Permafrost

Permafrost is permanently frozen ground that remains continuously below 0 °C for two or more years. The upper level of permafrost, the permafrost table, can occur within a centimeter of the ground surface or at a depth of several meters. The active layer, which thaws each summer, overlies permafrost. Permafrost underlies about a quarter of the northern hemisphere and can form in sediment or bedrock and on land or under the ocean. Permafrost forms incrementally and, in the regions where it is up to 1 km thick, permafrost can represent thousands of years of formation. Permafrost is present at high latitudes and high altitudes. In these regions, permafrost can be described as continuous, discontinuous, sporadic, or isolated. Continuous permafrost forms at mean annual air temperatures below -5 °C and is laterally continuous, regardless of surface aspect or material. Discontinuous permafrost forms where the mean annual air temperature is between -2 and -4 °C, allowing permafrost to persist in 50 to 90 percent of the landscape. Permafrost is sporadic where 10 to <50 percent of the landscape is underlain by permafrost and mean annual air temperature is between 0 and -2 °C. Permafrost is considered isolated where less than 10 percent of the landscape is underlain by permafrost. When it is present, permafrost creates unique conditions. Permafrost forms an impermeable layer beneath the active layer, for example, which limits the rooting depth of plants and prevents infiltration by water during the summer. The lack of deep infiltration can facilitate formation of extensive wetlands in high-latitude areas that receive relatively little precipitation. Permafrost degradation (thaw) creates diverse environmental hazards, including instability of the ground surface that affects infrastructure and fluxes of water, sediment, and organic matter entering rivers, lakes and oceans. Permafrost degradation releases frozen microbes, some of which are pathogens, and organic carbon. Permafrost degradation also influences the geographic range of plants and animals and thus ecosystem processes and biotic communities. The greatest concern with permafrost degradation at present, however, is the potential for releasing significant carbon into the atmosphere. Globally, soils are the largest terrestrial reservoir of carbon and permafrost soils are the single largest component of the carbon reservoir. Carbon released by degrading permafrost can enter the atmosphere as the greenhouse gases carbon dioxide and methane, or the carbon can be taken up by plants or transported by rivers to the ocean and buried in marine sediments. The balance among these different pathways is largely unknown, but carbon release to the atmosphere presents a serious threat as a mechanism to enhance global warming.

Stream Mitigation Banking

Stream mitigation banking is a market-based approach for managing negative impacts on fluvial systems under Section 404 of the US Clean Water Act. The core rationale of mitigation banking is that we should protect the environment not by preventing harm, but by pricing it: developers, public-works agencies, and other entities may damage a stream in one location as long as they pay for the restoration of a comparable stream elsewhere (see Compensatory Mitigation and Valuation of Ecosystem Services). That restoration is most often provided by a for-profit company that speculatively purchases rights to land with a degraded stream on it, then restores that stream to produce stream credits. The stream credits can then be purchased by developers or other entities to fulfill the permit obligations incurred by proposing to damage another stream somewhere else (see Stream Mitigation Banking in Practice). Along with its older sister, wetlands mitigation banking, stream mitigation banking is one of the oldest and most firmly established market-based approaches to environmental management in the world. As of 2018, there were nearly 3,500 mitigation banks in the United States, with sales estimated at least $1 billion per year. Mitigation banking has thus become a poster child for market-based (also referred to as neoliberal) approaches to conservation, inspiring comparable policies to tackle issues from endangered species to carbon dioxide emissions on every continent except Antarctica. However, there has been relatively little biophysical evaluation of whether stream mitigation banking actually leads to better outcomes for fluvial systems, and the data we do have are not promising (see Environmental Impacts).

Agroforestry: The North American Perspective

Agroforestry has a long, rich history that is rooted in activities practiced by indigenous communities around the world. Native peoples were known to gather fruits, nuts, and understory herbs from the forest, based on their deep ecological knowledge of the natural system, and they often cultivated preferred species. For modern applications, agroforestry can be defined as the intentional integration of trees and shrubs with crops or livestock to create a multifunctional system with a wide range of benefits. In temperate regions, agroforestry is characterized by six key practices: (1) alley cropping—planting rows of trees with a companion crop grown between the rows; (2) forest farming—growing high-value specialty crops in the shaded forest understory; (3) riparian buffers—protecting water resources such as streams with a zone of trees, shrubs, and herbaceous plants; (4) silvopasture—combining trees, forage, and livestock for multiple products; (5) windbreaks—planting rows of trees and shrubs to protect crops or livestock from wind and to reduce soil erosion; and (6) urban food forests—integrating trees, shrubs, and herbaceous plants that provide edible products for the good of the community. The environmental benefits of agroforestry have been widely studied and continue to be a source of great interest. Most recently, the potential for agroforestry to contribute to climate change adaption and mitigation is being explored. While the science of agroforestry has been influenced to a great extent by the field of ecology and related disciplines, social science dimensions are increasingly captured through the study of adoption behaviors, preferences, and cultural benefits. The investigation of the role of economic and policy drivers is critical to understanding strategies to motivate landowners to adopt these practices at a level that would expand agroforestry into the mainstream. Landscape-level planning and design could provide a vision and a pathway for a broader transformation to a system that encourages perennial habitats including specialty crops that supply edible products. Such a strategy could place agroforestry more directly into the growing call to support regional food systems and positive human health outcomes. This article focuses on the trends and directions in agroforestry research primarily in North America, with emphasis on developments in the early 21st century.

Agricultural Land Abandonment

Agricultural land abandonment is increasingly a global land-cover change phenomenon that has strong implications for the environment (e.g., biodiversity, carbon sequestration, novel ecosystems, wildfires) and societal well-being (livelihood, agricultural landscapes). Agricultural land abandonment is often referred to as the cessation of farming and giving away land for natural succession, such as grasses, shrubs, and trees on former agricultural lands, but may also result in land degradation. Agricultural land abandonment can be a more complex land-change transition, including the cessation of agricultural activity in favor of land uses other than agricultural ones, such as forestry, construction of dwellings, game reserves, and tourism. Studies have shown that agricultural land abandonment often is driven by rational decision-making and profit maximization, including weighing up opportunity costs and alternative livelihood strategies. However, the conditions of institutions, which are supposed to govern land use, and the personal characteristics of those involved in agricultural activities, are playing a vital role in the decision of abandonment. It should also be noted that the decision on abandonment or maintenance of farming can be quite complex and driven by non-economic factors, such as personal predisposition to farming, education, ethnicity, religion, age, and availability of successors. The progress of studying land abandonment and existing research gaps are highlighted in the text.

Non-Renewable Resource Depletion and Use

Modern society relies on an increasing number of minerals and metals, meaning that over time production of these commodities has significantly increased, especially within the last fifty years. However, metals and minerals are dominantly produced from ore or mineral deposits that are inherently non-renewable as the geological processes that form these resources (and if necessary exhume them to nearer surface environments where they can be exploited) occur at much slower rates (often over thousands or millions of years) than they are being consumed. This at a basic level indicates that at some point we will “run out” of these non-renewable resources. Although this may be true on a very long timescale, this simple view does not take into account a number of different factors, such as changes in the types, sizes, and grades of mineral deposits that are being exploited. Past changes in the mineral and mining sectors have led to a global increase in mineral and metal production throughout the 20th and 21st centuries that has been (more than) matched by an increase in global mineral and metal resources and reserves. This increase in the amount of material available for exploitation has reflected the decreasing cost of mining and energy, the development of new mining and mineral processing technologies, continued exploration success that has led to the discovery of new resources and reserves, and increasing demand, which in real terms has increased the prices of the majority of commodities. However, the potential lifespan of these historic patterns remains unclear, especially given that mineral resources are finite and other aspects that influence metal and mineral production, such as energy costs and environmental and social issues, are becoming increasingly important. This has led to recent concerns focused on a variety of metals and minerals considered to be at potential supply risk, including base metals such as zinc as well as a the so-called critical metals; metals that are associated with supply risk as a result of their concentration of supply, political instability in source countries, or production (and hence reliance) as by-products to primary metals such as Cu or Zn. These risks are compounded by the fact that these critical metals and minerals are essential for numerous often advanced technologies as well as defense and energy production requirements. This review focuses on the key considerations in estimating metal and mineral resources, aspects that need to be considered when estimating current resources and reserves and determining whether we can meet current and future demand. The dynamic nature of global metal and mineral resources means that an in-depth analysis of these data is not within the scope of this review, although the references provided form a comprehensive bibliography for this topic.

Environmental Health

Each day people are exposed to a wide variety of agents and stressors that have the potential to impact human health and well-being. Environmental health is the study of those environmental factors and how they may contribute to human health and disease. An individual’s environment is one of the most important contributors to one’s overall wellness and quality of life. Environmental factors play a role in at least 85 percent of all human diseases. More importantly, an individual’s environment is the most easily modified aspect of one’s overall health. Understanding the impact of the external environment, how it interacts with biological processes, and what can be done to eliminate or mitigate negative effects provides better protection for human populations from deleterious health outcomes. Traditionally, science has looked at environmental factors by using a risk-based approach. In this model, information on an agent’s potential to cause harm, as depicted by a dose-response relationship for a given adverse effect, is integrated with an individual’s potential to be exposed to that hazard in order to characterize the likelihood and severity of health risk. As we move into a new era of environmental-health research, scientists are thinking about environmental impacts on human health in new ways. It’s no longer as simple as “the dose makes the poison,” where high doses of a chemical are bad and lower doses are not as bad. While there are still many instances of high-concentration exposures to toxic heavy metals, pesticides, or other substances, a new understanding of how low-level exposures contribute to the development of common disorders such as diabetes, developmental delays, and other modern epidemics is changing the traditional paradigm of toxicology. Timing of exposure during fetal and early-childhood development, mixture effects from combined exposures, impacts on genetic and epigenetic gene regulation, and individual human susceptibilities can result in increased disease incidence or severity. Further, these effects are seen not only in exposed individuals, but also in their direct offspring and potentially subsequent generations. The study of environmental health provides opportunities to mitigate or prevent a wide range of human disease and disability from an individual, community, and policy perspective. We can’t change our genes, but we can change our environment, behaviors, and exposures. This article describes the ways we are exposed to stressors in our environment, the primary fields that contribute to our understanding of environmental health, and some emerging issues that require 21st-century approaches to promoting healthy environments and preventing human disease.

Erosion

Erosion is the movement of material from rest, as opposed to sediment transport, which is the ongoing movement of material, and deposition, which is the cessation of movement. The topics covered pertain to erosion of the land surface and related processes. It does not include subsurface movement of material that may occur by processes of dissolution or piping or subaqueous erosion may occur at the bottom of lakes and seas. The word “erosion” immediately conjures up a vast literature concerned with soil erosion, but it also refers to the movement of geologic materials in geomorphic systems such as rivers, dunes, beaches, glaciers, and landslides. Surface erosion may be caused by a variety of physical processes including water (fluvial erosion), wind (aeolian erosion or deflation), waves and tides (coastal erosion), ice (glacial erosion and nivation), gravitational failures (mass movement), and human movement of materials (anthropogenic erosion). The latter topic may be distinguished as accelerated as opposed to natural erosion. A broad view of erosion is important for many reasons including threats of property damage and public safety by geomorphic processes, chemical changes such as sequestration or release of carbon and toxins, and off-site damages of sedimentation. Studies of erosion covered in this article are grouped into two fundamental classes: soil erosion and geomorphic erosion. The first group of topics is concerned with soil erosion and includes agricultural sustainability, soil conservation, soil erosion processes, modeling soil erosion, and gully erosion. Much of the scientific literature on soil erosion has focused on (1) arable land because of the importance of soils to global food production or (2) other lands because of the ecosystem services that soils provide and the off-site costs of sediment. Erosion includes the removal of rock, beach, channel, and other geologic materials in addition to soils. Studies of erosion of these geologic materials are grouped by the various geomorphic systems in which they occur (e.g., channel erosion, aeolian [wind] erosion, coastal erosion [waves and tides], glacial erosion, and mass movements [landslides or gravitational failures]). This review ends with studies of anthropogenic erosion beyond the erosion of soil and geomorphic systems, such as mining and construction, and the effects of erosion on the global carbon budget and climate change. This article does not cover the large scientific literature on long-term denudation or landscape evolution studies, the history of soil erosion, or erosion following natural disturbances such as fire and pestilence. Nor does it attempt to cover the numerous regional studies of erosion or benefits of specific soil-conservation techniques. Nevertheless, a broad approach is taken to examine the erosion of soil and geologic materials in response to natural and anthropogenic processes.

Spatial Statistics

Spatial statistics (SS)—statistics that address and account for the correlations among georeferenced observations arising from their relative locations in geographic space (i.e., spatial autocorrelation [SA])—has a formal history dating to the mid-1900s, although conceptual awareness of it dates back to the very early 1900s. It is a special case of correlated data. It arises from a relaxation of the independent observations assumption of classical statistics. Its development emphasized the following three themes: point pattern analysis, spatial autoregression, and geostatistics. Point pattern analysis was a precursor to spatial autoregression, whereas these latter two themes evolved in parallel, with little cross-fertilization during most of their first 40 years of development, primarily because spatial autoregression was the preferred interest of English-speaking scholars, whereas geostatistics was the preferred interest of French-speaking scholars. Much of the early work treated point patterns; spatial autoregression and geostatistics began eclipsing this emphasis around 1990. SS is best understood after completion of a course in multivariate statistical analysis. Spatial Autocorrelation, the book by Cliff and Ord in 1973 initiated a popularizing of SS in the early 1970s; later, Spatial Econometrics, by Paelinck and Klaassen in 1979, extended it to spatial econometrics. The important message is that accounting for SA in georeferenced data really matters in the environmental sciences.

Marine Protected Areas

Marine Protected Areas (MPAs) are a key tool in ecosystem-based management, implementing a spatial approach to biodiversity conservation in the oceans. While the use of protected areas to conserve and/or protect resources has a long history, including centuries of royal hunting areas and traditionally managed areas, the modern conceptualization of protected areas dates to the late 19th century, with the designation of Yellowstone National Park in the United States in 1872. The first similar formally protected area with a marine component was the Royal National Park MPA in New South Wales, Australia, in 1879, although it also included a terrestrial component, as do many MPAs in coastal areas. The land/sea interface poses a challenge to delineating between terrestrial and marine parks, adding to a complex jurisdictional and legal landscape. Consequently, it is helpful to categorize MPAs based on the broad definition for protected areas offered by the IUCN (International Union for Conservation of Nature): a clearly defined geographical space, recognized, dedicated and managed, through legal or other effective means, to achieve the long-term conservation of nature with associated ecosystem services and cultural values. As evidenced in this definition, discussions surrounding MPAs have become more amenable to soft-law approaches and/or less formal legal designations, and they are also increasingly tied to the concept of ecosystem services (i.e., protecting systems that in turn provide people with services that would be costly to otherwise reproduce, such as the coastal protection provided by mangroves and coral reefs). Of course, there are also strong arguments for protecting nature for its own intrinsic value, as well as the value it holds for non-human species. In order to fully understand the promise and efficacy of MPAs, it is necessary to examine their legal basis, their effectiveness as tools, how they can work together as networks to achieve ecological objectives, and how the global community is using protected area targets and large-scale MPAs to maximize coverage. However, it is also important to consider the socioeconomic dimensions of MPAs, as these often lead to problems with their success, including concerns with equity and justice and how well they are governed. Looking forward, future work in the field of MPAs includes ensuring they are achieving their ecological objectives, by ensuring enough areas are closed to all extractive uses, and developing a regime for designating them in areas beyond national jurisdiction, on the high seas.

Human Impact on Historical Fluvial Sediment Dynamics in Europe

Rivers and floodplains are dynamic environments formed through hydrologic and geomorphic processes that are in turn governed by environmental conditions in the river catchment, such as climate, tectonics, lithology and soils, vegetation cover, and topography. At longer timescales, river landscapes are in a state of dynamic equilibrium with these controlling factors. However, over the last few thousand years, humans have become another important factor in controlling fluvial-landscape dynamics that in many regions has now overwhelmed the importance of natural controlling factors. This is in particular true for many river catchments in Europe that have a long history of human impact. Anthropogenic changes in land cover (e.g., deforestation and the rise in agriculture) have resulted in increased rates of hillslope erosion and amounts of sediment delivered to river systems. As a result, many floodplains have witnessed increased rates of aggradation. Furthermore, these changes also led to the development of typical anthropogenic fluvial landscapes. In temperate Europe, rivers often changed from anastomosing channels in wetland environments to single-thread meandering rivers with levees and overbank deposits. In Mediterranean Europe, many river valleys also became silted up, creating more-extensive, relatively flat valleys. With the increasing industrialization and urbanization of Europe since the 19th century, but also through the implementation of erosion control measures, many regions experienced a greening of the landscape and a return to less erosive land cover. Several rivers, often in mountainous environments, saw again a reduction in sediment loads and river incision and a return in fluvial style. Humans also impacted European river systems through direct interventions such as mining of river sands and gravel, damming of rivers, artificial straightening, or the hunting and later reintroduction of river-engineering animals such as the beaver. Changes in floodplain sedimentation and floodplain ecology following human impact also affected the amount of carbon sequestered and buried within European floodplains.