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date: 19 September 2021

Climate and Coast: Overview and Introductionfree

Climate and Coast: Overview and Introductionfree

  • Hans von Storch, Hans von StorchInstitute of Coastal Systems, Helmholtz Center Geesthacht and Meteorological Institute of the University of Hamburg
  • Katja Fennel, Katja FennelDalhousie University
  • Jürgen Jensen, Jürgen JensenUniversity of Siegen
  • Kristy A. Lewis, Kristy A. LewisUniversity of Central Florida
  • Beate Ratter, Beate RatterInstitute of Coastal Systems, Helmholtz-Zentrum Geesthacht Centre for Materials and Coastal Research
  • Torsten Schlurmann, Torsten SchlurmannThe Leibniz University Hannover
  • Thomas WahlThomas WahlUniversity of Central Florida
  •  and Wenyan ZhangWenyan ZhangInstitute of Coastal Systems, Helmholtz Center Geesthacht

Summary

Coasts are those regions of the world where the land has an impact on the state of the sea, and that part of the land is in turn affected by the sea. This land–sea interaction may take various forms—geophysical, biological, chemical, sociocultural, and economic. Coasts are conditioned by specific regional conditions. These unique characteristics result, in heavily fragmented regional and disciplinary research agendas, among them geographers, meteorologists, oceanographers, coastal engineers, and a variety of social and cultural sciences.

Coasts are regions where the effects and risks of climate impact societal and ecological life. Such occurrences as coastal flooding, storms, saltwater intrusion, invasive species, declining fish stocks, and coastal retreat and morphological change are challenging natural resource managers and local governments to mitigate these impacts. Societies are confronted with the challenge of dealing with these changes and hazards by developing appropriate cultural practices and technical measures.

Key aspects and concepts of these dimensions are presented here and will be examined in more detail in the future to expand on their characteristics and significance.

Subjects

  • Climate Impact: Marine Ecosystems
  • Climate Impact: Sea Level Rise
  • Climate and Coasts

Introduction

Global coasts are diverse in their geoscientific features but also in their human significance and utility. They are subject to considerable influences of climate and climate change and take part in the global carbon cycle. Thus, coasts are a legitimate subject in climate science. A characteristic of coasts are not only impacted by climate, but also affected by a broad range of human activities, such as coastal engineering, shipping, channelization, discharging various substances into the sea, fishing, aquaculture, and tourism. Decisions about how to use the coast is subject to political conflicts (Figure 1). Pressures are widely different in different parts of the world, and it is not surprising, given such variety, that the scientific community is fragmented in its responses to these conflicts.

Coasts are those land regions of the world that interact with the sea and those sea regions that are affected by the land. A strict definition of coasts makes little sense, since even the middle of the Sahara is influenced by the sea and the deep ocean is influenced by the land. In most cases, however, coastal science is focusing on the coastal seas, as is the case in this article.

Human populations are increasing at the coast due to migration and expanding urbanization in coastal areas. At the beginning of the 21st century, about one half of the world’s urban populations in cities over 100,000 inhabitants can be found in coastal areas that are less than 100 km from the sea (Barragán & de Andres, 2015). Coastal societies have historically evolved and developed culturally embedded relationships with their environment, which influences how, these societies experience and react to climate change impacts in their region.

When considering climate and climate change, the issues of climate change dynamics, climate hazards, mitigation of climate change, and “adaptation to climate change” receive the most attention.

In the dynamics of the climate “engine,” coasts do not play a key role, but they do modify the global carbon cycle.

Climate hazards in the coastal zones are related to flooding and winds. Flooding may be caused by regional sea level rise as well as regional storms. Also, violent ocean surface waves may be responsible for coastal damages, such as bursting cliffs.

Mitigation of climate change usually refers to the reduction of emissions of radiatively active gases and substances. Some geoengineering may also be considered a mitigation measure. Geoengineering is an actively studied field (e.g., Ott & Neuber, 2019), but concepts specific to coastal regions have not entered the scientific arena. There are a myriad of examples showcasing how coastal seas are used to generate alternative energy provisions,, mostly, but not only, by means of wind turbines. (Energy provisions deal with the drivers of climate change, thus changes toward carbon-free technology are part of mitigation.) Historically, coastal populations have adapted to climatic dangers of coastal regions developed technical measures to protect their physical and cultural integrity.

The most obvious factors of climate change affecting coastal seas are planet-scale warming and sea level rise. In addition, climate risks such as storms and river runoff are challenges for coastal stability and risk management and lead to changes in coastal morphology, including changing shorelines and retreating cliffs. Climate change exacerbates coastal risks through sea level rise, changing precipitation and snowmelt regimes, as well as changing storm patterns. Thus, adaptation is a key concept for coasts.

Figure 1. Usages of coasts. (A) Societal conflict. Protesting locals, displaying a banner stating: “The Wadden area is the base for our livelihood, not a playground for scientists.” © Holger Piening, Heide. (B) Coastal defense: breaking dikes in a storm surge.

Adapted from Petersen and Rohde (1977). (C) Coastal urban city (Hong Kong). © Hans von Storch. (D) Port activity (Ho Chi Minh City). © Hans von Storch. (E) Waste disposal. © Hans von Storch. (F) Offshore energy construction. © Friedrich von Storch.

Planet scale warming has consequences for coastal ecosystems, such as species migrations in order to stay in thermal comfort zones. Other implications to coastal ecosystems are hidden in the complexity of system dynamics and multiple human pressures. Indeed, separating the roles of different drivers for ecological change is a formidable challenge for ecological climate scientists.

The first two sections of this article (“Regional Technological Adaptation of Coast and Climate,” and “Perceptions of and Resilience to Coastal Climate Risks”) deal with adaptation to planet warming, first technologically, then in the socioeconomic dimension. The authors of these sections are rooted in different disciplinary traditions, ranging from coastal engineering, geographymarine ecology, to climate science. Climate change is considered a wicked problem, as the impacts are broad-reaching, perspectives of solutions differ among stakeholders, and often times, trade-offs must be agreed upon to find reconciliation (Rittel & Webber, 1973), This diversity of disciplines presented here highlights the need for various perspectives considering the complicated nature of climate’s impact on coastal areas.

Coastal science focuses mostly on regional issues. Attempts have been made to frame the issues in a global way in deriving indices of the state of coasts and suggestions for adaptation in particular or sea level rise (e.g., Wong et al., 2014). This localization led to the development of various scientific communities focusing on the issues important in their region. Thus, some readers may notice an overrepresentation of examples related to the North Sea and an underrepresentation of, say, the Rio de la Plata. However, given the many degrees of freedom or the variety of different coasts and utilities, such a bias seems unavoidable. Indeed, it is considered acceptable, as these are examples of the issues involved while the specifics related to concrete problems, with anchors in space and time, are not a key aspect.

The work presented in this article intends to expand the reach of this science to novices, by explaining how climate impacts various aspects of the coast. The sections that follow present a series of relevant aspects of the nexus of climate change, and coasts are introduced in some detail.

Regional Technological Adaptation of Coast and Climate

The potential for a natural hazard along coastlines to cause a disaster depends on how vulnerable an unprotected area is to such an event. Specific actions and measures can reduce the impact of a hazardous event.

As a prominent example, the case of the coastlines along the North Sea, including its islands, may serve to illustrate these adopted strategies. These coastlines have changed due to land losses related to severe storm surges and sea level rise in the past. Reconstructed shorelines from the past 10,000 years are shown in Figure 2, demonstrating enormous land losses (Meier, 2008).

Figure 2. Reconstructed state of the coastal region along the eastern German shoreline of the North Sea: 6500 bp, 1800 bp, and 1000 bp.

© Meier (2008).

These past changes were strongly correlated with storm surges and sea level rise, and human activities, such as diking, land reclamation, soil degradation (caused by burning peat for salt production), urbanization, and industrialization have also impacted coasts (Jensen & Müller-Navarra, 2008).

The most significant hazard has been storm surges followed by the associated hazard of coastal flooding. In the past, many storm surges and floods have resulted in hundreds of thousands of deaths (Figure 3) (Pranzini & Williams, 2013; von Storch et al., 2014).

Figure 3. Global distribution of coasts affected by storms and storm surges, with coastal mega cities.

© Ralf Weisse.

Even with an increase of about 20 cm in the 20th century, mean regional sea level rise has received little attention. This order of relevance will likely change in the 21st century, with accelerated sea level rise and minor changes in storminess (IPCC, 2018).

In the past, the dynamics of storm surges were not well understood. Such events were linked to divine responses to human misbehavior, say the loss of mythical Rungholt in 1362 in Northern Frisia. An intriguing question was whether coastal defenses should be allowed if storm surges were divine punishments (Jakubowski-Thiessen, 2003). The question was addressed by considering only the most severe storm surges as punishments, as they would fail to mitigate the events anyway. These surges were accepted as acts of God and are still considered as such in modern times, as documented by Evadzi et al. (2018) for the case of Ghana.

Germany and neighboring Netherlands have a long tradition of coastal engineering based on the fundamental requirements for survival of coastal inhabitants along endangered coastlines. Initial purely empirical knowledge has developed into a system providing the technical and scientific basis for coastal protection and engineering measures (e.g., Niemeyer et al., 1996).

Since medieval times, the development of various technical defense structures has evolved as follows:

Dikes, sea walls, and revetments: artificial earthen or concrete walls meant to prevent flooding of the hinterland (Figure 4a).

Figure 4a. Sketches of dikes, sea walls and revetments (MHW Mean High Water).

© Forschungsinstitut Wasser und Umwelt.

Groins: walls or barriers built out into the sea to interrupt currents and sand movement along a shore and to reduce wind wave heights (Figure 4b).

Figure 4b. Sketch of groins.

© Seawall modified by Forschungsinstitut Wasser und Umwelt.

Poldering and reclamation: a method of reclaiming land from the sea through the use of groins and creating new land by filling sections with sand or other material. Polders have various uses (e.g., agriculture) (Figure 4c).

Figure 4c. Polder development North Sea.

© Martina Nolte, Lizenz: Creative Commons.

Storm surge barriers: These barriers are large and fully or partly movable. They are used in estuaries, waterways, rivers, bays, or fiords which can be temporarily closed during a severe storm surge in order to protect vulnerable areas and cities against flooding (Figure 4d).

Figure 4d. The Oosterscheldekering (Eastern Scheldt storm surge barrier).

© Klaas Overbeek.

Figure 4e. Sketch of beach nourishment.

© Forschungsinstitut Wasser und Umwelt.

Beach nourishment: Beach nourishment (also referred to as beach replenishment or sand replenishment) is a process by which sediment, usually sand lost through longshore drift or erosion, is replaced from other sources (Figure 4e).

Climate change has been indicted for its effects on coastal risks of flooding since the early days of the climate change debate. Regarding the North Sea coast, Langenberg et al. (1999) and Woth (2005) published early papers. The issue is an integral part of coastal management (Schleswig-Holstein: Ministerium für Energiewende, Landwirtschaft, Umwelt und ländliche Räume, 2015). An example is the changing design of dikes (see Figure 5).

Figure 5. Changing dike design, in anticipation of additional risks related to climate change (after Schleswig-Holstein: Ministerium für Energiewende, Landwirtschaft, Umwelt und ländliche Räume, 2015).

Perceptions of and Resilience to Coastal Climate Risks

Coastal landscapes are shaped by human activity: by the history of settlement, agriculture, fisheries, land reclamation, coastal protection structures, and by maritime transport and shipping. The understanding of coastal regions reflects current ideas about the relationship between sea and land and between society and nature. Coastal landscapes are assigned different meanings in different places and at different times.

The ways in which individuals, societies, and policies respond to climate change are in many cases contingent on perceptions of its causes, consequences, and wider implications. An example is a case documented by Gee (2010) and Ratter and Gee (2012) on conflicting views about regional wind energy in Northern Germany. One group considered the presence of wind turbines as destructive of their aesthetics of the coastal landscape (Figure 6), whereas the other group valued the economic–ecological value of the wind farms higher than the aesthetical loss (von Storch et al., 2015). Obviously, this clash of perceptions presented a challenge for regional decision-making, with significance for the efforts of mitigating climate change.

Figure 6. Wind farm in Dithmarschen, Germany, at the North Sea coast.

© Rademaker (2019).

The study of climate change impacts, therefore, requires the understanding of place-specific perceptions of coasts and of coastal climate risks. These perceptions, along with other influencing factors such as economic interests and political will, form the societal resilience and response of a coastal community.

Resilience people’s ability to respond adequately to shocks and stressorsis place-dependent and closely connected to the existing political and institutional arrangements forming current governance structures. Resilience does not simply reflect the expected effects of quantifiable factors, such as level of assets, or even less quantifiable social processes, such as people’s experience, but is also determined by more subjective dimensions related to people’s perceptions of their ability to cope, adapt, or transform in the face of adverse events (Béné et al., 2016).

To understand and evaluate climate change adaptation strategies and measures, it is not only necessary to scrutinize climate change impacts threatening coastal societal livelihoods but also the societal frames of climate change adaptation policies. The focal questions are as follows: Which societal frames of climate change perception precondition adaptation? Which risks are perceived? How can climate change adaptation be constructed? How can disaster risk response (DRR) be integrated in climate change adaptation endeavors? Which cultural and political barriers hinder successful adaptation?

Public perceptions of climate change are known to differ between societies and to have fluctuated over time (e.g., Capstick et al., 2015; Döring & Ratter, 2017; Evadzi et al., 2018; Jakubowski-Thiessen, 2003). At the individual level, attitudes toward climate change may be influenced by a person’s experience, but they are also subject to wider sociocultural and political factors. Recent research in human geography and environmental psychology revealed the relevance of a place-based understanding of climate change which investigates the material geography as well as the intangible features of place impacting on the social framing of risks, adaptive options, aspects of vulnerability, and resilience. The analytical concepts of place and place-attachment have gained attention in the context of a more grounded study of societal relevance in climate change research (Devine-Wright, 2013; Döring & Ratter, 2017). Successful climate change adaptation processes are influenced by the social and cultural framing of society, considering the perceptions of those affected, and depending on cooperation based on trust and accountability. Addressing new forms of transformative, multilayer governance via participation and institutionalized multistakeholder partnership makes an important contribution toward rethinking coastal risk management within an inclusive and integrative perspective.

In recent times, the climate change adaptation (CCA) discourse, has found its way into Intergovernmental Panel on Climate Change (IPCC) narratives, and is fostered by discussion about new climate governance structures and the need for a transformation toward climate-resilient sustainable pathways (CRSP).

In a follow-up detailed paper (Ratter & Leyshon, 2021), these issues are considered in depth, specifically the following: Perception of coasts and coastal risks; relationship between adaptation and societal resilience; climate change adaptation as place-dependent; the need for integrating DRR and CCA; and climate-resilient sustainable pathways (transformative governance).

Coasts as a Region for Energy Provision

The Intergovernmental Panel on Climate Change (IPCC) SR1.5 report (IPCC, 2018) calls for a large-scale transformation of the global energy system toward renewable energy resources. In order to meet targets as set in the recent IPCC report, carbon emissions need to be limited by at least 49% of 2017 levels by 2030 and then achieve carbon neutrality by 2050, except for a minor amount, balanced by negative emissions.

This projection presumes that measures to meet the 1.5 °C goal include ramping up the installation of renewable energy systems to provide 70%–85% of the world’s electricity by 2050 (Tollefson, 2018). Recent years have seen record-breaking productions stemming from renewable energies so far, with 2017 the largest ever increase in renewable power capacity, mainly due to falling costs in installation and operation. Increases in investments and advances in enabling technologies on a global scale have driven estimated renewable energy shares of global electricity production (MWh) to only about 26.5% by the end of 2017 (REN21, 2018). Thus, there is still a huge gap to be met in order to achieve 70%–85% of global electricity production by the middle of the 21st century.

Figure 7. Wind farm in the North Sea.

©A. Götz, EnBW, with permission.

In the decade 2007–2017, wind power capacities continued to rise massively and accounted for an approximately sixfold increase in between 2007 and 2017, recently reaching 539 GW. Onshore wind power continues to account for 96% of global installed capacity, but offshore wind resources in nearshore coastal water (Figure 7) are on the rise and reached a maximum installation capacity of 18.8 GW in late 2017, which depicts an approximately 19-fold increase in between 2007 and 2017 (REN21, 2018).

In contrast, marine energy, related to tides and waves, remains a largely untapped renewable energy source, despite decades of research and development efforts, as only 529 Megawatts (MW) are in operation on a global scale (i.e., only 2.5% of currently installed offshore wind capacities). These technologies have not seen a real breakthrough or commercialization so far, although open-water technologies, such as tidal stream or wave energy converters, seem a promising and sustainable pathway toward renewable energy, but are still in an earlier stage of development with various prototypes deployed or piloted (REN21, 2018).

Buchsbaum (2018) pointed out the vast technological progress, the increased competitiveness, and, in parallel, the cost reductions of renewables, particularly for wind technologies with bottom-fixed offshore wind (BFOW) farms and floating offshore wind (FOW) installations. While BFOW farms have seen vast installation increases in the recent past in Northern European waters and off the coasts of China in the Taiwan Strait, floating offshore wind farms indicate better access to seasonal fluctuating wind resources and deep water depths, also in emerging markets and developing countries. Upcoming offshore wind farms will make use of far larger turbines beyond 10 MW of installed power and will exploit new spaces in even larger water depths up to 40–50 m in coastal seas (IEE, 2018a). The implementation of FOW farms is still in an early experimental phase in real coastal waters. Such farms, running floating wind turbine structures, will access the vast offshore wind of the global ocean (Buchsbaum, 2018). Moreover, FOW installations demonstrate a smaller environmental footprint due to a lower intrusiveness of FOW on marine life and habitats (ETIPWind, 2020).

By the end of 2017, wind energy sources of about 18.8 GW were in operation globally. Most of this energy (more than 85%) was generated in European waters (IEE, 2018b), reflecting an active, successful European Union policy (European Commission, 2018), which is regularly updated in anticipation of near-future technological progress.

Sustained and intensified attempts in offshore wind initiatives require new collaborations between research bodies and industry to push forward maritime energy in a joint and efficient approach. Strategic assessments of environmental impacts on a regional scale, and more detailed environmental impact assessments (EIA) of wind farms, or even on a scale of individual offshore wind energy converters (OWEC), must be conducted prior to design, installation, and operation of any new maritime infrastructure.

A sustainable development of maritime technologies for energy production on the coasts requires robust assessments of environmental impacts as well as consistent estimations of their structural designs and reliability under extreme met-ocean conditions. The likeliness of detrimental environmental impacts stemming from the installation and operation of marine renewables in the marine environment have repeatedly been reported. They may lead to irreversible impacts on marine wildlife in the formerly unaffected marine environment (Brandt et al., 2011; Carstensen et al., 2006; Madsen et al., 2006) by means of disruption or loss of ecosystems or ecosystem services. Those, in turn, have made necessary unconventional technological improvements, mainly during the installation process (e.g., Dähne et al., 2017). Other issues are the environmental impacts of typical offshore wind energy structures on the natural local flow field by additional turbulence and the imposing of extra sheer stresses on the surrounding mobile seafloor (Schendel et al., 2015), which are induced by so-called monopiles (i.e., vertical cylinders with diameters up to 12 m). In effect, sediment in the vicinity of the structure is eroded and successively entrained into the water column, leading to increased turbidit,y which affects the environment and, in turn, leads to the advection of sediments and deposition in the far-field of the structure under tidal currents (Schendel et al., 2018) and waves (Schendel et al., 2020). Of course, scouring might also affect the structural integrity of the whole marine infrastructure over time and has to be monitored or managed with additional scour protection systems (e.g., Schendel et al., 2016, 2017). More recently, Welzel et al. (2019) disproved the nonintrusiveness of so-called hydraulic-transparent jacket-type platforms on the surrounding mobile seafloor and quantified the entrainment and displacement processes of marine sands in the vicinity of the structure (Welzel et al., 2020).

Also, systemic knowledge gaps in structural design, lifetime, and system installation remain. Uncertainties exist regarding how to effectively address the broad spectrum of challenges in marine environments in the mid- and long term, and how to employ suitable maintenance concepts for marine renewable energy resources over the entire lifetime of the maritime structures, up to decommissioning in 20–25 years (Schlurmann, 2014). These issues demand new concepts to allow for parallel uses, interests, and functions in the exclusive economic zones of the coastal seas worldwide. Major research interest needs to follow global demands of renewable energy in coastal areas and address the development and exploitation of renewable energy resources to meet increased demands, as well as recently accumulated pressures in the development and optimization of maritime technologies.

Regional Sea Level

Sea level rise has been the main oceanographic driver for changes in coastal flood risk in the 20th and 21st centuries in most locations of the world (e.g., Vitousek et al., 2017), leading to more and higher extreme events, which can have dramatic societal impacts. Such extreme events always represent the superposition of different sea level components, comprising an underlying base water level (i.e., the mean sea level averaged over a certain period of time), astronomical tides, as well as surges and waves forced by low-pressure systems and strong winds. Figure 8 highlights how different sea level components interact to produce dangerous water levels leading to erosion and flooding.

Starting with the underlying mean sea level, significant spatio-temporal variability exists, which leads to much higher rates of rise in some regions compared to others. This effect is even more pronounced when factoring in vertical land motion from natural processes, such as glacial isostatic adjustment (GIA) or anthropogenic impacts, often related to the extraction of ground fluids and subsequent sediment compaction. Until the 5th Assessment Report of the Intergovernmental Panel on Climate Change (IPCC), the main scientific focus was on understanding past and projecting global mean sea level. Since then, research emphasis has shifted more toward regional sea level changes because this is what drives changes in flood risk for particular locations, and hence adaptation and migration decisions. Both models and observational data sets have improved, allowing for a much better understanding of the processes driving regional sea level changes (e.g., Frederikse et al., 2020).

Spatio-temporal variablity in relative mean sea level changes stems from different rates in vertical land motions, gravitational fingerprints of melting glaciers and ice sheets, changes in ocean circulation and uneven distribution of ocean warming, as well as variations in wind and atmospheric pressure over the oceans (e.g., Hamlington et al., 2020). The temporal variability in sea level is further often associated with natural variations in large-scale climate patterns, leading to periods where observed changes at the same location are much more pronounced compared to other time periods, where no or even negative changes occur (while there is still an underlying long-term trend). Consequently, projections of regional sea level changes are available but much more complex and surrounded by even larger uncertainties than projections of global mean sea level rise.

The next component shown in Figure 8 are ocean tides, which are deterministic and driven by gravitational forces in the Earth–Sun–Moon system. The resulting tidal range (i.e., the difference bewteen tidal high and low waters) can be neglible in marginal, almost fully enclosed seas such as the Mediteranean or Baltic Sea, and as high as 16 m in the Canadian Bay of Fundy due to funneling effects. In areas where the tidal range is large, tides play a key role in determining the timing and phasing of extreme events. For instance, a storm surge occurring around low tide rarely causes flooding in areas with large tidal range, whereas a moderate storm surge during a spring tide may lead to significant impacts.

Nontidal residuals (which encompass storm surges), the next component in Figure 8, pose a major risk for the majority of the world’s coastal settlements. They can be caused by extratropical or tropical cyclones and lead to water levels several meters higher than the predicted high tide. Hurricane Katrina, as an example, caused a storm surge of more than 8 m at the northern Gulf of Mexico coast, USA. Numerical models can be used to simulate and forecast storm surges or to produce hindcasts and long-term projections to assess past and future changes in storm surge statistics (e.g., Langenberg et al., 1999; Woth, 2005), associated with changes in the intensity, size, or frequency of storms (e.g., Muis et al., 2020). In coastal management and design applications, extreme value analysis plays an important role in order to parameterize modeled or observed extreme events and to infer water levels associated with certain return periods, or vice versa (e.g., Wahl et al., 2017).

Finally, depending on the coastal geomorphological setting and the presence or absence of protection measures, wind waves and ocean swell contribute to total water levels. They can generate wave setup, a vertical displacement of the still water level due to breaking waves, and lead to wave runup (the combination of wave setup and swash) on beaches, dunes, or dikes. This can result in overtopping and hence dune erosion and damage to other flood protection measures, such as sea walls and dikes, and ultimately, to flooding (van der Meer et al., 2018).

All of the sea level components shown in Figure 8 are linked in one way or another to the climate system and exhibit spatial and temporal variability. Importantly, they are also interacting with each other, leading to important feedbacks which can either escalate or offset changes in one or more of the components.

Figure 8. Sea level components contributing to total water levels with erosion and flooding potential (adapted from Moritz et al., 2015).

Storms

Strong wind episodes have been described since ancient times, due in part to their significance for coastal regions and seafaring. In Germanic languages, these events were named storms, and in the Latin tradition they were named tempests. In other language families, other terms are used. For instance, in Polish, part of the Slavic languages, they are called “burza.” For centuries, no reasonable causes for the emergence of such wind episodes were known; thus they were conceptualizd as “acts of God,” somtimes attributed to human sins.

In the 19th century, after much infighting among meteorologists, it became clear that there are different types of storms with different dynamic backgrounds. The large mid-latitude baroclinic (i.e., driven by the meridional temperature gradient) storms are one class of storms of great interest, and eventually, their dynamics were deciphered by the Bergen School of Meteorology (e.g., Friedmann, 1989) as being driven by thermal contrasts, leading in a gradient of the Coriolis force to baroclinic instability. The other large class of storms are tropical cyclones (e.g., Anduaga, 2019), which regionally bear a variety of names (e.g., typhoons, hurricanes). They are related to thermal instability in low latitudes, but not along the equator, with its vanishing Coriolis force.

Limited to certain regions are polar lows, Medicanes, and Australian east coast lows. Polar lows (e.g., Rasmusson & Turner, 2003) and their siblings, such as comma clouds, form mostly when cold air is advected from sea ice-covered areas over a relatively warm sea surface, exploiting convective processes.

Medicanes are storms with partially tropical characteristics that occur in the Mediterranean Sea (Reale & Atlas, 2001). They normally initially form as extratropical cyclones and at some stage in their evolution, they undergo a process of tropical transition. Medicanes are observed primarily in autumn and early winter in the Western Mediterranean Sea and in the Ionian Sea (Cavicchia et al., 2014).

East coast lows are cyclones forming in the Tasman Sea, near southeastern Australia. Severe east coast lows often have a stage of explosive deepening and cause major coastal hazards, including extreme winds, waves, or floods. East coast lows are influenced by both mid-latitude and tropical atmospheric processes. As a result, they can have a number of different dynamic structures, including those of baroclinic and hybrid tropical or extratropical storms (Dowdy et al., 2019).

Tropical storms are smaller than baroclinic mid-latitude storms. Polar lows and Medicanes are even considerably smaller than both tropical and mid-latitude baroclinic storms.

Dynamically, storms have the same “function,” namely, the alignment of strong gradients of temperature and water vapor. Polar lows transport cold arctic air to warmer ocean surfaces, where a mixing takes place. Baroclinic storms diminish the temperature gradient across the polar front. Thus, storms are an integral part of a dynamic climate system, and the present social practice of relating all storms to evolving human-caused climate change is a fallacy. Indeed, even if the question is intensely studied, clear answers as to whether storms have intensified or have become more frequent are not available. Also, the question of whether such changes in intensity and frequency are expected in the coming decades is open to debate.

Assessing changes in storminess is a methodological challenge, and several areas need to be recognized. Certainly, a physical good indicator of storminess is the statistics of seasonal or annual percentiles of wind speed. However, the physical surroundings of a meteorological station recording wind undergo many modifications over time due to natural processes, such as tree growth and human interference in the surroundings (e.g., buildings and fallen trees). This problem is referred to as “inhomogeneities” (e.g., Lindenberg et al., 2012).

Annual wind statistics, in this case annual mean wind speeds and 95% of wind speeds recorded at a cluster of coastal stations along the southeastern North Sea, demonstrate the problem (Lindenberg et al., 2012). Figure 9 shows the time series of annual mean wind speeds at the three island stations.

Figure 9. Inhomogeneities of wind records illustrated by the time series of annual mean wind speeds at a number of stations along the southeast coast of the North Sea (after Lindenberg et al., 2012).

Figure 10. Locations of wind recording on the Island of Helgoland. Left: until 1988, somewhat sheltered; right: beginning 1990 on the pier of the port (from Lindenberg et al., 2012).

In all three cases, sudden jumps happen from one year to the next. In the case of Helgoland, the movement of the instrument, caused by the building of a house, from a slightly sheltered position to an open position on the pier of the port caused the variations (Figure 10).

Much meta-ocean data were provided by ship reports, such as the COADS data set (Woodruff et al., 1987), which is currently known as ICOADS.1 Such data sets, made up of data reported by ships of opportunity, suffer from creeping inhomogeneities because of a tendency toward larger and higher ships and their willingness to pass through bad weather zones. Another type of inhomogeneity associated with global reanalysis is related to the changing density of weather reports through the addition or deletion of stations, instruments (e.g., satellites, radiosondes), or platforms (such as weather ships). An example is provided by Krüger et al. (2013).

For the determination of past changes in storminess, the analysis of non-wind data is meaningful. An example is sgeostrophic wind (Schmidt & von Storch, 1992). Also, modeling strategies may circumvent the problem of inhomogeneities (see Feser, 2018; Pryor & Hahmann, 2019).

Obviously, storm surges are among the most significant factors leading to coastal inundation events. In the case of tropical storms, this inundation often happens through a combination of sea water amassed along the coast and additional inland flowing related to excessive rainfall.

Coastal Morphology

Coastal morphology refers to the morphology and morphological development of the coast in response to a combined influence of atmospheric, oceanic, and anthropogenic forcing (Figure 11a). Coastal morphology comprises a wide variety of landforms (subaerial) and bedforms (subaqueous) manifested in a spectrum of spatial scales and shapes ranging from mildly sloping lower shoreface to steep cliffs, from large river deltas to small ripples (Figure 11b).

Figure 11. (a) Multiscale processes and factors involved in coastal morphological development. Ratios of spatial to temporal scales of the processes and factors marked in color deviate from others, suggesting that they introduce a second level of complexity into the system, which has to be solved in the modeling. GIA: Glacial isostatic adjustment. (b) Temporal and spatial scales of the morphological development of typical coastal systems.

Global warming causes ocean expansion and melting of ice shelves, leading to a rise of sea level that has significant impact on coastal morphological development. The observed sea level rise, which has been projected to continue for the 21st century, enlarges the inundation area of low-lying coastal regions, making them increasingly vulnerable to extreme events such as tropical cyclones, extratropical storms, and heavy precipitation. In response to a changing climate, adaptation of coastal morphology can be categorized into three basic states: erosional, stable, and accretionary. Each state may persist or iterate at any given part of a coast, even in the context of a persistently warming or cooling climate. It is the net change of sediment budget, namely, the difference between the incoming and the outgoing sediment volumes, that determines the morphological state of a coast. Coastal morphology is resilient to a changing climate only in the case that it is able to sustain physical dimension (elevation, width, and volume) and ecological function (vegetation growth). It has been found that 24% of the world’s sandy coasts are eroding at rates exceeding 0.5 m/year, while 48% are stable, and 28% are accreting (Luihendijk et al., 2018). However, it should be noted that the accreting and stable coasts are to a large extent due to anthropogenic protection, especially by beach nourishments that have been intensified in recent decades (Hanson et al., 2002). An earlier study by Bird (1985) suggested that 70% of the world’s sandy coasts had been under erosion till the early 1980s. An increase in coastal land loss in the upcoming decades would not be a surprise if no suitable additional protection measures are installed.

Coastal morphological development at small scales (101–103 m scale in space and 101–104 s scale in time) is much more variable than that at larger scales (Figure 11). Small-scale coastal morphological development is strongly steered by a variety of local state variables including wind velocity and direction, water level, waves, as well as the antecedent state of the coast (Anthony, 2013; Zhang et al., 2015), while large-scale development of the coastal morphology is primarily controlled by land uplift and subsidence (e.g., tectonics) and sediment supply modulated by processes at comparable scale, notably mean sea level, cyclones, tides, and regional wind and wave patterns (Zhang et al., 2011, 2014; see also Figure 11). The underlying lithology, namely, the physical properties of sediment at the coastline such as its resistance to erosion, is at least as important as external forcing in determining coastal morphology (Cooper et al., 2018). In addition, the relative level of human development imposes impact on coastal morphological development at a spatial scale that is far larger than the size of the engineering structure. It has been found that even local and modest levels of development (e.g., beach nourishment and pier construction) are able to influence several hundred kilometers-long coastline development that persists for decades (Hapke et al., 2013).

In the historical development of coastal morphological research, there have been long-existing debates on the validity of the equilibrium concept. Equilibrium in geology refers to a self-correcting balance of material and geometry. The most well-known equilibrium concept for coastal morphology was proposed by Bruun (1954, 1962). After examination of beach profiles in Denmark and the United States, Bruun (1954) found that the vertical shape of a cross-shore coastal profile can be approximated by the formulation h=Ay2/3, where h is water depth, A is a sediment scale parameter, and y is the cross-shore distance from the shoreline. Based on this, Bruun (1962) suggested a simple relationship between the coastal retreat rate Rand the sea-level rise rate S:R=SL/h+B, where L is the cross-shore width of the active profile, h is the closure depth (i.e., the most landward depth seaward of which there is no detectable change in bed elevation and net sediment transport between the nearshore and the offshore can be ignored for a given or characteristic time interval), and B is the elevation of the beach or dune crest. This relationship assumes a balance between the sediment yield Rh+B from the horizontal retreat of the profile and the sediment demand to fill the increased accommodation space, SL, from a vertical rise in the profile. Strictly, the Bruun rule describes an idealized coastal setting based on assumptions that sands are conserved in the system and no gradients exist in the longshore or cross-shore transport of sands (Dean et al., 2002). Because of these restrictions, its use to investigate real coasts was questioned by some other studies (e.g., Cooper & Pilkey, 2004; Stive, 2004) based on the fact that a beach never attains an equilibrium due to always changing hydrodynamic and boundary sediment supply conditions. Various modifications of the Bruun rule have been made to increase its applicability to real coasts (e.g., Deng et al., 2014; Rosati et al., 2013).

Climate Change Impacts on Coastal Ecosystems

Coastal ecosystems such as marshes, beaches, and coral reefs are the most dynamic regions on the planet, supporting tremendous ecological, biological, and human diversity. Both coastal ecosystems and the humans that live in these regions are susceptible to impacts from climate change. These impacts can range from temperature changes to sea level rise to weather anomalies and can have varying effects on different types of ecosystems. Climate change has the potential to greatly impact physical and biological characteristics of coastal ecosystems and the benefits they provide to both human and nonhuman communities.

Human activity may exacerbate the effects of climate change on these already vulnerable systems (Figure 12). Increasing temperatures, sea level rise, and changes in rainfall and weather will impact ecological processes, distribution of species, and food web interactions. Human development can further damage and stress these systems by altering the landscape, introducing nonnative species, and generating pollutants such as oil, gas, and fertilizer. In some habitats, such as islands, the impacts are obvious, while in other habitats, these changes can be more subtle and less understood.

Figure 12. Examples of coastal vulnerabilities that are exacerbated by the impacts from climate change.

Climate change may affect physical characteristics of the system, such as sediment and water supply, as well as biological characteristics, such as plant and animal communities. Biodiversity may be impacted through the loss of habitat for native species and the creation of habitat for nonnative and invasive species. As species migrate to previously unpopulated areas, competition between native and nonnative species will likely cause complex ecosystem changes. However, if the system is not monitored, these changes may go unnoticed for long periods of time. Scientific monitoring programs, such as periodic wildlife surveys and ongoing water quality measurements, are key components of evaluating how ecosystems respond to various disturbances and allow natural resource managers to predict changes they may see in the future.

The type and severity of climate change impacts on a coastal ecosystem depends on the type of system. Coastal zones can be broadly classified into tidal zones, shallow marine areas, and estuaries/wetlands. Within these classifications fall small-scale habitats such as rocky coasts and sandy beaches, hard and soft coral reefs, seagrass beds, kelp forests, oyster reefs, mangroves, coastal wetlands, and other submerged aquatic vegetation (SAV). The impacts of climate change among the various system types will be mediated by ongoing human activities and the natural resilience of the system.

Climate Impacts in the Tidal Zone

Rocky coasts. Rocky coasts, typically found on active margins of tectonic plates, are known for their captivating breaking waves against enormous rock formations. These coasts may be composed of any rock type, with the more erosion-resistant rock types creating more high-profile coastlines (e.g., Olympic National Park, Washington; Figure 13) and the more erodible rock type having a more gradual slope (e.g., Dry Tortugas National Park, Florida; Figure 14).

Figure 13. Shi Shi Beach at Olympic National Park, Washington.

Figure 14. Beach at Loggerhead Key in Dry Tortugas National Park, Florida.

Rocky coasts will be impacted by climate change in the following ways:

Temperature changes impact rocky coastlines through modifications to biotic communities. As each species has upper and lower temperature limits, small changes in overall temperature can impact survival, trophic interactions (feeding, predation), and reproduction. The intertidal communities of rocky coastlines experience temperature extremes with tidal exposure. Climate change may represent new thermal maxima for these environments and extreme swings in salinity through evaporation.

Precipitation changes impact rocky coastlines through alterations to the salinity of tidepool environments and the availability of freshwater to species within tidepool communities. Both evaporation (loss of freshwater, increasing salt content) and increased precipitation (increase of freshwater, decreasing salt content) can modify tidepool communities to the advantage of one species over another, with unknown consequences.

Changes in hydrology and sea level rise impact rocky coastlines through erosional processes. Increased water flow (e.g., increased precipitation or irrigation) through rocky coastlines, coupled with wave action, can increase rates of erosion, moving large volumes of sediment and rock.

Storms and floods increase the speed of both precipitation changes and changes to hydrology and sea level rise.

Sandy beaches and barrier islands. Barrier beaches and islands are common coastal formations found throughout the world. These formations tend to occur where there is a low to moderate tidal range, consistent wave action, and a steady supply of sand or sediment. The plant and animal communities that live in these dynamic habitats are adapted to extreme temperatures, high salinity, and tidal waters. Islands have high rates of endemism, supporting unique native species found nowhere else in the world.

Barrier beaches and islands provide a valuable ecosystem service by shielding inland areas against waves, wind, and storms. In addition, these areas are often tourist hotspots, attracting millions of visitors and generating billions of dollars in revenue each year.

Climate change will likely impact beaches and islands primarily through sea level rise and increasing frequency and severity of storms. Increases in the amount of water in the system will alter normal processes of removal and deposition of sediment, leading to changes in the shape, location, and height of beaches and islands, or resulting in their complete loss.

Notable barrier beaches and islands include Fraser Island in Australia, the largest sand island in the world (Figure 15); the Outer Banks in the United States, which attracts thousands of visitors each year (Figures 1618); and the islands Lido di Venezia and Pellestrina, which protects the iconic city of Venice in Italy.

Common ways in which climate change can impact these systems include the following:

Sea level rise may impact barrier beaches through substrate and sediment movement, resulting in coastline alteration, relocation, or complete overwash. As low-lying beaches are covered, ocean water may reach different sediment types, resulting in ecological changes in these areas.

Storms and floods are likely to hasten substrate and sediment movement, both in erosional and depositional processes. Increased storm frequency may also mitigate the “barrier” defensive capacity of island beaches, exposing coastal areas further inland to greater storm damage.

Figure 15. Ground view of Fraser Island, Australia.

Figure 16. Ground view of the Outer Banks, North Carolina, showing the beach and sand dunes.

Figure 17. Aerial view of the Outer Banks, North Carolina, depicting the turbulent waters of the Atlantic Ocean on the right and the calm waters of the sounds on the left.

Source: Wikipedia.

Figure 18. Satellite view of the Outer Banks, North Carolina, depicting the barrier between the sounds and the Atlantic Ocean.

Source: Wikipedia.

Climate Impacts to Shallow Marine Areas

Coral Reefs. Coral reefs are underwater ecosystems created by tiny animals, similar to jellyfish, that excrete calcium carbonate “homes” which collectively create hard coral outcroppings. The overlapping growth of coral skeletons creates a naturally elevated habitat along the seafloor, which supports a wide array of marine organisms. Coral reefs are well known for their high degree of biological diversity and are most commonly found in warm, shallow water areas (e.g., Great Barrier Reef, Australia, Figure 19, and Belize Barrier Reef, Belize, Figure 20), though they can be found in colder, deeper waters (e.g., Gray’s Reef National Marine Sanctuary, Georgia, Figure 21).

Figure 19. Corals on the Great Barrier Reef in Australia.

Figure 20. Belize Barrier Reef as seen from space.

Figure 21. Grey’s Reef National Marine Sanctuary. Limestone outcropping with hard and soft corals.

Coral reefs are highly sensitive to changing environmental conditions, making them particularly fragile ecosystems. Climate change can impact these ecosystems in the following ways:

Temperature changes are critical to shallow water coral reefs due to the limited thermal tolerance of the endosymbiotic algae that live within many types of coral. These algae allow corals to function as photosynthetic organisms, forming the basis of the food web. In addition, reef structures act as habitat for many other species. As temperatures rise, corals are more likely to expel the algae, a disruptive phenomenon known as coral bleaching.

Sea level rise is likely to impact shallow water coral reefs through the availability of light. As functional photosynthetic organisms, many corals require certain quantities of light. Increased depth through sea level rise will decrease light availability.

Storm frequency may be climate driven. Large cyclonic storms are capable of destroying significant areas of coral reefs. While it is still undetermined if climate change increases the frequency of large storms, it is important to understand that if the damage rates outpace the growth rates of coral, there will be a net loss of coral reefs.

Altered ocean currents can impact coral reefs through temperature and movement of particles. Currents in the ocean form large loops moving nutrients from cold, deep-water areas to warmer shallow areas. Proximity to currents can modify the temperature corals are exposed to, the availability of nutrient resources, and the movement of coral larvae to new areas to settle. As currents are driven by temperature differentials in the ocean, they are subject to change with altered climate.

Ocean acidification impacts coral reefs through a decreased ability to form the coral “skeleton.” As water becomes more acidic, the process of removing calcium carbonate from the water and secreting a skeleton or shell becomes more difficult for corals and other organisms. As these materials form the structural basis of many coral reefs, impairment of this ability will slow the potential for reef growth.

Seagrasses. Seagrasses are a type of submerged aquatic vegetation (SAV) that flowers and form underwater meadows, generally in full salinity seawater. These ecosystems are generally found in shallow coastal waters, where light can penetrate to the grass blades for their photosynthetic needs but can be found up to 60 m in depth (Unsworth et al., 2019). Seagrass beds are found in the waters of every major continent except Antarctica and provide important ecosystem services such as nursery habitat for marine organisms (Figure 22), sediment entrapment (i.e., clearing waters of excess sediment), and evidence is mounting that these ecosystems play an important role in climate stabilization through carbon storage (Duarte et al., 2013).

Seagrasses are experiencing declines worldwide. Given their close proximity to the coast they are threatened not only by a changing climate but from the impact of human activity in and around these ecosystems. Some of the primary impacts from climate change include:

Increased temperature will likely change their physiology, which will thus impact their ability to grow

Distribution of seagrasses will shift given the changing physical parameters of the world’s oceans

As the sea level rises, light availability for the seagrasses decreases, limiting their overall productivity

Figure 22. A fan mussel (Pinna nobilis) in a seagrass meadow in the Mediterranean Sea by Image Credit: Arnaud Abadie is licensed under CC BY 2.0.

Kelps. Kelps are a large brown macroalgae (up to 30m in length) that can be found in a quarter of coastal oceans around the world, from temperate to artic regions (Figure 23). Like seagrasses, kelps provide physical habitat for an abundance of marine organisms, offering increased opportunities for both foraging and hiding from predators. These brown algae are primary producers, using sunlight to grow and can be negatively impacted if the range of environmental factors for which it thrives are disrupted. That is, if there are changes, for example, to the amount of sunlight, water temperature, or the amount of nutrients in the water, kelps will stop growing and even decline in population. It is thought that declines in kelp populations are reaching almost 2% per year (Wernberg et al., 2019). Previous studies suggest that kelps are a sentinel species, providing an early warning that climate change will soon destabilize the entire system (e.g., Merzouk & Johnson, 2011), while other studies have challenged this notion using a recent warming event in comparison to long-term biological monitoring data (Reed et al., 2016), While the notion of how sensitive kelps can be to warmer temperatures, it is still accepted that with long-term changes to their physical environment, kelps will still susceptible to climate change pushing the algae outside of their optimum physicological thresholds (Smale, 2020).

Figure 23. Brown kelp forest as is displayed in the Monterey Bay Aquarium, California, USA. Image Credit: This file is made available under the Creative Commons CC0 1.0 Universal Public Domain Dedication.

Climate Impacts to Estuaries and Coastal Wetlands

An estuary is a coastal ecosystem found across the globe, defined as a partially enclosed body of water where rivers drain into the sea. The interaction of river currents and ocean tidal flow shapes the unique morphology of each estuary through hydrology and sediment transportation. Mixing of freshwater rivers and salty seas results in brackish or slightly salty water conditions. Specific species of plants and animals are adapted to survive in these ever-changing ecosystems. These species have adjusted to the fluctuations of salt within the estuary and benefit from estuaries’ sheltered habitat. Estuaries are diverse and support a variety of habitats, such as mangrove forests, oyster reefs, salt marshes, mud flats, and other submerged aquatic vegetation (SAV) across a range of salinites. These characteristics make estuaries both ecologically and economically important. Many migratory birds use estuaries as resting points in their migration, relying on estuarine shelter and food for survival. Nearly 75% of important fisheries, such as salmon and oysters, are dependent on estuaries for nursery habitat and shelter.

Fisheries are just one of the many ecosystem services or contributions to human well-being provided by estuaries. Estuaries provide natural filtration systems for river runoff and buffer zones for coastal flooding and storms. Rivers carry runoff, high in nutrients and pollutants from human activities, to our oceans. Estuarine plants and soils help absorb and recycle these hazardous materials before entering the ocean. When coastal flooding and storm events occur, an estuary acts as a sponge, soaking up the excess water and buffering wave energy with wetland structure. Climate change will impact ecosystem services and ecological processes, mainly through sea level rise and increased temperatures. Sea level rise changes landscape water flow, potentially reducing the absorption of floodwaters and increasing erosion of estuarine habitats. Rising temperatures can also cause shifts in estuarine plant communities, impacting other species that live within the estuary.

Estuaries are extremely diverse and can be classified as coastal plain estuaries, tectonic estuaries, bar-built estuaries, or fjord estuaries. These estuarine types are defined by their formation, either through sea level rise, tectonic shifts, barrier islands, or glacial movement. Estuaries are also dynamic ecosystems; for example, some estuaries can be all or mostly freshwater (note that to be considered an estuary, it must still be tidally influenced). In this case, rivers flow into a freshwater lake, with limited or seasonal connection to the ocean. A unique example of this is Laguna de Manialtepec, located along the west coast of Mexico. During the dry season, Laguna de Manialtepec is a freshwater lake surrounded by mangrove forests, fed by upland rivers (Figure 24). However, during the rainy season, water levels rise and connect the freshwater lake to the ocean through a small canal, making this estuary both a freshwater lake and a salty tidal estuary, depending on the season. Sea level rise could significantly impact this estuary by permanently connecting Laguna de Manialtepec to the ocean, losing its freshwater characteristics and increasing erosion along its banks. These changes will impact the local communities of Mexico, as they depend on Laguna de Manialtepec for food and income.

Estuaries will be impacted by climate change in the following ways:

Temperature changes may impact estuaries through changes in dominant plant communities. Mangroves, trees tolerant of salt water, are restricted by freezing temperatures. As the global area exposed to freezing temperatures decreases, mangrove species communities are moving into higher latitudes and outcompeting marsh grasses for habitat.

Changes in hydrology, including the rate and direction of waterflow across a landscape, impact estuarine habitats such as oyster reefs through changes to salinity, water retention, and sediment transport. These changes can impact oyster fisheries or the finfish fisheries that use the oyster reefs as habitat.

Sea level rise will impact low-elevation coastal areas first. Increased water volume will reduce water retention capacity of estuarine marshes—limiting the ability of these environments to absorb floodwaters. Increased water depth will also exclude the growth of mangrove forests, negating their ability to serve as a storm break.

Sea level rise will also impact various SAV species across a range of salinities into the upper reaches of the estuary. Changes to their distribution can alter how the ecosystem functions and decrease their ability to mitigate synergistic stressors such as storm events.

Erosion is likely to increase in some areas with changes to hydrology and sea level rise. Rate of water flow across a landscape may increase with increased precipitation, limiting the transport of sediment and deltaic accumulation. This may be linked to the loss of land as areas switch from depositional environments to erosional.

As dynamic systems with natural interactions between powerful biotic and abiotic forces, coastal systems are among the most resilient to perturbation. The capacity of mangrove forests and coral reefs to withstand and buffer coastal areas from cyclonic storms is well documented. As the impacts of climate change on coastal regions are examined, it should be acknowledged that coastal environments are among the least “fragile” on the planet, yet still face broadscale devastation through anthropogenic climate change.

Coastal Role in the Carbon Cycle

The global ocean holds the largest reservoir of mobile carbon in the Earth system and has sequestered about one third of cumulative anthropogenic CO2 (Sabine & Tanhua, 2010). Coastal regions are thought to play an important role in the global carbon cycle, which governs the partitioning of mobile carbon between Earth’s different reservoirs and may contribute significantly to the oceanic uptake of anthropogenic CO2.

Up to the end of the 1990s, the recognized mechanisms by which carbon is transported from the atmosphere to the ocean interior were the so-called biological and solubility pumps, the former driven by the gravitational sinking of organic matter, the latter by temperature-driven uptake of atmospheric CO2 in deep water formation regions (Volk & Hoffert, 1985). Neither was thought to be relevant to coastal regions or continental shelves, operationally defined as waters shallower than 200 m, because of their shallow depth. The influential paper by Tsunogai et al. (1999) first suggested that continental shelf regions play a disproportionate role in oceanic carbon uptake and introduced the concept of the continental shelf pump, a mechanism by which shelf water is enriched with carbon and then moved to the ocean interior by cross-shelf transport. Initially, this concept seemed to be confirmed by studies in other regions such as in the North Sea (Thomas et al., 2004), but a number of subsequent studies painted a more complicated picture with highly variable and regionalized patterns (Cai et al., 2006; Chen et al., 2013; Laruelle et al., 2014).

Two major pathways of carbon flow that are mediated by the coastal ocean are input of terrestrial carbon, in both inorganic and organic forms, and uptake of CO2 from the atmosphere, subsequent processing of this carbon within coastal waters (i.e., exchange between the organic and inorganic forms through primary production, respiration, and other transformations), and ultimately export to the open ocean, with smaller fractions buried in coastal sediments and accumulating in coastal waters. A quantification of these pathways is relevant for global and national carbon accounting and for assessment, and possibly mitigation, of coastal acidification trends.

A recent assessment of carbon fluxes in the coastal ocean around the North American continent shows that while collectively considered a net sink of atmospheric carbon, air–sea fluxes are heterogeneous, with large seasonal reversals and some regions acting as net sources of CO2 to the atmosphere in the annual mean (Fennell et al., 2019). The estimated uptake of atmospheric carbon of 160 ± 80 Tg C year−1 in North American coastal waters in this assessment amounts to 6.4% of the global ocean uptake of atmospheric CO2 of 2,500 Tg C year−1 (Le Quéré et al., 2015). Since the North American coastal region considered in this estimate makes up about 4% of the surface area of the global ocean, its uptake of CO2 is about 50% more efficient than the global average.

While the coastal ocean is now recognized as playing an important role in the global carbon cycle (Bauer et al., 2013; Regnier et al., 2013), its relative importance in taking up and sequestering carbon in comparison to the open ocean remains poorly quantified. Equally uncertain is the future trajectory of coastal carbon fluxes. Rising atmospheric CO2 levels and changes in weather patterns, ocean circulation and the hydrological cycle, and other anthropogenic pressures (including coastal eutrophication) have resulted in secular trends in inorganic carbon properties (dissolved inorganic carbon, alkalinity, and pH) and exchange fluxes, but the relative importance of these different factors can be highly regional and their collective impact is difficult to quantify. Even along well-observed ocean margins, observations are typically insufficient to completely describe the complex and highly localized interplay of these processes. Model analyses informed by the available observations are helpful but have to be regarded with some caution, as they cannot be fully validated given insufficient observational constraints.

Acknowledgments

The authors thank Frauke Feser and Leone Cavicchia for contributing to the description of Medicanes and east coast lows in the section “Storms.”

Further Reading

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