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Climate of the Free Troposphere and Mountain Peaks  

Stefan Brönnimann

The free troposphere is the location of important weather and climate processes. Here, horizontal and vertical transport of energy, mass, and momentum take place, and it holds greenhouse gases, water vapor, and clouds. The free troposphere therefore plays an important role in global climate feedback processes. Mountains provide important ecosystem services for a large lowland population. Mountain ecosystems may react particularly strongly to climatic changes. This is because mountains intersect important environmental and geoecological boundaries such as the snow line and the tree line. In a changing climate, these boundaries may shift. Climate change thus affects mountain glaciers, water resources, and mountain ecosystems. Climates of mountains and of the free troposphere have attracted scientists of the enlightenment and have been studied scientifically at least since the 18th century. High-altitude observatories were installed in the late 19th century, and upper-air measurements were started soon afterwards. However, even in the early 21st century, the climate observing systems do not well cover mountain regions and specifically mountain peaks. The temperature of the free troposphere is dominated by horizontal and vertical transport of sensible and latent heat, condensation and release of latent heat, and radiation to space. Mountain peaks sometimes reach into the free troposphere, but at the same time also share characteristics of surface climate. They are strongly influenced by radiative processes of the surrounding surface, while during the day they are often within the atmospheric boundary layer. With respect to climate change, temperature trends are amplified in the tropical upper-troposphere relative to the surface due to latent heat release, while in the Arctic the surface warms faster than the free atmosphere due to strong inversions and due to feedback processes operating at the surface. Mountain peaks may see both types of amplification. Several processes have been suggested to cause an elevation dependent warming, the most important of which arguably is the snow-albedo feedback. Elevation dependent warming is also seen in model studies and in observations, although detecting this signal in observations turns out rather difficult outside the tropics due to high variability and sometimes low-data quality. The observed climatic changes are expected to continue into the future.


Poland’s Climate in the Last Millennium  

Rajmund Przybylak

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.


Climate of the Mediterranean Region  

Ricardo García-Herrera and David Barriopedro

The Mediterranean is a semi-enclosed sea surrounded by Europe to the north, Asia to the east, and Africa to the south. It covers an area of approximately 2.5 million km2, between 30–46 °N latitude and 6 °W and 36 °E longitude. The term Mediterranean climate is applied beyond the Mediterranean region itself and has been used since the early 20th century to classify other regions of the world, such as California or South Africa, usually located in the 30º–40º latitudinal band. The Mediterranean climate can be broadly characterized by warm to hot dry summers and mild wet winters. However, this broad picture hides important variations, which can be explained through the existence of two geographical gradients: North/South, with a warmer and drier south, and West/East, more influenced by Atlantic/Asian circulation. The region is located at a crossroad between the mid-latitudes and the subtropical regimes. Thus, small changes in the Atlantic storm track may lead to dramatic changes in the precipitation of the northwestern area of the basin. The variability of the descending northern branch of the Hadley cell influences the climate of the southern margin, while the eastern border climate is conditioned by the Siberian High in winter and the Indian Summer Monsoon during summer. All these large-scale factors are modulated by the complex orography of the region, the contrasting albedo, and the moisture and heat supplied by the Mediterranean Sea. The interactions occurring among all these factors lead to a complex picture with some relevant phenomena characteristic of the Mediterranean region, such as heatwaves and droughts, Saharan dust intrusions, or specific types of cyclogenesis. Climate model projections generally agree in characterizing the region as a climate change hotspot, considering that it is one of the areas of the globe likely to suffer pronounced climate changes. Anthropogenic influences are not new, since the region is densely populated and is the home of some the oldest civilizations on Earth. This has produced multiple and continuous modifications in the land cover, with measurable impacts on climate that can be traced from the rich available documentary evidence and high-resolution natural proxies.


Quaternary Climate Variation in Eastern Africa  

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.


Climate Change Communication in Switzerland  

Heinz Bonfadelli

The contribution summarizes the topic of climate change communication in Switzerland. The development of the topic of “climate change” is described and located within the general area of environmental politics in Switzerland, based on the specifics of Switzerland as a small, federal state, and non-EU member with direct democratic political processes. Climate change communication then is analyzed based on the results of several content analyses, mostly of Swiss print media, which focus on intensity of coverage, topics, and media frames. In the last part, the perception of and attitudes towards environment and climate change are presented and compared to other countries, based on public opinion survey data.


Climate of Eastern Africa  

Pierre Camberlin

Eastern Africa, classically presented as a major dry climate anomaly region in the otherwise wet equatorial belt, is a transition zone between the monsoon domains of West Africa and the Indian Ocean. Its complex terrain, unequaled in the rest of Africa, results in a huge diversity of climatic conditions that steer a wide range of vegetation landscapes, biodiversity and human occupations. Meridional rainfall gradients dominate in the west along the Nile valley and its surroundings, where a single boreal summer peak is mostly observed. Bimodal regimes (generally peaking in April and November) prevail in the east, gradually shifting to a single austral summer peak to the south. The swift seasonal shift of the Intertropical Convergence Zone and its replacement in January–February and June–September by strong meridional, generally diverging low-level winds (e.g., the Somali Jet), account for the low rainfall. These large-scale flows interact with topography and lakes, which have their own local circulation in the form of mountain and lake breezes. This results in complex rainfall patterns, with a strong diurnal component, and a frequent asymmetry in the rainfall distribution with respect to the major relief features. Whereas highly organized rain-producing systems are uncommon, convection is partly modulated at intra-seasonal (about 30–60-day) timescales. Interannual variability shows a fair level of spatial coherence in the region, at least in July–September in the west (Ethiopia and Nile Valley) and October–December in the east along the Indian Ocean. This is associated with a strong forcing from sea-surface temperatures in the Pacific and Indian Oceans, and to a lesser extent the Atlantic Ocean. As a result, Eastern Africa shows some of the largest interannual rainfall variations in the world. Some decadal-scale variations are also found, including a drying trend of the March–May rainy season since the 1980s in the eastern part of the region. Eastern Africa overall mean temperature increased by 0.7 to 1 °C from 1973 to 2013, depending on the season. The strong, sometimes non-linear altitudinal gradients of temperature and moisture regimes, also contribute to the climate diversity of Eastern Africa.


Climate Change and Carbon Cycle Feedbacks  

Pierre Friedlingstein

Climate and carbon cycle are tightly coupled on many time scales, from the interannual to the multimillennial. Observation always shows a positive feedback between climate and the carbon cycle: elevated atmospheric CO2 leads to warming, but warming is expected to further release of carbon to the atmosphere, enhancing the atmospheric CO2 increase. Earth system models do represent these climate–carbon cycle feedbacks, always simulating a positive feedback over the 21st century; that is, climate change will lead to loss of carbon from the land and ocean reservoirs. These processes partially offset the increases in land and ocean carbon sinks caused by rising atmospheric CO2. As a result, more of the emitted anthropogenic CO2 will remain in the atmosphere. There is, however, a large uncertainty on the magnitude of this feedback. Recent studies now help to reduce this uncertainty. On short, interannual, time scales, El Niño years record larger-than-average atmospheric CO2 growth rate, with tropical land ecosystems being the main drivers. These climate–carbon cycle anomalies can be used as emerging constraint on the tropical land carbon response to future climate change. On a longer, centennial, time scale, the variability of atmospheric CO2 found in records of the last millennium can be used to constrain the overall global carbon cycle response to climate. These independent methods confirm that the climate–carbon cycle feedback is positive, but probably more consistent with the lower end of the comprehensive models range, excluding very large climate–carbon cycle feedbacks.


Countering Climate Science Denial and Communicating Scientific Consensus  

John Cook

Scientific agreement on climate change has strengthened over the past few decades, with around 97% of publishing climate scientists agreeing that human activity is causing global warming. While scientific understanding has strengthened, a small but persistent proportion of the public actively opposes the mainstream scientific position. A number of factors contribute to this rejection of scientific evidence, with political ideology playing a key role. Conservative think tanks, supported with funding from vested interests, have been and continue to be a prolific source of misinformation about climate change. A major strategy by opponents of climate mitigation policies has been to cast doubt on the level of scientific agreement on climate change, contributing to the gap between public perception of scientific agreement and the 97% expert consensus. This “consensus gap” decreases public support for mitigation policies, demonstrating that misconceptions can have significant societal consequences. While scientists need to communicate the consensus, they also need to be aware of the fact that misinformation can interfere with the communication of accurate scientific information. As a consequence, neutralizing the influence of misinformation is necessary. Two approaches to neutralize misinformation involve refuting myths after they have been received by recipients (debunking) or preemptively inoculating people before they receive misinformation (prebunking). Research indicates preemptive refutation or “prebunking” is more effective than debunking in reducing the influence of misinformation. Guidelines to practically implement responses (both preemptive and reactive) can be found in educational research, cognitive psychology, and a branch of psychological research known as inoculation theory. Synthesizing these separate lines of research yields a coherent set of recommendations for educators and communicators. Clearly communicating scientific concepts, such as the scientific consensus, is important, but scientific explanations should be coupled with inoculating explanations of how that science can be distorted.


Geographies of Climate Change Belief  

Debbie Hopkins and Ezra M. Markowitz

Despite scientific consensus on the anthropogenic causation of climate change, and ever-growing knowledge on the biophysical impacts of climate change, there is large variability in public perceptions of and belief in climate change. Public support for national and international climate policy has a strong positive association with certainty that climate change is occurring, human caused, serious, and solvable. Thus to achieve greater acceptance of national climate policy and international agreements, it is important to raise public belief in climate change and understandings of personal climate risk. Public understandings of climate change and associated risk perceptions have received significant academic attention. This research has been conducted across a range of spatial scales, with particular attention on large-scale, nationally representative surveys to gain insights into country-scale perceptions of climate change. Generalizability of nationally representative surveys allows some degree of national comparison; however, the ability to conduct such comparisons has been limited by the availability of comparative data sets. Consequently, empirical insights have been geographically biased toward Europe and North America, with less understanding of public perceptions of climate change in other geographical settings including the Global South. Moreover, a focus on quantitative surveying techniques can overlook the more nuanced, culturally determined factors that contribute to the construction of climate change perceptions. The physical and human geographies of climate change are diverse. This is due to the complex spatial dimensions of climate change and includes both the observed and anticipated geographical differentiation in risks, impacts, and vulnerabilities. While country location and national climate can impact upon how climate change is understood, so too will sociocultural factors such as national identity and culture(s). Studies have reported high variability in climate change perceptions, the result of a complex interplay between personal experiences of climate, social norms, and worldviews. Exploring the development of national-scale analyses and their findings over time, and the comparability of national data sets, may provide some insights into the factors that influence public perceptions of climate change and identify national-scale interventions and communications to raise risk perception and understanding of climate change.


The Role of the IPCC in Climate Science  

Gerald A. Meehl

The Intergovernmental Panel on Climate Change (IPCC) consists of about 190 governments that commission assessments performed by the international climate science community to determine the current state of human knowledge of climate and climate change. As such, the IPCC does not perform scientific research, but, rather, assesses research in the form of published papers addressing topics in climate science related to climate variability and change. However, as the IPCC assessments have evolved (from the first in 1990 to the sixth in 2021, so far), the IPCC has formed a symbiotic relationship with climate science. Even though the goal of the IPCC is to assess the scientific research that is taking place, its high profile, prestige, and interest from governments that fund climate science research has stimulated and arguably accelerated climate science research. This is particularly relevant for Earth system modeling (including the physical climate system plus the biogeochemical components) that will be addressed here to illustrate the influence of IPCC on climate science. One outcome is that enhanced observations of the Earth system from a number of field campaigns have been funded by countries to gather targeted observations to improve the understanding of crucial processes that need to be represented with fidelity in Earth system models. Governments that fund Earth system modeling research want to have results from their model appear prominently in the IPCC assessments to partially justify the funds being spent on developing, running, and analyzing these models. And just as important as getting a model into the IPCC assessment process are the analyses of the model outputs done by the scientists in the modeling groups and other scientists around the world. The products of this process are the papers describing cutting-edge results that use the models to advance knowledge of climate variability and change. Therefore, model developers are competing with other modeling groups around the world to have the best possible models producing climate simulations that are analyzed to produce papers of the highest quality that are assessed in the IPCC reports. An important part of this process is the international scientific coordination provided by the World Climate Research Programme’s Coupled Model Intercomparison Project (CMIP). CMIP involves climate scientists from around the world who volunteer their time to organize CMIP while also developing climate models in their respective countries. CMIP started in the mid-1990s for modeling groups to run exactly the same experiments so the response across the models could be directly compared to quantify uncertainty in their simulations of historical and future climate. Because these climate experiments are, by construction, the current state-of-the-art in climate modeling with the best representation of human understanding of the workings of the climate system, the papers that are written based on those model integrations are of primary interest for the IPCC assessments. CMIP has since evolved to include numerous climate science communities that interface with the modeling groups to perform model intercomparison projects to address various compelling climate science problems. Thus, there is a symbiosis between climate science/modeling, the scientific framework provided by CMIP for coordinated climate change experiments, and the IPCC process that assesses papers that emerge from the scientific research done by scientists who desire their work to be featured in those prestigious IPCC assessments.


Climate Change Scenarios and African Climate Change  

Kerry H. Cook

Accurate projections of climate change under increasing atmospheric greenhouse gas levels are needed to evaluate the environmental cost of anthropogenic emissions, and to guide mitigation efforts. These projections are nowhere more important than Africa, with its high dependence on rain-fed agriculture and, in many regions, limited resources for adaptation. Climate models provide our best method for climate prediction but there are uncertainties in projections, especially on regional space scale. In Africa, limitations of observational networks add to this uncertainty since a crucial step in improving model projections is comparisons with observations. Exceeding uncertainties associated with climate model simulation are uncertainties due to projections of future emissions of CO2 and other greenhouse gases. Humanity’s choices in emissions pathways will have profound effects on climate, especially after the mid-century. The African Sahel is a transition zone characterized by strong meridional precipitation and temperature gradients. Over West Africa, the Sahel marks the northernmost extent of the West African monsoon system. The region’s climate is known to be sensitive to sea surface temperatures, both regional and global, as well as to land surface conditions. Increasing atmospheric greenhouse gases are already causing amplified warming over the Sahara Desert and, consequently, increased rainfall in parts of the Sahel. Climate model projections indicate that much of this increased rainfall will be delivered in the form of more intense storm systems. The complicated and highly regional precipitation regimes of East Africa present a challenge for climate modeling. Within roughly 5º of latitude of the equator, rainfall is delivered in two seasons—the long rains in the spring, and the short rains in the fall. Regional climate model projections suggest that the long rains will weaken under greenhouse gas forcing, and the short rains season will extend farther into the winter months. Observations indicate that the long rains are already weakening. Changes in seasonal rainfall over parts of subtropical southern Africa are observed, with repercussions and challenges for agriculture and water availability. Some elements of these observed changes are captured in model simulations of greenhouse gas-induced climate change, especially an early demise of the rainy season. The projected changes are quite regional, however, and more high-resolution study is needed. In addition, there has been very limited study of climate change in the Congo Basin and across northern Africa. Continued efforts to understand and predict climate using higher-resolution simulation must be sustained to better understand observed and projected changes in the physical processes that support African precipitation systems as well as the teleconnections that communicate remote forcings into the continent.


Governance Arrangements for Adaptation to Climate Change  

Catrien Termeer, Arwin van Buuren, Art Dewulf, Dave Huitema, Heleen Mees, Sander Meijerink, and Marleen van Rijswick

Adaptation to climate change is not only a technical issue; above all, it is a matter of governance. Governance is more than government and includes the totality of interactions in which public as well as private actors participate, aiming to solve societal problems. Adaptation governance poses some specific, demanding challenges, such as the context of institutional fragmentation, as climate change involves almost all policy domains and governance levels; the persistent uncertainties about the nature and scale of risks and proposed solutions; and the need to make short-term policies based on long-term projections. Furthermore, adaptation is an emerging policy field with, at least for the time being, only weakly defined ambitions, responsibilities, procedures, routines, and solutions. Many scholars have already shown that complex problems, such as adaptation to climate change, cannot be solved in a straightforward way with actions taken by a hierarchic or monocentric form of governance. This raises the question of how to develop governance arrangements that contribute to realizing adaptation options and increasing the adaptive capacity of society. A series of seven basic elements have to be addressed in designing climate adaptation governance arrangements: the framing of the problem, the level(s) at which to act, the alignment across sectoral boundaries, the timing of the policies, the selection of policy instruments, the organization of the science-policy interface, and the most appropriate form of leadership. For each of these elements, this chapter suggests some tentative design principles. In addition to effectiveness and legitimacy, resilience is an important criterion for evaluating these arrangements. The development of governance arrangements is always context- and time-specific, and constrained by the formal and informal rules of existing institutions.


The Genesis and Evolution of European Union Framework Programmes on Climate Science  

Elisabeth Lipiatou and Anastasios Kentarchos

Although the first European Union Framework Programme (FP) for research and technological development was created in 1984, it was the second FP (FP2) in 1987 that devoted resources to climatological research for the first time. The start of FP2 coincided with the establishment of the Intergovernmental Panel on Climate Change in 1988, aimed at providing a comprehensive assessment on the state of knowledge of the science of climate change. FP-funded research was not an end in itself but a means for the European Union (EU) to achieve common objectives based on the principle of cross-border research cooperation and coordination to reduce fragmentation and effectively tackle common challenges. Since 1987, climate science has been present in all nine FPs (as of 2023) following an evolutionary process as goals, priority areas, and financial and implementation instruments have constantly changed to adapt to new needs. A research- and technology-oriented Europe was gradually created including in the area of climate science. There has historically been a strong intrinsic link between research and environmental and climate policies. Climate science under the FPs, focusing initially on oceans, the carbon cycle, and atmospheric processes, has increased tremendously both in scope and scale, encompassing a broad range of areas over time, such as climate modeling, polar research, ocean acidification, regional seas and oceans, impacts and adaptation, decarbonization pathways, socioeconomic analyses, sustainability, observations, and climate services. The creation and evolution of the EU’s FPs has played a critical role in establishing Europe’s leading position on climate science by means of promoting excellence, increasing the relevance of climate research for policymaking, and building long-lasting communities and platforms across Europe and beyond as international cooperation has been a key feature of the FPs. No other group of countries collaborates on climate science at such scale. Due to their inherited long-term planning and cross-national nature, the FPs have provided a stable framework for advancing climate science by incentivizing scientists and institutions with diverse expertise to work together, creating the necessary critical mass to tackle the increasing complex and interdisciplinary nature of climate science, rationalizing resource allocation, and setting norms and standards for scientific collaboration. It is hard to imagine in retrospect how a similar level of impact could have been achieved solely at a national level. Looking ahead and capitalizing on the rich experience and lessons learned since the 1980s, important challenges and opportunities need to be addressed. These include critical gaps in knowledge, even higher integration of disciplines, use of new technologies and artificial intelligence for state-of-the-art data analysis and modeling, capturing interlinkages with sustainable development goals, better coordination between national and EU agendas, higher mobility of researchers and ideas from across Europe and beyond, and stronger interactions between scientists and nonscientific entities (public authorities, the private sector, financial institutions, and civil society) in order to better communicate climate science and proactively translate new knowledge into actionable plans.


Atmosphere, Economy, and Their Holistic Framings in the Twentieth Century and Beyond  

Robert Luke Naylor

Despite apocalyptic discourse surrounding climate change since the 1970s, climate and weather have a longer history of being conceptualized as useful entities in the Anglophone world. The adversities of the Great Depression and hopes for a better postwar future led to climate being designated as a limitless resource—an object integral to the national economy that organizations, most notably governments, could draw upon to operate more effectively, especially against adversity. With a resurgence of neo-Malthusian perspectives in the 1970s, fears over resource scarcity reframed atmospheric resources as being strictly limited, and the concurrent rise of environmentalism challenged the idea that the atmosphere should be seen as a useful entity for industry. Instead, the economy–atmosphere relationship increasingly began to be framed through climate impact assessments, which analyzed the ability of climatic changes to perturb human systems. In addition, economic fragmentation, marketization, and privatization challenged the concept of national resources, meaning that by the end of the 1980s, the idea of the atmospheric resource had fallen from vogue. In the context of such marketization, the meteorological applications industry experienced rapid growth, leading some to advocate seeing the sector as a weather forecasting enterprise to encourage a renewed integrated perspective on weather impacts, forecasts, and policy. In contrast, in 2015, scholars identified how climate change has been reconstructed as a market transition by political and business elites, as climate change came to be seen as a market opportunity that was disconnected from goings-on in the material atmosphere. This disconnection can be seen as the culmination of a long process of conceptually disintegrating economy from the material atmosphere that began with the dismantling of the atmospheric resource concept.


Future Climate Change in the European Alps  

Andreas Gobiet and Sven Kotlarski

The analysis of state-of-the-art regional climate projections indicates a number of robust changes of the climate of the European Alps by the end of this century. Among these are a temperature increase in all seasons and at all elevations and a significant decrease in natural snow cover. Precipitation changes, however, are more subtle and subject to larger uncertainties. While annual precipitation sums are projected to remain rather constant until the end of the century, winter precipitation is projected to increase. Summer precipitation changes will most likely be negative, but increases are possible as well and are covered by the model uncertainty range. Precipitation extremes are expected to intensify in all seasons. The projected changes by the end of the century considerably depend on the greenhouse-gas scenario assumed, with the high-end RCP8.5 scenario being associated with the most prominent changes. Until the middle of the 21st century, however, it is projected that climate change in the Alpine area will only slightly depend on the specific emission scenario. These results indicate that harmful weather events in the Alpine area are likely to intensify in future. This particularly refers to extreme precipitation events, which can trigger flash floods and gravitational mass movements (debris flows, landslides) and lead to substantial damage. Convective precipitation extremes (thunderstorms) are additionally a threat to agriculture, forestry, and infrastructure, since they are often associated with strong wind gusts that cause windbreak in forests and with hail that causes damage in agriculture and infrastructure. Less spectacular but still very relevant is the effect of soil erosion on inclined arable land, caused by heavy precipitation. At the same time rising temperatures lead to longer vegetation periods, increased evapotranspiration, and subsequently to higher risk of droughts in the drier valleys of the Alps. Earlier snowmelt is expected to lead to a seasonal runoff shift in many catchments and the projected strong decrease of the natural snow cover will have an impact on tourism and, last but not least, on the cultural identity of many inhabitants of the Alpine area.


Regional Climate Modeling for the Baltic Sea Region  

Erik Kjellström and Ole Bøssing Christensen

Regional climate models (RCMs) are commonly used to provide detailed regional to local information for climate change assessments, impact studies, and work on climate change adaptation. The Baltic Sea region is well suited for RCM evaluation due to its complexity and good availability of observations. Evaluation of RCM performance over the Baltic Sea region suggests that: • Given appropriate boundary conditions, RCMs can reproduce many aspects of the climate in the Baltic Sea region. • High resolution improves the ability of RCMs to simulate significant processes in a realistic way. • When forced by global climate models (GCMs) with errors in their representation of the large-scale atmospheric circulation and/or sea surface conditions, performance of RCMs deteriorates. • Compared to GCMs, RCMs can add value on the regional scale, related to both the atmosphere and other parts of the climate system, such as the Baltic Sea, if appropriate coupled regional model systems are used. Future directions for regional climate modeling in the Baltic Sea region would involve testing and applying even more high-resolution, convection permitting, models to generally better represent climate features like heavy precipitation extremes. Also, phenomena more specific to the Baltic Sea region are expected to benefit from higher resolution (these include, for example, convective snowbands over the sea in winter). Continued work on better describing the fully coupled regional climate system involving the atmosphere and its interaction with the sea surface and land areas is also foreseen as beneficial. In this respect, atmospheric aerosols are important components that deserve more attention.


Theory and Modeling of the African Humid Period and the Green Sahara  

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.


Communicating about Fossil Fuel Divestment  

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.


Social Movements and Climate Change: “Climatizing” Society From Within  

James Goodman

Climate change brings profound challenges for social movements. The persistent failure to address climate challenges has driven a rapid “climatization” of politics. Spurred by the climate justice movement, social movements across a broad spectrum have become directly engaged with climate issues. Social movements are defined as groupings of people who act intentionally through an organization or via a network or even as a loose affiliation. They must have a collective identification and capacity for sustained action and participation. Their purpose often is to transform the conditions for social change as key agents in creating a “movement society” of mass political involvement. In doing so, social movements engage in a “metapolitics” of creating power and recreating society. Climate movements are increasingly being shown to have this effect. Recent research demonstrates that with climate change, there is a growing realignment in the social movement field to simultaneously address both climate concerns and social agendas. New forms of social agency are emerging under climate change, posing a new kind of climatized “movement society.” Arguably, as demonstrated by the limited efforts at developing international climate policy, mass mobilization on climate issues is a necessary element of any strategy to secure climate stability. Three broad fields of action are evident – politicising the impacts of climate change, contesting the causes, and advancing solutions. In each there is a widening field of agendas as climate concerns overwhelm existing social relations. Distinctive strategies emerge. First, there is growing collective identification among people affected by the impacts of climate change, now or anticipated, with a marked shift from climate advocacy to climate organizing, of acting “with,” not “for” those affected. Second, actions to challenge the legitimacy of the fossil fuel sector have escalated, materializing the causes of climate change in the fossil fuel cycle. With this, there is a move from abstract demands for emissions reduction to much more concrete demands for fossil fuel phase-out. Finally, in terms of solutions, there is a move from a focus on emission-reduction programs to wider policy agendas designed to transform social relations. Emissions reduction is no longer seen as a burden to be shared, but as part of wider social transformation, of benefit to all.


Future Climate Change in the Baltic Sea Region and Environmental Impacts  

Jouni Räisänen

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.