Annual Review of Environment and Resources - Volume 46, 2021

Human use of land has been transforming Earth's ecology for millennia. From hunting and foraging to burning the land to farming to industrial agriculture, increasingly intensive human use of land has reshaped global patterns of biodiversity, ecosystems, landscapes, and climate. This review examines recent evidence from archaeology, paleoecology, environmental history, and model-based reconstructions that reveal a planet largely transformed by land use over more than 10,000 years. Although land use has always sustained human societies, its ecological consequences are diverse and sometimes opposing, both degrading and enriching soils, shrinking wild habitats and shaping novel ones, causing extinctions of some species while propagating and domesticating others, and both emitting and absorbing the greenhouse gases that cause global climate change. By transforming Earth's ecology, land use has literally paved the way for the Anthropocene. Now, a better future depends on land use strategies that can effectively sustain people together with the rest of terrestrial nature on Earth's limited land.

Climate change worry, eco-anxiety, and ecological grief are concepts that have emerged in the media, public discourse, and research in recent years. However, there is not much literature examining and summarizing the ways in which these emotions are expressed, to what processes they are related, and how they are distributed. This narrative review aims to (a) summarize research about the relationships between, on the one hand, negative emotions in relation to climate change and other environmental problems and, on the other hand, mental well-being among people in different parts of the world and (b) examine studies that have explored the potentially constructive role of worry—for example, in the form of providing motivation to act. It is clear from this review that negative emotions regarding environmental problems are normal, and often constructive, responses.Yet, given the nature, range, and extent of these emotions, it is important to identify diverse place-based and culturally relevant strategies to help people cope.

Air conditioning and refrigeration services are increasing rapidly in developing countries due to improved living standards. The cooling services industry is currently responsible for over 10% of global greenhouse gas (GHG) emissions, so it is critical to investigate how the expansion of cooling services will impact future GHG emissions. In this article, we first examine the current status and expected expansion of cooling services worldwide and the associated GHG emissions. Then, we review potential improvements and innovations that could reduce future GHG emissions. Three approaches to reduce GHG emissions within the cooling sector include converting to alternative refrigerants, improving energy efficiency, and moving toward a lower-carbon electricity grid. In addition, we identify eight interventions that apply to the built environment or the food supply chain that would lead to additional GHG reductions in the cooling sector.

The past century has been a time of unparalleled changes in global climate and global biogeochemistry. At the forefront of the study of these changes are regular time-series observations at remote stations of atmospheric CO2, isotopes of CO2, and related species, such as O2 and carbonyl sulfide (COS). These records now span many decades and contain a wide spectrum of signals, from seasonal cycles to long-term trends. These signals are variously related to carbon sources and sinks, rates of photosynthesis and respiration of both land and oceanic ecosystems, and rates of air-sea exchange, providing unique insights into natural biogeochemical cycles and their ongoing changes. This review provides a broad overview of these records, focusing on what they have taught us about large-scale global biogeochemical change.

Global climate warming disproportionately affects high-latitude and mountainous terrestrial ecosystems. Warming is accompanied by permafrost thaw, shorter winters, earlier snowmelt, more intense soil freeze-thaw cycles, drier summers, and longer fire seasons. These environmental changes in turn impact surface water and groundwater flow regimes, water quality, greenhouse gas emissions, soil stability, vegetation cover, and soil (micro)biological communities. Warming also facilitates agricultural expansion, urban growth, and natural resource development, adding growing anthropogenic pressures to cold regions’ landscapes, soil health, and biodiversity. Further advances in the predictive understanding of how cold regions’ critical zone processes, functions, and ecosystem services will continue to respond to climate warming and land use changes require multiscale monitoring technologies coupled with integrated observational and modeling tools. We highlight some of the major challenges, knowledge gaps, and opportunities in cold region critical zone research, with an emphasis on subsurface processes and responses in both natural and agricultural ecosystems.

This article presents a critical assessment of 40 years of research that may be brought under the umbrella of energy efficiency, spanning different aggregations and domains—from individual producing and consuming agents to economy-wide effects to the role of innovation to the influence of policy. After 40 years of research, energy efficiency initiatives are generally perceived as highly effective. Innovation has contributed to lowering energy technology costs and increasing energy productivity. Energy efficiency programs in many cases have reduced energy use per unit of economic output and have been associated with net improvements in welfare, emission reductions, or both. Rebound effects at the macro level still warrant careful policy attention, as they may be nontrivial. Complexity of energy efficiency dynamics calls for further methodological and empirical advances, multidisciplinary approaches, and granular data at the service level for research in this field to be of greatest societal benefit.

Automation carries paradigm-shifting potential for urban transport and has critical sustainability dimensions for the future of our cities. This article examines the diverse environmental and energy-related dimensions of automated mobility at the city level by reviewing an emerging and increasingly diversified volume of literature for road, rail, water, and air passenger transport. The multimodal nature of this investigation provides the opportunity for a novel contribution that adds value to the literature in four distinctive ways. It reviews from a sustainability angle the state of the art underpinning the transition to a paradigm of automated mobility, identifies current knowledge gaps highlighting the scarcity of non-technical research outside the autonomous car's realm, articulates future directions for research and policy development, and proposes a conceptual model that contextualizes the automation-connectivity-electrification-sharing-multimodality nexus as the only way forward for vehicle automation to reach its pro-environmental and resource-saving potential.

The water, sanitation, and solid waste sectors are closely related and have many interactions between their respective service chains in low- and middle-income countries. Currently, these interactions mostly lead to cross-contamination, and opportunities for co-benefits are seldom realized. This review presents the key advancements within each of these three development sectors in the past two decades. We identify numerous similarities such as decentralization, resource recovery, community involved planning, and digitalization. Despite the potential for synergies and the opportunities to maximize positive interactions, there have been few attempts to break the existing sectoral silos in order to integrate these three service chains. We argue that, with the right enabling environment, an integrated approachto holistically planning and implementing water supply, sanitation, and solid waste management can create positive interactions resulting in co-benefits among complementary development goals.

Several hundred species are hunted for wild meat in the tropics, supporting the diets, customs, and livelihoods of millions of people. However, unsustainable hunting is one of the most urgent threats to wildlife and ecosystems worldwide and has serious ramifications for people whose subsistence and income are tied to wild meat. Over the past 18 years, although research efforts have increased, scientific knowledge has largely not translated into action. One major barrier to progress has been insufficient monitoring and evaluation, meaning that the effectiveness of interventions cannot be ascertained. Emerging issues include the difficulty of designing regulatory frameworks that disentangle the different purposes of hunting, the large scale of urban consumption, and the implications of wild meat consumption for human health. To address these intractable challenges, wepropose eight new recommendations for research and action for sustainable wild meat use, which would support the achievement of the United Nations Sustainable Development Goals.

More food and energy allow for more people who then require more food and energy, and so it has gone for centuries. At the same time, economic progress leads to a different lifestyle with an increasing demand for energy and food, also accelerating food waste. Fueling this food-energy-population dynamic is an ever-increasing conversion of unreactive dinitrogen (N2) to reactive N (Nr), which then results in a cascade of positive (food and energy for people) and negative (damage to people, climate, biodiversity, and environment) impacts as Nr is distributed throughout Earth systems. The most important step in reducing the environmental impacts of Nr is limiting its human-based creation. In this article, therefore, we focus on this most important first step: the conversion of N2 to Nr by human activities. Specifically, we examine Nr creation and use (they are different!) on a global and regional basis and Nr use on a global and regional per capita basis. In addition, we introduce the metric Nr Use Index (NUI), which can be used to track and project Nr use relative to a fixed point in time. We then assess the progress in Nr management over the past 20 years. Our article presents a case study of the Netherlands to show what one country, beset by Nr-relatedproblems that have led to an N crisis, did to address those problems and what worked and what didn't work. The article concludes with an assessment of what the future might hold with respect to Nr creation and use, including a review of other projections. We expect that NUI will increase especially in Asia, Latin America, and Africa. The other parts of the world are consolidating or even decreasing NUI. In Latin America and Asia, there is limited agricultural land, and by increasing NUI for food the risk of Nr pollution is very high. The Netherlands has shown not only what effects can be expected with increasing NUI but also what successful policies can be introduced to limit environmental losses. Our assessment shows that Nr creation needs to be limited to prevent local to global environmental impacts.

A series of international initiatives have set ambitious goals for restoring global forests. This review synthesizes natural and social science research on forest restoration (FR), with a focus on restoration on cleared land in low- and middle-income countries. We define restoration more broadly than reestablishing native forests, given that landholders might prefer other forest types. We organize the review loosely around ideas in the forest transition literature. We begin by examining recent trends in FR and forest transition indicators. We then investigate two primary parts of the forest transition explanation for forest recovery: wood scarcity, including its connection to restoration for climate change mitigation, and the dynamic relationships between migration and land use. Next, we review ecological and silvicultural aspects of restoration on cleared land. We conclude by discussing selected interventions to promote restoration and the challenge of scaling up restoration to achieve international initiatives' goals.

The availability and use of fresh water are critical for human health and for economic and ecosystem stability. But the growing mismatch between human demands and natural freshwater availability is contributing to water scarcity, affecting industrial and agricultural production and a wide range of social, economic, and political problems, including poverty, deterioration of ecosystem health, and violent conflicts. Understanding and addressing different forms of water scarcity are vital for moving toward more sustainable management and use of fresh water. We provide here a review of concepts and definitions of water scarcity, metrics and indicators used to evaluate scarcity together with strategies for addressing and reducing the adverse consequences of water scarcity, including the development of alternative sources of water, improvements in water-use efficiency, and changes in systems of water management and planning.

Decarbonizing power grids is an essential pillar of global efforts to mitigate climate change impacts. Renewable energy generation is expected to play an important role in electricity decarbonization, although its variability and uncertainty are creating new flexibility challenges for electric grid operators that must match supply with constantly changing demand. Distributed energy resources (DERs)—including distributed generation, demand response, and distributed energy storage—can play an important role in providing the flexibility needed to integrate high penetrations of renewable energy. This article examines federal and state enabling policies and regulations for DER, market strategies and business models that have facilitated DER expansion, and key emerging challenges for DER in the United States. Based on a review of the US experience, the article offers lessons for other countries, focusing on the role and limits of policy, the facilitative role of utility regulatory reform, the need to balance different interests in tariff design, the benefits of DER participation in wholesale markets, and the importance of proactive interconnection policies.

This article provides a systematic review of the literature on net-zero carbon cities, their objectives and key features, current efforts, and performance. We discuss how net-zero differs from low-carbon cities, how different visions of a net-zero carbon city relate to urban greenhouse gas accounting, deep decarbonization pathways and their application to cities and urban infrastructure systems, net-zero carbon cities in theory versus practice, lessons learned from net-zero carbon city plans and implementation, and opportunities and challenges in transitioning toward net-zero carbon cities across both sectors and various spatial fabrics within cities. We conclude that it is possible for cities to get to or near net-zero carbon, but this requires systemic transformation. Crucially, a city cannot achieve net-zero by focusing only on reducing emissions within its administrative boundaries. Cities must decarbonize key transboundary supply chains and use urban and regional landscapes to sequester carbon from the atmosphere. Because of carbon lock-in, and the complex interplay between urban infrastructure and behavior, strategic sequencing of mitigation action is essential for cities to achieve net-zero.

Environmental factors, particularly those related to climate change, are stranding or could strand assets across different sectors and geographies with significant implications for economies, companies, financial institutions, communities, and workers. In this review, we focus on physical climate change, biodiversity loss, and litigation related to environmental factors as causes of stranded assets. We also review the emerging literature on the consequences of asset stranding for society before turning to some of the key supervisory responses that are emerging to ensure that stranded assets are measured and managed, particularly by financial institutions. These are among the areas of the stranded assets literature that have been growing most rapidly since 2015, and we focus on the literature produced since then.

Coastal settlements, home to more than three billion people and growing rapidly, are highly vulnerable to climate change. Increasingly, there are calls for climate adaptation that goes beyond business-as-usual approaches, transforms socioeconomic systems, and addresses underlying drivers of vulnerability. Although calls for transformational adaptation are growing, greater clarity is needed on what transformation means in context in order to bridge the gap between theory and practice. This article reviews the theoretical literature on transformational adaptation, as well as practitioner frameworks and case studies of urban coastal adaptation. The article discusses specific challenges for transformational adaptation and its governance in coastal cities. In doing so, this review contributes to the growing debate about operationalizing the concept of transformational adaptation in the context of coastal cities and offers insights to ensure that transformation processes are inclusive and equitable.

The knowledge, values, and practices of Indigenous peoples and local communities offer ways to understand and better address social-environmental problems. The article reviews the state of the literature on this topic by focusing on six pathways by which Indigenous peoples and local communities engage with management of and relationships to nature. These are (a) undertaking territorial management practices and customary governance, (b) contributing to nature conservation and restoration efforts with regional to global implications, (c) co-constructing knowledge for assessments and monitoring, (d) countering the drivers of unsustainable resource use and resisting environmental injustices, (e) playing key roles in environmental governance across scales, and (f) offering alternative conceptualizations of the interrelations between people and nature. The review shows that through these pathways Indigenous peoples and local communities are making significant contributions to managing the health of local and regional ecosystems, to producing knowledge based in diverse values of nature, confronting societal pressures and environmental burdens, and leading and partnering in environmental governance. These contributions have local to global implications but have yet to be fully recognized in conservation and development polices, and by society at large.

Over the past few years, studies in political ecology and environmental justice have been increasingly connecting the commons and social movements empirically, giving shape to a new, distinctive body of research on commons movements. In our review, we first organize and synthesize empirical lessons from this body of literature. We then highlight recent theoretical efforts made by scholars to both bridge and transcend the gap between the theory of the commons and social movement theory. As we illustrate, movements can help create and strengthen commons institutions and discourses, as well as rescale them horizontally and vertically. This is particularly evident in the context of rural community-rights movements in the global South, as well as in new water and food commons movements and community energy movements in both the global South and North. Commons institutions, in turn, can serve as the basis of social mobilization and become a key frame for social movements, as shown in the context of local environmental justice and livelihoods conflicts and anti-privatization struggles. Tensions and contradictions of commons-movement dynamics also exist and reflect trade-offs between diversity versus uniformization and organizational closure versus expansion of discourses and practices. Theoretically, there is an opportunity to cross boundaries from the theory of the commons to social movementtheory and vice versa, e.g., by highlighting the role of political opportunities and framing, and biophysical factors and polycentricity, respectively. More importantly, a new commons movements theory is emerging focusing on cross-scalar organizations, the virtuous cycles between commons projects and mobilization, and the processes of commons-making.

Climate change threatens core dimensions of human security, including economic prosperity, food availability, and societal stability. In recent years, war-torn regions such as Afghanistan and Yemen have harbored severe humanitarian crises, compounded by climate-related hazards. These cases epitomize the powerful but presently incompletely appreciated links between vulnerability, conflict, and climate-related impacts. In this article, we develop a unified conceptual model of these phenomena by connecting three fields of research that traditionally have had little interaction: (a) determinants of social vulnerability to climate change, (b) climatic drivers of armed conflict risk, and (c) societal impacts of armed conflict. In doing so, we demonstrate how many of the conditions that shape vulnerability to climate change also increase the likelihood of climate–conflict interactions and, furthermore, that impacts from armed conflict aggravate these conditions. The end result may be a vicious circle locking affected societies in a trap of violence, vulnerability, and climate change impacts.