30 Ocean and Climate scientific items

The importance of the Ocean in climate change negotiations has been considered only until very recently. The following texts point to a change in mindset. They show that this planetary environment has at long last been given its rightful place in climate issues. Concerns such as the role that the ocean plays for the climate and the impacts of climate change on the ocean are addressed in this items. 

Ocean, heat reservoir

On our watery planet, the ocean is the primary regulator of global climate by continuous radiative, mechanical and gaseous exchanges with the atmosphere. In particular, the ocean absorbs, stores, and transports through its flow motion (i.e., currents) heat from the sun affecting atmospheric temperature and circulation around the world. Furthermore, seawater is the source of most precipitation. The ocean is much more efficient at storing heat (93% of the excess of energy resulting from the human induced Green House Gases content in the atmosphere) than the continents (3%) and the atmosphere (1%). As a result, the ocean is the slow component of the climate system and has a moderating effect on climate changes. However, consequent to the continuous absorption by the ocean of the human induced excess of heat, ocean waters are warming, which has consequences on the ocean’s properties and dynamics, on its exchanges with the atmosphere and on the habitats of marine ecosystems. For a long time, discussions of climate change did not take the oceans fully into account, simply because very little was known about them. Nonetheless, our ability to understand and anticipate what might happen to Earth’s climate in the future, depends on our understanding of the role of the ocean in climate.

 By Sabrina Speich, Gilles Reverdin, Herlé Mercier, Catherine Jeandel

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The ocean: a carbon pump

The ocean contains 50 times more carbon than the atmosphere and is exchanging large amounts of CO2 with the atmosphere every year. In the past decades, the ocean has slowed down the rate of climate change by absorbing about 30% of human emissions. While this absorption of anthropogenic CO2 is today the result of physical-chemical processes, marine biology is playing an important role in the ocean carbon cycle by sequestering carbon in the deep ocean. Changes in any of these physical, chemical and biological processes may result in climate feedbacks that either increase or decrease the rate of climate change, although knowledge of such interconnections is today still limited. The feedbacks between climate, the ocean, and its ecosystems require a better understanding in order to predict the co-evolution of atmospheric CO2 and climate change more reliably as well as to understand the characteristics of a future ocean.

 By Laurent Bopp*, Chris Bowler*, Lionel Guidi, Éric Karsenti, Colomban de Vargas *lead authors

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Sea level rise

Measurements from tide gauges and satellites have shown that the sea is rising globally at an average rate of about 1.7mm per year since the beginning of the 20th century, a direct consequence of human-driven global warming, although there is strong regional variability. This increase is mainly due to two factors: the increase in ocean temperature resulting in expansion of sea water, and the melting of continental ice sheets, glaciers and ice caps with an input of fresh water into the ocean. Despite uncertainties, proposed scenarios indicate that sea levels will continue to rise at a faster pace than during the 20th century, reaching an increase of more than 25cm (best case) and 82cm (worst case but likely underestimated) by 2100.

 By Benoit Meyssignac, Gilles Reverdin

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How does the ocean acquire its chemical composition?

Over the geological era, the chemical state of the ocean determines its ability to absorb atmospheric carbon dioxide gas, and therefore to contribute to the Earth climate regulation. Its state depends on the balance between sources and departures of any element delivered to the ocean, both terms extremely difficult and complex to quantify. This article reviews the state of knowledge on dissolved and particulate contributions to the ocean issued from the land-ocean interface (continental flux) on one hand and from the oceanic crust – ocean interface (hydrothermal flux) on the other hand.

By Valérie Chavagnac and Catherine Jeandel

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Ocean Acidification

Each day, the oceans absorb about a quarter of the CO2 produced by human activities, causing a chemical modification of seawater that results in ocean acidification. The disso- lution of CO2 in seawater causes an increase in acidity (decrease in pH) and a decrease in the availability of carbonate ions (CO32-) which are one of the building blocks required by marine plants and animals to make their skeletons, shells and other calcareous structures. Ocean acidity has increased by 30% in 250 years, and could triple by 2100. It threatens species such as oysters and mussels, and will also have an impact on marine food chains. Our understanding of the effects of ocean acidification on marine life is still only rudimentary.

By Jean-Pierre Gattuso, Lina Hansson

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The ocean is losing its breath

The decrease in oxygen content (deoxygenation) of coastal and oceanic waters wor- ldwide has worsened in recent decades. The main causes are climate change (warmer water holds less oxygen and causes increased stratification, which reduces ventilation, i.e. oxygen replenishment of the ocean interior and estuaries), and measurably higher nutrient concentrations (eutrophication) due to intensified human activities affecting coastal areas. Open-ocean deoxygenation, warming and ocean acidification are all driven by increased atmospheric carbon dioxide (CO2); they constitute multiple stressors for marine ecosystems, the socio-economic consequences of which are only just beginning to be appreciated.

 By Kirsten Isensee, Lisa Levin, Denise Breitburg, Marilaure Gregoire, Véronique Garçon, Luis Valdés

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The deep ocean: which climate issues?

The deep ocean (200m below the surface to 11,000m) represents over 98% of marine waters in volume. The image of a stable and homogeneous environment over vast areas, with low biological activity, does not actually reflect the diversity of deep-sea ecosystems nor their sensitivity to climate change. Even on the abyssal plains, variations in abundance of key species have been attributed to changes in the photosynthetic productivity at the surface of the ocean. Moreover, many biodiversity and productivity ‘hot-spots’ of the deep seafloor, and their foundation species such as deep-sea corals could be particularly vulnerable to the already observable changes at great depths, such as local or regional warming deep water, acidification and deoxygenation and modifications of the circulation of water masses. This vulnerability questions our ability to anticipate the consequences of climate change on poorly known ecosystems and the services they provide.

 By Nadine Le Bris

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The Southern Ocean

The southern part of the global ocean is furthest away from any industrial or human activity. Yet since several decades, many observers have reported significant changes within the ecosystems as well their indigenous species. In most cases, these disturbances have been connected to the consequences of human activities, whether indirect (increased temperature, changes in seasonal sea ice, consequences of the hole in the ozone layer, acidification) or direct (fisheries exploitation of living resources). The magnitude of these pressures varies across the different regions of the Southern Ocean. Although the image of an undiversified ocean is generally etched in the collective mind, the biogeographic atlas of the Southern Ocean (De Broyer et al., 2014) in which over 9064 species have been identified, shows that this is certainly not the case. Disturbances that are now being observed are presumed to modify the functioning of these ecosystems and trophic webs. This also concerns the modification of habitats of pelagic and benthic species, from primary producers to top predators, from coastal to deep-water species, from the sub-Antarctic ice-free areas to sea ice-covered zones. A well-known example is the modification of the sea ice regime around the Antarctic Peninsula although changes have been observed at variable degrees of intensity all around the continent. This ice is nonetheless necessary for the completion of the life cycle of many species, such as Antarctic krill whose exceptional biomass is at the basis of the diet of many predators including birds or marine mammals. Icebergs or the recent dislocation of large ice shelves are also known to have a major impact on benthic communities. Finally, the sub-Antarctic areas, at the northern boundary of the Southern Ocean might be the most affected by climate change. In this context, it is important to estimate how the biodiversity of this ocean, which has been accustomed to extreme conditions for almost 34 million years, will be able to adapt to these new conditions.

By Philippe Koubbi, Gabriel Reygondeau, Claude De Broyer, Andrew Constable and William W.L. Cheung

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The Arctic: issues, challenges and opportunities

The Arctic is often thought of as the land of polar bears and explorers. There are several industries operating in the Arctic, through the Arctic, or at the periphery of the Arctic Circle. Receding and thinning sea ice with climate change increases access to natural resources, shipping routes and touristic areas, thereby providing new opportunities for economic development in the Arctic. The rewards for operating in the Arctic are potentially extremely high and attractive, but at high financial, environmental and social costs in an environment which remains financially very risky. Some stakeholders have started securing access to Arctic resources, sowing the seeds for a ‘cold rush’. Such ‘cold rush’ has not materialised yet, slowed down mainly because of high economic costs and political sensitivity. The main political challenge is to successfully reconcile the different perspectives and interests in the Arctic. One option to facilitate this is to build up existing institutional capacity in line with the pace of economic development. There is certainly strong potential for creating shared economic wealth and well-being. Actual choices made by Arctic industries and countries for economic development, coordination and cooperation within the coming years will shape what the future Arctic will look like.

 By Emmanuelle Quillérou, Mathilde Jacquot, Annie Cudennec, Denis Bailly

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Ocean, Biodiversity and Climate

The marine environment has played a key role in the history of life and today’s ocean continues its primordial function in the evolution of life and climate. The recognized species diversity in the oceans does not exceed 13% of all currently described living species – fewer than 250,000 – but this can be due partly to our lack of knowledge, especially concerning deep zones of the oceans and microorganisms, and partly to the fact that marine ecosystems and the way of life in such a continuous medium disperse more easily species and they are less predisposed to endemism. In contrast, marine biomass can be considerable. Climate disturbance has a direct role in the loss of biological diversity, and this loss contributes in turn to the deregulation itself.

 By Gilles Bœuf

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Ecosystem services and marine conservation

Marine and land ecosystems provide many benefits to society. Marine ecosystems are under increased pressure because of climate change and expanding human population, needs and impacts. Increased pressures often render current management and associated policies inappropriate to mitigate or regulate such pressures and maintain the level of ecosystem benefits provided. Integrated approaches centred on ecosystems can help assess synergies and trade offs for delivery of benefits provided by ecosystems linked to management choices for different levels of usage and conservation. The ecosystem service framework can help structure the production of comprehensive assessments drawing from multi-disciplinary academic knowledge and management experience. Interaction and dialogue with stakeholders structured by a step-wise iterative ‘triage’ process can help ensure ecosystem assessment outputs are meaningful, salient (reflecting the interests of those involved), useful to management and policy concerns, needs and projects, and feasible under available knowledge and resources. The ecosystem service framework needs to be applied at regular intervals to gain an idea of how the benefits derived from ecosystems evolve in time. Using the ecosystem service framework in combination with the DPSIR framework based on identification of Drivers, Pressures, States, Impact, Response can provide very rich insights to discussions for establishment of management plans and policies for marine conservation, with delivery of healthy ecosystems and associated human well-being.

 By Denis Bailly, Rémi Mongruel, Linwood Pendleton, Emmanuelle Quillérou

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Coral reefs and climate change

Coral reefs are found in only a small percentage of global oceans, between 0.08 and 0.16,%, but they shelter about one third of the marine species known today. This ecological success is due to a symbiosis between a coral and an intracellular microalgae, commonly called zooxanthellae. “Organismic engineers”, they are the source of the largest biological constructions on the planet. Genuine oases of life, they support the direct sustenance of more than 500 million people in the world from fishing, but they engage human interest also for other reasons: protection of coasts against erosion, high value tourist areas… Ecological services from coral reefs are estimated at approximately 30 billion USD per year. Their growth depends on many factors (light, temperature, pH, nutrients, turbidity…). They are therefore extremely sensitive to the current changes in our environment: water temperature variability, ocean acidification, in addition to localized disruptions (pollution, sedimentation, coastal development, overfishing, marine shipping…). An increase of less than 1 degree above a threshold value is sufficient to cause bleaching. It breaks the coral symbiosis with their zooxanthellae throughout the populations, leading to the disappearance of the reef. Similarly, ocean acidification impedes the formation of a coral’s skeletons, and many other biological functions such as reproduction. We actually estimate that approximately 20% of the global coral reefs have already disappeared completely; 25% are in high danger; and 25% more will be threatened by 2050 if positive management action is not taken.

 By Denis Allemand

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©N.LeBescot/Tara Expéditions

©N.LeBescot/Tara Expéditions

Exploited marine biodiversity and climate change

Climate change is affecting the productivity of marine ecosystems and impacting fisheries, while the demand for fish for human consumption is increasing. Fish is the main source of animal protein for one billion people, and is one of the renewable resources most transacted in the world. Changes in physico-chemical characteristics of seawater affect the metabolism of individuals, the life cycles of species, relationships between predators and prey, and modification of habitats. Geographic distributions of fish (displacement rate towards the poles is 72.0 ± 13.5km/decade) and the dynamics of ecosystems could undergo profound disturbances in the coming decades, affecting fisheries globally and jeopardizing food security in many southern countries. The maintenance of healthy and productive marine ecosystems is a critical issue.

 By Philippe Cury

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Aquaculture and climate change

Aquaculture, a booming sector, now provides almost half of the fish and shellfish on world markets. Climate change will certainly affect aquaculture productions, however the scale is not presently quantifiable given the uncertainty of global models. Impacts will vary by region and type of production. Adaptation of production systems is potentially feasible through actions of all stakeholders involved. Direct impacts will be related to changes in production conditions in freshwater, brackish water and marine environments. The main indirect impact will probably be related to the dependence on an exogenous food supply for the cultivated organisms. However, the negative impacts (eutrophication of inland waters, ocean acidification…) and positive impacts (aquaculture activities in colder areas, better growth of farmed organisms…) could balance out.

 By Marc Metian

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©J.Girardot/Tara Expéditions

©J.Girardot/Tara Expéditions

Small islands, ocean and climate

The physical characteristics of small islands (limited land area, small plains, high exposure to unpredictable marine weather) and their human characteristics (strong dependence on subsistence activities and ecosystems) explain their potentially high vulnerability to environmental changes (i.e., changes in the ocean and sea-related hazards). They have become iconic figures representing the threats associated with climate change: rising sea levels, increase in cyclones, as well as ocean warming and acidification. Although a wide diversity of answers is to be expected from one island system to another, Small islands in general have to face significant threats: reduction in islands’surface area, increase in coastal erosion, degradation of coral reefs and mangroves. The impact on land (soil, water, flora and fauna) and marine resources (reefs and fisheries) will be major, hampering the future of human survival in many islands. Consequently, such societies have to face an extremely pressing challenge.

 By Virginie Duvat, Alexandre Magnan, Jean-Pierre Gattuso

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Informing climate investments priorities for coastal populations

Since the 1990s, the Intergovernmental Panel on Climate Change (IPCC) has used global level analyses of vulnerability to inform investment and action against the effects of climate change. Beyond the IPCC, the practice has been used widely to understand the vulnerability of coastal areas to a variety of hazards, including climate change. These analyses, however, have been driven by objectives that change from one assessment to the next, with very different conceptualisations of vulnerability. Over time these analyses have become increasingly data intensive and complex, drawing from an ever-expanding number of indicators. Such variations in objectives, conceptualisations and data have led to different and often contradictory rankings of priority areas for climate change action. The increased complexity makes it more difficult to disentangle the root causes of these different rankings and the degree to which climate change influences vulnerability rankings, compared to other factors such as local environment factors and the adaptive capacity of populations to deal with environmental change. If these global indicator analyses were deconstructed, climate decision-makers could use them as scoping studies rather than expect them to provide comprehensive and robust priorities for investment. Such scoping studies, if they are to be truly useful to climate decision-makers, need to be simplified and harmonised to isolate climate change specific drivers. They can help target the locations for more in-depth local level analyses and should be supplemented by global level analyses of costs of climate action including technical, social and economic factors.

By Adrien Comte, Linwood Pendleton, Emmanuelle Quillérou, Denis Bailly

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Lifestyles and attitudes in Tabiteuea: a dam against the Pacific?

Kiribati archipelago mythology teaches us that the world began with a crack in a rock, followed by a mixture between dry and moist matter and finally by the prolific efflorescence of a crowd of creative ancestors. Scientific facts in the western world warn that within less than 300 years, these Eastern Micronesian atolls will disappear as a result of rising sea level due to global warming. Ethnographic fieldwork demonstrates how these people of Oceania, who settled in the heart of the Pacific more than a thousand years ago, have the dignity to interpret present climate instability with remarkable philosophical intelligence. Climate change reveals the reality of this duel between the survival of so-called modern progress that continues to endanger the future of the planet on one hand, and the survival of traditional ways of living with the belief that Man evolves in a respected natural environment on the other hand. These two confronting issues will be presented at the coming Climate Conference in Paris. Is it such a utopian concept to expect the decision-makers to offer a sacrifice to Mother Nature?

By Guigone Camus

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