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Climate Science for Today’s World You are about to embark on a systematic study of climate, climate variability, and climate change. Earth is a mosaic of many climate types, each featuring a unique combination of physical, chemical, and biological characteristics. Differences in climate distinguish, for example, deserts from rainforests, temperate regions from glacier-bound polar localities, and treeless tundra from subtropical savanna. We will come to understand the spatial and temporal (time) variations in climate as a response to many interacting forcing agents or mechanisms both internal and external to the planetary system. At the same time we will become familiar with the scientific principles and basic understandings that underlie the operations and interactions of those forcing agents and mechanisms. This is climate science, the systematic study of the mean state of the atmosphere at a specified location and time period as governed by natural laws. Our study of climate science provides valuable insights into one of the most pressing environmental issues of our time: global climate change. We explore the many possible causes of climate change with special emphasis on the role played by human activity (e.g., burning fossil fuels, clearing vegetation). A thorough grounding in climate science enables us to comprehend the implications of anthropogenic climate change, how each of us contributes to the problem, and how each of us can be part of the solution to the problem. The essential value in studying climate science stems from the ecological and societal impacts of climate and climate change. Climate is the ultimate environmental control that governs our lives; for example, what crops can be cultivated, the supply of fresh water, and the average heating and cooling requirements for homes. By its very nature, climate science is interdisciplinary, drawing on principles and basic understandings of many scientific disciplines. We recognize climate as a system in which Earth’s major subsystems (i.e., atmosphere, hydrosphere, cryosphere, geosphere, and biosphere) individually and in concert function as controls of climate. Linking these subsystems are biogeochemical cycles (e.g., global carbon cycle, global water cycle), pathways for transfer of climate-sensitive materials (e.g., greenhouse gases) and energy among Earth-bound reservoirs. An easy and popular way of summarizing local or regional climate is in terms of the averages of weather elements, such as temperature and precipitation, derived from observations taken over a 1 span of many years. In this empirically-based context, climate is defined as weather (the state of the atmosphere) at some locality averaged over a specified time interval. Climate must be specified for a particular place and period because, like weather, climate varies both spatially and temporally. Thus, for example, the climate of Chicago differs from that of New Orleans, and winters in Chicago were somewhat milder in the 1980s and 1990s than in the 1880s and 1890s. In addition to average values of weather elements, the climate record includes extremes in weather. Climatic summaries typically tabulate extremes such as the coldest, warmest, driest, wettest, snowiest, or windiest day, month or year on record for some locality. Extremes are useful aspects of the climate record if only because what has happened in the past can happen again. For this reason, for example, farmers are interested in not only the average rainfall during the growing season but also the frequency of exceptionally wet or dry growing seasons. In essence, records of weather extremes provide a perspective on the variability of local or regional climate. Selection of an internationally agreed 30-year period for averaging weather data may be inappropriate for some applications because climate varies over a broad range of time scales and can change significantly in periods much shorter than 30 years. For example, El Niño refers to an inter-annual variation in climate involving air/sea interactions in the tropical Pacific and weather extremes in various parts of the world. The phenomenon typically lasts for 12 to 18 months and occurs about every 3 to 7 years. For some purposes, a 30-year period is a short-sighted view of climate variability. Compared to the long-term climate record, for example, the current 1971-2000 averaging period was unusually mild over much of the nation. In the United States, 30-year averages are computed for temperature, precipitation (rain plus melted snow and ice), and degree days and identified as normals. Averages of other climate elements such as wind speed and humidity are derived from the entire period of record or at least the period when observations were made at the same location. Other useful climate elements include average seasonal snowfall, length of growing season, percent of possible sunshine, and number of days with dense fog. Tabulation of extreme values of weather elements is usually also drawn from the entire period of the observational record. While the empirical definition of climate (in terms of statistical summaries) is informative and useful, the dynamic definition of climate is more fundamental. It addresses the nature and controls of Earth’s climate together with the causes of climate variability and change operating on all time scales. Climate differs from season to season and with those variations in climate, the array of weather patterns that characterize one season differs from the array of characteristic weather patterns of another season. The status of the planetary system (that is, the Earthatmosphere-land-ocean system) determines (or selects) the array of possible weather patterns for any season. In essence, this status constitutes boundary conditions (i.e., forcing agents and mechanisms) such as incoming solar radiation and the albedo (reflectivity) of Earth’s surface. Hence, in a dynamic context, climate is defined by the boundary conditions in the planetary system coupled with the associated typical weather patterns that vary with the seasons. For example, the higher Sun’s path across the local sky and the longer daylight length in Bismarck, ND during July increase the chance of warm weather and possible thunderstorms, whereas lower Sun angles and shorter daylight duration during January would mean colder weather and possible snow. Climatology is the study of climate, its controls, and spatial and temporal variability. Climatology is primarily a field science rather than a laboratory science. The field is the atmosphere and Earth’s surface where data are obtained by direct (in situ) measurement by instruments and remote sensing, mostly by sensors flown aboard Earth-orbiting satellites. The only scientific experiments routinely conducted by climate scientists involve manipulation of numerical climate models. Usually these global or regional models are used to predict the climatic consequences of change in the boundary conditions of Earth’s climate system. Furthermore, climatology is an interdisciplinary science that reveals how the various components of the natural world are interconnected. For example, the composition of the atmosphere is the end product of many processes where gases are emitted (e.g., via volcanic eruptions) or absorbed (e.g., gases dissolving in the ocean). The composition of the atmosphere, in turn, affects the ocean, living organisms, geological processes, and climate. Climate and Society Probably the single most important reason for studying climate science is the many linkages between climate and society. For one, climate imposes constraints on social and economic development. For example, the 2 abject poverty of North Africa’s Sahel in large measure is due to the region’s subtropical climate that is plagued by multi-decadal droughts. In other regions, climate provides resources that are exploited to the advantage of society. For example, some climates favor winter or summer recreational activities (e.g., skiing, boating) that attract vacationers and feed the local economy. Severe weather (e.g., tornadoes, hurricanes, floods, heat waves, cold waves, and drought) can cause deaths and injuries, considerable long-term disruption of communities, property damage, and economic loss. The impact of Hurricane Katrina on the Gulf Coast is still being felt many years after that weather system made landfall (August 2005). Regardless of a nation’s status as developed or developing, it is not possible to weather- or climateproof society to prevent damage to life and property. In the agricultural sector, for example, the prevailing strategy is to depend on technology to circumvent climate constraints. Where water supply is limited, farmers and ranchers routinely rely on irrigation water usually pumped from subsurface aquifers (e.g., the High Plains Aquifer in the central U.S.) or transferred via aqueducts and canals from other watersheds. Because of consumers’ food preferences and for economic reasons, this strategy is preferred to matching crops to the local or regional climate (e.g., dry land farming). Other strategies include construction of dams and reservoirs to control runoff and genetic manipulation to breed drought resistant crops. Although these strategies have some success, they have limitations and often require tradeoffs. For example, many rivers around the world lose so much of their flow to diversions (mostly for irrigation) that they are reduced to a trickle or completely dry up prior to reaching the sea at least during part of the year. Compounding the constraints of climate on society is the prospect of global climate change. The scientific evidence is now convincing that human activity is influencing climate on a global scale with significant consequences for society. Burning of fossil fuels (coal, oil, natural gas) and clearing of vegetation is responsible for a steady build-up of atmospheric carbon dioxide (CO2) and enhancement of Earth’s greenhouse effect. This enhancement is exacerbated by other human activities that are increasing the concentration of methane (CH4) and nitrous oxide (N2O), also greenhouse gases. Our understanding of the potential impact of climate and climate change on society requires knowledge of (1) the structure and function of Earth’s climate system, (2) interactions of the various components of that system, and (3) how human activities influence and are influenced by these systems. The Climate System What is the climate system and, more fundamentally, what is a system? A system is an entity whose components interact in an orderly manner according to the laws of physics, chemistry, and biology. A familiar example of a system is the human body, which consists of various identifiable subsystems including the nervous, respiratory, and reproductive systems, plus the input/output of energy and matter. In a healthy person, these subsystems function internally and interact with one another in regular and predictable ways that can be studied based upon analysis of the energy and mass budgets for the systems. Extensive observations and knowledge of a system enable scientists to predict how the system and its components are likely to respond to changing internal and external conditions. The ability to predict the future state(s) of a system is important, for example, in dealing with the complexities of global climate change and its potential impacts on Earth’s subsystems and society. The 1992 United Nations Framework Convention on Climate Change defines Earth’s climate system as the totality of the atmosphere, hydrosphere (including the cryosphere), biosphere and geosphere and their interactions. The view of Planet Earth in Figure 1, resembling a “blue marble,” shows all the major subsystems of the climate system. The ocean, the most prominent feature covering more than two-thirds of Earth’s surface, appears blue. Clouds obscure most of the ice sheets (the major part of the cryosphere) that cover much of Greenland and Antarctica. The atmosphere is made visible by swirling storm clouds over the Pacific Ocean near Mexico and the middle of the Atlantic Ocean. Viewed edgewise, the atmosphere appears as a thin, bluish layer. Land (part of the geosphere) is mostly green 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 Earth’s atmosphere is a relatively thin envelope of gases and tiny suspended particles surrounding the planet. But the thin atmospheric skin is essential for life and the orderly functioning of physical, chemical and biological processes on Earth. Nitrogen (N2) and oxygen (O2), the chief atmospheric gases, make up a uniform 78.08% and 20.95% by volume, respectively through most of the atmosphere. Not counting water vapor (with its highly variable concentration), the next most abundant gases are argon (0.93%) and carbon dioxide (0.038%). Many other gases occur in the atmosphere in trace concentrations, including ozone (O3) and methane (CH4) (Table 1). Unlike nitrogen and oxygen, the percent volume of some of these trace gases varies with time and location. TABLE 1 Some Gases Composing Dry Air in the Lower Atmosphere Gas % by volume 78.08 Nitrogen (N2) 20.95 Oxygen (O2) Argon (Ar) 0.93 Carbon Dioxide (CO2) 0.0388 0.00014 Methane (CH4) 0.00005 Nitrous Oxide (N2O) 0.000007 Ozone (O3) Parts per million 780,840.0 209,460.0 9,340.0 388.0 1.4 0.5 0.07 Aerosols, minute solid and liquid particles, suspended mainly in the lower atmosphere derive from wind erosion of soil, ocean spray, forest fires, volcanic eruptions, industrial chimneys, and the exhaust of motor vehicles. Although aerosol concentrations are relatively small, they participate in some important processes. Aerosols are nuclei for cloud formation, interact with incoming solar radiation and dust blown out over the tropical Atlantic Ocean from North Africa may affect the development of tropical cyclones (hurricanes and tropical storms). The significance of an atmospheric gas is not necessarily related to its concentration. Some atmospheric components that are essential for life occur in very low concentrations. For example, most water vapor is confined to the lowest kilometer or so of the atmosphere and is never more than about 4% by volume even in the most humid places on Earth (e.g., over tropical rainforests and seas). But without water vapor, the planet would have no water cycle, no rain or snow, no ocean, and no fresh water. Also, without water vapor, Earth would be much too cold for most forms of life to exist. Water vapor is the main greenhouse gas, one that interacts with infrared 4 radiation. Although comprising only 0.038% of the lower atmosphere, carbon dioxide is essential for photosynthesis. Without carbon dioxide, green plants and the food webs they support could not exist. While the atmospheric concentration of ozone (O3) is minute, the chemical reactions responsible for its formation (from oxygen) and dissociation (to oxygen) in the stratosphere (mostly at altitudes between 30 and 50 km) shield organisms on Earth’s surface from potentially lethal levels of solar UV radiation. Carbon dioxide and ozone are also greenhouse gases. The atmosphere is dynamic; the atmosphere continually circulates in response to different rates of heating and cooling within the rotating planetary system. Heat is conveyed from warmer locations to colder locations, from Earth’s surface to the atmosphere and from the tropics to higher latitudes. The global water cycle and accompanying phase changes of water play an important role in this planetary-scale transport of heat energy. HYDROSPHERE The hydrosphere is the water component of the climate system. Water continually cycles among reservoirs within the climate system. The ocean, by far the largest reservoir of water in the hydrosphere, covers about 70.8% of the planet’s surface and has an average depth of about 3.8 km (2.4 mi). About 96.4% of the hydrosphere is ocean salt water. The next largest reservoir in the hydrosphere is glacial ice (also considered the cryosphere), most of which covers much of Antarctica and Greenland. Ice and snow make up 2.1% of water in the hydrosphere. Considerably smaller quantities of water occur on the land surface (lakes, rivers), in the subsurface (soil moisture, groundwater), the atmosphere (water vapor, clouds, precipitation), and biosphere (plants, animals). The ocean and atmosphere are coupled such that the wind drives surface ocean currents. Winddriven currents are restricted to a surface ocean layer typically about 100 m (300 ft) deep and take a few months to years to cross an ocean basin. Ocean currents at much greater depths are more sluggish and more challenging to study than surface currents because of greater difficulty in taking measurements. Movements of deep-ocean waters are caused primarily by small differences in water density (mass per unit volume) arising from small differences in water temperature and salinity (a measure of dissolved salt content). Cold sea water, being denser than warm water, tends to sink whereas warm water, being less dense, is buoyed upward by (or floats on) colder water. Likewise, saltier water is denser than less salty water and tends to sink, whereas less salty water is buoyed upward. The combination of temperature and salinity determines whether a water mass remains at its original depth or sinks to the ocean bottom. Even though deep currents are relatively slow, they keep ocean waters well mixed so that the ocean has a nearly uniform chemical composition. The densest ocean waters form in polar or nearby subpolar regions. Salty waters become even saltier where sea ice forms at high latitudes because growing ice crystals exclude dissolved salts. Chilling of this salty water near Greenland and Iceland and in the Norwegian and Labrador Seas further increases its density so that surface waters sink and form a bottom current that flows southward under equatorial surface waters and into the South Atlantic as far south as Antarctica. Here, deep water from the North Atlantic mixes with deep water around Antarctica. Branches of that cold bottom current then spread northward into the Atlantic, Indian, and Pacific basins. Eventually, the water slowly diffuses to the surface, mainly in the Pacific, and then begins its journey on the surface through the islands of Indonesia, across the Indian Ocean, around South Africa, and into the tropical Atlantic. There, intense heating and evaporation make the water hot and salty. This surface water is then transported northward in the Gulf Stream thereby completing the cycle. This meridional overturning circulation (MOC) and its transport of heat energy and salt is an important control of climate. The hydrosphere is dynamic; water moves continually through different parts of Earth’s landatmosphere-ocean system and the ocean is the ultimate destination of all moving water. Water flowing in river or stream channels may take a few weeks to reach the ocean. Groundwater typically moves at a very slow pace through sediment, and the fractures and tiny openings in bedrock, and feeds into rivers, lakes, or directly into the ocean. The water of large, deep lakes moves even more slowly, in some cases taking centuries to reach the ocean via groundwater flow. CRYOSPHERE The frozen portion of the hydrosphere, the cryosphere, encompasses massive continental (glacial) ice sheets, much smaller ice caps and mountain glaciers, ice in permanently frozen ground (permafrost), and the pack ice and ice bergs floating at sea. All of these ice types except pack ice (frozen sea water) and undersea permafrost are fresh water. A glacier is a mass of ice that flows internally under the influence of gravity. The Greenland and Antarctic ice sheets in places are up to 3 km (1.8 mi) thick. The Antarctic ice sheet contains 90% of all ice on Earth. Much smaller glaciers (tens to hundreds of meters thick) primarily occupy the highest mountain valleys on all continents. At present, glacial ice covers about 5 10% of the planet’s land area but at times during the past 1.7 million years, glacial ice expanded over as much as 30% of the land surface, primarily in the Northern Hemisphere. As snow accumulates, the pressure exerted by the new snow converts underlying snow to ice. As the ice forms, it preserves traces of the original seasonal layering of snow and traps air bubbles. Chemical analysis of the ice layers and air bubbles in the ice provides clues to climatic conditions at the time the original snow fell. Ice cores extracted from the Greenland and Antarctic ice sheets yield information on changes in Earth’s climate and atmospheric composition extending as far back as hundreds of thousands of years—to 800,000 years or more in Antarctica. Under the influence of gravity, glacial ice flows slowly from sources at higher latitudes and higher elevations (where some winter snow survives the summer) to lower latitudes and lower elevations, where the ice either melts or flows into the nearby ocean. Around Antarctica, streams of glacial ice flow out to the ocean. Ice, being less dense than seawater, floats, forming ice shelves (typically about 500 m or 1600 ft thick). Thick masses of ice eventually break off the shelf edge, forming flat-topped icebergs that are carried by surface ocean currents around Antarctica. Likewise, irregularly shaped icebergs break off the glacial ice streams of Greenland and flow out into the North Atlantic Ocean, posing a hazard to navigation. Most sea ice surrounding Antarctica forms each winter through freezing of surface seawater. During summer most of the sea ice around Antarctica melts, whereas in the Arctic Ocean sea ice can persist for several years before flowing out through Fram Strait into the Greenland Sea, and eventually melting. This “multi-year” ice loses salt content with age as brine, trapped between ice crystals, melts downward, so that Eskimos can harvest this older, less salty ice for drinking water. How long is water frozen into glaciers? Glaciers normally grow (thicken and advance) and shrink (thin and retreat) slowly in response to changes in climate. Mountain glaciers respond to climate change on time scales of a decade. Until recently, scientists had assumed that the response time for the Greenland and Antarctic ice sheets is measured in millennia; however, two Greenland glaciers have exhibited significant changes in discharge in only a few years. Changes in ice surface elevation were detected by sensors onboard NASA’s Ice, Cloud, and Land Elevation Satellite (ICESat). Hence, ice sheet glaciers may behave more like mountain glaciers, raising questions regarding the long-term stability of polar ice sheets and their response to global climate change. GEOSPHERE The geosphere is the solid portion of the planet consisting of rocks, minerals, soil, and sediments. Surface geological processes encompass weathering and erosion occurring at the interface between Earth’s crust and the other Earth subsystems. Weathering entails the physical disintegration, chemical decomposition, or solution of exposed rock. Rock fragments produced by weathering become sediments. Water plays an important role in weathering by dissolving soluble rock and minerals, and participating in chemical reactions that decompose rock. Water’s unusual physical property of expanding while freezing can fragment rock when the water saturates tiny cracks and pore spaces. Often the water is not as confined and fragmentation is due to stress caused by the growth of ice lenses within the rock. The ultimate weathering product is soil, a mixture of organic (humus) and inorganic matter (sediment) on Earth’s surface that supports plants, also supplying nutrients and water. Soils derive from the weathering of bedrock or sediment, and vary widely in texture (particle size). Typical soil is 50% open space (pores), roughly equal proportions of air and water. Plants also participate in weathering via the physical action of their growing roots and the carbon dioxide they release to the soil. Erosion refers to the removal and transport of sediments by gravity, moving water, glaciers, and wind. Running water and glaciers are pathways in the global water cycle. Erosive agents transport sediments from source regions (usually highlands) to low-lying depositional areas (e.g., ocean, lakes). Weathering aids erosion by reducing massive rock to particles that are sufficiently small to be transported by agents of erosion. Erosion aids weathering by removing sediment and exposing fresh surfaces of rock to the atmosphere and weathering processes. Together, weathering and erosion work to reduce the elevation of the land. Internal geological processes counter surface geological processes by uplifting land through tectonic activity, including volcanism and mountain building. Most tectonic activity occurs at the boundaries between crustal plates. The overlying crust and rigid mantle is broken into a dozen massive plates (and many smaller ones) that are slowly driven (typically less than 20 cm per year) across the face of the globe by huge convection currents in Earth’s mantle. Continents are carried on the moving plates and ocean basins are formed by seafloor spreading. Plate tectonics probably has operated on the planet for at least 3 billion years, with continents periodically assembling into supercontinents and then splitting apart. The most recent supercontinent, called Pangaea (Greek for “all land”), broke apart about 200 6 million years ago and its constituent landmasses, the continents of today, slowly moved to their present locations. Plate tectonics explains such seemingly anomalous discoveries as glacial sediments in the Sahara and fossil coral reefs, indicative of tropical climates, in northern Wisconsin. Such discoveries reflect climatic conditions hundreds of millions of years ago when the continents were at different latitudes than they are today. Geological processes occurring at boundaries between plates produce large-scale landscape and ocean bottom features, including mountain ranges, volcanoes, deep-sea trenches, as well as the ocean basins themselves. Enormous stresses develop at plate boundaries, bending and fracturing bedrock over broad areas. Hot molten rock material, known as magma, wells up from deep in the crust or upper mantle and migrates along rock fractures. Some magma pushes into the upper portion of the crust where it cools and solidifies into massive bodies of rock, forming the core of mountain ranges (e.g., Sierra Nevada). Some magma feeds volcanoes or flows through fractures in the crust and spreads over Earth’s surface as lava flows (flood basalts) that cool and slowly solidify (e.g., Columbia River Plateau in the Pacific Northwest and the massive Siberian Traps). At spreading plate boundaries on the sea floor, upward flowing magma solidifies into new oceanic crust. Plate tectonics and associated volcanism are important in geochemical cycling, releasing to the atmosphere water vapor, carbon dioxide, and other gases that impact climate. BIOSPHERE All living plants and animals on Earth are components of the biosphere. They range in size from microscopic single-celled bacteria to the largest organisms (e.g., redwood trees and blue whales). Bacteria and other single-celled organisms dominate the biosphere, both on land and in the ocean. Organisms on land or in the atmosphere live close to Earth’s surface. However, marine organisms occur throughout the ocean depths and even inhabit rock fractures, volcanic vents, and the ocean floor. Certain organisms live in extreme environments at temperatures and pressures once considered impossible to support life. In fact, some scientists estimate that the mass of organisms living in fractured rocks on and below the ocean floor may vastly exceed the mass of organisms living on or above it. Photosynthesis and cellular respiration are essential for life near the surface of the Earth, and exemplify how the biosphere interacts with the other subsystems of the climate system. Photosynthesis is the process whereby green plants convert light energy from the Sun, carbon dioxide from the atmosphere, and water to sugars and oxygen (O2). The sugars, which contain a relatively large amount of energy and oxygen, are essential for cellular respiration. Through cellular respiration, an organism processes food and liberates energy for maintenance, growth, and reproduction, also releasing carbon dioxide, water, and heat energy to the environment. With few exceptions, sunlight is the originating source of energy for most organisms living on land and in the ocean’s surface waters. Dependency between organisms on one another (e.g., as a source of food) and on their physical and chemical environment (e.g., for water, oxygen, carbon dioxide, and habitat) is embodied in the concept of ecosystem. Ecosystems consist of plants and animals that interact with one another, together with the physical conditions and chemical substances in a specific geographical area. An ecosystem is home to producers (plants) which take nutrients to produce foods, consumers (animals) which consume the food to grow, and decomposers (bacteria, fungi) which return nutrients to the environment. Feeding relationships among organisms, called a food chain, can be quite simple or more complex as in a food web. In a food chain, each stage, a trophic (or feeding) level transfers only about 10% of the energy available to the next higher level, i.e. producers to consumers to decomposers. Because mass transfers are more easily measured than energy, biomass, the total weight or mass of organisms, is generally tracked through food chains or webs. Climate is the principal ecological control, largely governing the location and species composition of natural ecosystems such as deserts, rain forests, and tundra. A warmer climate would likely mean fewer days of arctic air and a northward shift of the boreal forest. What actually happens to the forest, however, could hinge on the rate of climate change. Relatively rapid warming may not only shift an ecosystem northward but also alter the ecosystem’s species composition and disturb the orderly internal operation of the ecosystem. For example, rapid climate change could disrupt long-established predator/prey relationships with implications for the stability of populations of plants and animals. Similar observations of close relationships between vegetation and climate variables on a global basis were made by the noted German climatologist Wladimir Köppen (1846-1940) in the early 20th century. This is a central aspect of his widely used climate classification system. Subsystem Interactions: Biogeochemical Cycles Biogeochemical cycles are the pathways along which 7 solids, liquids, and gases move among the various reservoirs on Earth, often involving physical or chemical changes to these substances. Accompanying these flows of materials are transfers and transformations of energy. Reservoirs in these cycles are found within the subsystems of the overall planetary system (atmosphere, hydrosphere, cryosphere, geosphere, and biosphere). Examples of biogeochemical cycles are the water cycle, carbon cycle, oxygen cycle, and nitrogen cycle. Earth is an open (or flow-through) system for energy, where energy is defined as the capacity for doing work. Earth receives energy from the Sun primarily and some from its own interior while emitting energy in the form of invisible infrared radiation to space. Along the way, energy is neither created nor destroyed, although it is converted from one form to another. This is the law of energy conservation (also known as the first law of thermodynamics). The Earth system is essentially closed for matter; that is, it neither gains nor loses matter over time (except for meteorites and asteroids). All biogeochemical cycles obey the law of conservation of matter, which states that matter can be neither created nor destroyed, but can change in chemical or physical form. When a log burns in a fireplace, a portion of the log is converted to ash and heat energy, while the rest goes up the chimney as carbon dioxide, water vapor, creosote and heat. In terms of accountability, all losses from one reservoir in a cycle can be accounted for as gains in other reservoirs of the cycle. Stated succinctly, for any reservoir: Input = Output + Storage The quantity of a substance stored in a reservoir depends on the rates at which the material is cycled into and out of the reservoir. This cycling will include gains to or losses from a reservoir through chemical reactions within the reservoir. If the input rate exceeds the output rate, the amount of material stored in the reservoir increases. If the input rate is less than the output rate, the amount stored decreases. Over the long term, the cycling rates of materials among the various global reservoirs are relatively stable; that is, equilibrium tends to prevail between the rates of input and output. Consider the global cycling of carbon as an illustration of a biogeochemical cycle that has important implications for climate (Figure 1.16). Through photosynthesis, carbon dioxide cycles from the atmosphere to green plants where carbon is incorporated into sugar (C6 H12O6). Plants use sugar to manufacture other organic compounds including fats, proteins, and other carbohydrates. As a byproduct of cellular respiration, plants and animals transform a portion of the carbon in these organic compounds into CO2 that is released to the atmosphere. In the ocean, CO2 is cycled into and out of marine organisms through photosynthesis and respiration. In addition to the uptake of CO2 via photosynthesis, marine organisms also use carbon for calcium carbonate (CaCO3) to make hard, protective shells. Furthermore, decomposer organisms (e.g., bacteria) act on the remains of dead plants and animals, releasing CO2 to the atmosphere and ocean through cellular respiration. When marine organisms die, their remains (shells and skeletons) slowly settle downward through ocean waters. In time, these organic materials reach the sea floor, accumulate, are compressed by their own weight and the weight of other sediments, and gradually transform into solid, carbonate rock. Common carbonate rocks are limestone (CaCO3) and Subsequently, tectonic dolostone (CaMg(CO3)2). processes uplift these marine rocks and expose them to the atmosphere and weathering processes. Rainwater contains dissolved atmospheric CO2 producing carbonic acid (H2CO3) that, in turn, dissolves carbonate rock releasing CO2. As part of the global water cycle, rivers and streams transport these weathering products to the sea where they settle out of suspension or precipitate as sediments that accumulate on the ocean floor. Over the millions of years that constitute geologic time, the formation and ultimate weathering and erosion of carbon-containing rocks have significantly altered the concentration of carbon dioxide in the atmosphere thereby changing the climate. From about 280 to 345 million years ago, the geologic time interval known as the Carboniferous period, trillions of metric tons of organic remains 8 (detritus) accumulated on the ocean bottom and in lowlying swampy terrain on land. The supply of detritus was so great that decomposer organisms could not keep pace. In some marine environments, plant and animal remains were converted to oil and natural gas. In swampy terrain, heat and pressure from accumulating organic debris concentrated carbon, converting the remains of luxuriant swamp forests into thick layers of coal. Today, when we burn coal, oil, and natural gas, collectively called fossil fuels, we are tapping energy that was originally locked in vegetation through photosynthesis hundreds of millions of years ago. During combustion, carbon from these fossil fuels combines with oxygen in the air to form carbon dioxide which escapes to the atmosphere. Another important biogeochemical cycle operating in the Earth system is the global water cycle (Chapter 5), which is closely linked to all other biogeochemical cycles. Reservoirs in the water cycle (hydrosphere, atmosphere, geosphere, biosphere) are also reservoirs in other cycles, for which water is an essential mode of transport. In the nitrogen cycle, for example, intense heating of air caused by lightning combines atmospheric nitrogen (N2), oxygen (O2), and moisture to form droplets of extremely dilute nitric acid (HNO3) that are washed by rain to the soil. In the process, nitric acid converts to nitrate (NO3-), an important plant nutrient that is taken up by plants via their root systems. Plants convert nitrate to ammonia (NH3), which is incorporated into a variety of compounds, including amino acids, proteins, and DNA. On the other hand, both nitrate and ammonia readily dissolve in water so that heavy rains can deplete soil of these important nutrients and wash them into waterways.