Chapter 4. The Ocean’s Role in the Hydrological Cycle
Contributors: Deirdre Byrne and Carlos Garcia-Soto (Convenors), Gordon Hamilton Eric Leuliette, LisanYu, Edmo Campos, Paul J. Durack, Giuseppe M.R. Manzella Kazuaki Tadokoro, Raymond W. Schmitt, Phillip Arkin, Harry Bryden, Leonard Nurse John Milliman, Lorna Inniss (Lead Member), Patricio Bernal (Co-Lead Member)
1. The interactions between the seawater and freshwater segments of th hydrological cycle
The global ocean covers 71 per cent of the Earth’s surface, and contains 97 per cent o all the surface water on Earth (Costello et al., 2010). Freshwater fluxes into the ocea include: direct runoff from continental rivers and lakes; seepage from groundwater runoff, submarine melting and iceberg calving from the polar ice sheets; melting of se ice; and direct precipitation that is mostly rainfall but also includes snowfall Evaporation removes freshwater from the ocean. Of these processes, evaporation precipitation and runoff are the most significant at the present time.
Using current best estimates, 85 per cent of surface evaporation and 77 per cent o surface rainfall occur over the oceans (Trenberth et al., 2007; Schanze et al., 2010) Consequently, the ocean dominates the global hydrological cycle. Water leaving th ocean by evaporation condenses in the atmosphere and falls as precipitation completing the cycle. Hydrological processes can also vary in time, and these tempora variations can manifest themselves as changes in global sea level if the net freshwate content of the ocean is altered.
Precipitation results from the condensation of atmospheric water vapour, and is th single largest source of freshwater entering the ocean (~530,000 km?/yr). The source o water vapour is surface evaporation, which has a maximum over the subtropical ocean in the trade wind regions (Yu, 2007). The equatorward trade winds carry the wate vapour evaporated in the subtropics to the Intertropical Convergence Zone (ITCZ) nea the equator, where the intense surface heating by the sun causes the warm moist air t rise, producing frequent convective thunderstorms and copious rain (Xie and Arkin 1997). The high rainfall and the high temperature support and affect life in the tropica rainforest (Malhi and Wright, 2011).
Evaporation is enhanced as global mean temperature rises (Yu, 2007). The water holding capacity of the atmosphere increases by 7 per cent for every degree Celsius o warming, as per the Clausius-Clapeyron relationship. The increased atmospheri moisture content causes precipitation events to change in intensity, frequency, an duration (Trenberth, 1999) and causes the global precipitation to increase by 2-3 pe cent for every degree Celsius of warming (Held and Soden, 2006).
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Direct runoff from the continents supplies about 40,000 km?/yr of freshwater to th ocean. Runoff is the sum of all upstream sources of water, including continenta precipitation, fluxes from lakes and aquifers, seasonal snow melt, and melting o mountain glaciers and ice caps. River discharge also carries a tremendous amount o solid sediments and dissolved nutrients to the continental shelves.
The polar ice sheets of Greenland and Antarctica are the largest reservoirs of freshwate on the planet, holding 7 m and 58 m of the sea-level equivalent, respectively (Vaugha et al., 2013). The net growth or shrinkage of such an ice sheet is a balance between th net accumulation of snow at the surface, the loss from meltwater runoff, and th calving of icebergs and submarine melting at tidewater margins, collectively known a marine ice loss. There is some debate about the relative importance of these in the cas of Greenland. Van den Broeke et al. (2009), show the volume transport to the ocean i almost evenly split between runoff of surface meltwater and marine ice loss. In a mor recent work, Box and Colgan (2013) estimate marine ice loss at about twice the volum of meltwater (see Figure 5 in that article), with both marine ice loss and particularl runoff increasing rapidly since the late 1990s. According to the Arctic Monitoring an Assessment Programme (AMAP, 2011), the annual mass of freshwater being added a the surface of the Greenland Ice Sheet (the surface mass balance) has decreased sinc 1990. Model reconstructions suggest a 40% decrease from 350 Gt/y (1970 - 2000) t 200 Gt/y in 2007. Accelerating ice discharge from outlet glaciers since 1995 - 2002 i widespread and has gradually moved further northward along the west coast o Greenland with global warming. According to AMAP (2011), the ice discharge ha increased from the pre-1990 value of 300 Gt/y to 400 Gt/y in 2005.
Antarctica’s climate is much colder, hence surface meltwater contributions ar negligible and mass loss is dominated by submarine melting and ice flow across th grounding line where this ice meets the ocean floor (Rignot and Thomas, 2002) Freshwater fluxes from ice sheets differ from continental river runoff in two importan respects. First, large fractions of both Antarctic ice sheets are grounded well below se level in deep fjords or continental shelf embayments; therefore freshwater is injecte not at the surface of the ocean but at several hundred meters water depth. This dee injection of freshwater enhances ocean stratification which, in turn, plays a role i ecosystem structure. Second, unlike rivers, which act as a point source for freshwate entering the ocean, icebergs calved at the grounding line constitute a distributed sourc of freshwater as they drift and melt in adjacent ocean basins (Bigg et al., 1997; Enderli and Hamilton, 2014).
Sea ice is one of the smallest reservoirs of freshwater by volume, but it exhibit enormous seasonal variability in spatial extent as it waxes and wanes over the pola oceans. By acting as a rigid cap, sea ice modulates the fluxes of heat, moisture an momentum between the atmosphere and the ocean. Summertime melting of Arctic se ice is an important source of freshwater flux into the North Atlantic, and episodes o enhanced sea ice export to warmer latitudes farther south give rise to rapid freshenin episodes, such as the Great Salinity Anomaly of the late 1960s (Gelderloos et al., 2012).
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The spatial distributions of these freshwater fluxes drive important patterns in regiona and global ocean circulation, which are discussed in Chapter 5.
The Southern Ocean (defined as all ocean area south of 60°S) deserves special mentio due to its role in the storage of heat (and carbon) for the entire planet. The Antarcti Circumpolar Current (ACC) connects the three major southern ocean basins (Sout Atlantic, South Pacific and Indian) and is the largest current by volume in the world. Th ACC flows eastward, circling the globe in a clockwise direction as viewed from the Sout Pole. In addition to providing a lateral connection between the major ocean basin (Atlantic, Indian, Pacific), the Southern Ocean also connects the shallow and deep part of the ocean through a mechanism known as the meridional overturning circulatio (MOC) (Gordon, 1986; Schmitz, 1996, see Figures I-90 and I-91). Because of its capacit to bring deep water closer to the surface, and surface water to depths, the Souther Ocean forms an important pathway in the global transport of heat. Although there is n observational evidence at present, (WG II AR5, 30.3.1, Hoegh-Guldberg, 2014) mode studies indicate with a high degree of confidence that the Southern Ocean will becom more stratified, weakening the surface-to-bottom connection that is the hallmark o present-day Southern Ocean circulation (WG | ARS 12.7.4.3, Collins et al., 2013).  similar change is anticipated in the Arctic Ocean and subarctic seas (WG | AR5 12.7.4.3 Collins et al., 2013), another region with this type of vertical connection between ocea levels (Wust, 1928). These changes will result in fresher, warmer surface ocean water in the polar and subpolar regions (WGII ARS 30.3.1, Hoegh-Guldberg, 2014; WG | AR 12.7.4.3, Collins et al., 2013), significantly altering their chemistry and ecosystems.
Imbalances in the freshwater cycle manifest themselves as changes in global sea level Changes in global mean sea level are largely caused by a combination of changes i ocean heat content and exchanges of freshwater between the ocean and continents When water is added to the ocean, global sea level adjusts, rapidly resulting in  relatively uniform spatial pattern for the seasonal ocean mass balance, as compared t the seasonal steric signal, which has very large regional amplitudes (Chambers, 2006) ‘Steric’ refers to density changes in seawater due to changes in heat content an salinity. On annual scales, the maximum exchange of freshwater from land to ocea occurs in the late Northern Hemisphere summer, and therefore the seasonal ocea mass signal is in phase with total sea level with an amplitude of about 7 mm (Chamber et al., 2004). Because most of the ocean is in the Southern Hemisphere, the seasona maximum in the steric component occurs in the late Southern Hemisphere summer when heat storage in the majority of the ocean peaks (Leuliette and Willis, 2011) Because globally averaged sea level variations due to heat content changes largel cancel out between the Northern and Southern Hemispheres, the size of the steri signal, globally averaged, is only 4mm.
Globally averaged sea level has risen at 3.2 mm/yr for the past two decades (Church e al., 2011), of which about a third comes from thermal expansion. The remainder is du to fluxes of freshwater from the continents, which have increased as the melting o continental glaciers and ice sheets responds to higher temperatures. Multi-decada fluctuations in equatorial and mid-latitude winds (Merrifield et al., 2012; Moon et al.,
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2013) cause regional patterns in sea-level trends which are reflected in the E Nifio/Southern Oscillation (ENSO) and the Pacific decadal oscillation (PDO) indices in th Pacific (Merrifield et al., 2012; Zhang and Church, 2012) and northern Australia (Whit et al., 2014). Interannual changes in global mean sea level relative to the observed tren are largely linked to exchanges of water with the continents due to changes i precipitation patterns associated largely with the ENSO; this includes a drop of 5 m during 2010-11 and rapid rebound in 2012-13 (Boening et al., 2012; Fasullo et al., 2013).
Some key alterations are anticipated in the hydrological cycle due to global warming an climate change. Changes that have been identified include shifts in the seasona distribution and amount of precipitation, an increase in extreme precipitation events changes in the balance between snow and rain, accelerated melting of glacial ice, and o course sea-level rise. Although a global phenomenon, it is the impact of sea-level ris along the world’s coastlines that has major societal implications. The impacts of thes changes are discussed in the next Section.
Changes in the rates of freshwater exchange between the ocean, atmosphere an continents have additional significant impacts. For example, spatial variations in th distribution of evaporation and precipitation create gradients in salinity and heat that i turn drive ocean circulation; ocean freshening also affects ecosystem structure. Thes aspects and their impacts are discussed in Sections 3 and 4.
Another factor potentially contributing to regional changes in the hydrological cycle ar changes in ocean surface currents. For example, the warm surface temperatures of th large surface currents flowing at the western boundaries of the ocean basins (th Agulhas, Brazil, East Australian, Gulf Stream, and Kuroshio Currents) provide significan amounts of heat and moisture to the atmosphere, with a profound impact on th regional hydrological cycle (e.g., Rouault et al., 2002). Ocean surface currents like thes are forced by atmospheric winds and sensitive to changes in them - stronger winds ca mean stronger currents and an intensification of their effects (WGII AR5 30.3.1, Hoegh Guldberg, 2014), as well as faster evaporation rates. Shifts in the location of winds ca also alter these currents, for example causing the transport of anomalously war waters (e.g., Rouault, 2009). However, despite a well-documented increase in globa wind speeds in the 1990s (Yu, 2007), the overall effect of climate change on winds i complex, and difficult to differentiate observationally from decadal-scale variability, an thus the ultimate effects of these currents on the hydrological cycle are difficult t predict with any high degree of confidence (WGIl ARS 30.3.1, Hoegh-Guldberg, 2014).
2. Environmental, economic and social implications of ocean warming
As a consequence of changes in the hydrological cycle, increases in runoff, flooding, an sea-level rise are expected, and their potential impacts on society and natura environment are among the most serious issues confronting humankind, according t the Fifth Assessment Report (ARS) of the United Nations Intergovernmental Panel on
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Climate Change (IPCC). This report indicates that it is very likely that extreme sea level have increased globally since the 1970s, mainly as a result of global mean sea-level ris due in part to anthropogenic warming causing ocean thermal expansion and glacie melting (WGI ARS 3.7.5, 3.7.6; WGI ARS 10.4.3). In addition, local sea-level changes ar also influenced by several natural factors, such as regional variability in oceanic an atmospheric circulation, subsidence, isostatic adjustment, and coastal erosion, amon others; combined with human perturbations by land-use change and coasta development (WGI AR5 5.3.2). A 4°C warming by 2100 (Betts et al., 2011; predicted b the high-end emissions scenario RPC8.5 in WGI AR5 FAQ12.1) leads to a median sea level rise of nearly 1 m above 1980-1999 levels (Schaeffer et al., 2012).
The vulnerability of human systems to sea-level rise is strongly influenced by economic social, political, environmental, institutional and cultural factors; such factors in turn wil vary significantly in each specific region of the world, making quantification  challenging task (Nicholls et al., 2007; 2009; Mimura, 2013). Three classes o vulnerability are identified: (i) early impacts (low-lying island states, e.g., Kiribati Maldives, Tuvalu, etc.); (ii) physically and economically vulnerable coastal communitie (e.g., Bangladesh); and (iii) physically vulnerable but economically "rich" coasta communities (e.g., Sydney, New York). Table 1 outlines the main effects of relative sea level rise on the natural system and provides examples of socio-economic syste adaptations.
It is widely accepted that relative trends in sea-level rise pose a significant threat t coastal systems and low-lying areas around the world, due to inundation and erosion o coastlines and contamination of freshwater reserves and food crops (Nicholls, 2010); i is also likely that sea-level effects will be most pronounced during extreme episodes such as coastal flooding arising from severe storm-induced surges, wave overtoppin and rainfall runoff, and increases in sea level during ENSO events. An increase in globa temperature of 4°C is anticipated to have significant socio-economic effects as sea-leve rise, in combination with increasingly frequent severe storms, will displace population (Field et al., 2012). These processes will also place pressure on existing freshwate resources through saltwater contamination (Nicholls and Cazenave, 2010). Figure  outlines in more detail the effects of sea-level rise on water resources of low-lyin coastal areas.
Small island countries, such as Kiribati, Maldives and Tuvalu, are particularly vulnerable Beyond this, entire identifiable coherent communities also face risk (e.g., Torres Strai Islanders; Green, 2006). These populations have nowhere to retreat to within thei country and thus have no alternative other than to abandon their country entirely. Th low level of economic activity also makes it difficult for these communities to bear th costs of adaptation. A shortage of data and local expertise required to assess risk related to sea-level rise further complicate their situation. Indeed the response of th island structure to sea-level rise is likely to be complex (Webb and Kench, 2010) Traditional customs are likely to be at risk and poorly understood by outside agencies Yet traditional knowledge is an additional resource that may aid adaptation in suc settings and should be carefully evaluated within adaptation planning. A significant part
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of the economy of many island nations is based on tourism; this too will be affected b sea-level rise through its direct effects on infrastructure and possibly also indirectly b the reduced availability of financial resources in the market (Scott et al., 2012).
Coastal regions, particularly some low-lying river deltas, have very high populatio densities. It is estimated that over 150 million people live within 1 metre of the high-tid level, and 250 million within 5 metres of high tide. Because of these high populatio densities (often combined with a lack of long-range urban planning), coastal cities i developing regions are particularly vulnerable to sea-level rise in concert with othe effects of climate change (World Bank, 2012).
Table 1. The main effects of relative sea-level rise on the natural system, interacting factors, and example of socio-economic system adaptations. Some interacting factors (for example, sediment supply) appea twice as they can be influenced both by climate and non-climate factors. Adaptation strategies: P  Protection; A = Accommodation; R = Retreat. Source: based on Nicholls and Tol, 2006.
Natural System Effects Interacting Factors Socio-economic System Adaptations  Climate Noe-climat 1. Inundation, a. Surge (sea) —wave/stormclimate | — sediment supply — dykes /surge barriers [P flood and storm — erosion —ficodmanagement | — building codes/floodwise buildings [A damage — sediment supply — erosion — land use planning/hazard delineation [A/R — land us b. Backwater effect runoff catchmen (rwer) man land us 2. Wetland loss (and change) CO, fertilization sediment supply land-use planning [A/R — sediment supply — migration space —Mmarniaged realignnent/forbid hard defence direct destruction nourishment/sediment managemen 3. Erosion (direct and indirect sediment supply sediment supply coast defences [P morphological change) —wave/stormclimate — nourishmen — building setbacks [R 4. Saltwater a. Surface Waters runoff catchment Saltwater intrusion barrier Intrusion management — change water abstraction [A/ land us b. Ground-water — rainfall — land use — freshwater injection [P — aquifer use — change water abstraction [A/R 5. Rising water tables/ impeded draii — rainfall — land use -u le drai system " wee — nun-off — aquifer use ~ pole ( mee  — catchment — change land use [A management — land use planning/hazard delineation WR )
Effects of sea-level rise are projected to be asymmetrical even within regions an countries. Nicholls and Tol (2006), extending the global vulnerability analysis o Hoozemans et al. (1993) on the impacts of and responses to sea-level rise with stor surges over the 21% century, show East Africa (including small island States an countries with extensive coastal deltas) as one of the problematic regions that coul experience major land loss. Dasgupta et al. (2009) undertook a comparative study o the impacts of sea-level rise with intensified storm surges on developing countrie globally in terms of its impacts on land area, population, agriculture, urban extent,
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major cities, wetlands, and local economies. They based their work on a 10 per cen future intensification of storm surges with respect to current 1-in-100-year storm-surg predictions. They found that Sub-Saharan African countries will suffer considerably fro the impacts. The study estimated that Mozambique, along with Madagascar, Mauritani and Nigeria account for more than half (9,600 km’) of the total increase in the region’ storm-surge zones.
Of the impacts projected for 31 developing countries, just ten cities account for two thirds of the total exposure to extreme floods. Highly vulnerable cities are found i Bangladesh, India, Indonesia, Madagascar, Mexico, Mozambique, the Philippines Venezuela and Viet Nam (Brecht et al., 2012). Because of the small population of smal islands and potential problems with implementing adaptations, Nicholls et al. (2011 conclude that forced abandonment of these islands seems to be a possible outcom even for small changes in sea level. Similarly, Barnett and Adger (2003) point out tha physical impact might breach a threshold that pushes social systems into complet abandonment, as institutions that could facilitate adaptation collapse.
=
Figure 1. Effects of sea-level rise on water resources of small islands and low-lying coastal areas. Source Based on Oude Essink et al. (1993); Hay and Mimura (2006).
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Impacts of climate change on the hydrological cycle, and notably on the availability o freshwater resources, have been observed on all continents and many islands. Glacier continue to shrink worldwide, affecting runoff and water resources downstream. Figur 2 shows the changes anticipated by the late 21st century in water runoff into rivers an streams. Climate change is the main driver of permafrost warming and thawing in bot high-latitude and high-elevation mountain regions (IPCC WGIIl AR518.3.1, 18.5). Thi thawing has negative implications for the stability of infrastructure in areas now covere with permafrost.
Projected heat extremes and changes in the hydrological cycle will in turn affec ecosystems and agriculture (World Bank, 2012). Tropical and subtropical ecoregions i Sub-Saharan Africa are particularly vulnerable to ecosystem damage (Beaumont et al. 2011). For example, with global warming of 4°C (predicted by the high-end emission scenario RPC8.5 in WGI ARS FAQ 12.1), between 25 per cent and 42 per cent of 5,19 African plant species studied are projected to lose all their suitable range by 208 (Midgley and Thuiller, 2011). Ecosystem damage would have the follow-on effect o reducing the ecosystem services available to human populations.
The Mediterranean basin is another area that has received a lot of attention in regard t the potential impacts of climate change on it. Several modelling groups are taking par in the MedCORDEX (www.medcordex.eu) international effort, in order to bette simulate the Mediterranean hydrological cycle, to improve the modelling tools available and to produce new climate impact scenarios. Hydrological model schemes must b improved to meet the specific requirements of semi-arid climates, accounting i particular for the related seasonal soil water dynamics and the complex surface subsurface interactions in such regions (European Climate Research Alliance, 2011).
Even the most economically resilient of States will be affected by sea-level rise, a adaptation measures will need to keep pace with ongoing sea-level rise (Kates et al. 2012). As a consequence, the impacts of sea-level rise will also be redistributed throug the global economic markets as insurance rates increase or become unviable and thes costs are passed on to other sectors of the economy (Abel et al., 2011).
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Change in Runoff (percent)
-40 -20 0 20 40
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Figure 2. Changes in water runoff into rivers and streams are another anticipated consequence of climat change by the late 21st Century. This map shows predicted increases in runoff in blue, and decreases i brown and red. (Map by Robert Simmon, using data from Milly et al., 2005; Graham et al., 2010; NAS Geophysical Fluid Dynamics Laboratory.)
3. Chemical composition of seawater
3.1 Salinity
Surface salinity integrates the signals of freshwater sources and sinks for the ocean, an if long-term (decadal to centennial) changes in salinity are considered, this provides  way to investigate associated changes in the hydrological cycle. Many studies hav assessed changes to ocean salinity over the long term; of these, four have considere changes on a global scale from the near-surface to the sub-surface ocean (Boyer et al. 2005; Hosoda et al., 2009; Durack and Wijffels, 2010; Good et al., 2013). These studie independently concluded that alongside broad-scale ocean warming associated wit climate change, shifts in ocean salinities have also occurred. These shifts, which ar calculated using methods such as objective analysis from the sparse historical observin system, suggest that at the surface, high-salinity subtropical ocean regions and th entire Atlantic basin have become more saline, and low-salinity regions, such as th western Pacific Warm Pool, and high-latitude regions have become even fresher ove the period of analysis (Figure 3). Significant regional-scale differences may be ascribe to the paucity of observational data, particularly in the pre-Argo era, the difference i temporal period over which each analysis was conducted, and differences i methodology and data selection criteria.
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Despite regional differences, the broad-scale patterns of change suggest that long-term coherent changes in salinity have occurred over the observed record, and thi conclusion is also supported by shifts in salinity apparent in the subsurface ocea (Figure 4). These subsurface changes also show that spatial gradients of salinity withi the ocean interior have intensified, and that at depth, salinity-minimum (intermediate waters have become fresher, and salinity-maximum waters have become saltier (Durac and Wijffels, 2010; Helm et al., 2010; Skliris et al., 2014). Taken together, this evidenc suggests intensification of the global hydrological cycle; this is consistent with what i expected from global warming (see Section 1). Actual changes in the hydrological cycl may be even more intense than indicated by patterns of surface salinity anomalies, a these may be spread out and reduced in intensity by being transported (advected) b ocean currents. For example, the work of Hosoda et al. (2009) and Nagano et al. (2014 indicates that large (ENSO-scale) salinity anomalies are rapidly transported from th central Pacific to the northwestern North Pacific (the Kuroshio Extension region).
Latitude
Latitude
0 60E 120E 180 120W 60w 00 60E 120E 180 120W 6ow  Longitude Longitude
-0.2 -0.15 -0.1 -0.05 0 0.05 0.1 0.15 0.2
The boundaries and names shown and the designations used on this map do not imply official endorsement or acceptance by the United Nations.
Figure 3. Four long-term estimates of global sea-surface salinity (SSS) change according to (A) Durac and Wijffels (2010; OAmerican Meteorological Society. Used with permission.), analysis period 1950 2008; (B) Boyer et al. (2005), analysis period 1955-1998; (C) Hosoda et al. (2009), analysis perio 1975-2005; and (D) Good et al. (2013), analysis period 1950-2012; all are scaled to represen equivalent magnitude changes over a 50-year period (PSS-78 50-year"). Black contours show th associated climatological mean SSS for the analysis period. Broad-scale similarities exist betwee each independent analysis of long-term change, and suggest an increase in spatial gradients o salinity has occurred over the period of analysis. However, regional-scale differences are due t differences in data sources, temporal periods of analysis, and analytical methodologies.
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Q 1500  rer |A Dwi0 B Bo 70S 50S 30S 10S 10N 30N 50N 70N 70S 50S 30S 10S 10N 30N 50N 70N 70S 50S 30S 10S 10N 30N 50N 70 Latitude Latitude Latitud -0.2 -0.15 -0.1 -0.05 0 0.05 0.1 0.15 0.2
The boundaries and names shown and the designations used on this map do not imply official endorsement or acceptance by the United Nations.
Figure 4. Three long-term estimates of global zonal mean subsurface salinity changes according to (A Durack and Wijffels (2010; OAmerican Meteorological Society. Used with permission.), analysi period 1950-2008; (B) Boyer et al. (2005), analysis period 1955-1998; and (C) Good et al (2013),analysis period 1950-2012; all scaled to represent equivalent magnitude changes over a 50 year period (PSS-78 50-year"). Black contours show the associated climatological mean subsurfac salinity for the analysis period. Broad-scale similarities also exist in the subsurface salinity changes which suggest a decreasing salinity in ocean waters fresher than the global average, and an increasin salinity in waters saltier than the global average. However, regional differences, particularly in th high-latitude regions, are due to limited data sources, different temporal periods of analysis an different analytical methodologies.
3.2 Nutrients
Many different nutrients are required as essential chemical elements that organism need to survive and reproduce in the ocean. Macronutrients, needed in large quantities include calcium, carbon, nitrogen, magnesium, phosphorus, potassium, silicon an sulphur; micronutrients like iron, copper and zinc are needed in lesser quantities (Smit and Smith, 1998). Macronutrients provide the bulk energy for an organism's metaboli system to function, and micronutrients provide the necessary co-factors for metabolis to be carried out. In aquatic systems, nitrogen and phosphorus are the two nutrient that most commonly limit the maximum biomass, or growth, of algae and aquatic plant (United Nations Environment Programme (UNEP) Global Environment Monitorin System (GEMS) Water Programme, 2008). Nitrate is the most common form of nitroge and phosphate is the most common form of phosphorus found in natural waters. On th other hand, one of arguably the most important groups of marine phytoplankton is th diatom. Recent studies, for example, Brzezinski et al., (2011), show that marine diatom are significantly limited by iron and silicic acid.
About 40 per cent of the world’s population lives within a narrow fringe of coastal lan (about 7.6 per cent of the Earth’s total land area; United Nations Environmen Programme, 2006). Land-based activities are the dominant source of marine nutrients,
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especially for fixed nitrogen, and include: agricultural runoff (fertilizer), atmospheri releases from fossil-fuel combustion, and, to a lesser extent, from agricultural fertilizers manure, sewage and industrial discharges (Group of Experts on the Scientific Aspects o Marine Environmental Protection, 2001; Figure 5).
An imbalance in the nutrient input and uptake of an aquatic ecosystem changes it structure and functions (e.g., Arrigo, 2005). Excessive nutrient input can seriousl impact the productivity and biodiversity of a marine area (e.g., Tilman et al., 2001) conversely, a large reduction in natural inputs of nutrients (caused by, e.g., dammin rivers) can also adversely affect the productivity of coastal waters. Nutrient enrichmen between 1960-1980 in the developed regions of Europe, North America, Asia an Oceania has resulted in major changes in adjacent coastal ecosystems.
Nitrogen flow into the ocean is a good illustration of the magnitude of changes i anthropogenic nutrient inputs since the industrial revolution. These flows hav increased 15-fold in North Sea watersheds, 11-fold in the North Eastern USA, 10-fold i the Yellow River basin, 5.7-fold in the Mississippi River basin, 5-fold in the Baltic Se watersheds, 4.1-fold in the Great Lakes/St Lawrence River basin, and 3.7-fold in South Western Europe (Millennium Ecosystem Assessment, 2005). It is expected that globa nitrogen exports by rivers to the oceans will continue to rise. Projections for 2030 sho an increase of 14 per cent compared to 1995. By 2030, global nitrogen exports by river are projected to be 49.7 Tg/yr; natural sources will contribute 57 per cent of the total agriculture 34 per cent, and sewage 9 per cent (Bouwman et al., 2005). An example o this is discussed in Box 1.
Box 1: Example — Nutrients in the Pacific region
The Pacific Ocean basins form the largest of the mid-latitude oceans. In addition, th subarctic North Pacific Ocean is one of the most nutrient-rich areas of the worl ocean; in 2013, the most recent year for which statistics have been compiled, th North Pacific (north of 40° N) provided 30% of the world's capture, by weight, o ocean fish (FAO, 2015). Many oceanographic experiments have been carried ou over the last half century in the North Pacific Ocean; studies based on these dataset reveal the decadal-scale variation of nutrient concentrations in the surface an subsurface (intermediate) layers, as seen in Figure 6.
A linearly increasing trend of nutrient concentrations (nitrate and phosphate) ha been observed in the intermediate waters in a broad area of the North Pacifi (Figure 6b); Ono et al., 2001; Watanabe et al., 2003; 2008; Tadokoro et al., 2009 Guo et al., 2012; Whitney et al., 2013). Conversely, the concentration of nutrients i the surface layer has decreased (Figure 6a; Freeland, 1997; Ono et al., 2002; 2008 Watanabe et al., 2005; 2008; Aoyama et al., 2008, Tadokoro et al., 2009; Whitney,
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2011). Surface nutrients are primarily supplied by the subsurface ocean through  process known as "vertical mixing", an exchange between surface and subsurfac waters. Vertical mixing is partly dependent on the differences in density betwee adjacent ocean layers: layers closer to one another in density mix more easily.
A significant increase in temperature and a corresponding decrease in salinity (se above) have been observed during the last half-century in the upper layer of th North Pacific (IPCC, 2013, WG1 ARS). These changes are in the direction o increased stratification in the upper ocean and thus it is possible that this increase stratification has caused a corresponding decrease in the vertical mixing rate.
Superimposed on the linear trends, nutrient concentrations in the ocean have als exhibited decadal-scale variability, which is evident in both surface and subsurfac waters (Figure 6c). Unlike the linear trends, the decadal-scale variability appeare synchronized between the surface and subsurface layers in the western Nort Pacific (Tadokoro et al., 2009). These relationships suggest that the mechanism producing the trends and more cyclical variability are different.
4. Environmental, economic and social implications of changes in salinity an nutrient content
4.1 Salinity
Although changes to ocean salinity do not directly affect humanity, changes in th hydrological cycle that are recorded in the changing patterns of ocean salinity certainl do. Due to the scarcity of hydrological cycle observations over the ocean, and th uncertainties associated with these measurements, numerous studies have linke salinity changes to the global hydrological cycle by using climate models (Durack et al. 2012; 2013; Terray et al., 2012) or reanalysis products (Skliris et al., 2014). However these studies only considered long-term salinity changes, and not changes that occur o interannual to decadal time-scales. These latter scales are strongly affected by climati variability (Yu, 2011; Vinogradova and Ponte, 2013). As mentioned in Section 3, thes studies collectively conclude that changes to the patterns of ocean salinity are likely du to the intensification of the hydrological cycle, in particular patterns of evaporation an rainfall at the ocean surface. This result concurs well with the “rich-get-richer mechanism proposed in earlier studies, suggesting that terrestrial “dry” zones wil become dryer and terrestrial “wet” zones will become wetter due to ongoing climat change (Chou and Neelin, 2004; Held and Soden, 2006).
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4.2 Nutrients
Marine environments are unsteady systems, whose response to climate-induced o anthropogenic changes is difficult to predict. As a result, no published studies quantif long-term trends in ocean nutrient concentrations. However, it is well understood tha imbalances in nutrient concentration cause widespread changes in the structure an functioning of ecosystems, which, in turn, have generally negative impacts on habitats food webs and species diversity, including economically important ones; such advers effects include: general degradation of habitats, destruction of coral reefs and sea-gras beds; alteration of marine food-webs, including damage to larval or other life stages mass mortality of wild and/or farmed fish and shellfish, and of mammals, seabirds an other organisms.
Among the effects of nutrient inputs into the marine environment it is important t mention the link with marine pH. The production of excess algae from increase nutrients has the effect, inter alia, to release CO2 from decaying organic matter derivin from eutrophication (Hutchins et al., 2009; Sunda and Cai, 2012). The effects of thes acidification processes, combined with those deriving from increasing atmospheric CO2 can reduce the time available to coastal managers to adopt approaches to avoid o minimize harmful effects on critical ecosystem services, such as fisheries and tourism Globally, the manufacture of nitrogen fertilizers has continued to increase (Galloway e al., 2008) accompanied by increasing eutrophication of coastal waters and degradatio of coastal ecosystems (Diaz and Rosenberg, 2008; Seitzinger et al., 2010; Kim et al. 2011), and amplification of CO2 drawdown (Borges and Gypens, 2010; Provoost et al. 2010). In addition, atmospheric deposition of anthropogenic fixed nitrogen may no account for up to about 3 per cent of oceanic new production, and this nutrient sourc is projected to increase (Duce et al., 2008).
Figure 5 (a 80 0.3 70 0.2 60 0.2 50 0.2 40 0.2 30 0.1 20 0.1 10 0.1 0 0.12
1960 1965 1970 1975 1980 1985 1990 1995 2000
Figure 5 (b)
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250 2020
1989-199 200 1959-1960
 oO = 15 g  ®  2 10  S =
50
World Sub-Saharan Latin West Asia South East Developing Develope total Africa America North Africa Asia Asia countries countries
Figure 5 (a) Trends in annual rates of application of nitrogenous fertilizer (N) expressed as mass of N and of phosphate fertilizer (P) expressed as mass of P2Os, for all States of the world except for man of the countries belonging to the United Nations regional group of Eastern European States and th former USSR (scale on the left in 10° metric tons), and trends in global total area of irrigated crop lan (H20) (scale on the right in 109 hectares ). Source: Tilman et al., 2001. Figure 5 (b) Estimated growt in fertilizer use, 1960-2020. From GESAMP (2001). Source: Bumb and Baanante, 1996.
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a
/
L f Geanteeennon tects 2 ayes 43
!"East China Sea 7, e kutoshio-Oyashio Central North Pacific 2.3 ~ ~Kuroshio-Oyashio Transitio transition water 5 OF pr a-~ 6 os cy
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The boundaries and names shown and the designations used on this map do not imply official endorsement or acceptance by the United Nations.
Figure 6. Synthesis of the decadal-scale change in nutrient concentrations in the North Pacific Ocea in the last fifty years. (a) The blue area shows the waters for which a decreasing trend in nutrien concentrations was reported in the surface layer. (b) The pink area shows the waters for which a increasing trend in nutrient concentrations was reported in the subsurface. (c) Example of th nutrient change in the North Pacific Ocean. Five-year running mean of the annual mea concentration (mmol m?) of Phosphate concentration in the surface and North Pacific Intermediat Water (NPIW) of the Oyashio and Kuroshio-Oyashio transition waters from the mid-1950s to earl 2010. (Time series from Tadokoro et al., 2009). Blue broken lines indicate statistically significan trends of PO,. Thin green broken lines represent the index of diurnal tidal strength represented b the sine curve of the 18.6-yr cycle.’ The numbers following each area name indicate the reference literature: (1) Freeland et al., (1997); (2) Ono et al., (2008); (3) Whitney (2011); (4) Ono et al., (2002) (5) Tadokoro et al., (2009); (6) Watanabe et al., (2005); (7) Aoyama et al., (2008); (8) Watanabe et al. (2008); (9) Ono et al., (2001); (10) Watanabe et al., (2003); (11) Guo et al., (2012).
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