inc. Precession FIGURE 14-21 Precession Earth wobbles as it orbits the Sun, much as a top wobbles as it spins. This wobble gradually rotates the poles in a circle. Precession changes the direction of Earth’s tilt about its axis, but not the amount of tilt. Rotation of the poles shifts them slightly toward or away from the Sun. A complete precession cycle takes 22,000 years. Milankovitch Cycles—cyclic changes in axis tilt, orbital eccentricity, and precession cycles lead to cycles of colder climate (glacial maximums) and warmer climate (interglacial periods) © 2011 Pearson Education, Inc. Tectonic Processes and Climate • Continent size and distribution • The size and distribution of these landmasses directly affect circulation patterns • Affect the concentration of CO2 in the atmosphere • Influence on the chemical weathering of silicate minerals • Atmosphere transfers dissolved CO2 to the geosphere through rain • Reacts with water to give carbonic acid (H2CO3) • Chemical-weathering processes break down silicate minerals • Reactions work to decrease atmospheric CO2 concentrations • Silicate weathering is likely to be more extensive: • Continents split into smaller fragments surrounded by oceans—wetter • Continentals are located at lower latitudes—warmer conditions © 2011 Pearson Education, Inc. Tectonic Processes and Climate (cont.) • Mountain ranges • Direct influences on atmosphere circulation and precipitation • Increase exposure of silicate minerals to chemical weathering • Decrease CO2 concentrations in the atmosphere • Volcanoes • Volcanic gases (e.g., SO2) reach high altitudes—cool Earth • May release enough CO2 to affect global climate for long periods of time. © 2011 Pearson Education, Inc. Average Past Global Temperatures FIGURE 14-22 Average Global Temperature during the Phanerozoic Over the last 540 million years, Earth was mostly warmer than it is now. Earth’s geologic record shows evidence of past climates both colder and warmer than today. These past climates are related to interactions involving geosphere processes, greenhouse gas cycles, the distribution and size of continents and oceans, and variations in the amount of solar radiation Earth receives. © 2011 Pearson Education, Inc. Paleoclimatology—Studying Past Climates • Sedimentary records • • • • Evaporites (salt-rich sediments) indicate arid conditions Coal seams commonly indicate warm, moist conditions Tillite indicates cold, glacial conditions Cores of deep-sea sediments with continental rock debris (from icebergs) • Fossils • Remains of organisms help determine environmental conditions • Zooplankton and algae—indicators of water temperatures • Fossil plant spores and pollen © 2011 Pearson Education, Inc. Paleoclimatology—Studying Past Climates (cont.) • Oxygen isotopes • • • • Almost all oxygen has an atomic mass of 16 (16O, or O-16 atoms) Two other stable oxygen isotopes are: O-17 and O-18. O-18 is five times more abundant than O-17—easier to use O-18 is slightly heavier than O-16 • Water containing O-18 does not evaporate as easily as water with O-16 • The escaping water vapor is slightly enriched in O-16 • Seawater left behind is slightly enriched in O-18 • During cold periods • O-16-enriched water vapor is added to ice sheets • Ratio of O-18 to O-16 in seawater increases • Marine organism incorporate this O-18 : O-16 into shells and skeletons • Locking in ratio for time they grew • Expanding ice sheets—higher O-18 to O-16 ratios • Contracting ice sheets—decreasing O-18 to O-16 ratios © 2011 Pearson Education, Inc. Oxygen-16 in Glacial Ice FIGURE 14-25 How Oxygen-16 Becomes More Concentrated in Glacial Ice (a) Because O-16 is lighter than O-18, water vapor evaporated from the oceans is slightly enriched in O-16. Because most of this water eventually finds its way back to the oceans in rain and runoff, however, the ratio of the two isotopes in seawater is largely unaffected. (b) During times of glaciation, water vapor enriched in O-16 becomes trapped in ice accumulations and doesn’t return to the oceans. As a result, the relative concentration of O-18 in seawater increases. © 2011 Pearson Education, Inc. The Phanerozoic Oxygen Isotope Record of Seawater FIGURE 14-26 The Phanerozoic Oxygen Isotope Record of Seawater The variation in the ratio of O-18 to O-16 is determined from measurements of oxygen isotopes in fossil marine shells and skeletons. A greater O-18 to O-16 ratio indicates colder periods. © 2011 Pearson Education, Inc. Paleoclimatology—Studying Past Climates (cont.) • Atmosphere samples in ice cores • Air is trapped in snow each year on ice sheets • Snow becomes compressed under the weight of new snow and turns into ice • Enclosed air becomes trapped in the ice as bubbles of ancient air • Bubbles can be analyzed for their content of greenhouse gases • Determine prehistoric atmospheric concentrations of methane and CO2 © 2011 Pearson Education, Inc. Paleoclimatology—Studying Past Climates (cont.) • Sea level history • Shifts from cold to warm climate marked a change from expanding permanent ice sheets to melting ice sheets. • Major cold periods make major continental-scale ice sheets • Trap tremendous volumes of water evaporated from the oceans • Lower sea level • Warm periods melt the great ice sheets • Send tremendous amounts of water back to the oceans • Sea level rises © 2011 Pearson Education, Inc. Atmospheric Gases Trapped in Ice at Vostok Station, Antarctica FIGURE 14-27 Atmospheric Gases Trapped in Ice at Vostok Station, Antarctica Higher concentrations of methane (a) and carbon dioxide (b) in air bubbles trapped in ice cores indicate warmer interglacial times. Earth entered a warming interglacial period 20,000 years ago. © 2011 Pearson Education, Inc. Shifts between Warm and Cold Periods during Last 3 Million Years FIGURE 14-30 Shifts between Warm and Cold Periods during the Last 3 Million Years The plots are based on deep-sea sediment oxygen isotope data. Up to 0.9 million years ago, shifts from warm to cold climates occurred every 41,000 years (the axis tilt cycle). Since then the shifts have been every 100,000 years. © 2011 Pearson Education, Inc. The Last 10,000 Years of Greenhouse Gas Concentrations FIGURE 14-31 The Last 10,000 Years of Greenhouse Gas Concentrations The curves represent the concentrations of atmospheric CO2 (a) and methane (CH4) (b) as determined from ice cores. Note the shift from the longer-term natural trends at about 7000 years ago for CO2 and at 5000 years ago for methane. Could this be related to an increase in farming practices? If data for the last 200 years were included, the CO2 concentration would go off the chart—CO2 concentrations increased to 380 ppm during this time! © 2011 Pearson Education, Inc. Climate Models—IPCC • A scientific model is a physical or mathematical representation of system relationships and processes • General Circulation Models, GCMs, represent the interactions of the major components of climate system: solar radiation, atmosphere, oceans, land, and ice © 2011 Pearson Education, Inc. Climate Models—IPCC (cont.) • GCMs share characteristics of all models, including: • They are incomplete. Models represent what scientists consider to be the most important components: ocean temperature, solar radiation levels, atmosphere and ocean circulation patterns, and greenhouse gas concentrations, for example • They require assumptions. Scientists must assume many quantitative aspects of the climate system components and their relationships to one another—for example, the rate at which CO2 is being added to the atmosphere each year. They base such assumptions on what they can reasonably project from past experience and on the best existing information • They incorporate variable parameters. The components of the climate system vary in their physical character through space and time; for example, ocean water temperatures vary around the world © 2011 Pearson Education, Inc. Climate Models—Feedback Loops • Sophisticated models take changes into consideration as they simulate future conditions. Feedback mechanisms respond to a change in the climate system and either amplify or diminish the change. • positive feedbacks—amplify the change • negative feedbacks—diminish the change • Increasing ocean temperatures decrease the amount of CO2 oceans can absorb. This in turn increases atmospheric CO2 concentrations and their greenhouse gas effect causes additional ocean temperature warming. This is an example of a positive feedback. © 2011 Pearson Education, Inc. Climate Models—Feedback Loops (cont.) • With climate warming, ice sheets melt and reflect less solar radiation. This in turn causes more surface warming, a positive feedback • Increasing air temperatures enable more water vapor to be stored in the atmosphere. As water vapor has a significant greenhouse gas effect, more water vapor increases surface and air temperatures, a positive feedback. • If increased air temperatures lead to increased water vapor in the atmosphere, an outcome can be increased cloudiness. Clouds reflect incoming solar radiation and decrease surface and air temperatures, negative feedback © 2011 Pearson Education, Inc. FIGURE 14-35 Atmospheric CO2 Concentrations Increased Very Rapidly during the Last 200 Years The change from about 280 ppm in 1800 to 380 ppm today reflects the impact of the Industrial Revolution and the increased use of fossil fuels. © 2011 Pearson Education, Inc. FIGURE 14-37 The Global Climate System Is Very Complex This figure shows some of the many interlocking feedback mechanisms that affect global climate. (You don’t need to worry about the details—the purpose of the figure is just to suggest why the system is so challenging to model.) © 2011 Pearson Education, Inc. IPCC Story Lines and Scenarios • Define the scope of possible changes the world may experience by 2100: • A1—very rapid economic growth, low population growth, and the rapid introduction of new and more efficient technologies. Themes are convergence among regions, capacity building, and increased cultural and social interactions, with a substantial reduction in regional differences in per capita income. • A2—very heterogeneous world. Theme is self-reliance and preservation of local identities. High population growth. Economic development is regionally oriented, and per capita economic growth and technological change are more fragmented and slower than in other story lines. • B1—convergent world with the low population growth (like A1), but with rapid changes toward a service and information economy, with reductions in material intensity, and the introduction of clean and resource-efficient technologies. The emphasis is on global solutions to economic, social, and environmental sustainability, including improved equity, but without additional climate initiatives. • B2—emphasis is on local solutions to economic, social, and environmental sustainability. Moderate population growth, intermediate levels of economic development, and less rapid and more diverse technological change. © 2011 Pearson Education, Inc. FIGURE 14-36 Average Global Temperature Rise over the Past Century Instrument-measured air temperatures are compared to the average from 1961 to 1990 (the zero line) to calculate the observed temperature anomaly. © 2011 Pearson Education, Inc. The IPCC 2007 Assessment • Warming of the climate system is unequivocal, as is now evident from observations of increases in global average air and ocean temperatures, widespread melting of snow and ice, and rising global average sea level. • Global GHG (greenhouse gas) emissions due to human activities have grown since preindustrial times. • Most of the observed increase in global average temperatures since the mid-20th century is very likely due to the observed increase in anthropogenic GHG concentrations. • It is likely that there has been significant warming caused by human activities over the past 50 years averaged over each continent (except Antarctica). © 2011 Pearson Education, Inc. The IPCC 2007 Assessment (cont.) • There is high agreement and much evidence that with current climate change mitigation policies and related sustainable development practices, global GHG emissions will continue to grow over the next few decades. • Continued GHG emissions at or above current rates would cause further warming and induce many changes in the global climate system during the 21st century— would very likely be larger than those observed during the 20th century. • A wide array of adaptation options are available, but more extensive adaptation than is occurring is required to reduce vulnerability to climate change. © 2011 Pearson Education, Inc. FIGURE 14-40 Examples of Impacts Associated with Different Amounts of Global Average Temperature Change Projected here by the IPCC. © 2011 Pearson Education, Inc. Kyoto Protocol and Carbon Sequestration • The Kyoto Protocol • Identified CO2 emission reduction goals for all participating countries. • As of May 2008, 181 countries are parties to the agreement • Developing countries are not required to limit their emissions • Countries with a lot of CO2 emissions are not participating • Carbon sequestration © 2011 Pearson Education, Inc. Kyoto Protocol and Carbon Sequestration 9 (cont.) • Capturing and storing CO2 emitted by hydrocarbon burning • • • • Physical membranes that separate CO2 from exhaust streams Use of sorbents that chemically react with emitted CO2 A bigger challenge is storing CO2 after it’s captured. Reacting CO2 with Ca, Mg, Fe to form stable carbonate minerals • Storage in underground geologic reservoirs—geosequestration • Injects captured CO2 into permeable reservoirs • Depleted oil fields, saline water-filled formations • Overlain by impermeable layers to trap CO2 and prevent leaking • Making of methanol (wood alcohol, CH3OH) © 2011 Pearson Education, Inc. FIGURE 14-43 An Example of Carbon Capture Technology CO2 emissions from a large point source first combine with solid sodium carbonate in the carbonation reactor. The resulting sodium bicarbonate is heated in the decarbonation reactor. This releases concentrated CO2 and regenerates sodium carbonate that is recycled back to the carbonation reactor. © 2011 Pearson Education, Inc. SUMMARY • The atmosphere supports life and supplies useful elements (e.g., O, N, Ar). • The atmosphere absorbs gamma rays, X-rays, and ultraviolet radiation. • Atmospheric pollutants include NOx, SO2, VOCs, low-level ozone, CO, CO2. • Greenhouse gases include CO2, methane, water vapor, and N2O. • Smog = reactions with VOCs and NOx that produce lowlevel ozone. • Acid rain forms from reactions between NOx and SO2 and H2O vapor. • Stable Cl- and BR-bearing compounds, CFCs, are emitted to the atmosphere. © 2011 Pearson Education, Inc. SUMMARY (cont.) • Greenhouse gases in Earth’s atmosphere absorb heat radiated by Earth, trap it in the atmosphere, and reradiate it back to Earth, causing surface and ocean temperature increases. • Variations in solar radiation caused by cyclic changing of the tilt of Earth’s axis, Earth’s distance from the Sun due to orbital eccentricity, and Earth’s wobble as it orbits the Sun are significant controls on global climate. • Shifts between warm and cold climates have occurred for large parts of Earth’s history (e.g., 40–50 cycles during the last 2 to 3 million years) • The last glacial maximum occurred 20,000 years ago—incoming solar radiation increased between 20,000 and 10,000 years ago, the ice sheets melted, and sea level rose some 110 to 125 meters (360 to 410 ft). • Addition of greenhouse gases has led to global warming. © 2011 Pearson Education, Inc.