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Global Environmental Change (GEC) refers to a multitude of environmental changes occurring at the global scale. Such changes include alterations to global bio-geochemical cycles, including carbon, nitrogen, and hydrological cycles; widespread alterations to land use and cover in multiple locations across the world with far-reaching consequences for soils, ecology, economy, and human health; and increasing losses of biological diversity globally. GEC is distinguished by some as systemic (changes that operate globally, such as global warming) or cumulative (local effects that accumulate until the overall impact is global, such as land use/cover change or biodiversity loss). Although the environmental movement faults human activity for the majority of GEC, many of the changes in question have both natural and human drivers and consequences. There is growing consensus, however, regarding the increasingly significant role that human societies have played in altering the structure and function of the planet’s biosphere in recent centuries and decades.
Earth System Science
Much of the current research on global environmental change adopts an earth system science perspective. This perspective involves the recognition that the earth’s oceans, land, and atmosphere constitute an intricately coupled system with its terrestrial and marine biota, and employs an interdisciplinary and integrative approach to studying its components, their interactions and systemic change and variability over time. Definition, characterization, and understanding of GEC are contingent upon spatial and temporal scale, since earth system processes span a range of such scales.
For instance, plate tectonic movements occur over large spatial extents (tens of thousands of kilometers) and long time scales (millions of years). On the other hand, seasonal variations in primary productivity in a temperate deciduous forest biome occur over a spatial extent of hundreds of kilometers and relatively short time scales (months). The definition of a system’s mean behavior depends upon the choice of spatial and temporal scale over which to average that behavior; thus affecting conclusions about system change or variability. A change in a system is generally perceived as unidirectional, sometimes irreversible, whereas variation implies some form of oscillation or fluctuation around a mean value. Systems differ with regard to their stability (ability to retain system characteristics such as structure and function in the face of an externally induced perturbation), or resilience (a measure of a system’s ability to return to its initial state following a perturbation). In addition, systems may be characterized as approximating an equilibrium state (homeostasis), typically involving negative feedbacks, or a non-equilibrium state characterized by stochastic and/or nondeterministic processes of change.
The International Geosphere-Biosphere Program (IGBP) was founded in 1986 by the International Council of Scientific Unions (ICSU). The IGBP constitutes an international, interdisciplinary scientific approach to pose and answer questions about the nature of the earth system and its biogeochemical cycles, its structure, function, and response to human-induced alterations (forcing functions); whether we can or should return to the system state preceding current episodes of human-induced system forcing, such as greenhouse-gas emissions led climate change; and how human societies and economies can achieve such challenges. The IGBP helps coordinate and synthesize research that elaborates key aspects of the earth’s hydrological and biochemical cycles, quantifies rates and patterns of change within them and identifies critical drivers and consequences of those changes.
The Hydrological Cycle
An important component of the earth system is the hydrological cycle-the movement of water through the distinct spheres of the earth-including the lithosphere, atmosphere and surface, and groundwater. This movement of water is driven by solar energy and the processes of evaporation, transpiration, precipitation, surface runoff, infiltration, and subsurface flow, and may be accompanied with changes of phase (solid ice or snow, liquid water, and gaseous water vapor). Approximately 97 percent of the earth’s water is stored in oceans, 2 percent in ice caps and glaciers, and the remainder in ground and surface water reservoirs, the atmosphere, and the earth’s biota. Most of the water in the atmosphere derives from evaporation, the solar-driven conversion of water from terrestrial or marine water sources into water vapor.
A smaller portion of the atmosphere’s water derives from plant transpiration, the loss of water through leaf stomatal openings after it is drawn up from the soil by plant roots by the process of osmosis. Taken together, evaporation and transpiration (evapotranspiration) account for a large part of water loss from vegetated ecosytems and watersheds; however, the kind of vegetation greatly influences evapotranspiration rates. Water vapor in the atmosphere undergoes condensation in water droplets or ice/snow crystals and may move to different locations in the atmosphere by the process of advection. It may be precipitated over oceans or land. Precipitation over land results in surface runoff, some degree of water infiltration into the soil depending upon soil properties, and storage in artificial or natural reservoirs, as well as subsurface flow. Human activities can interrupt the global hydrological cycle through a number of activities, including: surface and groundwater withdrawals for basic water supply for increasing populations in urban and rural areas, agricultural diversion of freshwater, the construction of artificial reservoirs such as dams, and land cover changes such as deforestation and reforestation that alter evapotranspiration and condensation rates. Over half of the earth’s freshwater is estimated to be directly or indirectly used by humans. Some functional aspects of the water cycle remain incompletely understood. Water vapor, clouds, and rainfall, for instance, alter local and regional rates of atmospheric heating and cooling, exerting an important influence on circulation and precipitation and, therefore, regional and global climate. Such dynamics are not well captured in global climate models.
Nitrogen
Nitrogen is an essential element in amino acids and proteins, a component of nucleic acids such as DNA and RNA and of chlorophyll, thereby playing a critical role in the photosynthetic pathway. Approximately 78 percent of the earth’s nitrogen is found in the atmosphere in gaseous form. In order for living organisms to be able to use nitrogen, however, it must first be converted to a usable form, or fixed. Some nitrogen fixation from the atmosphere occurs in lightning strikes [approximately 10 teragrams (Tg) per year globally; 1 Tg = 1,012 g, or roughly 1 million U.S. tons]. Biological nitrogen fixation (approximately 100 Tg) is completed by free-living and symbiotic bacteria that convert gaseous nitrogen into ammonium ions, and subsequently into nitrite and nitrate ions through nitrification. Symbiotic bacteria form mutualistic associations with specific plant species such as legumes, living in root systems, fixing nitrogen in return for carbohydrates and able to increase nitrite and nitrate concentrations in their immediate soil environment. The large-scale agricultural cultivation of legumes thus releases nitrogen into soils (approximately 30 Tg). Nitrogen is also fixed naturally in marine environments (approximately 5-20 Tg). Nitrogen is fixed industrially during the production of ammonia fertilizer (approximately 80 Tg) and released in the combustion of fossil fuels (approximately 25 Tg). It is now estimated that the rate of human-driven fixation of nitrogen through fertilizer production, legume cultivation, and fossil fuel combustion exceeds that of natural pathways (Vitousek 1994). Additional nitrogen may be released by humans through land conversions such as biomass burning and the draining of wetlands.
Much of this excess fixed nitrogen finds its way into groundwater through the process of leaching following rainfall or irrigation. Increased concentrations of nitrogen, a limiting nutrient in many ecosystems, may lead to eutrophication, precipitate dramatic changes in ecological structure, composition, and function. For instance, increased nitrogen may favor the dominance of nitrogen-demanding species, thus reducing species heterogeneity and richness. It may increase productivity and biomass in certain ecosystems. It may drive local declines in abundance and distribution of particular species by affecting populations of consumers, predators, symbionts, decomposers, and parasites in addition to those of primary producers, and even drive species extinctions and forest diebacks, such as in Europe.
The Carbon Cycle
The carbon cycle is likely the most debated aspect of the earth’s changing biogeochemistry, and most significant to debates over climate change and global warming. The carbon cycle consists of movements between the principal carbon reservoirs: terrestrial biota, sediments (including fossil fuels), the ocean, and the atmosphere. The carbon budget denotes the exchange of carbon among the reservoirs, the balance of inputs and outputs to each reservoir and, thus, whether a reservoir acts as an effective source of sink of carbon in the global cycle. Atmospheric carbon is predominantly in the form of carbon dioxide, which forms approximately 0.04 percent of the atmosphere, and is a greenhouse gas akin to methane and chlorofluorocarbons (CFCs). Carbon is fixed (sequestered) from the atmosphere through the process of photosynthesis or primary production; productivity rates are highest in young, growing forested ecosystems.
Accurate records of atmospheric concentrations of carbon dioxide maintained since 1957 at Mauna Loa, Hawaii indicate two principal patterns. An annual fluctuation reflects the seasonal growth pattern of northern forests, and is superimposed upon a steady upward trend over the longer term. Atmospheric carbon dioxide (CO2) concentrations over the past 2,000 years have been recreated by analyzing air bubbles trapped in the Greenland and Antarctic ice caps. The ice core data fit smoothly into the dataset beginning in 1957, and indicate that global carbon dioxide concentrations were relatively constant until the 19th century. A sharp upswing in concentrations since the 1800s coincides with the Industrial Revolution and the dramatic increase in the combustion of fossil fuels for power plants and internal combustion engines. Industrial metabolism is one of the most significant proximate (immediate) anthropogenic sources of GEC. Radiocarbon dating confirms that most of the CO2 increase is attributable to fossil fuel consumption [approximately 10 petagrams (Pg) per year globally; 1 Tg = 1,015 grams], and not deforestation-related CO2 release. The missing carbon sink problem arises because the rate of increase in atmospheric CO2 (approximately 3.5 Pg) does not match that of fossil fuel combustion.
Changing Concentrations
Changing global concentrations of methane (increase of over 30 percent since preindustrial period) and CO2 (increase of approximately 150 percent since preindustrial period) track well with fluctuations in mean annual and or longer-period averaged temperatures, indicating support for greenhouse-gas driven global warming. Global mean surface temperature has increased by 0.6±0.2 degrees C since the late 1800s, and is projected by climate mod els to increase by 1.4 to 5.8 degrees C from 1990 to 2100. Other explanations for observed warming trends hypothesize that the warming is part of natural variation or upswing following the conclusion of the Little Ice Age, or forced externally by solar radiance. Direct data on global temperatures from thermometer readings date to the mid-1800s; temperatures prior to that period are reconstructed from proxies such as width of tree rings, amount of snowfall over glaciers, and isotope records in various glacial and reef systems and calibrated with recent observational data. These longer-term data indicate a warming during the Medieval Warm Period (10th to 14th centuries), and a cooling during the Little Ice Age (14th to 19th centuries), although the global nature of these trends is in question.
Other research has used the Vostok ice core data to examine the anomalous (increasing instead of declining or stabilizing) trends in CO2 and methane concentrations in the Holocene interglacial period relative to the previous three interglacial periods in the past 400,000 years, linking these trends to the only difference in climate forcing during that time, the human-led clearing of land for agriculture. Ruddiman also made the controversial suggestion that cooling during the Little Ice Age was too large to be accounted for by external (solar/orbital) forcings, but was driven by forest regrowth after outbreaks of bubonic plague.
Debate Over Global Effects
The Intergovernmental Panel on Climate Change (IPCC) was established in 1988 by the World Meteorological Organization and the United Nations Environment Program to assess the risk of humaninduced climate change, its potential ecological and human impacts, and options for adaptation and mitigation. The general objectivity of the IPCC’s assessments-which are primarily conducted through the analysis and compendium of peer-reviewed scientific publications-and the IPCC’s emissions scenarios in particular have been questioned by some climate scientists who regard the panel as unduly influenced by political considerations and/or prone to overstate the rate of change in global temperatures.
The effects of increased atmospheric concentrations of CO2 have inspired much lively debate, owing in no small part to the complexity of the earth system and difficulty of representing it realistically and precisely in climate models. In particular the regional variations in projected effects of climate change are large, making global generalizations suspect or less than useful from a policy perspective. Besides increasing global mean temperatures, other commonly considered effects of climate change include sea level rise and changes in rainfall patterns. Global warming trends are projected to cause melting of polar ice sheets and the expansion of water in the oceans, leading to rising sea levels. Models predict that a warming of 1.5-4.5 degrees C will lead to a rise in sea level of 15-95 centimeters. Positive feedback loops can exacerbate the consequences. For instance, melting ice sheets result in reduced albedo, increasing absorption of solar radiation by darker ocean waters and leading to further warming of the oceans and melting of ice sheets. Increased global mean temperatures may release methane trapped in Siberian peat bogs formerly under permafrost, in creasing greenhouse gas concentrations and causing further global warming. Increased respiration from terrestrial ecosystems as a response to increased temperatures may release CO2 to the atmosphere in another positive feedback. Global warming is projected to increase global mean precipitation over the 21st century, though regional variations are significant. According to the IPCC, global climate models project increases in winter precipitation in northern latitudes and over Antarctica, while lower latitudes will experience both increases and decreases in distinct regions, as well as increased variability from year to year in those regions. Another postulated effect of climate change is a link to increased frequency and intensity of extreme events such as hurricanes, although those results are highly debated in the climate science community.
Additional effects pertain to the impacts of climate change on ecosystems, human economies, and health. The effect of increased CO2 on terrestrialbiota, and the feedback effects to the global climate system, was the overall focus of the IGBP’s Global Change in Terrestrial Ecosystems (GCTE) research effort that came to a close in 2003. GCTE focused specifically on ecosystem physiology and the drivers of terrestrial carbon fluxes and pools; changes in ecosystem structure and the relations between vegetation dynamics and landscape pattern and process; impacts of climate change on food production systems and major crops such as wheat and rice; and the relationships among biodiversity and ecosystem function, including ecosystem resilience and stability with respect to natural and human-induced disturbances. Increased CO2 concentrations may cause a fertilization effect, increasing carbon sequestration rates in terrestrial ecosystems. The efficiency of response to the higher CO2 levels varies by photosynthetic pathway; C3 plants stand to gain a relative advantage over C4 plants, with significant implications for community and competitive dynamics. Plants with rapid growth rates stand to gain more than slower-growing species; yet, they would produce leaf tissue with lower nutrient content under such circumstances, with consequences for the health and abundances of herbivore populations and other members of the trophic system as well as system nutrient cycles overall.
Changing regional climate regimes (CO2 enrichment, temperature, and precipitation) can have significant impacts on regional economies by affecting agriculture, forestry, and other production-related human activities and resource management systems. Despite predicted global increases in agricultural yields due to CO2 fertilization effects and increased efficiency of water use, scenarios vary by crop and region. Northern latitudes may experience greater benefits to agricultural in general, and economies that are dependent on rain-fed agriculture will be more vulnerable because of the possibility of prolonged droughts. The effects of climate change on weed development and pest outbreaks can further affect agricultural yields and viability.
Increased temperatures can affect human health directly through reduced cold-related or increased heat-related health problems and mortality. Climate change can significantly affect the abundance and distribution of vectors of infectious diseases such as malaria, dengue, and rift valley fever. Such effects are likely to be felt disproportionately in less-developed countries that tend to be located in lower latitudes and are less equipped economically and administratively to prevent and/or respond to health crises.
Policy Response
The policy response to climate change has focused on two main courses of action: mitigation and adaptation. Mitigation strategies aim at reducing the extent or rate of global warming by reducing fossil fuel use and greenhouse gas emissions through conservation and alternative energy sources such as solar, hydrothermal, and wind energy. Mitigation has also focused on increasing CO2 uptake or carbon sequestration, including mechanisms for emission trading and carbon taxes.
The Kyoto Protocol to the United Nations Framework Convention on Climate Change (UNFCCC) is the international policy instrument to deal with climate change. Countries that ratify the treaty commit to reducing their emissions of greenhouse gases-including CO2 -or participate in carbon taxes (a tax on CO2 emissions) and/or emissions trading. Emissions trading is a market mechanism for dealing with global reductions in greenhouse gases that allows the financial exchange of rights to greenhouse gas emissions between countries that expect to emit more than their allocated share under the Kyoto protocol with those that are under their allocated emissions quotas. The protocol was negotiated in the late 1990s and came into force in 2005 after ratification by Russia.
Adaptive Strategy
Adaptation to global warming, on the other hand, focuses not on reducing or stopping the change itself, but on blocking or responding to the change in a manner that reduces its negative effects on human or natural systems. Adaptation thus refers to the process of reducing the vulnerability to the negative effects of environmental change. The U.S. National Academy of Science (NAS) as well as the IPCC caution that adaptive strategies need to complement efforts at mitigation. Adaptation can be either engineered (planned) or endogenously generated; human societies have adapted to environmental changes in the past several centuries through population resettlement, changes in resources use patterns.
Adaptive strategies might include different modes of agricultural production, including alternative choices of crops and crop varieties, irrigation practices, policies that foster increased food security, the building of higher-capacity stormwater systems and levees in coastal/urban areas, increased use of air conditioning, improved building codes and land use planning, access to appropriate insurance mechanisms, and public health infrastructure to accommodate the negative human health impacts of GEC. Notably, developing nations that are lacking in resources, adaptive capacity, and strong institutions are also the most vulnerable to the effects of GEC and global warming.
Human Impact
By overwhelming consensus, changes to the earth’s biophysical (land) cover as a result of intensifying and diversifying human land uses constitute the most significant component of GEC. Land cover refers to the biophysical condition of the land: its soils, water, and vegetation; while land use refers to its intended human use. Thus, one type of land cover, such as a forest, may accommodate multiple uses such as forestry, recreation, and wildlife conservation. The same land use, on the other hand, may give rise to land covers that are distinguishable from one another, as when agriculture within a forested landscape uses plots in different stages of fallow rotations, generating fields of crops as well as various stages of secondary succession. Land use/ cover change (LUCC) affects multiple biomes and ecosystems, including soils, forests, grasslands, wetlands, terrestrial, marine, and coastal areas, as well as global biogeochemical cycles and global climate. Most anthropogenic LUCC to date has been a consequence of the expansion and intensification of agriculture and pasture, and by forces of urbanization. Over 32 percent of the earth’s surface is currently under productive use by humans.
Over the past 300 years, global extents of forests and woodlands have declined by over 18 percent and that of grasslands and pasture by approximately 1 percent. The world’s croplands, in the meantime, have increased by over 466 percent in that period. Urbanization has also increased dramatically over the past centuries and decades. The Population Reference Bureau (2006) estimates that 47 percent of the present population of the world is urban; this population occupies 1 percent of the earth’s surface and 6 percent of its settled lands. The biophysical transformations associated with urban form have large-scale implications for surface runoff, alterations to regional climate (temperature and precipitation regimes) and air quality.
Changes to land uses and covers can have significant consequences for ecosystems, climate, and human societies, often reaching far beyond the areas directly transformed. The reduction and fragmentation of habitat along with altered disturbance regimes results in changes to local ecosystem structure and function, declines in species and genetic diversity, and increases in the spread of invasive species. Invasive species often share physiological and life history traits that enable them to take advantage of LUCC as well as other aspects of GEC, such as increased CO concentrations and nitrogen deposition. Invaders and exotic species can have devastating ecological as well as economic impact; for instance, an estimated 42 percent of the species listed as endangered or threatened in the United States are at risk primarily because of exotic invaders. The 50,000 invasive species present in the United States collectively cause annual environmental and economic damages in the order of $120 billion. Habitat fragmentation due to LUCC also leads to altered biophysical environments in proximate areas, generating new areas of edges, or ecotones, between the contrasting environments. In 1988, the area of deforestation in the Amazon basin was exceeded by the area within a 1-kilometer distance of existing deforestation, testament to the rising significance of edge effects and their ecological consequences.
Changes in land cover, such as deforestation, can alter solar reflectance patterns by reducing albedo, thereby altering local climate by increasing local temperatures and decreasing humidity. Such increases in regional temperatures in areas of LUCC can influence regional climate and vector populations; for instance, temperature can affect rates of mosquito development, feeding, and infection and incubation times. Urbanization has also been linked to increased average surface temperatures (the urban heat island effect) and decreased diurnal ranges in temperature over urban areas. A 0.27 degrees C (later corrected to 0.35 degrees C) per century increase in surface temperatures was found over all meteorological stations located at heights below 500 meters in the United States, attributable to urbanization and other land use changes. Using two sets of decadal comparisons over 1960s-70s and 1980s-90s, they further found a statistically significant difference in mean temperature increases between urban and rural stations, with urban stations reporting the larger increase in mean temperature.
International Research Program
In recognition of LUCC’s far-reaching impacts and of the necessity of an interdisciplinary approach to the problem, the IGBP and the International Human Dimensions Program (IHDP) jointly founded the international research program on LUCC. The LUCC program initiated and consolidated research on empirical studies of changing land cover patterns, case-study derived understanding of the land use dynamics underlying the changing land covers; the development of regional/global datasets and protocols for land cover classifications; an analysis of scalar dynamics (e.g., how cover and use patterns and their drivers vary across scales), and the use of this information in regional and global models of LUCC and GEC.
Proximate and Driving Forces
LUCC researchers often decompose anthropogenic activity implicated in land change into two broad suites of factors: proximate sources and driving forces. Proximate sources of LUCC refer to the immediate human activity and intended land use(s) causing alterations to the earth’s land cover; these include, for example, agricultural or urban expansion, conversion of grasslands, forests, woodlands or other ecosystems to pasture, land cover changes due to the expansion of infrastructure-such as for road construction-and draining or filling of wetlands for development.
Driving forces, on the other hand, underlie such proximate sources of LUCC and emanate from fundamental social, political, economic, and cultural dynamics. Demographic, technological, socioeconomic, political/institutional, and cultural factors encompass the broad suites of driving forces that may act directly at local scales, or prevail at national/global scales but indirectly affect local areas.
Popular Explanation
One popular explanation of GEC impacts dates to the 1971 formulation of the IPAT hypothesis (Environmental Impact = Population Affluence Technology) by Paul Ehrlich and John Holdren. When treating the earth as a closed system, the IPAT formulation may serve as a shorthand proxy for explaining GEC at the global scale as a function of population size, per-capita consumption levels (affluence or poverty), and technological efficiency and appropriateness. The empirical validity of IPAT often breaks down at regional and local scales of analysis, however, since environmental transformations at these scales are the results of human agents operating within social structures, and involve complex effects of policies, markets, tenure, and other institutions as well as cultural beliefs and practices.
Economic driving forces of LUCC thus include market penetration and growth, relationships between production and consumption, growth in industrial and other sectors, and trade, foreign exchange and other indices of links to international markets and policies. Policy/institutional forces encompass land tenure and property regimes, state and local policies governing resource access, land management and economic development, and include policy instruments such as credits and subsidies, as well as considerations of policy failures. Technological factors include the technical and managerial strategies employed in production in agricultural, forestry, and other land use sectors, including concerns of efficiency and the allocation of labor and capital to the production process. Cultural factors pertain to a household, group or population’s attitudes, values and beliefs; and demographic factors include population increase and density, age structure, and other variables.
The static formulation for IPAT does not take into consideration flows of materials, energy, people and economic resources between linked social and environmental systems. Furthermore, several suites of driving forces may interact at multiple scales, structuring how environmental resources are produced and consumed, and generating complex pathways to land transformations. In 2002, H.J. Geist and E.F. Lambin, for instance, undertook a meta-analysis of 152 subnational case studies of tropical deforestation (a significant proximate source of GEC) and found regional patterns of interacting causal drivers, contradicting the conventional wisdom that previously faulted population growth (driving force) and shifting cultivation (proximate source) as the main culprits. Economic factors were most frequently cited driving forces at the global scale when case studies from Asia, Africa, and Latin America were pooled, followed in order by institutional/ policy, technological and sociocultural factors, with demographic factors cited least frequently. These global “average” trends, however, belie important regional differences. Institutional/policy factors are most frequently cited in case studies focused in Asia, while demographic factors prevail in African case studies, and economic factors dominate in studies based in Latin America.
Going Forward
The LUCC program concluded in 2005; however, its insights and evolving research questions now inform the newly established Global Land Project (GLP). The GLP is the latest IGBP-IHDP collaborative project on linked human-land systems, and merges the agendas and insights from over a decade of research on the relationship of GEC to terrestrial ecology (GCTE) and to dynamics and models of LUCC. The use of models has been particularly instrumental in the understanding and prediction of the dynamics and impacts of LUCC/GEC. While models span a wide range of purposes, analytical techniques and disciplinary/methodological traditions, they can be combined to test and formalize various theories in order to improve our explanatory and predictive power, ability to be generalized, accuracy, and precision. Challenges to present and future modeling approaches to studying land change include the integration of social and natural factors and interactions at multiple scales, the incorporation of qualitative information in models, and overall integration of epistemological, conceptual, and methodological integration in modeling linked social-environmental systems.
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