Masahiro Sugiyama, Atsushi Ishii, Shinichiro Asayama, and Takanobu Kosugi
Climate engineering, a set of techniques proposed to intervene directly in the climate system to reduce risks from climate change, presents many novel governance challenges. Solar radiation management (SRM), particularly the use of stratospheric aerosol injection (SAI), is one of the most discussed proposals. It has been attracting more and more interest, and its pertinence as a potential option for responding to the threats from climate change may be set to increase because of the long-term temperature goal (well below 2°C or 1.5°C) in the 2015 Paris Agreement. Initial research has demonstrated that SAI would cool the climate system and reduce climate risks in many ways, although it is mired in unknown environmental risks and various sociopolitical ramifications. The proposed techniques are in the early stage of research and development (R&D), providing a unique opportunity for upstream public engagement, long touted as a desirable pathway to more plural and inclusive governance of emergent technologies by opening up social choices in technology. Solar geoengineering governance faces various challenges. One of the most acute of these is how to situate public engagement in international governance discourse; the two topics have been studied separately. Another challenge relates to bridging the gap between the social choices at hand and assessment of the risks and benefits of SRM. Deeper integration of knowledge across disciplines and stakeholder and public inputs is a prerequisite for enabling responsible innovation for the future of our climate.
Timothy M. Shanahan
West Africa is among the most populated regions of the world, and it is predicted to continue to have one of the fastest growing populations in the first half of the 21st century. More than 35% of its GDP comes from agricultural production, and a large fraction of the population faces chronic hunger and malnutrition. Its dependence on rainfed agriculture is compounded by extreme variations in rainfall, including both droughts and floods, which appear to have become more frequent. As a result, it is considered a region highly vulnerable to future climate changes. At the same time, CMIP5 model projections for the next century show a large spread in precipitation estimates for West Africa, making it impossible to predict even the direction of future precipitation changes for this region. To improve predictions of future changes in the climate of West Africa, a better understanding of past changes, and their causes, is needed. Long climate and vegetation reconstructions, extending back to 5−8 Ma, demonstrate that changes in the climate of West Africa are paced by variations in the Earth’s orbit, and point to a direct influence of changes in low-latitude seasonal insolation on monsoon strength. However, the controls on West African precipitation reflect the influence of a complex set of forcing mechanisms, which can differ regionally in their importance, especially when insolation forcing is weak. During glacial intervals, when insolation changes are muted, millennial-scale dry events occur across North Africa in response to reorganizations of the Atlantic circulation associated with high-latitude climate changes. On centennial timescales, a similar response is evident, with cold conditions during the Little Ice Age associated with a weaker monsoon, and warm conditions during the Medieval Climate Anomaly associated with wetter conditions. Land surface properties play an important role in enhancing changes in the monsoon through positive feedback. In some cases, such as the mid-Holocene, the feedback led to abrupt changes in the monsoon, but the response is complex and spatially heterogeneous. Despite advances made in recent years, our understanding of West African monsoon variability remains limited by the dearth of continuous, high- resolution, and quantitative proxy reconstructions, particularly from terrestrial sites.
The East African Rift System (EARS) transecting the high-elevation East African plateau is one of the most outstanding rift systems on earth. Rifting was caused by a huge uprising mantle plume under East Africa. Two distinct rift branches are distinguished: an older, volcanically very active Eastern Branch and a younger, much less volcanic Western Branch. The Eastern Branch is generally characterized by high elevation, whereas the Western Branch comprises a number of deep rift lakes (e.g., Lake Tanganyika, Lake Malaŵi). These differences reflect different plate strengths, the latter of which are largely governed by differences in how the mantle plume interacted with the East African lithosphere. Much of the topography forming the East African plateau has been caused by the uprising mantle plume. The onset of topographic uplift in the EARS is poorly dated but preceded graben development, the latter of which commenced at ~24 Ma in the Ethiopian Rift, at ~12 Ma in Kenya, and at ~10 Ma in the Western Branch. Increased uplift of the East African plateau since ~15–10 Ma might be connected to climate change in East Africa and human evolution. East Africa experienced cooling starting at 15.5–12.5 Ma that heralded profound faunal changes at 8–5 Ma, when the hominin lineage split from the chimpanzee lineage. The Pliocene is characterized by warm and wet climate between 5.3 and 3.3 Ma transitioning into a period of cooler and more arid conditions after ~3 Ma. The climate in the EARS is controlled by westerly monsoonal flow over equatorial West Africa and easterly monsoonal flow over the Indian Ocean. The uplifting East African plateau intercepted those winds and contributed to the increased aridification of East Africa.
Kerry H. Cook
Accurate projections of climate change under increasing atmospheric greenhouse gas levels are needed to evaluate the environmental cost of anthropogenic emissions, and to guide mitigation efforts. These projections are nowhere more important than Africa, with its high dependence on rain-fed agriculture and, in many regions, limited resources for adaptation. Climate models provide our best method for climate prediction but there are uncertainties in projections, especially on regional space scale. In Africa, limitations of observational networks add to this uncertainty since a crucial step in improving model projections is comparisons with observations. Exceeding uncertainties associated with climate model simulation are uncertainties due to projections of future emissions of CO2 and other greenhouse gases. Humanity’s choices in emissions pathways will have profound effects on climate, especially after the mid-century.
The African Sahel is a transition zone characterized by strong meridional precipitation and temperature gradients. Over West Africa, the Sahel marks the northernmost extent of the West African monsoon system. The region’s climate is known to be sensitive to sea surface temperatures, both regional and global, as well as to land surface conditions. Increasing atmospheric greenhouse gases are already causing amplified warming over the Sahara Desert and, consequently, increased rainfall in parts of the Sahel. Climate model projections indicate that much of this increased rainfall will be delivered in the form of more intense storm systems.
The complicated and highly regional precipitation regimes of East Africa present a challenge for climate modeling. Within roughly 5º of latitude of the equator, rainfall is delivered in two seasons—the long rains in the spring, and the short rains in the fall. Regional climate model projections suggest that the long rains will weaken under greenhouse gas forcing, and the short rains season will extend farther into the winter months. Observations indicate that the long rains are already weakening.
Changes in seasonal rainfall over parts of subtropical southern Africa are observed, with repercussions and challenges for agriculture and water availability. Some elements of these observed changes are captured in model simulations of greenhouse gas-induced climate change, especially an early demise of the rainy season. The projected changes are quite regional, however, and more high-resolution study is needed. In addition, there has been very limited study of climate change in the Congo Basin and across northern Africa. Continued efforts to understand and predict climate using higher-resolution simulation must be sustained to better understand observed and projected changes in the physical processes that support African precipitation systems as well as the teleconnections that communicate remote forcings into the continent.
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.
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.
Philipp Pattberg and Oscar Widerberg
In 1992, when the international community agreed on the United Nations Framework Convention on Climate Change (UNFCCC), the science of climate change was under development, global greenhouse gas (GHG) emissions were by and large produced by developed countries, and the concentrations of CO2 in the atmosphere had just surpassed 350 ppm. Some 25 years later, climate change is scientifically uncontested, China has overtaken the United States as the world’s biggest emitter of CO2, and concentrations are now measured above 400 ppm. Against this background, states have successfully concluded a new global agreement under the UNFCCC, the 2015 Paris Agreement. Prior to the Paris Agreement, the climate regime focused on allocating emission reduction commitments among (a group of) countries. However, the new agreement has turned the climate regime on its feet by introducing an approach based on Nationally Determined Contributions (NDCs). Under this approach, states decide their ambition levels independently instead of engaging in negotiations about “who does what.” The result is a more flexible system that for the first time includes all countries in the quest to reduce GHG emissions to keep temperature increase below 2°C compared to preindustrial levels. Moreover, the international climate regime has transformed into a regime complex, denoting the broad activities of smaller groups of states as well as non-party actors, such as cities, regions, companies, and non-governmental organizations along with United Nations agencies.
Southern Africa extends from the equator to about 34°S and is essentially a narrow, peninsular land mass bordered to its south, west, and east by oceans. Its termination in the mid-ocean subtropics has important consequences for regional climate, since it allows the strongest western boundary current in the world ocean (warm Agulhas Current) to be in close proximity to an intense eastern boundary upwelling current (cold Benguela Current). Unlike other western boundary currents, the Agulhas retroflects south of the land mass and flows back into the South Indian Ocean, thereby leading to a large area of anomalously warm water south of South Africa which may influence storm development over the southern part of the land mass. Two other unique regional ocean features imprint on the climate of southern Africa—the Angola-Benguela Frontal Zone (ABFZ) and the Seychelles-Chagos thermocline ridge (SCTR). The former is important for the development of Benguela Niños and flood events over southwestern Africa, while the SCTR influences Madden-Julian Oscillation and tropical cyclone activity in the western Indian Ocean. In addition to South Atlantic and South Indian Ocean influences, there are climatic implications of the neighboring Southern Ocean.
Along with Benguela Niños, the southern African climate is strongly impacted by ENSO and to lesser extent by the Southern Annular Mode (SAM) and sea-surface temperature (SST) dipole events in the Indian and South Atlantic Oceans. The regional land–sea distribution leads to a highly variable climate on a range of scales that is still not well understood due to its complexity and its sensitivity to a number of different drivers. Strong and variable gradients in surface characteristics exist not only in the neighboring oceans but also in several aspects of the land mass, and these all influence the regional climate and its interactions with climate modes of variability.
Much of the interior of southern Africa consists of a plateau 1 to 1.5 km high and a narrow coastal belt that is particularly mountainous in South Africa, leading to sharp topographic gradients. The topography is able to influence the track and development of many weather systems, leading to marked gradients in rainfall and vegetation across southern Africa.
The presence of the large island of Madagascar, itself a region of strong topographic and rainfall gradients, has consequences for the climate of the mainland by reducing the impact of the moist trade winds on the Mozambique coast and the likelihood of tropical cyclone landfall there. It is also likely that at least some of the relativity aridity of the Limpopo region in northern South Africa/southern Zimbabwe results from the location of Madagascar in the southwestern Indian Ocean.
While leading to challenges in understanding its climate variability and change, the complex geography of southern Africa offers a very useful test bed for improving the global models used in many institutions for climate prediction. Thus, research into the relative shortcomings of the models in the southern African region may lead not only to better understanding of southern African climate but also to enhanced capability to predict climate globally.
Historic discussions of climate often suggested that it caused societies to have certain qualities. In the 19th-century, imperial representations of the world environment frequently “determined” the fate of peoples and places, a practice that has frequently been used to explain the largest patterns of political rivalry and the fates of empires and their struggles for dominance in world politics. In the 21st century, climate change has mostly reversed the causal logic in the reasoning about human–nature relationships and their geographies. The new thinking suggests that human decisions, at least those made by the rich and powerful with respect to the forms of energy that are used to power the global economy, are influencing future climate changes. Humans are now shaping the environment on a global scale, not the other way around. Despite the widespread acceptance of the 2015 Paris Agreement on climate-change action, numerous arguments about who should act and how they should do so to deal with climate change shape international negotiations. Differing viewpoints are in part a matter of geographical location and whether an economy is dependent on fossil-fuels revenue or subject to increasingly severe storms, droughts, or rising sea levels. These differences have made climate negotiations very difficult in the last couple of decades. Partly in response to these differences, the Paris Agreement devolves primary responsibility for climate policy to individual states rather than establish any other geopolitical arrangement. Apart from the outright denial that humanity is a factor in climate change, arguments about whether climate change causes conflict and how security policies should engage climate change also partly shape contemporary geopolitical agendas. Despite climate-change deniers, in the Trump administration in particular, in the aftermath of the Paris Agreement, climate change is understood increasingly as part of a planetary transformation that has been set in motion by industrial activity and the rise of a global fossil-fuel-powered economy. But this is about more than just climate change. The larger earth-system science discussion of transformation, which can be encapsulated in the use of the term “Anthropocene” for the new geological circumstances of the biosphere, is starting to shape the geopolitics of climate change just as new political actors are beginning to have an influence on climate politics.
The warming of the global climate is expected to continue in the 21st century, although the magnitude of change depends on future anthropogenic greenhouse gas emissions and the sensitivity of climate to them. The regional characteristics and impacts of future climate change in the Baltic Sea countries have been explored since at least the 1990s. Later research has supported many findings from the early studies, but advances in understanding and improved modeling tools have made the picture gradually more comprehensive and more detailed. Nevertheless, many uncertainties still remain.
In the Baltic Sea region, warming is likely to exceed its global average, particularly in winter and in the northern parts of the area. The warming will be accompanied by a general increase in winter precipitation, but in summer, precipitation may either increase or decrease, with a larger chance of drying in the southern than in the northern parts of the region. Despite the increase in winter precipitation, the amount of snow is generally expected to decrease, as a smaller fraction of the precipitation falls as snow and midwinter snowmelt episodes become more common. Changes in windiness are very uncertain, although most projections suggest a slight increase in average wind speed over the Baltic Sea. Climatic extremes are also projected to change, but some of the changes will differ from the corresponding change in mean climate. For example, the lowest winter temperatures are expected to warm even more than the winter mean temperature, and short-term summer precipitation extremes are likely to become more severe, even in the areas where the mean summer precipitation does not increase.
The projected atmospheric changes will be accompanied by an increase in Baltic Sea water temperature, reduced ice cover, and, according to most studies, reduced salinity due to increased precipitation and river runoff. The seasonal cycle of runoff will be modified by changes in precipitation and earlier snowmelt. Global-scale sea level rise also will affect the Baltic Sea, but will be counteracted by glacial isostatic adjustment. According to most projections, in the northern parts of the Baltic Sea, the latter will still dominate, leading to a continued, although decelerated, decrease in relative sea level. The changes in the physical environment and climate will have a number of environmental impacts on, for example, atmospheric chemistry, freshwater and marine biogeochemistry, ecosystems, and coastal erosion. However, future environmental change in the region will be affected by several interrelated factors. Climate change is only one of them, and in many cases its effects may be exceeded by other anthropogenic changes.