Bjørn H. Samset
Among the factors that affect the climate, few are as diverse and challenging to understand as aerosols. Minute particles suspended in the atmosphere, aerosols are emitted through a wide range of natural and industrial processes, and are transported around the globe by winds and weather. Once airborne, they affect the climate both directly, through scattering and absorption of solar radiation, and indirectly, through their impact on cloud properties. Combining all their effects, anthropogenic changes to aerosol concentrations are estimated to have had a climate impact over the industrial era that is second only to CO2. Their atmospheric lifetime of only a few days, however, makes their climate effects substantially different from those of well-mixed greenhouse gases.
Major aerosol types include sea salt, dust, sulfate compounds, and black carbon—or soot—from incomplete combustion. Of these, most scatter incoming sunlight back to space, and thus mainly cool the climate. Black carbon, however, absorbs sunlight, and therefore acts as a heating agent much like a greenhouse gas. Furthermore, aerosols can act as cloud condensation nuclei, causing clouds to become whiter—and thus more reflecting—further cooling the surface. Black carbon is again a special case, acting to change the stability of the atmosphere through local heating of the upper air, and also changing the albedo of the surface when it is deposited on snow and ice, for example.
The wide range of climate interactions that aerosols have, and the fact that their distribution depends on the weather at the time and location of emission, lead to large uncertainties in the scientific assessment of their impact. This in turn leads to uncertainties in our present understanding of the climate sensitivity, because while aerosols have predominantly acted to oppose 20th-century global warming by greenhouse gases, the magnitude of aerosol effects on climate is highly uncertain.
Finally, aerosols are important for large-scale climate events such as major volcanoes, or the threat of nuclear winter. The relative ease with which they can be produced and distributed has led to suggestions for using targeted aerosol emissions to counteract global warming—so-called climate engineering.
Precipitation levels in southern Africa exhibit a marked east–west gradient and are characterized by strong seasonality and high interannual variability. Much of the mainland south of 15°S exhibits a semiarid to dry subhumid climate. More than 66 percent of rainfall in the extreme southwest of the subcontinent occurs between April and September. Rainfall in this region—termed the winter rainfall zone (WRZ)—is most commonly associated with the passage of midlatitude frontal systems embedded in the austral westerlies. In contrast, more than 66 percent of mean annual precipitation over much of the remainder of the subcontinent falls between October and March. Climates in this summer rainfall zone (SRZ) are dictated by the seasonal interplay between subtropical high-pressure systems and the migration of easterly flows associated with the Intertropical Convergence Zone. Fluctuations in both SRZ and WRZ rainfall are linked to the variability of sea-surface temperatures in the oceans surrounding southern Africa and are modulated by the interplay of large-scale modes of climate variability, including the El Niño-Southern Oscillation (ENSO), Southern Indian Ocean Dipole, and Southern Annular Mode.
Ideas about long-term rainfall variability in southern Africa have shifted over time. During the early to mid-19th century, the prevailing narrative was that the climate was progressively desiccating. By the late 19th to early 20th century, when gauged precipitation data became more readily available, debate shifted toward the identification of cyclical rainfall variation. The integration of gauge data, evidence from historical documents, and information from natural proxies such as tree rings during the late 20th and early 21st centuries, has allowed the nature of precipitation variability since ~1800 to be more fully explored.
Drought episodes affecting large areas of the SRZ occurred during the first decade of the 19th century, in the early and late 1820s, late 1850s–mid-1860s, mid-late 1870s, earlymid-1880s, and mid-late 1890s. Of these episodes, the drought during the early 1860s was the most severe of the 19th century, with those of the 1820s and 1890s the most protracted. Many of these droughts correspond with more extreme ENSO warm phases.
Widespread wetter conditions are less easily identified. The year 1816 appears to have been relatively wet across the Kalahari and other areas of south central Africa. Other wetter episodes were centered on the late 1830s–early 1840s, 1855, 1870, and 1890. In the WRZ, drier conditions occurred during the first decade of the 19th century, for much of the mid-late 1830s through to the mid-1840s, during the late 1850s and early 1860s, and in the early-mid-1880s and mid-late 1890s. As for the SRZ, markedly wetter years are less easily identified, although the periods around 1815, the early 1830s, mid-1840s, mid-late 1870s, and early 1890s saw enhanced rainfall. Reconstructed rainfall anomalies for the SRZ suggest that, on average, the region was significantly wetter during the 19th century than the 20th and that there appears to have been a drying trend during the 20th century that has continued into the early 21st. In the WRZ, average annual rainfall levels appear to have been relatively consistent between the 19th and 20th centuries, although rainfall variability increased during the 20th century compared to the 19th.
This is an advance summary of a forthcoming article in the Oxford Research Encyclopedia of Climate Science. Please check back later for the full article.
Historical assumptions that climates shape human societies in particular places are widespread. But these modes of thinking are being dramatically reversed as it is recognized that human activities are now on such a scale that they are influencing the global climate. Many so-called climate deniers still refuse to accept these new insights into the human situation, and at least implicitly insist that the geographical context for human affairs is simply a given set of conditions. But despite this opposition, the recognition that human choices and decisions about which economic systems will be built in coming generations will have dramatic effects on the future of global climate finally shaped the first nearly universally accepted climate change agreement in Paris in December 2015.
How climate is invoked in political discussion is tied into diverse cultures and hence comes to be part of public discourse in numerous ways. How state policymakers and political advocates of diverse ideological stripes situate “their” state in the world and in relationship to other states and within a wider world system is a crucial part of how political identities are constructed and appropriate courses of action rendered legitimate. These processes are key to how geopolitics works and how politicians and public opinion shape policies.
Despite the widespread acceptance of the 2015 Paris agreement on climate change action, numerous arguments about who should act and how to deal with climate change persist. Such differences are in part a matter of geographical location, and a matter of whether an economy is dependent on fossil fuels revenue or subject to increasingly severe meteorological hazards and rising sea levels. Partly in response to these differences, the Paris agreement devolves primary responsibility for climate policy to individual states.
These differences are also connected to political rivalries among states and disputed claims about historical legacies of colonization and injustice, who should lead in international affairs, and how world order should be structured. Such formulations impute responsibilities within the international system in ways that former colonial and industrial powers frequently refuse to accept. Policy responses to climate change draw on these different geographical representations of the world, with various sources of climate danger and opportunity specified in terms of how they affect “our” society.
From this follow political arguments about how societies and governments ought to behave on the basis of their understandings of the identities in particular places in the world and their role in the larger patterns of progress, rivalry, and history. These matters of geopolitical culture shape political responses to climate change profoundly, and in turn have large consequences for the future configuration of the planet’s climate.
The topic of climate change and migration attracts a strong following from the media and produces an increase in academic literature and reports from international governmental institutions and NGOs. It poses questions that point to the core of social and environmental developments of the 21st century, such as environmental and climate justice as well as North–South relations.
This article examines the main features of the debate and presents a genealogy of the discussion on climate change and migration since the 1980s. It presents an analysis of different framings and lines of argument, such as the securitization of climate change and connections to development studies and adaptation research. This article also presents methodological and conceptual questions, such as how to conceive interactions between migration and climate change. As legal aspects have played a crucial role since the beginning of the debate, different legal strands are considered here, including soft law and policy-oriented approaches. These approaches relate to questions of voluntary or forced migration and safeguarding the rights of environmental migrants.
This article introduces theoretical concepts that are prompted by analyzing climate change as an “imaginative resource” and by questioning power relations related to climate-change discourses, politics, and practices. This article recommends a re-politicization of the debate, questions the often victimizing, passive picture of the “drowning” climate-change migrant, and criticizes alarmist voices that can trigger perceived security interests of countries of the Global North. Decolonizing and critical perspectives analyze facets of the debate that have racist, depoliticizing, or naturalizing tendencies or exoticize the “other.”
This is an advance summary of a forthcoming article in the Oxford Research Encyclopedia of Climate Science. Please check back later for the full article.
In debates surrounding policy options for mitigating emissions of greenhouse gasses (GHG), economists of various political stripes are near unanimous in their advocacy of putting a price on carbon emissions, whether through a tax or emissions trading program. Politically, however, few politicians have been brave enough to impose costs on emissions occurring within their own borders. This state of affairs, and the more recent diffusion of market-based instruments across political jurisdictions around the world raise new and important questions regarding how best to communicate the benefits of carbon taxes and cap-and-trade programs. How do advocates and opponents of carbon pricing frame their arguments around carbon pricing? How do experts, journalists, political leaders, and members of the public understand these policy instruments; and what specific approaches have been most successful in persuading policy makers and the public to support a price on carbon? In places that have yet to implement a carbon price, what can communication strategists learn from other jurisdictions that have successfully implemented a carbon price? In places where carbon taxes or emissions trading programs exist, how are the benefits of these policies best communicated to ensure the durability of carbon pricing policies over time?
Scientific agreement on climate change has strengthened over the past few decades, with around 97% of publishing climate scientists agreeing that human activity is causing global warming. While scientific understanding has strengthened, a small but persistent proportion of the public actively opposes the mainstream scientific position. A number of factors contribute to this rejection of scientific evidence, with political ideology playing a key role. Conservative think tanks, supported with funding from vested interests, have been and continue to be a prolific source of misinformation about climate change. A major strategy by opponents of climate mitigation policies has been to cast doubt on the level of scientific agreement on climate change, contributing to the gap between public perception of scientific agreement and the 97% expert consensus. This “consensus gap” decreases public support for mitigation policies, demonstrating that misconceptions can have significant societal consequences. While scientists need to communicate the consensus, they also need to be aware of the fact that misinformation can interfere with the communication of accurate scientific information. As a consequence, neutralizing the influence of misinformation is necessary. Two approaches to neutralize misinformation involve refuting myths after they have been received by recipients (debunking) or preemptively inoculating people before they receive misinformation (prebunking). Research indicates preemptive refutation or “prebunking” is more effective than debunking in reducing the influence of misinformation. Guidelines to practically implement responses (both preemptive and reactive) can be found in educational research, cognitive psychology, and a branch of psychological research known as inoculation theory. Synthesizing these separate lines of research yields a coherent set of recommendations for educators and communicators. Clearly communicating scientific concepts, such as the scientific consensus, is important, but scientific explanations should be coupled with inoculating explanations of how that science can be distorted.
Margaret M. Skutsch
The clean development mechanism of the Kyoto Protocol did not cover projects to reduce emissions from deforestation in developing countries. The reasons were in part technical (the difficulty of accounting for leakage) but mainly the result of fears of many Parties to the United Nations Framework Convention on Climate Change (UNFCCC) that this was a soft (and cheap) option that would discourage interventions for mitigation of emissions from fossil fuels. The alternative idea of a national, performance-based approach to reduced emissions from deforestation (RED) was first developed by research institutes in Brazil and proposed to the UNFCCC in a submission by Papua New Guinea and Costa Rica with technical support from the Environmental Defense Fund in 2005/2006. The idea was to reward countries financially for any decreases in annual rates of deforestation at a national level compared to a baseline that reflected historical rates of loss, through the sale of carbon credits, which as in the case of the Clean Development Mechanism (CDM) would be used as offsets by developed countries to meet their international obligations for emission reduction.
REDD+ as it is now included in the Paris Agreement of 2015 (Article 5) has evolved from this rather simple concept into something much more complex and far-reaching. Degradation was added early on in the negotiation process (REDD) and very soon conservation, sustainable management of forests, and enhancement of forest carbon stocks were also included, hence the “+” in REDD+. The idea of “safeguards” (social, environmental) is now also firmly embedded, and the importance of non-carbon benefits is being underlined in official policy. In the absence of legally binding emission reduction targets in developed countries, the notion of a market approach and offsets is no longer the only or even the main route envisaged. Instead, countries are being encouraged to coordinate financial support from a range of public, private, bilateral, and multilateral sources. The mechanism is still, however, seen as a results-based instrument, although this may not be so clear in alternative policy approaches, such as “joint mitigation and adaptation,” also included in the Paris Agreement.
Outside of the official policy negotiations, there has been a move away from operationalizing REDD+ as a purely forest-based mechanism toward developing a more holistic, landscape-based approach, given that many of the drivers of deforestation and degradation lie outside the forest itself. Countries in the vanguard of REDD+ implementation, such as Mexico, as well as several CGIAR organizations are visualizing REDD+ essentially as sustainable rural development. The central role of communities in the implementation of REDD+, and the importance of secure land tenure in this, have to a large extent been incorporated through the adoption of safeguards, but there remain a few lobbies of indigenous groups that are opposed to the whole nature of REDD+. The challenge of measurability, of both carbon and of non-carbon benefits, is addressed in this article.
Courtney Plante, Johnie J. Allen, and Craig A. Anderson
Given the dire nature of many researchers’ predictions about the effects of global climate change (e.g., rising sea levels, droughts, more extreme weather), it comes as little surprise that less attention has been paid to the subtler, less direct outcomes of rapid climate change: psychological, sociological, political, and economic effects. In this chapter we explore one such outcome in particular: the effects of rapid climate change on aggression. We begin by exploring the potential for climate change to directly affect aggression in individuals, focusing on research showing the relationship between uncomfortably hot ambient temperature and aggression. Next, we review several lines of research illustrating ways that climate change can indirectly increase aggression in individuals. We then shift our focus from individuals to the effects of climate change on group-level aggression. We finish by addressing points of contention, including the challenge that the effects of climate change on aggression are too remote and too small to be considered relevant.
Martin Claussen, Anne Dallmeyer, and Jürgen Bader
There is ample evidence from palaeobotanic and palaeoclimatic reconstructions that during early and mid-Holocene between some 11,700 years (in some regions, a few thousand years earlier) and some 4200 years ago, subtropical North Africa was much more humid and greener than today. This African Humid Period (AHP) was triggered by changes in the orbital forcing, with the climatic precession as the dominant pacemaker. Climate system modeling in the 1990s revealed that orbital forcing alone cannot explain the large changes in the North African summer monsoon and subsequent ecosystem changes in the Sahara. Feedbacks between atmosphere, land surface, and ocean were shown to strongly amplify monsoon and vegetation changes. Forcing and feedbacks have caused changes far larger in amplitude and extent than experienced today in the Sahara and Sahel. Most, if not all, climate system models, however, tend to underestimate the amplitude of past African monsoon changes and the extent of the land-surface changes in the Sahara. Hence, it seems plausible that some feedback processes are not properly described, or are even missing, in the climate system models.
Perhaps even more challenging than explaining the existence of the AHP and the Green Sahara is the interpretation of data that reveal an abrupt termination of the last AHP. Based on climate system modeling and theoretical considerations in the late 1990s, it was proposed that the AHP could have ended, and the Sahara could have expanded, within just a few centuries—that is, much faster than orbital forcing. In 2000, paleo records of terrestrial dust deposition off Mauritania seemingly corroborated the prediction of an abrupt termination. However, with the uncovering of more paleo data, considerable controversy has arisen over the geological evidence of abrupt climate and ecosystem changes. Some records clearly show abrupt changes in some climate and terrestrial parameters, while others do not. Also, climate system modeling provides an ambiguous picture.
The prediction of abrupt climate and ecosystem changes at the end of the AHP is hampered by limitations implicit in the climate system. Because of the ubiquitous climate variability, it is extremely unlikely that individual paleo records and model simulations completely match. They could do so in a statistical sense, that is, if the statistics of a large ensemble of paleo data and of model simulations converge. Likewise, the interpretation regarding the strength of terrestrial feedback from individual records is elusive. Plant diversity, rarely captured in climate system models, can obliterate any abrupt shift between green and desert state. Hence, the strength of climate—vegetation feedback is probably not a universal property of a certain region but depends on the vegetation composition, which can change with time. Because of spatial heterogeneity of the African landscape and the African monsoon circulation, abrupt changes can occur in several, but not all, regions at different times during the transition from the humid mid-Holocene climate to the present-day more arid climate. Abrupt changes in one region can be induced by abrupt changes in other regions, a process sometimes referred to as “induced tipping.” The African monsoon system seems to be prone to fast and potentially abrupt changes, which to understand and to predict remains one of the grand challenges in African climate science.