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The scientific community has made the urgent need to mitigate climate change clear and, with the ratification of the Paris Agreement under the United Nations Framework Convention on Climate Change, the international community has formally accepted ambitious mitigation goals. However, a wide gap remains between the aspirational emissions reduction goals of the Paris Agreement and the real-world pledges and actions of nations that are party to it. Closing that emissions gap can only be achieved if a similarly wide gap between scientific and societal understanding of climate change is also closed.

Several fundamental aspects of climate change make clear both the need for education and the opportunity it offers. First, addressing climate change will require action at all levels of society, including individuals, organizations, businesses, local, state, and national governments, and international bodies. It cannot be addressed by a few individuals with privileged access to information, but rather requires transfer of knowledge, both intellectually and affectively, to decision-makers and their constituents at all levels. Second, education is needed because, in the case of climate change, learning from experience is learning too late. The delay between decisions that cause climate change and their full societal impact can range from decades to millennia. As a result, learning from education, rather than experience, is necessary to avoid those impacts.

Climate change and sustainability represent complex, dynamic systems that demand a systems thinking approach. Systems thinking takes a holistic, long-term perspective that focuses on relationships between interacting parts, and how those relationships generate behavior over time. System dynamics includes formal mapping and modeling of systems, to improve understanding of the behavior of complex systems as well as how they respond to human or other interventions. Systems approaches are increasingly seen as critical to climate change education, as the human and natural systems involved in climate change epitomize a complex, dynamic problem that crosses disciplines and societal sectors.

A systems thinking approach can also be used to examine the potential for education to serve as a vehicle for societal change. In particular, education can enable society to benefit from climate change science by transferring scientific knowledge across societal sectors. Education plays a central role in several processes that can accelerate social change and climate change mitigation. Effective climate change education increases the number of informed and engaged citizens, building social will or pressure to shape policy, and building a workforce for a low-carbon economy. Indeed, several climate change education efforts to date have delivered gains in climate and energy knowledge, affect, and/or motivation. However, society still faces challenges in coordinating initiatives across audiences, managing and leveraging resources, and making effective investments at a scale that is commensurate with the climate change challenge. Education is needed to promote informed decision-making at all levels of society.

Aerosols (tiny solid or liquid particles suspended in the atmosphere) have been in the forefront of environmental and climate change sciences as the primary atmospheric pollutant and external force affecting Earth’s weather and climate. There are two dominant mechanisms by which aerosols affect weather and climate: aerosol-radiation interactions (ARIs) and aerosol-cloud interactions (ACIs). ARIs arise from aerosol scattering and absorption, which alter the radiation budgets of the atmosphere and surface, while ACIs are connected to the fact that aerosols serve as cloud condensation nuclei and ice nuclei. Both ARIs and ACIs are coupled with atmospheric dynamics to produce a chain of complex interactions with a large range of meteorological variables that influence both weather and climate. Elaborated here are the impacts of aerosols on the radiation budget, clouds (microphysics, structure, and lifetime), precipitation, and severe weather events (lightning, thunderstorms, hail, and tornadoes). Depending on environmental variables and aerosol properties, the effects can be both positive and negative, posing the largest uncertainties in the external forcing of the climate system. This has considerably hindered the ability to project future climate changes and make accurate numerical weather predictions.

Along with ceramics production, sedentism, and herding, agriculture is a major component of the Neolithic as it is defined in Europe. Therefore, the agricultural system of the first Neolithic societies and the dispersal of exogenous cultivated plants to Europe are the subject of many scientific studies. To work on these issues, archaeobotanists rely on residual plant remains—crop seeds, weeds, and wild plants—from archaeological structures like detritic pits, and, less often, storage contexts. To date, no plant with an economic value has been identified as domesticated in Western Europe except possibly opium poppy. The earliest seeds identified at archaeological sites dated to about 5500–5200 bc in the Mediterranean and Temperate Europe. The cultivated plants identified were cereals (wheat and barley), oleaginous plant (flax), and pulses (peas, lentils, and chickpeas). This crop package originated in the Fertile Crescent, where it was clearly established around 7500 bc (final Pre-Pottery Neolithic B), after a long, polycentric domestication process. From the middle of the 7th millennium bc, via the Balkan Peninsula, the pioneer Neolithic populations, with their specific economies, rapidly dispersed from east to west, following two main pathways. One was the maritime route over the northwestern basin of the Mediterranean (6200–5300 bc), and the other was the terrestrial and fluvial route in central and northwestern continental Europe (5500–4900 bc). On their trajectory, the agropastoral societies adapted the Neolithic founder crops from the Middle East to new environmental conditions encountered in Western Europe.

The Neolithic pioneers settled in an area that had experienced a long tradition of hunting and gathering. The Neolithization of Europe followed a colonization model. The Mesolithic groups, although exploiting plant resources such as hazelnut more or less intensively, did not significantly change the landscape. The impact of their settlements and their activities are hardly noticeable through palynology, for example. The control of the mode of reproduction of plants has certainly increased the prevalence of Homo sapiens, involving, among others, a demographic increase and the ability to settle down in areas that were not well adapted to year-round occupation up to that point. The characterization of past agricultural systems, such as crop plants, technical processes, and the impact of anthropogenic activities on the landscape, is essential for understanding the interrelation of human societies and the plant environment. This interrelation has undoubtedly changed deeply with the Neolithic Revolution.

The possibility that native peoples in eastern North America had cultivated plants prior to the introduction of maize was first raised in 1924. Scant evidence was available to support this speculation, however, until the “flotation revolution” of the 1960s and 1970s. As archaeologists involved in large-scale projects began implementing flotation, paleoethnobotanists soon had hundreds of samples and thousands of seeds that demonstrated that indigenous peoples grew a suite of crops, including cucurbit squashes and gourds, sunflower, sumpweed, and chenopod, which displayed signs of domestication. The application of accelerator mass spectrometry (AMS) dating to cucurbit rinds and seeds in the 1980s placed the domestication of these four crops in the Late Archaic period 5000–3800 bp. The presence of wild cucurbits during earlier Archaic periods lent weight to the argument that native peoples in eastern North America domesticated these plants independently of early cultivators in Mesoamerica. Analyses of DNA from chenopods and cucurbits in the 2010s definitively demonstrated that these crops developed from local lineages.

With evidence in hand that refuted notions of the diffusion of plant domestication from Mesoamerica, models developed in the 1980s for the transition from foraging to farming in the Eastern Woodlands emphasized the coevolutionary relationship between people and these crop plants. As Archaic-period groups began to occupy river valleys more intensively, in part due to changing climatic patterns during the mid-Holocene that created more stable river systems, their activities created disturbed areas in which these weedy plants thrive. With these useful plants available as more productive stands in closer proximity to base camps, people increasingly used the plants, which in turn responded to people’s selection. Critics noted that these models left little room for intentionality or innovation on the part of early farmers.

Models derived from human behavioral ecology explore the circumstances in which foragers choose to start using these small-seeded plants in greater quantities. In contrast to the resource-rich valley settings of the coevolutionary models, human behavioral ecology models posit that foragers would only use these plants, which provide relatively few calories per time spent obtaining them, when existing resources could no longer support growing populations. In these scenarios, Late Archaic peoples cultivated these crops as insurance against shortages in nut supplies.

Despite their apparent differences, current iterations of both models recognize humans as agents who actively change their environments, with intentional and unintentional results. Both also are concerned with understanding the social and ecological contexts within which people began cultivating and eventually domesticating plants.

The “when” and “where” questions of domestication in eastern North America are relatively well established, although researchers continue to fill significant gaps in geographic data. These primarily include regions where large-scale contract archaeology projects have not been conducted. Researchers are also actively debating the “how” and “why” of domestication, but the cultural ramifications of the transition from foraging to farming have yet to be meaningfully incorporated into the archaeological understanding of the region. The significance of these native crops to the economies of Late Archaic and subsequent Early and Middle Woodland peoples is poorly understood and often woefully underestimated by researchers. The socioeconomic roles of these native crops to past peoples, as well as the possibilities for farmers and cooks to incorporate them into their practices in the early 21st century, are exciting areas for new research.

Agriculture is a significant source of methane, contributing about 12% of the global anthropogenic methane emissions. Major sources of methane from agricultural activities are fermentation in the reticulo-rumen of ruminant animals (i.e., enteric methane), fermentation in animal manure, and rice cultivation. Enteric methane is the largest agricultural source of methane and is mainly controlled by feed dry matter intake and composition of the animal diet (i.e., fiber, starch, lipids). Processes that lead to generation of methane from animal manure are similar to those taking place in the reticulo-rumen. Methane emissions from manure, however, are greatly influenced by factors such as manure management system and ambient temperature. Systems that handle manure as a liquid generate much more methane than systems in which manure is handled as a solid. Low ambient temperatures drastically decrease methane emissions from manure. Once applied to soil, animal manure does not generate significant amounts of methane. Globally, methane emissions from rice cultivation represent about 10% of the total agricultural greenhouse gas emissions. In the rice plant, methane dissolves in the soil water surrounding the roots, diffuses into the cell-wall water of the root cells, and is eventually released through the micropores in the leaves. Various strategies have been explored to mitigate agricultural methane emissions. Animal nutrition, including balancing dietary nutrients and replacement of fiber with starch or lipids; alternative sinks for hydrogen; manipulation of ruminal fermentation; and direct inhibition of methanogenesis have been shown to effectively decrease enteric methane emissions. Manure management solutions include solid-liquid separation, manure covers, flaring of generated methane, acidification and cooling of manure, and decreasing manure storage time before soil application. There are also effective mitigation strategies for rice that can be categorized broadly into selection of rice cultivars, water regime, and fertilization. Alternate wetting and drying and mid-season drainage of rice paddies have been shown to be very effective practices for mitigating methane emissions from rice production.

Nutrient pollution can have a negative impact on the aquatic environment, with loss of biodiversity, toxic algal blooms, and a deficiency in dissolved oxygen in surface waters. Agricultural production is one of the main contributors to these problems; this article provides an overview of and background for the main biogeochemical processes causing agricultural nutrient pollution of surface waters. It discusses the main features of the agricultural impact on nutrient loads to surface waters, focusing on nitrogen and phosphorus, and describes some of the main characteristics of agricultural management, including processes and pathways from soil to surface waters. An overview of mitigation measures to reduce pollution, retention in the landscape, and challenges regarding quantification of nutrient losses are also dealt with. Examples are presented from different spatial scales, from field and catchment to river basin scale.

The Basin of Mexico is a key world region for understanding agricultural intensification and the development of ancient and historic cities and states. Archaeologists working in the region have had a long-standing interest in understanding the dynamics of interactions between society and environment and their research has been at the forefront of advances in both method and theory. The Basin of Mexico was the geopolitical core of the Aztec empire, the largest state in the history of Mesoamerica. Its growth was sustained by a complex economy that has been the subject of much research.

Two themes underlie a broad interest in the pre-Hispanic agriculture of the Basin of Mexico. First, how with a Neolithic technology did the Aztecs and their predecessors sustain the growth of large cites, dense rural populations, and the largest state system in the history of pre-Hispanic Mesoamerica? Second, what is the relationship of agricultural intensification and urbanization and state formation? Mesoamerica is the only world region where primary civilizations developed that lacked domestic herbivores for either food or transportation. Their farming depended entirely on human labor and hand tools but sustained large cities, dense populations, and complex social institutions. Intensive agriculture began early and was promoted by risk, ecological diversity, and social differentiation, and included irrigation, terracing, and drained fields (chinampas). Most farming was managed by smallholder households and local communities, which encouraged corporate forms of governance and collective action. Environmental impacts included erosion and deposition, but were limited compared with the degradation that took place in the colonial period.

Worldwide, governments subsidize agriculture at the rate of approximately 1 billion dollars per day. This figure rises to about twice that when export and biofuels production subsidies and state financing for dams and river basin engineering are included. These policies guide land use in numerous ways, including growers’ choices of crop and buyers’ demand for commodities. The three types of state subsidies that shape land use and the environment are land settlement programs, price and income supports, and energy and emissions initiatives. Together these subsidies have created perennial surpluses in global stores of cereal grains, cotton, and dairy, with production increases outstripping population growth. Subsidies to land settlement, to crop prices, and to processing and refining of cereals and fiber, therefore, can be shown to have independent and largely deleterious effect on soil fertility, fresh water supplies, biodiversity, and atmospheric carbon.

The ancient Near East was one of the earliest centers of agriculture in the world, giving rise to domesticated herd animals, cereals, and legumes that today have become primary agricultural staples worldwide. Although much attention has been paid to the origins of agriculture, identifying when, where, and how plants and animals were domesticated, equally important are the social and environmental consequences of agriculture. Shortly after the advent of domestication, agricultural economies quickly replaced hunting and gathering across Mesopotamia, the Levant, and Anatolia. The social and environmental context of this transition has profound implications for understanding the rise of social complexity and incipient urbanism in the Near East.

Economic transformation accompanied the expansion of agriculture throughout small-scale societies of the Near East. These farmsteads and villages, as well as mobile pastoral groups, formed the backbone of agricultural production, which enabled tradable surpluses necessary for more expansive, community-scale economic networks. The role of such economies in the development of social complexity remains debated, but they did play an essential role in the rise of urbanism. Cities depended on agricultural specialists, including farmers and herders, to feed urban populations and to enable craft and ritual specializations that became manifest in the first cities of southern Mesopotamia. The environmental implications of these agricultural systems in the Mesopotamian lowlands, especially soil salinization, were equally substantial. The environmental implications of Mesopotamian agriculture are distinct from those accompanying the spread of agriculture to the Levant and Anatolia, where deforestation, erosion, and loss of biodiversity can be identified as the hallmarks of agricultural expansion.

Agriculture is intimately connected with the rise of territorial empires across the Near East. Such empires often controlled agricultural production closely, for both economic and strategic ends, but the methods by which they encouraged the production of specific agricultural products and the adoption of particular agricultural strategies, especially irrigation, varied considerably between empires. By combining written records, archaeological data from surveys and excavation, and paleoenvironmental reconstruction, together with the study of plant and animal remains from archaeological sites occupied during multiple imperial periods, it is possible to reconstruct the environmental consequences of imperial agricultural systems across the Near East. Divergent environmental histories across space and time allow us to assess the sustainability of the agricultural policies of each empire and to consider how resulting environmental change contributed to the success or failure of those polities.

South Asia possesses a unique Neolithic transition to agricultural domestication. India has received far less attention in the quest for evidence of early agriculture than other regions of the world traditionally recognized as “centers of domestication” such as southwest Asia, western Asia, China, Mesoamerica, South America, New Guinea, and Africa. Hunter-gatherers with agricultural production appeared around the middle of the Holocene, 4000 to 1500 bce, with the cultivation of domesticates and a correspondingly more sedentary lifestyle emerging at this time. Two thousand years ago South Asia was inhabited by farmers, with densely populated river valleys, coastal plains, urban populations, states, and even empires. While some of the crops that supported these civilizations had been introduced from other regions of the world, a large proportion of these crops had local origins from wild plants native to the subcontinent.

As a case study for the origins of agriculture, South Asia has much to offer archaeologists and environmental scientists alike for understanding domestication processes and local transitions from foraging to farming as well as the ways in which early farmers adapted to and transformed the environment and regional vegetation. Information exchange from distant farmers from other agricultural centers into the subcontinent cannot be ruled out. However, it is clear that local agricultural origins occurred via a series of processes, including the dispersal of pastoral and agro-pastoral peoples across regions, the local domestication of animals and plants and the adoption by indigenous hunter-gatherers of food production techniques from neighboring cultures. Indeed, it is posited that local domestication events in India were occurring alongside agricultural dispersals from other parts of the world in an interconnected mosaic of cultivation, pastoralism, and sedentism. As humans in South Asia increasingly relied on a more restricted range of plant species, they became entangled in an increasingly fixed trajectory that allowed greater food production levels to sustain larger populations and support their developing social, cultural and food traditions.

Agroecology can be defined as scientific research on ecological sustainability of food systems.

In addressing food production and consumption systems in their entirety, the focus of agroecology is on interactions and processes that are relevant for transitioning and maintaining ecological, economic, political, and social-cultural sustainability.

As a field of sustainability science, agroecology explores agriculture and food with explicit linkages to two other widespread interpretations of the concept of agroecology: environmentally sound farming practices and social movements for food security and food sovereignty. In the study of agroecology as science, both farming practices and social movements emerge as integrated components of agroecological research and development.

In the context of agroecology, the concept of ecology refers not only to the science of ecology as biological research but also to environmental and social sciences with research on social systems as integrated social and ecological systems. In agroecological theory, all these three are merged so that agroecology can broadly be defined as “human food ecology” or “the ecology of food systems.”

Since the last decades of the 20th century many developments have led to an increased emphasis on agroecology in universities, nonprofit organizations, movements, government programs, and the United Nations. All of these have raised a growing attention to ecological, environmental, and social dimensions of farming and food, and to the question of how to make the transition to sustainable farming and food systems.

One part of the foundation of agroecology was built during the 1960s when ecologically oriented environmental research on agriculture emerged as the era of optimism about component research began to erode. Largely, this took place as a reaction to unexpected and unwanted ecological and social consequences of the Green Revolution, a post–World War II scaling-up, chemicalization, and mechanization of agriculture. Another part of the foundation was a nongovernmental movement among thoughtful farmers wanting to develop sustainable and more ecological/organic ways of production and the demand by consumers for such food products. Finally, a greater societal acceptance, demand for research and education, and public funding for not only environmental ecology but also for wider sustainability in food and agriculture was ignited by an almost sudden high-level political awakening to the need for sustainable development by the end of 1980s.

Agroecology as science evolved from early studies on agricultural ecosystems, from research agendas for environmentally sound farming practices, and from concerns about addressing wider sustainability; all these shared several forms of systems thinking. In universities and research institutions, agroecologists most often work in faculties of food and agriculture. They share resources and projects among scientists having disciplinary backgrounds in genetics (breeding of plants and animals), physiology (crop science, animal husbandry, human nutrition), microbiology or entomology (crop protection), chemistry and physics (soil science, agricultural and food chemistry, agricultural and food technology), economics (of agriculture and food systems), marketing, behavioral sciences (consumer studies), and policy research (agricultural and food policy).

While agroecologists clearly have a mandate to address ecology of farmland, its biodiversity, and ecosystem services, one of the greatest added values from agroecology in research communities comes from its wider systems approach. Agroecologists complement reductionist research programs where scientists seek more detailed understanding of detail and mechanisms and put these into context by developing a broader appreciation of the whole. Those in agroecology integrate results from disciplinary research and increase relevance and adoption by introducing transdisciplinarity, co-creation of information and practices, together with other actors in the system. Agroecology is the field in sustainability science that is devoted to food system transformation and resilience.

Agroecology uses the concept of “agroecosystem” in broad ecological and social terms and uses these at multiple scales, from fields to farms to farming landscapes and regions. Food systems depend on functioning agroecosystems as one of their subsystems, and all the subsystems of a food system interact through positive and negative feedbacks, in their complex biophysical, sociocultural, and economic dimensions. In embracing wholeness and connectivity, proponents of agroecology focus on the uniqueness of each place and food system, as well as solutions appropriate to their resources and constraints.

Air pollution is one of the grand environmental challenges in developing countries, especially those with high population density like China. High concentrations of primary and secondary trace gases and particulate matter (PM) are frequently observed in the industrialized and urbanized regions, causing negative effects on the health of humans, plants, and the ecosystem.

Meteorological conditions are among the most important factors influencing day-to-day air quality. Synoptic weather and boundary layer dynamics control the dispersion capacity and transport of air pollutants, while the main meteorological parameters, such as air temperature, radiation, and relative humidity, influence the chemical transformation of secondary air pollutants at the same time. Intense air pollution, especially high concentration of radiatively important aerosols, can substantially influence meteorological parameters, boundary layer dynamics, synoptic weather, and even regional climate through their strong radiative effects.

As one of the main monsoon regions, with the most intense human activities in the world, East Asia is a region experiencing complex air pollution, with sources from anthropogenic fossil fuel combustion, biomass burning, dust storms, and biogenic emissions. A mixture of these different plumes can cause substantial two-way interactions and feedbacks in the formation of air pollutants under various weather conditions. Improving the understanding of such interactions needs more field measurements using integrated multiprocess measurement platforms, as well as more efforts in developing numerical models, especially for those with online coupled processes. All these efforts are very important for policymaking from the perspectives of environmental protection and mitigation of climate change.

Satellite reconnaissance of the Earth’s surface provides critical information about the state of human interaction with the natural environment. The strongest impact is agricultural, reflecting land-use approaches to food production extending back to the dawn of civilization. To variable degrees, depending upon location, regional field patterns result from traditional farming practices, surveying methods, regional histories, policies, political agendas, environmental circumstances, and economic welfare. Satellite imaging in photographic true or false color is an important means of evaluating the nature and implications of agricultural practices and their impacts on the surrounding world. Important platforms with publicly accessible links to satellite image sets include those of the European Space Agency, U.S. National Aeronautics and Space Administration, the Centre D’etudes Spatiales, Airbus, and various other governmental programs. Reprocessing of data worldwide in scope by commercial concerns including Digital Globe, Terrametrics, and GoogleEarth in the 21st century enable ready examination of most of the Earth’s surface in great detail and natural colors. The potential for monitoring and improving understanding of agriculture and its role in the Earth system is considerable thanks to these new ways of viewing the planet.

Space reconnaissance starkly reveals the consequences of unique land surveys for the rapid development of agriculture and political control in wilderness areas, including the U.S. Public Land Survey and Tierras Bajas systems. Traditional approaches toward agriculture are clearly shown in ribbon farms, English enclosures and medieval field systems, and terracing in many parts of the world. Irrigation works, some thousands of years old, may be seen in floodplains and dryland areas, notably the Maghreb and the deep Sahara, where center-pivot fields have recently appeared in areas once considered too dry to cultivate. Approaches for controlling erosion, including buffer zones, shelter belts, strip and contour farming, can be easily identified. Also evident are features related to field erosion and soil alteration that have advanced to crisis stage, such as badland development and widespread salinization. Pollution related to farm runoff, and the piecemeal (if not rapid) loss of farmlands due to urbanization can be examined in ways favoring more comprehensive evaluation of human impacts on the planetary surface. Developments in space technologies and observational platforms will continue indefinitely, promising ever-increasing capacity to understand how humans relate to the environment.

Arid environments cover about one third of the Earth’s surface, comprising the most extensive of the terrestrial biomes. Deserts show considerable individual variation in climate, geomorphic surface expression, and biogeography. Climatically, deserts range from dry interior environments, with large temperature ranges, to humid and relatively cool coastal environments, with small temperature ranges. What all deserts share in common is a consistent deficit of precipitation relative to water loss by evaporation, implying that the biological availability of water is very low. Deserts develop because of climatic (persistent high-pressure cells), topographic (mountain ranges that cause rain shadow effects), and oceanographic (cold currents) factors that limit the amount of rain or snowfall that a region receives. Most global deserts are subtropical in distribution.

There is a large range of geomorphic surfaces, including sand sheets and sand seas (ergs), stone pavements, bedrock outcrops, dry lakebeds, and alluvial fans. Vegetation cover is generally sparse, but may be enhanced in areas of groundwater seepage or along river courses. The limited vegetation cover affects fluvial and slope processes and results in an enhanced role for the wind. While the majority of streams in deserts are ephemeral features, both intermittent and perennial rivers develop in response to snowmelt in nearby mountains or runoff from distant, more well-watered regions. Most drainage is endoreic, meaning that it flows internally into closed basins and does not reach the sea, being disposed of by seepage and evaporation.

The early study of deserts was largely descriptive. More process-based studies commenced with the study of North American deserts in the mid- to late-1800s. Since the late 20th century, research has expanded into many areas of the world, with notable contributions coming from China, but our knowledge of deserts is still more compete in regions such as North America, Australia, Israel, and southern Africa, where access and funding have been more consistently secure. The widespread availability of high-quality remotely sensed images has contributed to the spread of study into new global field areas. The temporal framework for research has also improved, benefiting from improvements in geochronological techniques. Geochronological controls are vital to desert research because most arid regions have experienced significant climatic changes. Deserts have not only expanded or contracted in size, but have experienced changes in the dominant geomorphic processes and biogeographic environment. Contemporary scientific work has also benefited from improvements in technology, notably in surveying techniques, and from the use of quantitative modeling.

Atmospheric brown clouds (ABCs) are widespread pollution clouds that can at times span an entire continent or an ocean basin. ABCs extend vertically from the ground upward to as high as 3 km, and they consist of both aerosols and gases. ABCs consist of anthropogenic aerosols such as sulfates, nitrates, organics, and black carbon and natural dust aerosols. Gaseous pollutants that contribute to the formation of ABCs are NOx (nitrogen oxides), SOx (sulfur oxides), VOCs (volatile organic compounds), CO (carbon monoxide), CH4 (methane), and O3 (ozone). The brownish color of the cloud (which is visible when looking at the horizon) is due to absorption of solar radiation at short wavelengths (green, blue, and UV) by organic and black carbon aerosols as well as by NOx. While the local nature of ABCs around polluted cities has been known since the early 1900s, the widespread transoceanic and transcontinental nature of ABCs as well as their large-scale effects on climate, hydrological cycle, and agriculture were discovered inadvertently by The Indian Ocean Experiment (INDOEX), an international experiment conducted in the 1990s over the Indian Ocean. A major discovery of INDOEX was that ABCs caused drastic dimming at the surface. The magnitude of the dimming was as large as 10–20% (based on a monthly average) over vast areas of land and ocean regions. The dimming was shown to be accompanied by significant atmospheric absorption of solar radiation by black and brown carbon (a form of organic carbon). Black and brown carbon, ozone and methane contribute as much as 40% to anthropogenic radiative forcing. The dimming by sulfates, nitrates, and carbonaceous (black and organic carbon) species has been shown to disrupt and weaken the monsoon circulation over southern Asia. In addition, the ozone in ABCs leads to a significant decrease in agriculture yields (by as much as 20–40%) in the polluted regions. Most significantly, the aerosols (in ABCs) near the ground lead to about 4 million premature mortalities every year. Technological and regulatory measures are available to mitigate most of the pollution resulting from ABCs. The importance of ABCs to global environmental problems led the United Nations Environment Programme (UNEP) to form the international ABC program. This ABC program subsequently led to the identification of short-lived climate pollutants as potent mitigation agents of climate change, and in recognition, UNEP formed the Climate and Clean Air Coalition to deal with these pollutants.

In 2018 barley accounts for only 5% of the cereal production worldwide, and regionally for up to 40% of cereal production. The cereal represents the oldest crop species and is one of the best adapted crop plants to a broad diversity of climates and environments.

Originating from the wild progenitor species Hordeum vulgare ssp. spontaneum, biogeographically located in the Fertile Crescent of the Near East, the domesticated form developed as a founder crop in aceramic Neolithic societies 11,000 years ago, was cultivated in monocultures in Bronze Age Mesopotamia, entered the New World after 1492 ce, reached a state of global distribution in the 1950s and had reached approximately 200 accepted botanical varieties by the year 2000.

Its stress tolerance in response to increased aridity and salinity on one hand and adaptability to cool climates on the other, partially explains its broad range of applications for subsistence and economy across different cultures, such as for baking, cooking, beer brewing and as an animal feed.

Although the use of fermented starch for producing alcoholic beverages and foods is globally documented in archaeological contexts dating from at least the beginning of the Holocene era, it becomes concrete only in societies with a written culture, such as Bronze Age Mesopotamia and Egypt, where beer played a considerable role in everyday diet and its production represented an important sector of productivity.

In 2004 approximately 85% of barley production was destined for feeding animals. However, as a component of the human diet, studies on the health benefits of the micronutrients in barley have found that it has a positive effect on blood cholesterol and glucose levels, and in turn impacts cardiovascular health and diabetes control. The increasing number of barley-breeding programs worldwide focus on improving the processing characteristics, nutritional value, and stress tolerance of barley within the context of global climate change.