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.
Sharon E. Nicholson
Classic paradigms describing meteorological phenomena and climate have changed dramatically over the last half-century. This is particularly true for the continent of Africa. Our understanding of its climate is today very different from that which prevailed as recently as the 1960s or 1970s. This article traces the development of relevant paradigms in five broad areas: climate and climate classification, tropical atmospheric circulation, tropical rain-bearing systems, climatic variability and change, and land surface processes and climate. One example is the definition of climate. Originally viewed as simple statistical averages, it is now recognized as an environmental variable with global linkages, multiple timescales of variability, and strong controls via earth surface processes. As a result of numerous field experiments, our understanding of tropical rainfall has morphed from the belief in the domination by local thunderstorms to recognition of vast systems on regional to global scales. Our understanding of the interrelationships with land surface processes has also changed markedly. The simple Charney hypothesis concerning albedo change and the related concept of desertification have given way to a broader view of land–atmosphere interaction. In summary, there has been a major evolution in the way we understand climate, climatic variability, tropical rainfall regimes and rain-bearing systems, and potential human impacts on African climate. Each of these areas has evolved in complexity and understanding, a result of an explosive growth in research and the availability of such investigative tools as satellites, computers, and numerical models.
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.
For several decades, the Sahelian countries have been facing continuing rainfall shortages, which, coupled with anthropogenic factors, have severely disrupted the great ecological balance, leading the area in an inexorable process of desertification and land degradation. The Sahel faces a persistent problem of climate change with high rainfall variability and frequent droughts, and this is one of the major drivers of population’s vulnerability in the region. Communities struggle against severe land degradation processes and live in an unprecedented loss of productivity that hampers their livelihoods and puts them among the populations in the world that are the most vulnerable to climatic change. In response to severe land degradation, 11 countries of the Sahel agreed to work together to address the policy, investment, and institutional barriers to establishing a land-restoration program that addresses climate change and land degradation. The program is called the Pan-Africa Initiative for the Great Green Wall (GGW). The initiative aims at helping to halt desertification and land degradation in the Sahelian zone, improving the lives and livelihoods of smallholder farmers and pastoralists in the area and helping its populations to develop effective adaptation strategies and responses through the use of tree-based development programs. To make the GGW initiative successful, member countries have established a coordinated and integrated effort from the government level to local scales and engaged with many stakeholders. Planning, decision-making, and actions on the ground is guided by participation and engagement, informed by policy-relevant knowledge to address the set of scalable land-restoration practices, and address drivers of land use change in various human-environmental contexts. In many countries, activities specific to achieving the GGW objectives have been initiated in the last five years.
The Sahel of Africa has been identified as having the strongest land–atmosphere (L/A) interactions on Earth. The Sahelian L/A interaction studies started in the late 1970s. However, due to controversies surrounding the early studies, in which only a single land parameter was considered in L/A interactions, the credibility of land-surface effects on the Sahel’s climate has long been challenged. Using general circulation models and regional climate models coupled with biogeophysical and dynamic vegetation models as well as applying analyses of satellite-derived data, field measurements, and assimilation data, the effects of land-surface processes on West African monsoon variability, which dominates the Sahel climate system at intraseasonal, seasonal, interannual, and decadal scales, as well as mesoscale, have been extensively investigated to realistically explore the Sahel L/A interaction: its effects and the mechanisms involved.
The Sahel suffered the longest and most severe drought on the planet in the 20th century. The devastating environmental and socioeconomic consequences resulting from drought-induced famines in the Sahel have provided strong motivation for the scientific community and society to understand the causes of the drought and its impact. It was controversial and under debate whether the drought was a natural process, mainly induced by sea-surface temperature variability, or was affected by anthropogenic activities. Diagnostic and modeling studies of the sea-surface temperature have consistently demonstrated it exerts great influence on the Sahel climate system, but sea-surface temperature is unable to explain the full scope of the Sahel climate variability and the later 20th century’s drought. The effect of land-surface processes, especially land-cover and land-use change, on the drought have also been extensively investigated. The results with more realistic land-surface models suggest land processes are a first-order contributor to the Sahel climate and to its drought during the later 1960s to the 1980s, comparable to sea surface temperature effects. The issues that caused controversies in the early studies have been properly addressed in the studies with state-of-the-art models and available data.
The mechanisms through which land processes affect the atmosphere are also elucidated in a number of studies. Land-surface processes not only affect vertical transfer of radiative fluxes and heat fluxes but also affect horizontal advections through their effect on the atmospheric heating rate and moisture flux convergence/divergence as well as horizontal temperature gradients.
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.
D. B. Tindall, Mark C.J. Stoddart, and Candis Callison
This article considers the relationship between news media and the sociopolitical dimensions of climate change. Media can be seen as sites where various actors contend with one another for visibility, for power, and for the opportunity to communicate, as well as where they promote their policy preferences. In the context of climate change, actors include politicians, social movement representatives, scientists, business leaders, and celebrities—to name a few.
The general public obtain much of their information about climate change and other environmental issues from the media, either directly or indirectly through sources like social media. Media have their own internal logic, and getting one’s message into the media is not straightforward. A variety of factors influence what gets into the media, including media practices, and research shows that media matter in influencing public opinion.
A variety of media practices affect reporting on climate change─one example is the journalistic norm of balance, which directs that actors on both sides of a controversy be given relatively equal attention by media outlets. In the context of global warming and climate change, in the United States, this norm has led to the distortion of the public’s understanding of these processes. Researchers have found that, in the scientific literature, there is a very strong consensus among scientists that human-caused (anthropogenic) climate change is happening. Yet media in the United States often portray the issue as a heated debate between two equal sides.
Subscription to, and readership of, print newspapers have declined among the general public; nevertheless, particular newspapers continue to be important. Despite the decline of traditional media, politicians, academics, NGO leaders, business leaders, policymakers, and other opinion leaders continue to consume the media. Furthermore, articles from particular outlets have significant readership via new media access points, such as Facebook and Twitter.
An important concept in the communication literature is the notion of framing. “Frames” are the interpretive schemas individuals use to perceive, identify, and label events in the world. Social movements have been important actors in discourse about climate change policy and in mobilizing the public to pressure governments to act. Social movements play a particularly important role in framing issues and in influencing public opinion. In the United States, the climate change denial countermovement, which has strong links to conservative think tanks, has been particularly influential. This countermovement is much more influential in the United States than in other countries. The power of the movement has been a barrier to the federal government taking significant policy action on climate change in the United States and has had consequences for international agreements and processes.
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.
Judith L. Lean
Emergent in recent decades are robust specifications and understanding of connections between the Sun’s changing radiative energy and Earth’s changing climate and atmosphere. This follows more than a century of contentious debate about the reality of such connections, fueled by ambiguous observations, dubious correlations, and lack of plausible mechanisms. It derives from a new generation of observations of the Sun and the Earth made from space, and a new generation of physical climate models that integrate the Earth’s surface and ocean with the extended overlying atmosphere. Space-based observations now cover more than three decades and enable statistical attribution of climate change related to the Sun’s 11-year activity cycle on global scales, simultaneously with other natural and anthropogenic influences. Physical models that fully resolve the stratosphere and its embedded ozone layer better replicate the complex and subtle processes that couple the Sun and Earth.
An increase of ~0.1% in the Sun’s total irradiance, as observed near peak activity during recent 11-year solar cycles, is associated with an increase of ~0.1oC in Earth’s global surface temperature, with additional complex, time-dependent regional responses. The overlying atmosphere warms more, by 0.3oC near 20 km. Because solar radiation impinges primarily at low latitudes, the increased radiant energy alters equator-to-pole thermal gradients, initiating dynamical responses that produce regions of both warming and cooling at mid to high latitudes. Because solar energy deposition depends on altitude as a result of height-dependent atmospheric absorption, changing solar radiation establishes vertical thermal gradients that further alter dynamical motions within the Earth system.
It remains uncertain whether there are long-term changes in solar irradiance on multidecadal time scales other than due to the varying amplitude of the 11-year cycle. If so the magnitude of the additional change is expected to be comparable to that observed during the solar activity cycle. Were the Sun’s activity to become anomalously low, declining during the next century to levels of the Maunder Minimum (from 1645 to 1715), the expected global surface temperature cooling is less than a few tenths oC. In contrast, a scenario of moderate greenhouse gas increase with climate forcing of 2.6 W m−2 over the next century is expected to warm the globe 1.5 to 1.9oC, an order of magnitude more than the hypothesized solar-induced cooling over the same period.
Future challenges include the following: securing sufficiently robust observations of the Sun and Earth to elucidate changes on climatological time scales; advancing physical climate models to simulate realistic responses to changing solar radiation on decadal time scales, synergistically at the Earth’s surface and in the ocean and atmosphere; disentangling the Sun’s influence from that of other natural and anthropogenic influences as the climate and atmosphere evolve; projecting past and future changes in the Sun and Earth’s climate and atmosphere; and communicating new understanding across scientific disciplines, and to political and societal stakeholders.
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.
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.