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Pastoralists around the world are exposed to climate change and increasing climate variability. Various downscaled regional climate models in Africa support community reports of rising temperatures as well as changes in the seasonality of rainfall and drought. In addition to climate, pastoralists have faced a second exposure to unsupportive policy environments. Dating back to the colonial period, a lack of knowledge about pastoralism and a systemic marginalization of pastoral communities influenced the size and nature of government investments in pastoral lands. National governments prioritized farming communities and failed to pay adequate attention to drylands and pastoral communities. The limited government interventions that occurred were often inconsistent with contemporary realities of pastoralism and pastoral communities. These included attempts at sedentarization and modernization, and in other ways changing the priorities and practices of pastoral communities.
The survival of pastoral communities in Africa in the context of this double exposure has been a focus for scholars, development practitioners, as well as national governments in recent years. Scholars initially drew attention to pastoralists’ drought-coping strategies, and later examined the multiple ways in which pastoralists manage risk and exploit unpredictability. It has been learned that pastoralists are rational land managers whose experience with variable climate has equipped them with the skills needed for adaptation. Pastoralists follow several identifiable adaptation paths, including diversification and modification of their herds and herding strategies; adoption of livelihood activities that did not previously play a permanent role; and a conscious decision to train the next generation for nonpastoral livelihoods. Ongoing government interventions around climate change still prioritize cropping over herding. Sometimes, such nationally supported adaptation plans can undermine community-based adaptation practices, autonomously evolving within pastoral communities. Successful adaptation hinges on recognition of the value of autonomous adaptation and careful integration of such adaptation with national plans.
Ronald D. Brunner and Amanda H. Lynch
Adaptive governance is defined by a focus on decentralized decision-making structures and procedurally rational policy, supported by intensive natural and social science. Decentralized decision-making structures allow a large, complex problem like global climate change to be factored into many smaller problems, each more tractable for policy and scientific purposes. Many smaller problems can be addressed separately and concurrently by smaller communities. Procedurally rational policy in each community is an adaptation to profound uncertainties, inherent in complex systems and cognitive constraints, that limit predictability. Hence planning to meet projected targets and timetables is secondary to continuing appraisal of incremental steps toward long-term goals: What has and hasn’t worked compared to a historical baseline, and why? Each step in such trial-and-error processes depends on politics to balance, if not integrate, the interests of multiple participants to advance their common interest—the point of governance in a free society. Intensive science recognizes that each community is unique because the interests, interactions, and environmental responses of its participants are multiple and coevolve. Hence, inquiry focuses on case studies of particular contexts considered comprehensively and in some detail.
Varieties of adaptive governance emerged in response to the limitations of scientific management, the dominant pattern of governance in the 20th century. In scientific management, central authorities sought technically rational policies supported by predictive science to rise above politics and thereby realize policy goals more efficiently from the top down. This approach was manifest in the framing of climate change as an “irreducibly global” problem in the years around 1990. The Intergovernmental Panel on Climate Change (IPCC) was established to assess science for the Conference of the Parties (COP) to the U.N. Framework Convention on Climate Change (UNFCCC). The parties negotiated the Kyoto Protocol that attempted to prescribe legally binding targets and timetables for national reductions in greenhouse gas emissions. But progress under the protocol fell far short of realizing the ultimate objective in Article 1 of the UNFCCC, “stabilization of greenhouse gas concentrations in the atmosphere at a level that would prevent dangerous anthropogenic interference in the climate system.” As concentrations continued to increase, the COP recognized the limitations of this approach in Copenhagen in 2009 and authorized nationally determined contributions to greenhouse gas reductions in the Paris Agreement in 2015.
Adaptive governance is a promising but underutilized approach to advancing common interests in response to climate impacts. The interests affected by climate, and their relative priorities, differ from one community to the next, but typically they include protecting life and limb, property and prosperity, other human artifacts, and ecosystem services, while minimizing costs. Adaptive governance is promising because some communities have made significant progress in reducing their losses and vulnerability to climate impacts in the course of advancing their common interests. In doing so, they provide field-tested models for similar communities to consider. Policies that have worked anywhere in a network tend to be diffused for possible adaptation elsewhere in that network. Policies that have worked consistently intensify and justify collective action from the bottom up to reallocate supporting resources from the top down. Researchers can help realize the potential of adaptive governance on larger scales by recognizing it as a complementary approach in climate policy—not a substitute for scientific management, the historical baseline.
Deborah R. Coen
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.
The advent of climate science refers to the historical emergence of a research program to study climate according to a modern definition of climate. Climate in this sense (a) refers not simply to the average state of the atmosphere, but also to its variability; (b) is multiscalar, concerned with phenomena ranging from the very small and fast to the very large and slow; and (c) is understood to be influenced by the oceans, lithosphere, cryosphere, and biosphere. While the history of climate science is just beginning to be written, existing narratives tend to focus on the development of computerized general circulation models since World War II. However, following this definition, the advent of climate science occurred well before the computer age. Important precedents emerged in the 17th and 18th centuries, and it was in the late 19th century that the science of climate coalesced into a unified research program.
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.
Anthony Leiserowitz and Nicholas Smith
Affective imagery, or connotative meanings, play an important role in shaping public risk perceptions, policy support, and broader responses to climate change. These simple “top-of-mind” associations and their related affect help reveal how diverse audiences understand and interpret global warming. And as a relatively simple set of measures, they are easily incorporated into representative surveys, making it possible to identify, measure, and monitor how connotative meanings are distributed throughout a population and how they change over time. Affective image analysis can help identify distinct interpretive communities of like-minded individuals who share their own set of common meanings and interpretations. The images also provide a highly sensitive measure of changes in public discourse. As scientists, political elites, advocates, and the media change the frames, images, icons, and emotions they use to communicate climate change, they can influence the interpretations of the larger public. Likewise, as members of the public directly or vicariously experience specific events or learn more about climate risks, they construct their own connotative meanings, which can in turn influence larger currents of public discourse. This article traces the development of affective imagery analysis, reviews the studies that have implemented it, examines how affective images influence climate change risk perceptions and policy support, and charts several future directions of research.
Social scientists and media critics have often been befuddled about how and why news coverage of important issues takes the shapes that it does. While some issues seem to behave according to well-established patterns, others don’t. The issue of climate change is one that has been explained in numerous ways, often from a cyclical perspective. This perspective suggests that news attention naturally varies up and down, often cued by certain focusing events that draw attention for a time, after which attention wanes again. These observations are usually matched with the perspective that attention should normatively not be cyclical, that the issue is one that deserves continuous attention until it is resolved.
All of this is in the context that there are significant doubts about the objective role of newsmakers in this process. Climate change is an issue that has cut across a period of news evolution in which objectively neutral news has become even less prominent than it once was, if it ever was. News outlets with specific ideological agendas, a plethora of bloggers and websites with an axe to grind, and a variety of conspiracy theories about climate have obscured how news can even hope to cover this issue. With “belief” in climate change now becoming an important token of how one identifies oneself politically, we can wonder whether the issue can ever receive a fair hearing from a scientific perspective.
Linda S. Prokopy, Wendy-Lin Bartels, Gary Burniske, and Rebecca Power
Agricultural extension has evolved over the last 200 years from a system of top-down dissemination of information from experts to farmers to a more complex system, in which a diversity of knowledge producers and farmers work together to co-produce information. Following a detailed history of the evolution of extension in the United States, this article describes an example from the southeastern United States that illustrates how innovative institutional arrangements enable land-grant universities to actively engage farmers and extension agents as key partners in the knowledge generation process. A second U.S. example shows that private retailers are more influential than extension in influencing large-scale farmers’ farm management decisions in the midwestern United States. However, these private retailers trust extension as a source of climate change information and thus partnerships are important for extension. Nongovernmental organizations (NGOs) have been an important source of extension services for smallholder farmers across the world, and examples from the NGO CARE indicate that a participatory and facilitative approach works well for climate change communication. Collectively, these examples emphasize that the role of agricultural extension in climate change communication is essential in the context of both developed and developing countries and with both smallholder farmers and large-scale farmers. These case studies illustrate the effectiveness of a co-production approach, the importance of partners and donors, and the changing landscape of agricultural extension delivery.
The European Alps feature a unique situation with the densest network of long-term instrumental climate observations and anthropogenic emission sources located in the immediate vicinity of glaciers suitable for ice core studies. To archive atmospheric changes in an undisturbed sequence of firn and ice layers, ice core drilling sites require temperatures low enough to minimize meltwater percolation. In the Alps, this implies a restriction to the highest summit glaciers of comparatively small horizontal and vertical extension (i.e., with typical ice thickness not much exceeding 100 m). As a result, Alpine ice cores offer either high-resolution or long-term records, depending on the net snow accumulation regime of the drilling site. High-accumulation Alpine ice cores have been used with great success to study the anthropogenic influence on aerosol-related atmospheric impurities over the last 100 years or so. However, respective long-term reconstructions (i.e., substantially exceeding the instrumental era) from low-accumulation sites remain comparatively sparse. Accordingly, deciphering Alpine ice cores as long-term climate records deserves special emphasis. Certain conditions must exist for Alpine ice cores to serve as climate archives, and this is important in particular regarding the challenges and achievements that have significance for ice cores from other mountain areas: (a) a reliable chronology is the fundamental prerequisite for interpreting any ice core proxy time series. Advances in radiometric ice dating and annual layer counting offer the tools to crucially increase dating precision in the preinstrumental era. (b) Glacier flow effects and spatio-seasonal snow deposition variability challenge linking the ice core proxy signals to the respective atmospheric variability (e.g., of temperature, mineral dust, and impurity concentrations). Here, assistance comes from combining multiple ice cores from one site and from complementary meteorological, glaciological, and geophysical surveys. (c) As Alpine ice cores continue to advance their contribution to Holocene climate science, exploring the link to instrumental, historical, and other natural climate archives gains increasing importance.
Julia B. Corbett and Brett Clark
The communication strategy of simply sharing more scientific information has not effectively engaged and connected people to climate change in ways that facilitate understanding and encourage action. In part, this is because climate change is a so-called wicked problem, given that it is socially complex, has many interdependencies, and lacks simple solutions. For many people, climate change is generally seen as something abstract and distant—something that they know about, but do not “feel.” The arts and humanities can play an important role in disrupting the social and cultural worldviews that filter climate information and separate the public from the reality of climate change. Whether it is the visual arts, dance, theater, literature, comedy, or film, the arts and humanities present engaging stories, corporally sensed and felt experiences, awareness of interdependency with the world, emotional meanings, and connection with place. Climate stories, especially those based on lived experiences, offer distinct ways to engage a variety of senses. They allow the “invisibility” of climate change to be seen, felt, and imagined in the past, present, and future. They connect global issues to conditions close to home and create space to grieve and experience loss. They encourage critical reflection of existing social structures and cultural and moral norms, thus facilitating engagement beyond the individual level. The arts and humanities hold great potential to help spur necessary social and cultural change, but research is needed on their reach and efficacy.
Stefano Tibaldi and Franco Molteni
The atmospheric circulation in the mid-latitudes of both hemispheres is usually dominated by westerly winds and by planetary-scale and shorter-scale synoptic waves, moving mostly from west to east. A remarkable and frequent exception to this “usual” behavior is atmospheric blocking. Blocking occurs when the usual zonal flow is hindered by the establishment of a large-amplitude, quasi-stationary, high-pressure meridional circulation structure which “blocks” the flow of the westerlies and the progression of the atmospheric waves and disturbances embedded in them. Such blocking structures can have lifetimes varying from a few days to several weeks in the most extreme cases. Their presence can strongly affect the weather of large portions of the mid-latitudes, leading to the establishment of anomalous meteorological conditions. These can take the form of strong precipitation episodes or persistent anticyclonic regimes, leading in turn to floods, extreme cold spells, heat waves, or short-lived droughts. Even air quality can be strongly influenced by the establishment of atmospheric blocking, with episodes of high concentrations of low-level ozone in summer and of particulate matter and other air pollutants in winter, particularly in highly populated urban areas.
Atmospheric blocking has the tendency to occur more often in winter and in certain longitudinal quadrants, notably the Euro-Atlantic and the Pacific sectors of the Northern Hemisphere. In the Southern Hemisphere, blocking episodes are generally less frequent, and the longitudinal localization is less pronounced than in the Northern Hemisphere.
Blocking has aroused the interest of atmospheric scientists since the middle of the last century, with the pioneering observational works of Berggren, Bolin, Rossby, and Rex, and has become the subject of innumerable observational and theoretical studies. The purpose of such studies was originally to find a commonly accepted structural and phenomenological definition of atmospheric blocking. The investigations went on to study blocking climatology in terms of the geographical distribution of its frequency of occurrence and the associated seasonal and inter-annual variability. Well into the second half of the 20th century, a large number of theoretical dynamic works on blocking formation and maintenance started appearing in the literature. Such theoretical studies explored a wide range of possible dynamic mechanisms, including large-amplitude planetary-scale wave dynamics, including Rossby wave breaking, multiple equilibria circulation regimes, large-scale forcing of anticyclones by synoptic-scale eddies, finite-amplitude non-linear instability theory, and influence of sea surface temperature anomalies, to name but a few. However, to date no unique theoretical model of atmospheric blocking has been formulated that can account for all of its observational characteristics.
When numerical, global short- and medium-range weather predictions started being produced operationally, and with the establishment, in the late 1970s and early 1980s, of the European Centre for Medium-Range Weather Forecasts, it quickly became of relevance to assess the capability of numerical models to predict blocking with the correct space-time characteristics (e.g., location, time of onset, life span, and decay). Early studies showed that models had difficulties in correctly representing blocking as well as in connection with their large systematic (mean) errors.
Despite enormous improvements in the ability of numerical models to represent atmospheric dynamics, blocking remains a challenge for global weather prediction and climate simulation models. Such modeling deficiencies have negative consequences not only for our ability to represent the observed climate but also for the possibility of producing high-quality seasonal-to-decadal predictions. For such predictions, representing the correct space-time statistics of blocking occurrence is, especially for certain geographical areas, extremely important.
Guoxiong Wu, Anmin Duan, and Yimin Liu
With an average elevation of 4 kilometers, a combined area of more than 2.5 million square kilometers, and a variety of complicated landscapes, the Tibetan Plateau (TP) constitutes the highest and largest terrain on earth. The Tibetan and Iranian Plateau (TIP) form a dynamically coupled system that exerts a tremendous impact on the regional and global climate.
The TIP’s geographic location in the subtropics of central and eastern Asia, along with its altitude, size, and steep terrain on the southern and eastern slopes, make this climate impact particularly unique. In winter, the TIP reacts to the impinging subtropical westerly flow, producing a strong negative mountain torque and forming a prominent stationary circulation dipole with a huge anticyclone circulation to its north and cyclone circulation to its south in the tropics. A specific winter climate pattern over Asia is thus formed.
Due to its high elevation, the total mass of the air column over the TP is much smaller than over the neighboring regions, as the solar radiation heating in this region is more efficient. The atmospheric heating source (AHS) over the TP is negative in winter and strongly positive in summer. On this elevated terrain there is also a large number of intersecting isentropic surfaces in the lower troposphere. Along its sloping surfaces, the cooling in winter causes the near-surface air to slide downward and diverge toward its surroundings, whereas the surface heating of the slope in summer results in near-surface air ascent, causing the surrounding air to converge toward the plateau. More significantly, due to its huge size, the surface-sensible heating of the TIP produces a large-scale surface cyclonic circulation and works as an immense sensible-heat-driven air pump (SHAP), which transports abundant water vapor from ocean to land to support the Asian continental monsoon. In addition, the plateau’s heating produces a subtropical monsoonal meridional circulation and creates a large-scale air ascent background in subtropical Asia. Therefore, the Asian monsoon is the consequence of the seasonal change not only in land-sea thermal contrast but also in the thermal forcing of large-scale mountains.
Since the 1980s, the near surface atmospheric warming amplitude over the TP has grown much larger than the global mean, and the changes in climate and AHS over the TP have already influenced the water resources, ecosystem services, mountain hazards, and livelihoods across and around the TP. Understanding the climate effect of the TP’s AHS is not only a key issue for climate dynamics, but can also help us to recognize the thermal forcing of other large-scale topographies, such as the Rockies and Andes Mountains, on the global climate in the framework of land-air-sea interaction.
This article will introduce the effect of the TP’s AHS on the regional climate, with emphasis on the East Asian summer monsoon (EASM) and South Asian summer monsoon (SASM), in terms of the climatology, intra-seasonal oscillation, interannual variability, and decadal change. Controversies, challenges, and future perspectives on this topic will also be presented. Its informative content can be used as a professional reference for research scientists and professionals in the fields of meteorology, climate dynamics, environmental science, geography and geology, hydrology, and paleo-climatology. Most material presented here can also be helpful for non-specialists.
Donald W. Hine, Wendy J. Phillips, Aaron B. Driver, and Mark Morrison
Scientists and policy makers face significant challenges when attempting to engage the public about climate change. An important first step is to understand the number and nature of the audiences one plans to target—a process known as audience segmentation. Segmentation involves identifying, within an audience or target population, homogenous subgroups that share similar demographic and/or psychographic profiles. After segmenting an audience, climate change communicators can target their messages based on the unique characteristics of each subgroup. For example, to stimulate engagement and behavior change, messages aimed at audiences that are skeptical about climate change may require different content and framing than messages aimed at audiences already deeply concerned about climate change.
The notion of matching message content to audience characteristics has a long history, dating back to the Ancient Greeks. More recently, audience segmentation has played a central role in targeted advertising and also social marketing, which uses marketing principles to help “sell” ideas and behaviors that benefit society. Applications to climate change communication are becoming more common, with major segmentation and communication initiatives being implemented across the globe.
Messages crafted to meet the needs of specific audience segments are more likely to be read, understood, and recalled than generic ones, and are also more likely to change behavior. However, despite these successes, the approach has not been uniformly embraced. Controversies have emerged related to the cost effectiveness of segmentation strategies, choice of segmentation variables, potential effects related to social stigmatization, whether segmentation encourages shallow (as opposed to deep) change, the extent to which segments are “found” as opposed to socially constructed by researchers, and whether interindividual differences are best conceptualized in terms of categories or dimensions.
Ralf Weisse and Birgit Hünicke
A multitude of geophysical processes contribute to and determine variations and changes in the height of the Baltic Sea water surface. These processes act on a broad range of characteristic spatial and timescales ranging from a few seconds to millennia. On very long timescales, the northern parts of the Baltic are uplifting due to the still ongoing visco-elastic response of the Earth to the last deglaciation, and mean sea level is decreasing in these regions. Over centuries, the Baltic Sea responds to changes in global and North Atlantic mean sea level. Processes affecting global mean sea level, such as warming of the world ocean or melting of glaciers and of polar ice sheets, do have an imprint on Baltic Sea levels. Over decades, variations and changes in atmospheric circulation affect transport through the Danish Straits connecting the Baltic and North seas. As a result, the amount of water in the Baltic Sea and the height of the sea level vary. Similarly, atmospheric variability on shorter timescales down to a few days cause shorter period variations of transport through the Danish Straits and Baltic Sea level. On even shorter timescales, the Danish Straits act as a low pass filter, and high frequency variations of the water surface within the Baltic Sea such as storm surges, wind waves, or seiches are solely caused internally. All such processes have undergone considerable variations and changes in the past. Similarly, they are expected to show variations and changes in the future and across a broad range of scales, leaving their imprint on observed and potential future Baltic Sea level and its variability.
Julie Doyle, Nathan Farrell, and Michael K. Goodman
Since the mid-2000s, entertainment celebrities have played increasingly prominent roles in the cultural politics of climate change, ranging from high-profile speeches at UN climate conferences, and social media interactions with their fans, to producing and appearing in documentaries about climate change that help give meaning to and communicate this issue to a wider audience. The role afforded to celebrities as climate change communicators is an outcome of a political environment increasingly influenced by public relations and attuned toward the media’s representation of political ideas, policies, and sentiments. Celebrities act as representatives of mass publics, operating within centers of elite political power. At the same time, celebrities represent the environmental concerns of their audiences; that is, they embody the sentiments of their audiences on the political stage. It is in this context that celebrities have gained their authority as political, social, and environmental “experts,” and the political performances of celebrities provide important ways to engage electorates and audiences with climate change action.
More recently, celebrities offer novel engagements with climate change that move beyond scientific data and facilitate more emotional and visceral connections with climate change in the public’s everyday lives. Contemporary celebrities, thus, work to shape how audiences and publics ought to feel about climate change in efforts to get them to act or change their behaviors. These “after data” moments are seen very clearly in Leonardo DiCaprio’s documentary Before the Flood. Yet, with celebrities acting as our emotional witnesses, they not only might bring climate change to greater public attention, but they expand their brand through neoliberalism’s penchant for the commoditization of everything including, as here, care and concern for the environment. As celebrities build up their own personal capital as eco-warriors, they create very real value for the “celebrity industrial complex” that lies behind their climate media interventions. Climate change activism is, through climate celebrities, rendered as spectacle, with celebrities acting as environmental and climate pedagogues framing for audiences the emotionalized problems and solutions to global environmental change. Consequently, celebrities politicize emotions in ways that that remain circumscribed by neoliberal solutions and actions that responsibilize audiences and the public.
In equatorial East Africa, glaciers still exist on Mount Kenya, Kilimanjaro, and Ruwenzori. The decreasing ice extent has been documented by field reports since the end of the 19th century and a series of mappings. For Mount Kenya, the mappings are of 1947, 1963, 1987, 1993, and 2004, with more detailed mappings of Lewis Glacier in 1934, 1958, 1963, 1974, 1978, 1982, 1985, 1986, 1990, and 1993. For Kilimanjaro, the sequence is 1912, 1953, 1976, 1989, and 2000. For Ruwenzori (for which information is more scarce), the information is from 1906, 1955, and 1990. Photographs are valuable complementary evidence. At Lewis Glacier on Mount Kenya, measurements of mass budget and ice flow have been conducted over decades. The climatic forcing of ice recession in East Africa at the onset in the 1880s was radiationally controlled, affecting the most exposed locations. Later warming caused further ice shrinkage, except on the summit plateau of Kilimanjaro, above the freezing level. Whereas the ice recession in the Ecuadorian Andes and New Guinea began in the middle of the 19th century, plausibly caused by warming, the late onset in East Africa should be appreciated in the context of large-scale circulation changes evidenced by the historical ship observations in the equatorial Indian Ocean.
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.
Water, not temperature, governs life in West Africa, and the region is both temporally and spatially greatly affected by rainfall variability. Recent rainfall anomalies, for example, have greatly reduced crop productivity in the Sahel area. Rainfall indices from recent centuries show that multidecadal droughts reoccur and, furthermore, that interannual rainfall variations are high in West Africa. Current knowledge of historical rainfall patterns is, however, fairly limited. A detailed rainfall chronology of West Africa is currently only available from the beginning of the 19th century. For the 18th century and earlier, the records are still sporadic, and an interannual rainfall chronology has so far only been obtained for parts of the Guinea Coast. Thus, there is a need to extend the rainfall record to fully understand past precipitation changes in West Africa.
The main challenge when investigating historical rainfall variability in West Africa is the scarcity of detailed and continuous data. Readily available meteorological data barely covers the last century, whereas in Europe and the United States for example, the data sometimes extend back two or more centuries. Data availability strongly correlates with the historical development of West Africa. The strong oral traditions that prevailed in the pre-literate societies meant that only some of the region’s history was recorded in writing before the arrival of the Europeans in the 16th century. From the 19th century onwards, there are, therefore, three types of documents available, and they are closely linked to the colonization of West Africa. These are: official records started by the colonial governments continuing to modern day; regular reporting stations started by the colonial powers; and finally, temporary nongovernmental observations of various kinds. For earlier periods, the researcher depends on noninstrumental observations found in letters, reports, or travel journals made by European slave traders, adventurers, and explorers. Spatially, these documents are confined to the coastal areas, as Europeans seldom ventured inland before the mid-1800s. Thus, the inland regions are generally poorly represented. Arabic chronicles from the Sahel provide the only source of information, but as historical documents, they include several spatiotemporal uncertainties. Climate researchers often complement historical data with proxy-data from nature’s own archives. However, the West African environment is restrictive. Reliable proxy-data, such as tree-rings, cannot be exploited effectively. Tropical trees have different growth patterns than trees in temperate regions and do not generate growth rings in the same manner. Sediment cores from Lake Bosumtwi in Ghana have provided, so far, the best centennial overview when it comes to understanding precipitation patterns during recent centuries. These reveal that there have been considerable changes in historical rainfall patterns—West Africa may have been even drier than it is today.
Benjamin F. Zaitchik
Humans have understood the importance of climate to human health since ancient times. In some cases, the connections appear to be obvious: a flood can cause drownings, a drought can lead to crop failure and hunger, and temperature extremes pose a risk of exposure. In other cases, the connections are veiled by complex or unobserved processes, such that the influence of climate on a disease epidemic or a conflict can be difficult to diagnose. In reality, however, all climate impacts on health are mediated by some combination of natural and human dynamics that cause individuals or populations to be vulnerable to the effects of a variable or changing climate.
Understanding and managing negative health impacts of climate is a global challenge. The challenge is greater in regions with high poverty and weak institutions, however, and Africa is a continent where the health burden of climate is particularly acute. Observed climate variability in the modern era has been associated with widespread food insecurity, significant epidemics of infectious disease, and loss of life and livelihoods to climate extremes. Anthropogenic climate change is a further stress that has the potential to increase malnutrition, alter the distribution of diseases, and bring more frequent hydrological and temperature extremes to many regions across the continent.
Skillful early warning systems and informed climate change adaptation strategies have the potential to enhance resilience to short-term climate variability and to buffer against negative impacts of climate change. But effective warnings and projections require both scientific and institutional capacity to address complex processes that are mediated by physical, ecological, and societal systems. Here the state of understanding climate impacts on health in Africa is summarized through a selective review that focuses on food security, infectious disease, and extreme events. The potential to apply scientific understanding to early warning and climate change projection is also considered.
Jürgen Scheffran, Peter Michael Link, and Janpeter Schilling
Climate change was conceived as a “risk multiplier” that could exacerbate security risks and conflicts in fragile regions and hotspots where poverty, violence, injustice, and social insecurity are prevalent. The linkages have been most extensively studied for the African continent, which is affected by both climate change and violent conflict. Together with other drivers, climate change can undermine human security and livelihoods of vulnerable communities in Africa through different pathways. These include variability in temperature and precipitation; weather extremes and natural disasters, such as floods and droughts; resource problems through water scarcity, land degradation, and food insecurity; forced migration and farmer–herder conflict; and infrastructure for transport, water, and energy supply. Through these channels, climate change may contribute to humanitarian crises and conflict, subject to local conditions for the different regions of Africa. While a number of statistical studies find no significant link between reduced precipitation and violent conflict in Africa, several studies do detect such a link, mostly in interaction with other issues. The effects of climate change on resource conflicts are often indirect, complex, and linked to political, economic, and social conflict factors, including social inequalities, low economic development, and ineffective institutions.
Regions dependent on rainfed agriculture are more sensitive to civil conflict following droughts. Rising food prices can contribute to food insecurity and violence. Water scarcity and competition in river basins are partly associated with low-level conflicts, depending on socioeconomic variables and management practices. Another conflict factor in sub-Saharan Africa are shifting migration routes of herders who need grazing land to avoid livestock losses, while farmers depend on land for growing their harvest. Empirical findings reach no consensus on how climate vulnerability and violence interact with environmental migration, which also could be seen as an adaptation measure strengthening community resilience. Countries with a low human development index (HDI) are particularly vulnerable to the double exposure to natural disasters and armed conflict. Road and water infrastructures influence the social and political consequences of climate stress. The high vulnerabilities and low adaptive capacities of many African countries may increase the probability of violent conflicts related to climate change impacts.
Climate and simulation have become interwoven concepts during the past decades because, on the one hand, climate scientists shouldn’t experiment with real climate and, on the other hand, societies want to know how climate will change in the next decades. Both in-silico experiments for a better understanding of climatic processes as well as forecasts of possible futures can be achieved only by using climate models. The article investigates possibilities and problems of model-mediated knowledge for science and societies. It explores historically how climate became a subject of science and of simulation, what kind of infrastructure is required to apply models and simulations properly, and how model-mediated knowledge can be evaluated. In addition to an overview of the diversity and variety of models in climate science, the article focuses on quasiheuristic climate models, with an emphasis on atmospheric models.