sign in sign up
<<
Home , Biogeochemical cycle, Chemical decomposition, Oxygen cycle

Climate Science for Today’s World

You are about to embark on a systematic study of span of many years. In this empirically-based context, climate, climate variability, and . climate is defined as weather (the state of the is a mosaic of many climate types, each featuring a ) at some locality averaged over a specified unique combination of physical, chemical, and time interval. Climate must be specified for a biological characteristics. Differences in climate particular place and period because, like weather, distinguish, for example, deserts from rainforests, climate varies both spatially and temporally. Thus, for temperate regions from glacier-bound polar localities, example, the climate of Chicago differs from that of and treeless tundra from subtropical savanna. We will New Orleans, and winters in Chicago were somewhat come to understand the spatial and temporal (time) milder in the 1980s and 1990s than in the 1880s and variations in climate as a response to many interacting 1890s. forcing agents or mechanisms both internal and In addition to average values of weather external to the planetary system. At the same time we elements, the climate record includes extremes in will become familiar with the scientific principles and weather. Climatic summaries typically tabulate basic understandings that underlie the operations and extremes such as the coldest, warmest, driest, wettest, interactions of those forcing agents and mechanisms. snowiest, or windiest day, month or year on record for This is climate science, the systematic study of the some locality. Extremes are useful aspects of the mean state of the atmosphere at a specified location climate record if only because what has happened in and time period as governed by natural laws. the past can happen again. For this reason, for Our study of climate science provides example, farmers are interested in not only the average valuable insights into one of the most pressing rainfall during the growing season but also the environmental issues of our time: global climate frequency of exceptionally wet or dry growing seasons. change. We explore the many possible causes of In essence, records of weather extremes provide a climate change with special emphasis on the role perspective on the variability of local or regional played by human activity (e.g., burning fossil fuels, climate. clearing vegetation). A thorough grounding in climate Selection of an internationally agreed 30-year science enables us to comprehend the implications of period for averaging weather data may be inappropriate anthropogenic climate change, how each of us for some applications because climate varies over a contributes to the problem, and how each of us can be broad range of time scales and can change significantly part of the solution to the problem. in periods much shorter than 30 years. For example, El The essential value in studying climate Niño refers to an inter-annual variation in climate science stems from the ecological and societal impacts involving air/sea interactions in the tropical Pacific and of climate and climate change. Climate is the ultimate weather extremes in various parts of the world. The environmental control that governs our ; for phenomenon typically lasts for 12 to 18 months and example, what crops can be cultivated, the supply of occurs about every 3 to 7 years. For some purposes, a fresh water, and the average heating and cooling 30-year period is a short-sighted view of climate requirements for homes. variability. Compared to the long-term climate record, By its very nature, climate science is for example, the current 1971-2000 averaging period interdisciplinary, drawing on principles and basic was unusually mild over much of the nation. understandings of many scientific disciplines. We In the United States, 30-year averages are recognize climate as a system in which Earth’s major computed for temperature, precipitation (rain plus subsystems (i.e., atmosphere, , cryosphere, melted snow and ice), and degree days and identified geosphere, and ) individually and in concert as normals. Averages of other climate elements such function as controls of climate. Linking these as wind speed and humidity are derived from the entire subsystems are biogeochemical cycles (e.g., global period of record or at least the period when cycle, global ), pathways for transfer observations were made at the same location. Other of climate-sensitive materials (e.g., greenhouse gases) useful climate elements include average seasonal and energy among Earth-bound reservoirs. snowfall, length of growing season, percent of possible An easy and popular way of summarizing sunshine, and number of days with dense fog. local or regional climate is in terms of the averages of Tabulation of extreme values of weather elements is weather elements, such as temperature and usually also drawn from the entire period of the precipitation, derived from observations taken over a observational record.

1 While the empirical definition of climate (in abject poverty of North Africa’s Sahel in large measure terms of statistical summaries) is informative and is due to the region’s subtropical climate that is useful, the dynamic definition of climate is more plagued by multi-decadal droughts. In other regions, fundamental. It addresses the nature and controls of climate provides resources that are exploited to the Earth’s climate together with the causes of climate advantage of society. For example, some climates variability and change operating on all time scales. favor winter or summer recreational activities (e.g., Climate differs from season to season and with those skiing, boating) that attract vacationers and feed the variations in climate, the array of weather patterns that local economy. Severe weather (e.g., tornadoes, characterize one season differs from the array of hurricanes, floods, heat waves, cold waves, and characteristic weather patterns of another season. The drought) can cause and injuries, considerable status of the planetary system (that is, the Earth- long-term disruption of communities, property damage, atmosphere-land- system) determines (or selects) and economic loss. The impact of Hurricane Katrina the array of possible weather patterns for any season. on the Gulf Coast is still being felt many years after In essence, this status constitutes boundary conditions that weather system made landfall (August 2005). (i.e., forcing agents and mechanisms) such as incoming Regardless of a nation’s status as developed or solar radiation and the albedo (reflectivity) of Earth’s developing, it is not possible to weather- or climate- surface. Hence, in a dynamic context, climate is proof society to prevent damage to and property. defined by the boundary conditions in the planetary In the agricultural sector, for example, the prevailing system coupled with the associated typical weather strategy is to depend on technology to circumvent patterns that vary with the seasons. For example, the climate constraints. Where water supply is limited, higher Sun’s path across the local sky and the longer farmers and ranchers routinely rely on irrigation water daylight length in Bismarck, ND during July increase usually pumped from subsurface aquifers (e.g., the the chance of warm weather and possible High Plains Aquifer in the central U.S.) or transferred thunderstorms, whereas lower Sun angles and shorter via aqueducts and canals from other watersheds. daylight duration during January would mean colder Because of consumers’ food preferences and for weather and possible snow. economic reasons, this strategy is preferred to Climatology is the study of climate, its matching crops to the local or regional climate (e.g., controls, and spatial and temporal variability. dry land farming). Other strategies include Climatology is primarily a field science rather than a construction of dams and reservoirs to control runoff laboratory science. The field is the atmosphere and and genetic manipulation to breed drought resistant Earth’s surface where data are obtained by direct (in crops. Although these strategies have some success, situ) measurement by instruments and remote sensing, they have limitations and often require tradeoffs. For mostly by sensors flown aboard Earth-orbiting example, many rivers around the world lose so much of satellites. their flow to diversions (mostly for irrigation) that they The only scientific experiments routinely are reduced to a trickle or completely dry up prior to conducted by climate scientists involve manipulation reaching the sea at least during part of the year. of numerical climate models. Usually these global or Compounding the constraints of climate on regional models are used to predict the climatic society is the prospect of global climate change. The consequences of change in the boundary conditions of scientific evidence is now convincing that human Earth’s climate system. Furthermore, climatology is an activity is influencing climate on a global scale with interdisciplinary science that reveals how the various significant consequences for society. Burning of fossil components of the natural world are interconnected. fuels (, oil, natural gas) and clearing of vegetation For example, the composition of the atmosphere is the is responsible for a steady build-up of atmospheric end product of many processes where gases are emitted (CO2) and enhancement of Earth’s (e.g., via volcanic eruptions) or absorbed (e.g., gases greenhouse effect. This enhancement is exacerbated dissolving in the ocean). The composition of the by other human activities that are increasing the atmosphere, in turn, affects the ocean, living concentration of methane (CH4) and nitrous organisms, geological processes, and climate. (N2O), also greenhouse gases. Our understanding of the potential impact of Climate and Society climate and climate change on society requires knowledge of (1) the structure and function of Earth’s Probably the single most important reason for studying climate system, (2) interactions of the various climate science is the many linkages between climate components of that system, and (3) how human and society. For one, climate imposes constraints on activities influence and are influenced by these social and economic development. For example, the systems.

2 The Climate System

What is the climate system and, more fundamentally, subsystems and society. what is a system? A system is an entity whose The 1992 United Nations Framework components interact in an orderly manner according to Convention on Climate Change defines Earth’s the laws of physics, chemistry, and . A familiar climate system as the totality of the atmosphere, example of a system is the human body, which consists hydrosphere (including the cryosphere), biosphere and of various identifiable subsystems including the geosphere and their interactions. The view of Planet nervous, respiratory, and reproductive systems, plus the Earth in Figure 1, resembling a “blue marble,” shows input/output of energy and matter. In a healthy person, all the major subsystems of the climate system. The these subsystems function internally and interact with ocean, the most prominent feature covering more than one another in regular and predictable ways that can be two-thirds of Earth’s surface, appears blue. Clouds studied based upon analysis of the energy and mass obscure most of the ice sheets (the major part of the budgets for the systems. Extensive observations and cryosphere) that cover much of Greenland and knowledge of a system enable scientists to predict how Antarctica. The atmosphere is made visible by the system and its components are likely to respond to swirling storm clouds over the Pacific Ocean near changing internal and external conditions. The ability Mexico and the middle of the Atlantic Ocean. Viewed to predict the future state(s) of a system is important, edgewise, the atmosphere appears as a thin, bluish for example, in dealing with the complexities of global layer. Land (part of the geosphere) is mostly green climate change and its potential impacts on Earth’s because of vegetative cover (biosphere).

Figure 1. Planet Earth, viewed from space by satellite, appears as a “blue marble” with its surface mostly ocean water and partially obscured by swirling masses of clouds. [Courtesy of NASA, Goddard Space Flight Center]

3 ATMOSPHERE radiation. Earth’s atmosphere is a relatively thin Although comprising only 0.038% of the envelope of gases and tiny suspended particles lower atmosphere, carbon dioxide is essential for pho- surrounding the planet. But the thin atmospheric skin tosynthesis. Without carbon dioxide, green and is essential for life and the orderly functioning of the food webs they support could not exist. While the physical, chemical and biological processes on Earth. atmospheric concentration of (O3) is minute, the (N ) and (O ), the chief atmospheric chemical reactions responsible for its formation (from 2 2 oxygen) and dissociation (to oxygen) in the gases, make up a uniform 78.08% and 20.95% by (mostly at altitudes between 30 and 50 volume, respectively through most of the atmosphere. km) shield organisms on Earth’s surface from Not counting water vapor (with its highly variable potentially lethal levels of solar UV radiation. Carbon concentration), the next most abundant gases are argon dioxide and ozone are also greenhouse gases. (0.93%) and carbon dioxide (0.038%). Many other The atmosphere is dynamic; the atmosphere gases occur in the atmosphere in trace concentrations, continually circulates in response to different rates of including ozone (O ) and methane (CH ) (Table 1). 3 4 heating and cooling within the rotating planetary Unlike nitrogen and oxygen, the percent volume of system. Heat is conveyed from warmer locations to some of these trace gases varies with time and location. colder locations, from Earth’s surface to the atmosphere and from the tropics to higher latitudes. TABLE 1 The global water cycle and accompanying phase Some Gases Composing Dry Air in the Lower changes of water play an important role in this Atmosphere planetary-scale transport of heat energy.

Gas % by volume Parts per million HYDROSPHERE Nitrogen (N2) 78.08 780,840.0 The hydrosphere is the water component of Oxygen (O2) 20.95 209,460.0 the climate system. Water continually cycles among Argon (Ar) 0.93 9,340.0 reservoirs within the climate system. The ocean, by far Carbon Dioxide (CO2) 0.0388 388.0 the largest reservoir of water in the hydrosphere, covers Methane (CH4) 0.00014 1.4 about 70.8% of the planet’s surface and has an average Nitrous Oxide (N2O) 0.00005 0.5 depth of about 3.8 km (2.4 mi). About 96.4% of the Ozone (O3) 0.000007 0.07 hydrosphere is ocean salt water. The next largest reservoir in the hydrosphere is glacial ice (also Aerosols, minute solid and liquid particles, considered the cryosphere), most of which covers suspended mainly in the lower atmosphere derive from much of Antarctica and Greenland. Ice and snow make wind erosion of soil, ocean spray, forest fires, volcanic up 2.1% of water in the hydrosphere. Considerably eruptions, industrial chimneys, and the exhaust of smaller quantities of water occur on the land surface motor vehicles. Although aerosol concentrations are (lakes, rivers), in the subsurface (soil moisture, relatively small, they participate in some important groundwater), the atmosphere (water vapor, clouds, processes. Aerosols are nuclei for cloud formation, precipitation), and biosphere (plants, ). interact with incoming solar radiation and dust blown The ocean and atmosphere are coupled such out over the tropical Atlantic Ocean from North Africa that the wind drives surface ocean currents. Wind- may affect the development of tropical cyclones driven currents are restricted to a surface ocean layer (hurricanes and tropical storms). typically about 100 m (300 ft) deep and take a few The significance of an atmospheric gas is not months to years to cross an ocean basin. Ocean necessarily related to its concentration. Some currents at much greater depths are more sluggish and atmospheric components that are essential for life more challenging to study than surface currents occur in very low concentrations. For example, most because of greater difficulty in taking measurements. water vapor is confined to the lowest kilometer or so of Movements of deep-ocean waters are caused primarily the atmosphere and is never more than about 4% by by small differences in water density (mass per unit volume even in the most humid places on Earth (e.g., volume) arising from small differences in water tem- over tropical rainforests and seas). But without water perature and salinity (a measure of dissolved salt vapor, the planet would have no water cycle, no rain or content). Cold sea water, being denser than warm snow, no ocean, and no fresh water. Also, without water, tends to sink whereas warm water, being less water vapor, Earth would be much too cold for most dense, is buoyed upward by (or floats on) colder water. forms of life to exist. Water vapor is the main Likewise, saltier water is denser than less salty water greenhouse gas, one that interacts with infrared and tends to sink, whereas less salty water is buoyed

4 upward. The combination of temperature and salinity 10% of the planet’s land area but at times during the determines whether a water mass remains at its original past 1.7 million years, glacial ice expanded over as depth or sinks to the ocean bottom. Even though deep much as 30% of the land surface, primarily in the currents are relatively slow, they keep ocean waters Northern Hemisphere. well mixed so that the ocean has a nearly uniform As snow accumulates, the pressure exerted by chemical composition. the new snow converts underlying snow to ice. As the The densest ocean waters form in polar or ice forms, it preserves traces of the original seasonal nearby subpolar regions. Salty waters become even layering of snow and traps air bubbles. Chemical saltier where sea ice forms at high latitudes because analysis of the ice layers and air bubbles in the ice growing ice crystals exclude dissolved salts. Chilling provides clues to climatic conditions at the time the of this salty water near Greenland and Iceland and in original snow fell. Ice cores extracted from the the Norwegian and Labrador Seas further increases its Greenland and Antarctic ice sheets yield information density so that surface waters sink and form a bottom on changes in Earth’s climate and atmospheric current that flows southward under equatorial surface composition extending as far back as hundreds of waters and into the South Atlantic as far south as thousands of years—to 800,000 years or more in Antarctica. Here, deep water from the North Atlantic Antarctica. mixes with deep water around Antarctica. Branches of Under the influence of gravity, glacial ice that cold bottom current then spread northward into the flows slowly from sources at higher latitudes and Atlantic, Indian, and Pacific basins. Eventually, the higher elevations (where some winter snow survives water slowly diffuses to the surface, mainly in the the summer) to lower latitudes and lower elevations, Pacific, and then begins its journey on the surface where the ice either melts or flows into the nearby through the islands of Indonesia, across the Indian ocean. Around Antarctica, streams of glacial ice flow Ocean, around South Africa, and into the tropical out to the ocean. Ice, being less dense than seawater, Atlantic. There, intense heating and evaporation make floats, forming ice shelves (typically about 500 m or the water hot and salty. This surface water is then 1600 ft thick). Thick masses of ice eventually break transported northward in the Gulf Stream thereby off the shelf edge, forming flat-topped icebergs that are completing the cycle. This meridional overturning carried by surface ocean currents around Antarctica. circulation (MOC) and its transport of heat energy and Likewise, irregularly shaped icebergs break off the salt is an important control of climate. glacial ice streams of Greenland and flow out into the The hydrosphere is dynamic; water moves North Atlantic Ocean, posing a hazard to navigation. continually through different parts of Earth’s land- Most sea ice surrounding Antarctica forms atmosphere-ocean system and the ocean is the ultimate each winter through freezing of surface seawater. destination of all moving water. Water flowing in river During summer most of the sea ice around Antarctica or stream channels may take a few weeks to reach the melts, whereas in the Arctic Ocean sea ice can persist ocean. Groundwater typically moves at a very slow for several years before flowing out through Fram pace through sediment, and the fractures and tiny Strait into the Greenland Sea, and eventually melting. openings in bedrock, and feeds into rivers, lakes, or This “multi-year” ice loses salt content with age as directly into the ocean. The water of large, deep lakes brine, trapped between ice crystals, melts downward, moves even more slowly, in some cases taking so that Eskimos can harvest this older, less salty ice for centuries to reach the ocean via groundwater flow. drinking water. How long is water frozen into glaciers? CRYOSPHERE Glaciers normally grow (thicken and advance) and The frozen portion of the hydrosphere, the shrink (thin and retreat) slowly in response to changes cryosphere, encompasses massive continental (glacial) in climate. Mountain glaciers respond to climate ice sheets, much smaller ice caps and mountain change on time scales of a decade. Until recently, glaciers, ice in permanently frozen ground (per- scientists had assumed that the response time for the mafrost), and the pack ice and ice bergs floating at sea. Greenland and Antarctic ice sheets is measured in All of these ice types except pack ice (frozen sea millennia; however, two Greenland glaciers have water) and undersea permafrost are fresh water. A exhibited significant changes in discharge in only a few glacier is a mass of ice that flows internally under the years. Changes in ice surface elevation were detected influence of gravity. The Greenland and Antarctic ice by sensors onboard NASA’s Ice, Cloud, and Land sheets in places are up to 3 km (1.8 mi) thick. The Elevation Satellite (ICESat). Hence, ice sheet glaciers Antarctic ice sheet contains 90% of all ice on Earth. may behave more like mountain glaciers, raising Much smaller glaciers (tens to hundreds of meters questions regarding the long-term stability of polar ice thick) primarily occupy the highest mountain valleys sheets and their response to global climate change. on all continents. At present, glacial ice covers about

5 GEOSPHERE million years ago and its constituent landmasses, the The geosphere is the solid portion of the continents of today, slowly moved to their present loca- planet consisting of rocks, , soil, and tions. explains such seemingly sediments. Surface geological processes encompass anomalous discoveries as glacial sediments in the weathering and erosion occurring at the interface Sahara and fossil coral reefs, indicative of tropical between Earth’s and the other Earth subsystems. climates, in northern Wisconsin. Such discoveries Weathering entails the physical disintegration, reflect climatic conditions hundreds of millions of chemical , or solution of exposed rock. years ago when the continents were at different Rock fragments produced by weathering become latitudes than they are today. sediments. Water plays an important role in Geological processes occurring at boundaries weathering by dissolving soluble rock and minerals, between plates produce large-scale landscape and and participating in chemical reactions that decompose ocean bottom features, including mountain ranges, rock. Water’s unusual physical property of expanding volcanoes, deep-sea trenches, as well as the ocean while freezing can fragment rock when the water basins themselves. Enormous stresses develop at plate saturates tiny cracks and pore spaces. Often the water boundaries, bending and fracturing bedrock over broad is not as confined and fragmentation is due to stress areas. Hot molten rock material, known as magma, caused by the growth of ice lenses within the rock. wells up from deep in the crust or upper mantle and The ultimate weathering product is soil, a migrates along rock fractures. Some magma pushes mixture of organic (humus) and inorganic matter into the upper portion of the crust where it cools and (sediment) on Earth’s surface that supports plants, also solidifies into massive bodies of rock, forming the core supplying nutrients and water. Soils derive from the of mountain ranges (e.g., Sierra Nevada). Some weathering of bedrock or sediment, and vary widely in magma feeds volcanoes or flows through fractures in texture (particle size). Typical soil is 50% open space the crust and spreads over Earth’s surface as lava flows (pores), roughly equal proportions of air and water. (flood basalts) that cool and slowly solidify (e.g., Plants also participate in weathering via the physical Columbia River Plateau in the Pacific Northwest and action of their growing roots and the carbon dioxide the massive Siberian Traps). At spreading plate they release to the soil. boundaries on the sea floor, upward flowing magma Erosion refers to the removal and transport of solidifies into new oceanic crust. Plate tectonics and sediments by gravity, moving water, glaciers, and associated volcanism are important in geochemical wind. Running water and glaciers are pathways in the cycling, releasing to the atmosphere water vapor, global water cycle. Erosive agents transport sediments carbon dioxide, and other gases that impact climate. from source regions (usually highlands) to low-lying depositional areas (e.g., ocean, lakes). Weathering aids BIOSPHERE erosion by reducing massive rock to particles that are All living plants and animals on Earth are sufficiently small to be transported by agents of components of the biosphere. They range in size from erosion. Erosion aids weathering by removing microscopic single-celled to the largest sediment and exposing fresh surfaces of rock to the organisms (e.g., redwood trees and blue whales). atmosphere and weathering processes. Together, Bacteria and other single-celled organisms dominate weathering and erosion work to reduce the elevation of the biosphere, both on land and in the ocean. the land. Organisms on land or in the atmosphere live close to Internal geological processes counter surface Earth’s surface. However, marine organisms occur geological processes by uplifting land through tectonic throughout the ocean depths and even inhabit rock activity, including volcanism and mountain building. fractures, volcanic vents, and the ocean floor. Certain Most tectonic activity occurs at the boundaries between organisms live in extreme environments at crustal plates. The overlying crust and rigid mantle is temperatures and pressures once considered impossible broken into a dozen massive plates (and many smaller to support life. In fact, some scientists estimate that the ones) that are slowly driven (typically less than 20 cm mass of organisms living in fractured rocks on and per year) across the face of the globe by huge below the ocean floor may vastly exceed the mass of convection currents in Earth’s mantle. Continents are organisms living on or above it. carried on the moving plates and ocean basins are and cellular respiration are formed by seafloor spreading. essential for life near the surface of the Earth, and Plate tectonics probably has operated on the exemplify how the biosphere interacts with the other planet for at least 3 billion years, with continents subsystems of the climate system. Photosynthesis is periodically assembling into supercontinents and then the process whereby green plants convert light energy splitting apart. The most recent supercontinent, called from the Sun, carbon dioxide from the atmosphere, and Pangaea (Greek for “all land”), broke apart about 200 water to sugars and oxygen (O2). The sugars, which

6 contain a relatively large amount of energy and solids, liquids, and gases move among the various oxygen, are essential for cellular respiration. Through reservoirs on Earth, often involving physical or cellular respiration, an organism processes food and chemical changes to these substances. Accompanying liberates energy for maintenance, growth, and repro- these flows of materials are transfers and duction, also releasing carbon dioxide, water, and heat transformations of energy. Reservoirs in these cycles energy to the environment. With few exceptions, are found within the subsystems of the overall sunlight is the originating source of energy for most or- planetary system (atmosphere, hydrosphere, ganisms living on land and in the ocean’s surface cryosphere, geosphere, and biosphere). Examples of waters. biogeochemical cycles are the water cycle, carbon Dependency between organisms on one cycle, , and . another (e.g., as a source of food) and on their physical Earth is an open (or flow-through) system for and chemical environment (e.g., for water, oxygen, energy, where energy is defined as the capacity for carbon dioxide, and ) is embodied in the concept doing work. Earth receives energy from the Sun of . consist of plants and primarily and some from its own interior while animals that interact with one another, together with emitting energy in the form of invisible infrared the physical conditions and chemical substances in a radiation to space. Along the way, energy is neither specific geographical area. An ecosystem is home to created nor destroyed, although it is converted from producers (plants) which take nutrients to produce one form to another. This is the law of energy foods, consumers (animals) which consume the food to conservation (also known as the first law of grow, and (bacteria, fungi) which return thermodynamics). nutrients to the environment. The Earth system is essentially closed for Feeding relationships among organisms, matter; that is, it neither gains nor loses matter over called a , can be quite simple or more time (except for meteorites and asteroids). All complex as in a . In a food chain, each stage, biogeochemical cycles obey the law of conservation of a trophic (or feeding) level transfers only about 10% of matter, which states that matter can be neither created the energy available to the next higher level, i.e. nor destroyed, but can change in chemical or physical producers to consumers to decomposers. Because form. When a log burns in a fireplace, a portion of the mass transfers are more easily measured than energy, log is converted to ash and heat energy, while the rest , the total weight or mass of organisms, is goes up the chimney as carbon dioxide, water vapor, generally tracked through food chains or webs. creosote and heat. In terms of accountability, all losses Climate is the principal ecological control, from one reservoir in a cycle can be accounted for as largely governing the location and species composition gains in other reservoirs of the cycle. Stated of natural ecosystems such as deserts, rain forests, and succinctly, for any reservoir: tundra. A warmer climate would likely mean fewer days of arctic air and a northward shift of the boreal Input = Output + Storage forest. What actually happens to the forest, however, could hinge on the rate of climate change. Relatively The quantity of a substance stored in a rapid warming may not only shift an ecosystem reservoir depends on the rates at which the material is northward but also alter the ecosystem’s species cycled into and out of the reservoir. This cycling will composition and disturb the orderly internal operation include gains to or losses from a reservoir through of the ecosystem. For example, rapid climate change chemical reactions within the reservoir. If the input could disrupt long-established predator/prey rate exceeds the output rate, the amount of material relationships with implications for the stability of stored in the reservoir increases. If the input rate is less populations of plants and animals. than the output rate, the amount stored decreases. Over Similar observations of close relationships the long term, the cycling rates of materials among the between vegetation and climate variables on a global various global reservoirs are relatively stable; that is, basis were made by the noted German climatologist equilibrium tends to prevail between the rates of input Wladimir Köppen (1846-1940) in the early 20th and output. century. This is a central aspect of his widely used Consider the global cycling of carbon as an climate classification system. illustration of a that has important implications for climate (Figure 1.16). Subsystem Interactions: Biogeochemical Through photosynthesis, carbon dioxide cycles from Cycles the atmosphere to green plants where carbon is incorporated into sugar (C6 H12O6). Plants use sugar to

Biogeochemical cycles are the pathways along which manufacture other organic compounds including fats, proteins, and other carbohydrates. As a byproduct of

7 cellular respiration, plants and animals transform a por- () accumulated on the ocean bottom and in low- tion of the carbon in these organic compounds into CO2 lying swampy terrain on land. The supply of detritus that is released to the atmosphere. In the ocean, CO2 is was so great that organisms could not keep cycled into and out of marine organisms through pace. In some marine environments, and photosynthesis and respiration. In addition to the remains were converted to oil and natural gas. In uptake of CO2 via photosynthesis, marine organisms swampy terrain, heat and pressure from accumulating also use carbon for (CaCO3) to make organic debris concentrated carbon, converting the hard, protective shells. Furthermore, decomposer remains of luxuriant swamp forests into thick layers of organisms (e.g., bacteria) act on the remains of dead coal. Today, when we burn coal, oil, and natural gas, plants and animals, releasing CO2 to the atmosphere collectively called fossil fuels, we are tapping energy and ocean through cellular respiration. that was originally locked in vegetation through When marine organisms die, their remains photosynthesis hundreds of millions of years ago. (shells and skeletons) slowly settle downward through During combustion, carbon from these fossil fuels ocean waters. In time, these organic materials reach combines with oxygen in the air to form carbon dioxide the sea floor, accumulate, are compressed by their own which escapes to the atmosphere. weight and the weight of other sediments, and Another important biogeochemical cycle gradually transform into solid, carbonate rock. operating in the Earth system is the global water cycle Common carbonate rocks are limestone (CaCO3) and (Chapter 5), which is closely linked to all other dolostone (CaMg(CO3)2). Subsequently, tectonic biogeochemical cycles. Reservoirs in the water cycle processes uplift these marine rocks and expose them to (hydrosphere, atmosphere, geosphere, biosphere) are the atmosphere and weathering processes. Rainwater also reservoirs in other cycles, for which water is an contains dissolved atmospheric CO2 producing essential mode of transport. In the nitrogen cycle, for carbonic acid (H2CO3) that, in turn, dissolves carbonate example, intense heating of air caused by lightning rock releasing CO2. As part of the global water cycle, combines atmospheric nitrogen (N2), oxygen (O2), and rivers and streams transport these weathering products moisture to form droplets of extremely dilute nitric to the sea where they settle out of suspension or acid (HNO3) that are washed by rain to the soil. In the - precipitate as sediments that accumulate on the ocean process, nitric acid converts to nitrate (NO3 ), an floor. Over the millions of years that constitute important plant nutrient that is taken up by plants via geologic time, the formation and ultimate weathering their root systems. Plants convert nitrate to ammonia and erosion of carbon-containing rocks have (NH3), which is incorporated into a variety of significantly altered the concentration of carbon compounds, including amino acids, proteins, and DNA. dioxide in the atmosphere thereby changing the On the other hand, both nitrate and ammonia readily climate. dissolve in water so that heavy rains can deplete soil of From about 280 to 345 million years ago, the these important nutrients and wash them into geologic time interval known as the Carboniferous waterways. period, trillions of metric tons of organic remains

8