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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.
Toby Bolsen and Matthew A. Shapiro
The importance of framing as a concept is reflected by the massive amount of attention it has received from scholars across disciplines. As a communicative process, framing involves making certain considerations salient as a way to simplify or shape the way in which an audience understands a particular problem and its potential solutions. As recently as the early 2000s, social scientists began to examine how strategic frames in a communication affect both individuals’ beliefs about climate change and the actions they are willing to support to mitigate the likely effects. Research on the effects of how strategic frames influence the attitudes, beliefs, and preferences of individuals in this domain primarily builds on insights from framing theory, which explains that an individual’s attitude or preference in any given context depends on the available, accessible, and most applicable (i.e., perceived strongest) considerations. But it is much more than theory: frames related to the effects and potential solutions for climate change have been employed strategically by various actors in an effort to shape public opinion and public policy.
Perceptions of scientific consensus on climate change are thought to play an important role in determining support for policy actions. Consequently, strategic actors promote a particular agenda by accentuating the inherent uncertainty of climate science, thus casting doubt on the scientific consensus. This has contributed to partisan polarization on climate change and the rise of protective forms of information processing and reasoning in this domain. Strategic messages and frames that resonate with particular subgroups have no effect, or may even backfire, on other segments of the population. Additionally, as individuals who possess different partisan identities become more knowledgeable and numerate, they become increasingly likely to accept information and messages that bolster their existing group loyalties and to reject communications that challenge those identities. Science communicators are thus presented with a considerable barrier to building consensus among the public for action on climate change. In response, scholars have begun to identify strategies and approaches for addressing audiences with the kinds of messages that are most likely to resonate with individuals possessing a diverse range of values and political identities. Further research must identify ways to overcome partisan motivated reasoning on climate change and the persistent and deleterious effects that have resulted from the politicization of climate science.
Forecasting severe convective weather remains one of the most challenging tasks facing operational meteorology today, especially in the mid-latitudes, where severe convective storms occur most frequently and with the greatest impact. The forecast difficulties reflect, in part, the many different atmospheric processes of which severe thunderstorms are a by-product. These processes occur over a wide range of spatial and temporal scales, some of which are poorly understood and/or are inadequately sampled by observational networks. Therefore, anticipating the development and evolution of severe thunderstorms will likely remain an integral part of national and local forecasting efforts well into the future.
Modern severe weather forecasting began in the 1940s, primarily employing the pattern recognition approach throughout the 1950s and 1960s. Substantial changes in forecast approaches did not come until much later, however, beginning in the 1980s. By the start of the new millennium, significant advances in the understanding of the physical mechanisms responsible for severe weather enabled forecasts of greater spatial and temporal detail. At the same time, technological advances made available model thermodynamic and wind profiles that supported probabilistic forecasts of severe weather threats.
This article provides an updated overview of operational severe local storm forecasting, with emphasis on present-day understanding of the mesoscale processes responsible for severe convective storms, and the application of recent technological developments that have revolutionized some aspects of severe weather forecasting. The presentation, nevertheless, notes that increased understanding and enhanced computer sophistication are not a substitute for careful diagnosis of the current meteorological environment and an ingredients-based approach to anticipating changes in that environment; these techniques remain foundational to successful forecasts of tornadoes, large hail, damaging wind, and flash flooding.
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.
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.
Mental models are the sets of causal beliefs we “run” in our minds to infer what will happen in a given event or situation. Mental models, like other models, are useful simplifications most of the time. They can, however, lead to mistaken or misleading inferences, for example, if the analogies that inform them are misleading in some regard. The coherence and consistency of mental models a person employs to solve a given problem are a function of that person’s expertise. The less familiar and central a problem is, the less coherent and consistent the mental models brought to bear on that problem are likely to be. For problems such as those posed by anthropogenic climate change, most people are likely to recruit multiple mental models to make judgments and decisions.
Common types of mental models of climate change and global warming include: (a) a carbon emissions model, in which global warming is a result of burning fossil fuels thereby emitting CO2, and of deforestation, which both releases sequestered CO2 and decreases the possible sinks that might take CO2 out of the atmosphere; (b) a stratospheric ozone depletion mental model, which conflates stratospheric ozone depletion with global warming; (c) an air pollution mental model, in which global warming is viewed as air pollution; and (d) a weather change model, in which weather and climate are conflated. As social discourse around global warming and climate change has increased, mental models of climate change have become more complex, although not always more coherent. One such complexity is the belief that climate changes according to natural cycles and due to factors beyond human control, in addition to changes resulting from human activities such as burning fossil fuels and releasing other greenhouse gases.
As our inference engines, mental models play a central role in problem solving and subjective projections and are hence at the heart of risk perceptions and risk decision-making. However, both perceiving and making decisions about climate change and the risks thereof are affective and social processes foremost.
Jill E. Hopke and Luis E. Hestres
Divestment is a socially responsible investing tactic to remove assets from a sector or industry based on moral objections to its business practices. It has historical roots in the anti-apartheid movement in South Africa. The early-21st-century fossil fuel divestment movement began with climate activist and 350.org co-founder Bill McKibben’s Rolling Stone article, “Global Warming’s Terrifying New Math.” McKibben’s argument centers on three numbers. The first is 2°C, the international target for limiting global warming that was agreed upon at the United Nations Framework Convention on Climate Change 2009 Copenhagen conference of parties (COP). The second is 565 Gigatons, the estimated upper limit of carbon dioxide that the world population can put into the atmosphere and reasonably expect to stay below 2°C. The third number is 2,795 Gigatons, which is the amount of proven fossil fuel reserves. That the amount of proven reserves is five times that which is allowable within the 2°C limit forms the basis for calls to divest.
The aggregation of individual divestment campaigns constitutes a movement with shared goals. Divestment can also function as “tactic” to indirectly apply pressure to targets of a movement, such as in the case of the movement to stop the Dakota Access Pipeline in the United States. Since 2012, the fossil fuel divestment movement has been gaining traction, first in the United States and United Kingdom, with student-led organizing focused on pressuring universities to divest endowment assets on moral grounds.
In partnership with 350.org, The Guardian launched its Keep it in the Ground campaign in March 2015 at the behest of outgoing editor-in-chief Alan Rusbridger. Within its first year, the digital campaign garnered support from more than a quarter-million online petitioners and won a “campaign of the year” award in the Press Gazette’s British Journalism Awards. Since the launch of The Guardian’s campaign, “keep it in the ground” has become a dominant frame used by fossil fuel divestment activists.
Divestment campaigns seek to stigmatize the fossil fuel industry. The rationale for divestment rests on the idea that fossil fuel companies are financially valued based on their resource reserves and will not be able to extract these reserves with a 2°C or lower climate target. Thus, their valuation will be reduced and the financial holdings become “stranded assets.” Critics of divestment have cited the costs and risks to institutional endowments that divestment would entail, arguing that to divest would go against their fiduciary responsibility. Critics have also argued that divesting from fossil fuel assets would have little or no impact on the industry. Some higher education institutions, including Princeton and Harvard, have objected to divestment as a politicization of their endowments. Divestment advocates have responded to this concern by pointing out that not divesting is not a politically neutral act—it is, in fact, choosing the side of fossil fuel corporations.
John A. Alic
Stabilizing atmospheric greenhouse gases will require very large reductions in energy-related carbon dioxide emissions. This can be achieved only through continuous innovation, aggressive and ongoing. Fast-paced innovation, in turn, depends on rapid and widespread diffusion, adoption, adaptation—in short, on technological learning. These processes are integrally linked, as virtuous circles, through feedback loops embedded in economic markets. The overall dynamics are fundamentally incremental.
Pundits and policymakers, nonetheless, sometimes seem to hope that “breakthroughs” will emerge to sweep existing energy technologies aside. Such hopes are misplaced, for two reasons. If breakthroughs are construed as something “new under the sun,” they are rare and unpredictable, and policymakers have few tools to foster them. Energy technologies, after all, have been intensively explored over the past two centuries: the physical constraints are well understood and there are few reasons to expect research to lead to anything fundamentally new. Infant technologies, second, tend to perform poorly, and to be quite costly. Improvements come over time though technological learning. Inputs to this sort of learning range from field service experience to “just-in-time” research. Economic competition provides much of the driving force.
The dynamics just sketched are broadly representative of the evolutionary paths traced by past energy technologies—wind and steam power, gas turbines, nuclear power, and solar photovoltaic (PV) cells and systems. Similar paths will be followed if prospective innovations such as carbon capture and storage, small nuclear reactors, or schemes for tapping the energy of the world’s oceans begin to mature and diffuse. Over the next several decades, the world should expect to work with existing technologies in various stages of maturation that can and will—because this is inherent in the process of innovation—advance on technical measures of performance (e.g., energy conversion efficiency) and come down in costs (in most cases) through continuous improvement.
This sort of innovation is first and foremost the work of profit-seeking businesses, enterprises that conceive, develop, introduce, and market new technologies. These firms exploit publically funded R&D; just as important historically, government procurements have created initial markets, including the first PV cells and also the gas turbines that many utilities now buy for electric power generation, the early versions of which were based on designs for military aircraft. A major task for energy-climate policy is to create similarly viable market segments in which new and emerging technologies can gain a foothold, as a number of governments have done for battery-electric vehicles. Direct and indirect subsidies—financial preferences as provided in some countries for battery-electric vehicles, and market set-asides, as for biofuels in Europe, Brazil, and the United States—insulate firms from potential competition, creating opportunities to push forward technologically, overcoming early handicaps, such as high costs and poor performance, associated with emerging technologies. The implication: Effective innovation policies must provide powerful incentives for profit-seeking businesses. This is true worldwide, although mechanisms will differ from country to country.
Art Dewulf, Daan Boezeman, and Martinus Vink
Climate change communication in the Netherlands started in the 1950s, but it was not until the late 1970s that the issue earned a place on the public agenda, as an aspect of the energy problem, and in the shadow of controversy about nuclear energy. Driven largely by scientific reports and political initiatives, the first climate change wave can be observed in the period from 1987 to 1989, as part of a broader environmental consciousness wave. The Netherlands took an active role in international climate change initiatives at the time but struggled to achieve domestic emission reductions throughout the 1990s. The political turmoil in the early 2000s dominated Dutch public debate, until An Inconvenient Truth triggered the second climate change wave in 2006–2007, generating peak media attention and broad societal activity. The combination of COP15 and Climategate in late 2009 marked a turning point in Dutch climate change communication, with online communication and climate-sceptic voices gaining much more prominence. Climate change mitigation was pushed down on the societal and political agenda in the 2010s. Climate change adaptation had received much attention during the second climate change wave and had been firmly institutionalized with respect to flood defense and other water management issues. By 2015 a landmark climate change court case and the Paris Agreement at COP21 were fueling climate change communication once again.
An orbitally induced increase in summer insolation during the last glacial-interglacial transition enhanced the thermal contrast between land and sea, with land masses heating up compared to the adjacent ocean surface. In North Africa, warmer land surfaces created a low-pressure zone, driving the northward penetration of monsoonal rains originating from the Atlantic Ocean. As a consequence, regions today among the driest of the world were covered by permanent and deep freshwater lakes, some of them being exceptionally large, such as the “Mega” Lake Chad, which covered some 400 000 square kilometers. A dense network of rivers developed.
What were the consequences of this climate change on plant distribution and biodiversity? Pollen grains that accumulated over time in lake sediments are useful tools to reconstruct past vegetation assemblages since they are extremely resistant to decay and are produced in great quantities. In addition, their morphological character allows the determination of most plant families and genera.
In response to the postglacial humidity increase, tropical taxa that survived as strongly reduced populations during the last glacial period spread widely, shifting latitudes or elevations, expanding population size, or both. In the Saharan desert, pollen of tropical trees (e.g., Celtis) were found in sites located at up to 25°N in southern Libya. In the Equatorial mountains, trees (e.g., Olea and Podocarpus) migrated to higher elevations to form the present-day Afro-montane forests. Patterns of migration were individualistic, with the entire range of some taxa displaced to higher latitudes or shifted from one elevation belt to another. New combinations of climate/environmental conditions allowed the cooccurrences of taxa growing today in separate regions. Such migrational processes and species-overlapping ranges led to a tremendous increase in biodiversity, particularly in the Saharan desert, where more humid-adapted taxa expanded along water courses, lakes, and wetlands, whereas xerophytic populations persisted in drier areas.
At the end of the Holocene era, some 2,500 to 4,500 years ago, the majority of sites in tropical Africa recorded a shift to drier conditions, with many lakes and wetlands drying out. The vegetation response to this shift was the overall disruption of the forests and the wide expansion of open landscapes (wooded grasslands, grasslands, and steppes). This environmental crisis created favorable conditions for further plant exploitation and cereal cultivation in the Congo Basin.