Forest transitions take place when trends over time in forest cover shift from deforestation to reforestation. These transitions are of immense interest to researchers because the shift from deforestation to reforestation brings with it a range of environmental benefits. The most important of these would be an increased volume of sequestered carbon, which if large enough would slow climate change. This anticipated atmospheric effect makes the circumstances surrounding forest transitions of immediate interest to policymakers in the climate change era. This encyclopedia entry outlines these circumstances. It begins by describing the socio-ecological foundations of the first forest transitions in western Europe. Then it discusses the evolution of the idea of a forest transition, from its introduction in 1990 to its latest iteration in 2019. This discussion describes the proliferation of different paths through the forest transition. The focus then shifts to a discussion of the primary driver of the 20th-century forest transitions, economic development, in its urbanizing, industrializing, and globalizing forms. The ecological dimension of the forest transition becomes the next focus of the discussion. It describes the worldwide redistribution of forests toward more upland settings. Climate change since 2000, with its more extreme ecological events in the form of storms and droughts, has obscured some ongoing forest transitions. The final segment of this entry focuses on the role of the state in forest transitions. States have become more proactive in managing forest transitions. This tendency became more marked after 2010 as governments have searched for ways to reduce carbon emissions or to offset emissions through more carbon sequestration. The forest transitions by promoting forest expansion would contribute additional carbon offsets to a nation’s carbon budget. For this reason, the era of climate change could also see an expansion in the number of promoted forest transitions.
Article
V. Kerry Smith
Geologists’ reframing of the global changes arising from human impacts can be used to consider how the insights from environmental economics inform policy under this new perspective. They ask a rhetorical question. How would a future generation looking back at the records in the sediments and ice cores from today’s activities judge mankind’s impact? They conclude that the globe has entered a new epoch, the Anthropocene. Now mankind is the driving force altering the Earth’s natural systems. This conclusion, linking a physical record to a temporal one, represents an assessment of the extent of current human impact on global systems in a way that provides a warning that all policy design and evaluation must acknowledge that the impacts of human activity are taking place on a planetary scale. As a result, it is argued that national and international environmental policies need to be reconsidered. Environmental economics considers the interaction between people and natural systems. So it comes squarely into conflict with conventional practices in both economics and ecology. Each discipline marginalizes the role of the other in the outcomes it describes. Market and natural systems are not separate. This conclusion is important to the evaluation of how (a) economic analysis avoided recognition of natural systems, (b) the separation of these systems affects past assessments of natural resource adequacy, and (c) policy needs to be redesigned in ways that help direct technological innovation that is responsive to the importance of nonmarket environmental services to the global economy and to sustaining the Earth’s living systems.
Article
Juha Merilä and Ary A. Hoffmann
Changing climatic conditions have both direct and indirect influences on abiotic and biotic processes and represent a potent source of novel selection pressures for adaptive evolution. In addition, climate change can impact evolution by altering patterns of hybridization, changing population size, and altering patterns of gene flow in landscapes. Given that scientific evidence for rapid evolutionary adaptation to spatial variation in abiotic and biotic environmental conditions—analogous to that seen in changes brought by climate change—is ubiquitous, ongoing climate change is expected to have large and widespread evolutionary impacts on wild populations. However, phenotypic plasticity, migration, and various kinds of genetic and ecological constraints can preclude organisms from evolving much in response to climate change, and generalizations about the rate and magnitude of expected responses are difficult to make for a number of reasons.
First, the study of microevolutionary responses to climate change is a young field of investigation. While interest in evolutionary impacts of climate change goes back to early macroevolutionary (paleontological) studies focused on prehistoric climate changes, microevolutionary studies started only in the late 1980s. The discipline gained real momentum in the 2000s after the concept of climate change became of interest to the general public and funding organizations. As such, no general conclusions have yet emerged. Second, the complexity of biotic changes triggered by novel climatic conditions renders predictions about patterns and strength of natural selection difficult. Third, predictions are complicated also because the expression of genetic variability in traits of ecological importance varies with environmental conditions, affecting expected responses to climate-mediated selection.
There are now several examples where organisms have evolved in response to selection pressures associated with climate change, including changes in the timing of life history events and in the ability to tolerate abiotic and biotic stresses arising from climate change. However, there are also many examples where expected selection responses have not been detected. This may be partly explainable by methodological difficulties involved with detecting genetic changes, but also by various processes constraining evolution.
There are concerns that the rates of environmental changes are too fast to allow many, especially large and long-lived, organisms to maintain adaptedness. Theoretical studies suggest that maximal sustainable rates of evolutionary change are on the order of 0.1 haldanes (i.e., phenotypic standard deviations per generation) or less, whereas the rates expected under current climate change projections will often require faster adaptation. Hence, widespread maladaptation and extinctions are expected. These concerns are compounded by the expectation that the amount of genetic variation harbored by populations and available for selection will be reduced by habitat destruction and fragmentation caused by human activities, although in some cases this may be countered by hybridization. Rates of adaptation will also depend on patterns of gene flow and the steepness of climatic gradients. Theoretical studies also suggest that phenotypic plasticity (i.e., nongenetic phenotypic changes) can affect evolutionary genetic changes, but relevant empirical evidence is still scarce. While all of these factors point to a high level of uncertainty around evolutionary changes, it is nevertheless important to consider evolutionary resilience in enhancing the ability of organisms to adapt to climate change.
Article
Celia McMichael
Global environmental change amplifies and creates pressures that shape human migration. In the 21st century, there has been increasing focus on the complexities of migration and environmental change, including forecasts of the potential scale and pace of so-called environmental migration, identification of geographic sites of vulnerability, policy implications, and the intersections of environmental change with other drivers of human migration. Migration is increasingly viewed as an adaptive response to climatic and environmental change, particularly in terms of livelihood vulnerability and risk diversification. Yet the adaptive potential of migration will be defined in part by health outcomes for migrating populations. There has been limited examination, however, of the health consequences of migration related to environmental change.
Migration related to environmental change includes diverse types of mobility, including internal migration to urban areas, cross-border migration, forced displacement following environmental disaster, and planned relocation—migration into sites of environmental vulnerability; much-debated links between environmental change, conflict, and migration; immobile or “trapped” populations; and displacement due to climate change mitigation and decarbonization action. Although health benefits of migration may accrue, such as increased access to health services or migration away from sites of physical risk, migration—particularly irregular (undocumented) migration and forced displacement—can amplify vulnerabilities and present risks to health and well-being. For diverse migratory pathways, there is the need to anticipate, respond to, and ameliorate population health burdens among migrants.
Article
Encarna Esteban
The increased pressure on groundwater has resulted in a major deterioration of the overall status of this resource. Despite efforts to control the degradation of underground water bodies, most aquifers worldwide experience serious quality and quantity problems. New emerging issues around groundwater resources have become relevant and pose additional protection and management challenges. Climate change, with predictable impacts on temperature and precipitation, will cause considerable fluctuations in aquifer recharge levels and subsequent problems in the status of these water bodies. Expected reductions in water availability will increase groundwater withdrawals not just for irrigation but also for urban and industrial water use. Declines in stored water will have an impact on many freshwater ecosystems whose survival depends on the status of groundwater bodies. Furthermore, land subsidence, as a side effect of aquifer overexploitation, involves land collapse and deformation that are especially harmful for urban areas and deteriorate physical and hydrological water systems. All these new challenges require integrated planning strategies and multisectorial solutions to curtail the deterioration of these resources. Although these issues have been studied, in-depth analyses of the economic, social, and policy implications of groundwater management strategies are still necessary.
Article
It’s complicated. Tropical diseases have unusually intricate life cycles because most of them involve not only a human host and a pathogen, but also a vector host. The diseases are predominantly tropical due to their sensitivity to local ecology, usually due to the vector organism. The differences between the tropical diseases mean that they respond to environmental degradation in various ways that depend on local conditions. Urbanization and water pollution tend to limit malaria, but deforestation and dams can exacerbate malaria and schistosomiasis. Global climate change, the largest environmental change, will likely extend the range of tropical climate conditions to higher elevations and near the limits of the tropics, spreading some diseases, but will make other areas too dry or hot for the vectors. Nonetheless, the geographical range of tropical diseases will be primarily determined by public health efforts more than climate. Early predictions that malaria will spread widely because of climate change were flawed, and control efforts will probably cause it to diminish further. The impact of human disease on economic development is hard to pin down with confidence. It may be substantial, or it may be misattributed to other influences. A mechanism by which tropical disease may have large development consequences is its deleterious effects on the cognitive development of infants, which makes them less productive throughout their lives.
Article
Richard G. Lawford and Sushel Unninayar
The global water cycle concept has its roots in the ancient understanding of nature. Indeed, the Greeks and Hebrews documented some of the most some important hydrological processes. Furthermore, Africa, Sri Lanka, and China all have archaeological evidence to show the sophisticated nature of water management that took place thousands of years ago. During the 20th century, a broader perspective was taken and the hydrological cycle was used to describe the terrestrial and freshwater component of the global water cycle. Data analysis systems and modeling protocols were developed to provide the information needed to efficiently manage water resources. These advances were helpful in defining the water in the soil and the movement of water between stores of water over land surfaces. Atmospheric inputs to these balances were also monitored, but the measurements were much more reliable over countries with dense networks of precipitation gauges and radiosonde observations.
By the 1960s, early satellites began to provide images that gave a new perception of Earth processes, including a more complete realization that water cycle components and processes were continuous in space and could not be fully understood through analyses partitioned by geopolitical or topographical boundaries. In the 1970s, satellites delivered quantitative radiometric measurements that allowed for the estimation of a number of variables such as precipitation and soil moisture. In the United States, by the late 1970s, plans were made to launch the Earth System Science program, led by the National Aeronautics and Space Agency (NASA). The water component of this program integrated terrestrial and atmospheric components and provided linkages with the oceanic component so that a truly global perspective of the water cycle could be developed. At the same time, the role of regional and local hydrological processes within the integrated “global water cycle” began to be understood.
Benefits of this approach were immediate. The connections between the water and energy cycles gave rise to the Global Energy and Water Cycle Experiment (GEWEX)1 as part of the World Climate Research Programme (WCRP). This integrated approach has improved our understanding of the coupled global water/energy system, leading to improved prediction models and more accurate assessments of climate variability and change. The global water cycle has also provided incentives and a framework for further improvements in the measurement of variables such as soil moisture, evapotranspiration, and precipitation. In the past two decades, groundwater has been added to the suite of water cycle variables that can be measured from space. New studies are testing innovative space-based technologies for high-resolution surface water level measurements. While many benefits have followed from the application of the global water cycle concept, its potential is still being developed. Increasingly, the global water cycle is assisting in understanding broad linkages with other global biogeochemical cycles, such as the nitrogen and carbon cycles. Applications of this concept to emerging program priorities, including the Sustainable Development Goals (SDGs) and the Water-Energy-Food (W-E-F) Nexus, are also yielding societal benefits.
Article
Timo Kuuluvainen
Boreal countries are rich in forest resources, and for their area, they produce a disproportionally large share of the lumber, pulp, and paper bound for the global market. These countries have long-standing strong traditions in forestry education and institutions, as well as in timber-oriented forest management. However, global change, together with evolving societal values and demands, are challenging traditional forest management approaches. In particular, plantation-type management, where wood is harvested with short cutting cycles relative to the natural time span of stand development, has been criticized. Such management practices create landscapes composed of mosaics of young, even-aged, and structurally homogeneous stands, with scarcity of old trees and deadwood. In contrast, natural forest landscapes are characterized by the presence of old large trees, uneven-aged stand structures, abundant deadwood, and high overall structural diversity. The differences between managed and unmanaged forests result from the fundamental differences in the disturbance regimes of managed versus unmanaged forests. Declines in managed forest biodiversity and structural complexity, combined with rapidly changing climatic conditions, pose a risk to forest health, and hence, to the long-term maintenance of biodiversity and provisioning of important ecosystem goods and services. The application of ecosystem management in boreal forestry calls for a transition from plantation-type forestry toward more diversified management inspired by natural forest structure and dynamics.
Article
John A. Sorrentino
Carbon has been part of the Earth since its beginning, and the carbon cycle is well understood. However, its abundance in the atmosphere has become a problem. Those who propose solutions in decentralized market economies often prefer economic incentives to direct government regulation. Carbon cap-and-trade programs and carbon tax programs are the prime candidates to rein in emissions by altering the economic conditions under which producers and consumers make decisions. Under ideal conditions with full information, they can seamlessly remove the distortion caused by the negative externality and increase a society’s welfare. This distortion is caused by overproduction and underpricing of carbon-related goods and services. The ideal level of emissions would be set under cap-and-trade, or be the outcome of an ideally set carbon tax. The ideal price of carbon permits would result from demand generated by government decree meeting an ideal fixed supply set by the government. The economic benefit of using the ideal carbon tax or the ideal permit price occurs because heterogeneous decision-makers will conceptually reduce emissions to the level that equates their marginal (incremental) emissions-reduction cost to the tax or permit price. When applying the theory to the real world, ideal conditions with full information do not exist. The economically efficient levels of emissions, the carbon tax, and the permit price cannot be categorically determined. The targeted level of emissions is often proposed by non-economists. The spatial extent and time span of the emissions target need to be considered. The carbon tax is bound to be somewhat speculative, which does not bode well for private-sector decision-makers who have to adjust their behavior, and for the achievement of a particular emissions target. The permit price depends on how permits are initially distributed and how well the permit market is designed. The effectiveness of either program is tied to monitoring and enforcement. Social justice considerations in the operation of tax programs often include the condition that they be revenue-neutral. This is more complicated in the permit scheme as much activity after the initial phase is among the emitters themselves.
Based on global measurement of greenhouse gases, several models have been created that attempt to explain how emissions transform into concentrations, how concentrations imply radiative forcing and global warming potential, how the latter cause ecological and economic impacts, and how mitigation and/or adaptation can influence these impacts. Scenarios of the uncertain future continue to be generated under myriad assumptions in the quest for the most reliable. Several institutions have worked to engender sustained cooperation among the parties of the “global commons.” The balance of theory and empirical observation is intended to generate normative and positive policy recommendations. Cap-and-trade and carbon tax programs have been designed and/or implemented by various countries and subnational jurisdictions with the hope of reducing carbon-related emissions. Many analysts have declared that the global human society will reach a “tipping point” in the 21st century, with irreversible trends that will alter life on Earth in significant ways.
