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Oceanic mixing is one of the major determinants of the ocean circulation and its climatological influences. Existing distributions of mixing properties determine the rates of storage and redistribution within the climate system of fundamental scalar tracers including heat, fresh water, oxygen, carbon, and others. Observations have overturned earlier concepts that mixing rates might be approximately uniform throughout the ocean volume, with profound implications for determining the circulation and its properties. Inferences about past and potential future oceanic circulations and the resulting climate influence require determination of changed energy inputs and the expected consequent adjustment of mixing processes and their influence.
Communication campaigns play a key role in shaping what people think, feel, and do about climate change, and help shape public agendas at the local, national, and international levels. As more people around the world gain regular access to the Internet, online and social media are becoming significant contexts in which they come into contact with—or fail to come into contact with—news, debates, action, and social input related to climate change. This makes it important to understand the campaigning that takes place online. Many actors make concerted efforts to engage publics on climate change and go online to do so. These include businesses; governments and international organizations; scientists and scientific institutions; organizations, groups and individuals in civil society; public intellectuals and political, religious and entertainment leaders. Not all are ultimately concerned with climate change or engaging publics as such. Nevertheless, most campaigns involve at least one of four goals: to inform, raise awareness, and shape public understanding about the science, problems, and politics of climate change; to change consumer and citizen behavior; to network and connect concerned publics; to visibly mobilize consumers or citizens to put pressure on decision-makers. Online climate change campaigns are an emerging phenomenon and field of study. The campaigns appeared on broad front around the turn of the millennium, and have since become increasingly complex. In addition to the elements that produce variance in offline campaigns, scholars examine the role of online and social media in how campaigners render the issues and pursue their campaigns, how publics respond, and what this means for the development of the broader public discourse. Core debates concern the capacity and impact of online campaigning in the areas of informing, activating and including publics, and the ambivalences inherent in leveraging technology to engage publics on climate change.
Participation by citizens and stakeholder groups is an important aspect of climate governance at the regional, national, international, and global levels. Increasing awareness of anthropogenic causes of climate change has fueled calls for democratic action and renewal that promise to enrich both existing and emerging forms of political engagement. Participation is not a panacea, however, and has many limitations. Three substantial critiques of participatory and deliberative approaches to climate change hinge on questions of power, authority, and opportunities for dissent. The climate system itself poses unique challenges to democratic governance. Accelerating rates of environmental change associated with climate change make past experience less applicable to current situations and complicate predicting the future even further. As such, participatory and deliberative approaches may need to be reconfigured to respond adequately to the challenges of climate change. Systems approaches broaden the scope of participation and deliberation, and innovative participatory methods are increasingly moving beyond narrow framings of climate change. As deliberative and participatory initiatives become more common, it is no longer a question of supporting or rejecting participatory forms of climate governance. Rather, questions need to address what kinds of consequences will occur and in whose interests certain participatory processes operate. Which social views and values are supported and which are marginalized, and what are the consequences of collective responses to this pressing environmental and social issue?
Deborah Lynn Guber
Despite an accumulation of scientific evidence on both the causes and consequences of climate change, U.S. public opinion on the subject has splintered sharply along party lines. While a vast majority of Democrats now believe that global warming is real, that its effects will happen within their lifetime, and that human activity is the dominant cause, Republicans have grown increasingly skeptical, creating a yawning gap that complicates efforts to communicate the urgency of the problem and the need for aggressive action.
When attitudes harden and diverge, it is often driven by the behavior of political elites, who shape the frames and mental models that people use to interpret events. Scholars have long observed that people resort instinctively to heuristics to ease the burden of making decisions, especially on issues like climate, where there is an obvious disconnect between scientific understanding and mass competence. Those cues, however, are often unreliable and prone to cognitive bias. When voters act upon signals provided by their preferred political party and by selective exposure to preferred media outlets, they may do so mechanically, with little regard for the accuracy of the evidence that they receive, or they may ignore and distort information in a way that reinforces preexisting assumptions.
In the end, beliefs about climate change are as complex as the issue itself, which suggests that awareness of the problem and an understanding of its effects will not translate automatically—or even easily—into increased concern, issue salience, or policy preferences. The “pictures in our heads,” to borrow Walter Lippmann’s famous phrase, are shaped less by factual knowledge than by a variety of other factors more difficult to control—by personal experience and assorted real-world cues (such as the weather), but also by opinion leaders, media narratives, and political rhetoric, each of which provides a competing frame of reference with the power to filter and mislead. Because climate change has become so heavily laden with values and so absorbed into partisan identity, it will be nearly impossible to build social consensus through conventional means. Once a “hard” issue for all, which seemed to demand sophisticated calculation or technical expertise, it has now become an “easy” one for many, where the reactions that it prompts are familiar, stable, and symbolic, increasingly polarized, immune to rational argument, and vulnerable to manipulation by elites.
Rachel I. McDonald
There is mounting scientific evidence linking extreme weather events such as wildfires in the western United States and Hurricane Sandy with anthropogenic climate change. However, those in developed nations that contribute the most to carbon emissions associated with anthropogenic climate change are the least likely to suffer severe consequences. This asymmetry presents a challenge for climate change communication, given that those who most need to act on climate change (the largest emitters) are those likely to be the most removed from the impacts of carbon emissions, and may thus be less convinced of the critical need to act.
The psychological distance that people perceive from the impacts of climate change may also have implications for their belief in, concern about, and willingness to act on climate change. Psychological distance is the extent to which an object is perceived as distant from the self in time, space, certainty, or social similarity. Though climate change may be perceived by those in developed nations as distant on any or all of these dimensions, temporal and geographic distance are likely to be particularly important in the context of climate change, given the global nature of the problem and the long time horizons associated with predicted impacts.
Considering the potential distancing of climate change from the self, this review examines how perceptions of temporal and geographic distance impact public opinion about climate change, in terms of belief in anthropogenic climate change, concern, and support for action. Many researchers have suggested that the distal nature of climate change is a key reason for failures to engage in widespread mitigation action. However, studies of temporal and geographic distance reveal mixed effects on belief in climate change and support for action. For example, support for climate change action varies as a function of impacts being described as affecting near versus distant victims, and these effects vary as a function of political ideology, with Democrats more likely to support action when exposed to distal victims, and Republicans more supportive when victims are closer to them. Similarly, another study indicated that perceptions of global risk are associated with policy support, whereas perceptions of risk in one’s local area is linked to individual intentions to take action. These findings reveal that it may not always be ideal to perceive climate change as psychologically close in order to promote support for climate action, and a number of studies examined here explore when psychological distance and closeness may help or hinder climate change action.
Christopher P. Borick and Barry G. Rabe
The factors that determine individual perceptions of climate change have been a focus of social science research for many years. An array of studies have found that individual-level characteristics, such as partisan affiliation, ideological beliefs, educational attainment, and race, affect one’s views on the existence of global warming, as well as the levels of concern regarding this matter. But in addition to the individual-level attributes that have been shown to affect perceptions of climate change, a growing body of literature has found that individual experiences with weather can shape a variety of views and beliefs that individuals maintain regarding climate change. These studies indicate that direct experiences with extreme weather events and abnormal seasonal temperature and precipitation levels can affect the likelihood that an individual will perceive global warming to be occurring, and in some cases their policy preferences for addressing the problem. The emerging literature on this relationship indicates that individuals are more likely to express skepticism regarding the existence of global warming when experiencing below average temperatures or above average snowfall in the period preceding an interview on their views. Conversely, higher temperatures and various extreme weather events can elevate acceptance of global warming’s existence.
A number of studies also find that individuals are more likely to report weather conditions such as drought and extreme heat affected their acceptance of global warming when such conditions were occurring in their region. For example, the severe drought that has encompassed much of the western United States between 2005 and 2016 has increasingly been cited by residents of the region as the primary reason for their belief that climate change is occurring. What remains unclear at this point is whether the weather conditions are actually changing opinions regarding climate change or if the preexisting opinions are causing individuals to see the weather events in a manner consistent with those opinions.
Notably, the relationship between weather experiences and beliefs regarding climate change appear to be multidirectional in nature. Numerous studies have found that not only do weather experiences shape the views of individuals regarding global warming, but also individuals’ views on the existence of global warming can affect their perceptions of the weather that they have experienced. In particular, recent research has shown that individuals who are skeptical about the existence of global warming are less likely to report the weather recorded in their area accurately than individuals who believe global warming is happening.
The reconstruction of climate in Poland in the past millennium, as measured by several kinds of proxy data, is more complete than that of many other regions in Europe and the world. In fact, the methods of climate reconstruction used here are commonly utilized for other regions. Proxy data available for Poland (whether by documentary, biological, or geothermal evidence) mainly allow for reconstructions of three meteorological variables: air temperature, ground-surface temperature, and precipitation. It must be underlined however, that air temperature reconstructions are possible only for certain times of the year. This is particularly characteristic of biological proxies (e.g., tree rings measure January–April temperature, chironomids provide data for August temperature, chrysophyte cysts identify cold seasons, etc.). Potentially, such limitation has no corresponding documentary evidence. In Poland these data are available only for climate reconstructions covering mainly the last 500 years because the number of historical sources pre-1500 is usually too small. Geothermal data allow for reconstruction of mean annual ground surface temperature generally for the last 500 years. Reconstructions of air temperature that cover the entire, or almost the entire, millennium and have high time resolution are only available from biological proxies (tree rings, chironomids, diatoms, etc.).
At present, the best source of information about climate in Poland in the last millennium is still documentary evidence. This evidence defines a Medieval Warm Period (MWP), which was present in the 11th century and probably ended in the 14th or early 15th century. Air temperature in the MWP was probably about 0.5–1.0°C warmer than contemporary conditions on average, and the climate was characterized by the greatest degree of oceanity throughout the entire millennium. A Little Ice Age (LIA) can be also distinguished in Poland’s climate history. Data show that it clearly began around the mid-16th century and probably ended in the second half of the 19th century. In this LIA, winters were 1.5–3.0°C colder than present conditions, while summers tended to be warmer by about 0.5°C. As a result, the continentality of the climate in the LIA was the greatest for the entire millennium. Mean annual air temperature was probably lower than the modern temperature by about 0.9–1.5°C. The average rise of air temperature since the mid-19th century, which is often called the Contemporary Warming Period (CWP), is equal to about 1°C and is in line with the results of reconstructions using geothermal and dendrochronological methods. The reconstruction of precipitation in Poland is much more uncertain than the reconstruction of air temperature. There was probably considerably higher average precipitation in the 12th century (and particularly in the second half of this century), in the first half of the 16th century, and also in the first half of the 18th century. The second half of the 13th century and the first half of the 19th century were drier than average. In other periods, precipitation conditions were close to average, including for the entire CWP period.
Emily K. Vraga
Political participation on the issue of climate change can encompass many different forms of individual and collective actions designed to affect governmental policies. At the most basic level, issue-specific political participation occurs when individuals directly attempt to influence governmental actors or policies on climate change—most notably by voting, but also through donating money and communicating with public officials. These types of participation tend to be relatively rare, limited to a small subset of deeply committed individuals. In contrast, personal action on climate change is more widely dispersed, especially if one includes impact-oriented actions (e.g., actions that influence the environment but are primarily undertaken for other reasons, like convenience or saving money) rather than purely intention-based actions, which occur when individuals adopt behaviors with the goal of addressing climate change. Additionally, opportunities to engage in expressive participation, largely online, create new spaces for individuals to build networks to engage in political action, as well as potentially to reach unengaged groups that are less likely to seek out information on the issue.
A number of forces can contribute to whether an individual chooses to participate on the issue of climate change. Individual characteristics, like perceptions of impersonal and personal risks associated with climate change, knowledge of the issues, and environmental values all tend to produce people more likely to participate—especially when these attitudes become part of an individual’s identity as an opinion leader or activist. As a global issue, social norms play a particularly powerful role; when individuals believe others support and are likely to take action themselves, it tends to foster a sense of efficacy that such behaviors will be effective in producing change. Individual choices about media sources also intersect with media coverage and framing of the issue to influence perceptions of the issue and likelihood of taking action. Such media framing can exacerbate or mitigate the heightened political polarization on the issue of climate change that has erected barriers to effective political action in many democratic societies in recent years, most notably in the United States. New forms of political participation may create opportunities to encourage more participation on the issue of climate change, but they also raise ethical questions about inequality and participatory divides that privilege some groups over others.
Climate change specifically and the environment more generally are becoming increasingly central features in much of contemporary persuasive messages. From World Wildlife Fund public service announcements showing the Earth as a melting scoop of ice cream to advertisements for environmentally friendly hybrid cars set against backdrops of lush, green fields, climate change and the environment are closely linked to strategic communication and consumer behavior. This growing focus on the connection between climate change and consumption represents a wide and varied field of study, underscoring the ways in which the two can at once be symbiotic and yet also antagonistic.
Meaningful academic attention to environmental cues in advertising can be thought of as occurring in two waves. In the first wave, peaking in the 1990s, research was concerned primarily with content analyses of advertising containing environmental appeals. Questions about deceptive environmental claims, often referred to as greenwashing, were a primary concern during this phase. Climate change specifically was not a central element, and instead, issues of environmental preservation and conservation dominated. In the second wave, which emerged in the late 2000s and continues unabated, researchers have broadened their focus to examine not only how the environment was depicted in advertising messages but also how audiences understood them. Attention was paid to message factors, like framing, source cues, and visual depictions, as well as individual-level factors, such as environmental concern, political ideology and regulatory focus.
While concerns about greenwashing and deceptive advertising continue to plague green advertising, a collection of new critiques has emerged, including questions about the implications of emphasizing consumer behavior as a source of climate change mitigation, of relying on nature as a commodity to be sold and used, and of engaging individuals as consumers rather than as citizens in attempts to effect environmental change.
Post-glacial aquatic ecosystems in Eurasia and North America, such as the Baltic Sea, evolved in the freshwater, brackish, and marine environments that fringed the melting glaciers. Warming of the climate initiated sea level and land rise and subsequent changes in aquatic ecosystems. Seminal ideas on ancient developing ecosystems were based on findings in Swedish large lakes of species that had arrived there from adjacent glacial freshwater or marine environments and established populations which have survived up to the present day. An ecosystem of the first freshwater stage, the Baltic Ice Lake initially consisted of ice-associated biota. Subsequent aquatic environments, the Yoldia Sea, the Ancylus Lake, the Litorina Sea, and the Mya Sea, are all named after mollusc trace fossils. These often convey information on the geologic period in question and indicate some physical and chemical characteristics of their environment. The ecosystems of various Baltic Sea stages are regulated primarily by temperature and freshwater runoff (which affects directly and indirectly both salinity and nutrient concentrations). Key ecological environmental factors, such as temperature, salinity, and nutrient levels, not only change seasonally but are also subject to long-term changes (due to astronomical factors) and shorter disturbances, for example, a warm period that essentially formed the Yoldia Sea, and more recently the “Little Ice Age” (which terminated the Viking settlement in Iceland).
There is no direct way to study the post-Holocene Baltic Sea stages, but findings in geological samples of ecological keystone species (which may form a physical environment for other species to dwell in and/or largely determine the function of an ecosystem) can indicate ancient large-scale ecosystem features and changes. Such changes have included, for example, development of an initially turbid glacial meltwater to clearer water with increasing primary production (enhanced also by warmer temperatures), eventually leading to self-shading and other consequences of anthropogenic eutrophication (nutrient-rich conditions). Furthermore, the development in the last century from oligotrophic (nutrient-poor) to eutrophic conditions also included shifts between the grazing chain (which include large predators, e.g., piscivorous fish, mammals, and birds at the top of the food chain) and the microbial loop (filtering top predators such as jellyfish). Another large-scale change has been a succession from low (freshwater glacier lake) biodiversity to increased (brackish and marine) biodiversity. The present-day Baltic Sea ecosystem is a direct descendant of the more marine Litorina Sea, which marks the beginning of the transition from a primeval ecosystem to one regulated by humans. The recent Baltic Sea is characterized by high concentrations of pollutants and nutrients, a shift from perennial to annual macrophytes (and more rapid nutrient cycling), and an increasing rate of invasion by non-native species. Thus, an increasing pace of anthropogenic ecological change has been a prominent trend in the Baltic Sea ecosystem since the Ancylus Lake.
Future development is in the first place dependent on regional factors, such as salinity, which is regulated by sea and land level changes and the climate, and runoff, which controls both salinity and the leaching of nutrients to the sea. However, uncertainties abound, for example the future development of the Gulf Stream and its associated westerly winds, which support the sub-boreal ecosystems, both terrestrial and aquatic, in the Baltic Sea area. Thus, extensive sophisticated, cross-disciplinary modeling is needed to foresee whether the Baltic Sea will develop toward a freshwater or marine ecosystem, set in a sub-boreal, boreal, or arctic climate.
Florian Sévellec and Bablu Sinha
The Atlantic meridional overturning circulation (AMOC) is a large, basin-scale circulation located in the Atlantic Ocean that transports climatically important quantities of heat northward. It can be described schematically as a northward flow in the warm upper ocean and a southward return flow at depth in much colder water. The heat capacity of a layer of 2 m of seawater is equivalent to that of the entire atmosphere; therefore, ocean heat content dominates Earth’s energy storage. For this reason and because of the AMOC’s typically slow decadal variations, the AMOC regulates North Atlantic climate and contributes to the relatively mild climate of Europe. Hence, predicting AMOC variations is crucial for predicting climate variations in regions bordering the North Atlantic. Similar to weather predictions, climate predictions are based on numerical simulations of the climate system. However, providing accurate predictions on such long timescales is far from straightforward. Even in a perfect model approach, where biases between numerical models and reality are ignored, the chaotic nature of AMOC variability (i.e., high sensitivity to initial conditions) is a significant source of uncertainty, limiting its accurate prediction.
Predictability studies focus on factors determining our ability to predict the AMOC rather than actual predictions. To this end, processes affecting AMOC predictability can be separated into two categories: processes acting as a source of predictability (periodic harmonic oscillations, for instance) and processes acting as a source of uncertainty (small errors that grow and significantly modify the outcome of numerical simulations). To understand the former category, harmonic modes of variability or precursors of AMOC variations are identified. On the other hand, in a perfect model approach, the sources of uncertainty are characterized by the spread of numerical simulations differentiated by the application of small differences to their initial conditions. Two alternative and complementary frameworks have arisen to investigate this spread. The pragmatic framework corresponds to performing an ensemble of simulations, by imposing a randomly chosen small error on the initial conditions of individual simulations. This allows a probabilistic approach and to statistically characterize the importance of the initial condition by evaluating the spread of the ensemble. The theoretical framework uses stability analysis to identify small perturbations to the initial conditions, which are conducive to significant disruption of the AMOC.
Beyond these difficulties in assessing the predictability, decadal prediction systems have been developed and tested through a range of hindcasts. The inherent difficulties of operational forecasts span from developing efficient initialization methods to setting accurate radiative forcing to correcting for model drift and bias, all these improvements being estimated and validated through a range of specifically designed skill metrics.
Wansuo Duan and Mu Mu
This article retrospects the studies of the predictability of El Niño-Southern Oscillation (ENSO) events within the framework of error growth dynamics and reviews the results of previous studies. It mainly covers (a) the advances in methods for studying ENSO predictability, especially those of optimal methods associated with initial errors and model errors; and (b) the applications of these optimal methods in the studies of “spring predictability barrier” (SPB), optimal precursors for ENSO events (or the source of ENSO predictability) and target observations for ENSO predictions. In this context, some of major frontiers and challenges remaining in ENSO predictability are addressed.
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 tropical Indian Ocean is unique in several aspects. Unlike the Pacific and the Atlantic Oceans, the Indian Ocean is bounded to the north by a large landmass, the Eurasian continent. The large thermal heat contrast between the ocean in the south and the land in the north induces the world’s strongest monsoon systems in South and East Asia, in response to the seasonal migration of solar radiation. The strong and seasonally reversing surface winds generate large seasonal variations in ocean currents and basin-wide meridional heat transport across the equator. In contrast to the tropical Pacific and the Atlantic, where easterly trade winds prevail throughout the year, westerly winds (albeit with a relatively weak magnitude) blow along the equatorial Indian Ocean, particularly during the boreal spring and autumn seasons, generating the semi-annual Yoshida-Wyrtki eastward equatorial ocean currents. As a consequence of the lack of equatorial upwelling, the tropical Indian Ocean occupies the largest portion of the warm water pool (with Sea Surface Temperature [SST] being greater than 28 °C) on Earth. The massive warm water provides a huge potential energy available for deep convections that significantly affect the weather-climate over the globe. It is therefore of vital importance to discover and understand climate variabilities in the Indian Ocean and to further develop a capability to correctly predict the seasonal departures of the warm waters and their global teleconnections.
The Indian Ocean Dipole (IOD) is the one of the recently discovered climate variables in the tropical Indian Ocean. During the development of the super El Niño in 1997, the climatological zonal SST gradient along the equator was much reduced (with strong cold SST anomalies in the east and warm anomalies in the west). The surface westerly winds switched to easterlies, and the ocean thermocline became shallow in the east and deep in the west. These features are reminiscent of what are observed during El Niño years in the Pacific, representing a typical coupled process between the ocean and the atmosphere. The IOD event in 1997 contributed significantly to floods in eastern Africa and severe droughts and bushfires in Indonesia and southeastern Australia. Since the discovery of the 1997 IOD event, extensive efforts have been made to lead the rapid progress in understanding the air-sea coupled climate variabilities in the Indian Ocean; and many approaches, including simple statistical models and comprehensive ocean-atmosphere coupled models, have been developed to simulate and predict the Indian Ocean climate.
Essential to the discussion are the ocean-atmosphere dynamics underpinning the seasonal predictability of the IOD, critical factors that limit the IOD predictability (inter-comparison with El Niño-Southern Oscillation [ENSO]), observations and initialization approaches that provide realistic initial conditions for IOD predictions, models and approaches that have been developed to simulate and predict the IOD, the influence of global warming on the IOD predictability, impacts of IOD-ENSO interactions on the IOD predictability, and the current status and perspectives of the IOD prediction at seasonal to multi-annual timescales.
H.E. Markus Meier and Sofia Saraiva
In this article, the concepts and background of regional climate modeling of the future Baltic Sea are summarized and state-of-the-art projections, climate change impact studies, and challenges are discussed. The focus is on projected oceanographic changes in future climate. However, as these changes may have a significant impact on biogeochemical cycling, nutrient load scenario simulations in future climates are briefly discussed as well. The Baltic Sea is special compared to other coastal seas as it is a tideless, semi-enclosed sea with large freshwater and nutrient supply from a partly heavily populated catchment area and a long response time of about 30 years, and as it is, in the early 21st century, warming faster than any other coastal sea in the world. Hence, policymakers request the development of nutrient load abatement strategies in future climate. For this purpose, large ensembles of coupled climate–environmental scenario simulations based upon high-resolution circulation models were developed to estimate changes in water temperature, salinity, sea-ice cover, sea level, oxygen, nutrient, and phytoplankton concentrations, and water transparency, together with uncertainty ranges. Uncertainties in scenario simulations of the Baltic Sea are considerable. Sources of uncertainties are global and regional climate model biases, natural variability, and unknown greenhouse gas emission and nutrient load scenarios. Unknown early 21st-century and future bioavailable nutrient loads from land and atmosphere and the experimental setup of the dynamical downscaling technique are perhaps the largest sources of uncertainties for marine biogeochemistry projections. The high uncertainties might potentially be reducible through investments in new multi-model ensemble simulations that are built on better experimental setups, improved models, and more plausible nutrient loads. The development of community models for the Baltic Sea region with improved performance and common coordinated experiments of scenario simulations is recommended.
Ole Bøssing Christensen and Erik Kjellström
The ecosystems and the societies of the Baltic Sea region are quite sensitive to fluctuations in climate, and therefore it is expected that anthropogenic climate change will affect the region considerably. With numerical climate models, a large amount of projections of meteorological variables affected by anthropogenic climate change have been performed in the Baltic Sea region for periods reaching the end of this century.
Existing global and regional climate model studies suggest that:
• The future Baltic climate will get warmer, mostly so in winter. Changes increase with time or increasing emissions of greenhouse gases. There is a large spread between different models, but they all project warming. In the northern part of the region, temperature change will be higher than the global average warming.
• Daily minimum temperatures will increase more than average temperature, particularly in winter.
• Future average precipitation amounts will be larger than today. The relative increase is largest in winter. In summer, increases in the far north and decreases in the south are seen in most simulations. In the intermediate region, the sign of change is uncertain.
• Precipitation extremes are expected to increase, though with a higher degree of uncertainty in magnitude compared to projected changes in temperature extremes.
• Future changes in wind speed are highly dependent on changes in the large-scale circulation simulated by global climate models (GCMs). The results do not all agree, and it is not possible to assess whether there will be a general increase or decrease in wind speed in the future.
• Only very small high-altitude mountain areas in a few simulations are projected to experience a reduction in winter snow amount of less than 50%. The southern half of the Baltic Sea region is projected to experience significant reductions in snow amount, with median reductions of around 75%.
Emily H. Ho, David V. Budescu, and Han Hui Por
The overwhelming majority of the scientific community agrees that climate change (CC) is occurring and is caused by anthropogenic, or human-caused, forcing. The global populace is aware of this phenomenon but appears to be unconcerned about CC and is slow to adopt potential mitigative actions. CC is a unique and complex phenomenon affected by various kinds of uncertainty, rendering communicative efforts particularly challenging. The compound and, potentially, conflicting uncertainties inherent in CC engender public ambivalence about the issue. The treatment of uncertainty in the Intergovernmental Panel on Climate Change’s (IPCC’s) reports have been shown to be confusing to policymakers and the general public, further confounding public outreach efforts. Given diverse communication styles and the multifaceted nature of CC, an assortment of strategies has been recommended to maximize understanding and increase salience. In particular, using evidence-based approaches to communicate about probabilistic outcomes in CC increases communicative efficiency.
Nathaniel Geiger, Brianna Middlewood, and Janet Swim
Given the severity of the threat posed by climate change, why is large-scale societal action to decarbonize our energy systems not more widespread? The present article examines four categories of psychological barriers to accurate risk perceptions and engagement with this topic by the public. First, psychological barriers such as (a) not personally experiencing the threat, (b) not hearing people talk about climate change, (c) being limited by cultural narratives, and (d) not understanding how climate change works can lead to misperception of the threat posed by climate change. Second, individuals may lack knowledge or perceived ability about how to address the threat. Third, social barriers such as social norms not to act and socio-structural barriers can discourage climate change engagement. Finally, worldviews such as neoliberal ideology and conspiratorial worldviews can conflict with climate change engagement.
Jaime Gilden and Ellen Peters
It is a widely accepted scientific fact that our climate is changing and that this change is caused by human activity. Despite the scientific consensus, many individuals in the United States fail to grasp the extent of the consensus and continue to deny both the existence and cause of climate change; the proportion of the population holding these beliefs has been stable in recent history. Most of the American public also believe they know a lot about climate change although knowledge tests do not always reflect their positive perceptions. There are two frequent hypotheses about public knowledge and climate change beliefs: (a) providing the public with more climate science information, thus making them more knowledgeable, will bring the beliefs of the public closer to those of climate scientists and (b) individuals with greater cognitive ability (e.g., scientific literacy or numeracy) will have climate change beliefs more like those of experts. However, data do not always support this proposed link between knowledge, ability, and beliefs. A better predictor of beliefs in the United States is political identity. For example, compared to liberals, conservatives consistently perceive less risk from climate change and, perhaps as a result, are less likely to hold scientifically accurate climate change beliefs, regardless of their cognitive abilities. And greater knowledge and ability, rather than being related to more accurate climate change beliefs, tend to relate to increased polarization across political identities, such that the difference in beliefs between conservatives and liberals with high cognitive ability is greater than the difference in beliefs between conservatives and liberals with low cognitive ability.
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.
Thomas C. Johnson
The people of East Africa are particularly vulnerable to the whims of their regional climate. A rapidly growing population depends heavily on rain-fed agriculture, and when the rains deviate from normal, creating severe drought or flooding, the toll can be devastating in terms of starvation, disease, and political instability. Humanity depends upon climate models to ascertain how the climate will change in the coming decades, in response to anthropogenic forcing, to better comprehend what lies in store for East African society, and how they might best cope with the circumstances. These climate models are tested for their accuracy by comparing their output of past climate conditions against what we know of how the climate has evolved. East African climate has undergone dramatic change, as indicated by lake shorelines exposed several tens of meters above present lake levels, by seismic reflection profiles in lake basins displaying submerged and buried nearshore sedimentary sequences, and by the fossil and chemical records preserved in lake sediments, which indicate dramatic past change in lake water chemistry and biota, both within the lakes and in their catchments, in response to shifting patterns of rainfall and temperature. This history, on timescales from decades to millennia, and the mechanisms that account for the observed past climate variation, are summarized in this article. The focus of this article is on paleoclimate data and not on climate models, which are discussed thoroughly in an accompanying article in this volume. Very briefly, regional climate variability over the past few centuries has been attributed to shifting patterns of sea surface temperature in the Indian Ocean. The Last Glacial Maximum (LGM) was an arid period throughout most of East Africa, with the exception of the coastal terrain), and the region did not experience much wetter conditions until around 15,000 years ago (15 ka). A brief return to drier times occurred during the Younger Dryas (YD) (12.9–11.7 ka), and then a wet African Humid Period until about 5 ka, after which the region, at least north of Lake Malawi at ~10º S latitude, became relatively dry again. The penultimate ice age was much drier than the LGM, and such megadroughts occurred several times over the previous 1.3 million years. While the African continent north of the equator experienced, on average, progressively drier conditions over the past few million years, unusually wet periods occurred around 2.7–2.5, 1.9–1.7, and 1.1–0.7 million years ago. By contrast, the Lake Malawi basin at ~10º—14º S latitude has undergone a trend of progressively wetter conditions superimposed on a glacial–dry, interglacial–wet cycle since the Mid-Pleistocene Transition at ~900 ka.