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Meridional overturning circulation conveys fast acidification to the deep Atlantic Ocean

DOI:10.1038/nature25493
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Fiz F. Perez
Fiz F. Perez
Fiz F. Perez
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Marcos Fontela
Marcos Fontela
Marcos Fontela
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Maribel I. García-Ibáñez at Institut de Ciències del Mar
Maribel I. García-Ibáñez
Herlé Mercier at CNRS
Herlé Mercier
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Abstract and Figures

Since the Industrial Revolution, the North Atlantic Ocean has been accumulating anthropogenic carbon dioxide (CO2) and experiencing ocean acidification, that is, an increase in the concentration of hydrogen ions (a reduction in pH) and a reduction in the concentration of carbonate ions. The latter causes the 'aragonite saturation horizon'-below which waters are undersaturated with respect to a particular calcium carbonate, aragonite-to move to shallower depths (to shoal), exposing corals to corrosive waters. Here we use a database analysis to show that the present rate of supply of acidified waters to the deep Atlantic could cause the aragonite saturation horizon to shoal by 1,000-1,700 metres in the subpolar North Atlantic within the next three decades. We find that, during 1991-2016, a decrease in the concentration of carbonate ions in the Irminger Sea caused the aragonite saturation horizon to shoal by about 10-15 metres per year, and the volume of aragonite-saturated waters to reduce concomitantly. Our determination of the transport of the excess of carbonate over aragonite saturation (xc[CO32-])-an indicator of the availability of aragonite to organisms-by the Atlantic meridional overturning circulation shows that the present-day transport of carbonate ions towards the deep ocean is about 44 per cent lower than it was in preindustrial times. We infer that a doubling of atmospheric anthropogenic CO2 levels-which could occur within three decades according to a 'business-as-usual scenario' for climate change-could reduce the transport of xc[CO32-] by 64-79 per cent of that in preindustrial times, which could severely endanger cold-water coral habitats. The Atlantic meridional overturning circulation would also export this acidified deep water southwards, spreading corrosive waters to the world ocean.
Anthropogenic CO2 along the 2016 Ovide section Vertical distribution of anthropogenic CO2 (colour bar and white isolines; in μmol kg⁻¹) along the Ovide section of the North Atlantic (Fig. 1b) in 2016. The black line marks the isopycnal that delimits the upper and lower limbs of the AMOC. Dots represent samples. Dark shaded area represents the sea bed.
Anthropogenic CO2 along the 2016 Ovide section Vertical distribution of anthropogenic CO2 (colour bar and white isolines; in μmol kg⁻¹) along the Ovide section of the North Atlantic (Fig. 1b) in 2016. The black line marks the isopycnal that delimits the upper and lower limbs of the AMOC. Dots represent samples. Dark shaded area represents the sea bed.
… 
Projections of atmospheric CO2 and temperature changes as modelled by the five shared socioeconomic pathways (SSPs)²⁴ At present (bottom left of each graph), Earth has experienced a 1 °C warming with respect to preindustrial temperatures, and has an atmospheric CO2 concentration ([CO2atm]) of 400 p.p.m. (ref. 21); this represents an atmospheric CO2 excess (xc[CO2atm]) of 120 p.p.m. with respect to preindustrial times (xc[CO2atm] = [CO2atm] − 280 p.p.m., where 280 p.p.m. is the preindustrial [CO2atm]). If atmospheric Cant were to double, so too would xc[CO2atm] (to 480–520 p.p.m.; dotted red line in main graph); this is within the projection range for a warming of 2 °C. According to SSP5 (pink shaded area), this temperature could be reached in 2040–2052 (see insets). SSP5 is the only one of these five scenarios that results in a radiative forcing as high as that projected by the previous IPCC business-as-usual scenario (RCP8.5)⁵⁴.
Projections of atmospheric CO2 and temperature changes as modelled by the five shared socioeconomic pathways (SSPs)²⁴ At present (bottom left of each graph), Earth has experienced a 1 °C warming with respect to preindustrial temperatures, and has an atmospheric CO2 concentration ([CO2atm]) of 400 p.p.m. (ref. 21); this represents an atmospheric CO2 excess (xc[CO2atm]) of 120 p.p.m. with respect to preindustrial times (xc[CO2atm] = [CO2atm] − 280 p.p.m., where 280 p.p.m. is the preindustrial [CO2atm]). If atmospheric Cant were to double, so too would xc[CO2atm] (to 480–520 p.p.m.; dotted red line in main graph); this is within the projection range for a warming of 2 °C. According to SSP5 (pink shaded area), this temperature could be reached in 2040–2052 (see insets). SSP5 is the only one of these five scenarios that results in a radiative forcing as high as that projected by the previous IPCC business-as-usual scenario (RCP8.5)⁵⁴.
… 
Evolution of ocean acidification from preindustrial to the doubling of atmospheric Cant scenario a–c, Along the Ovide section (Fig. 1b) are shown H⁺ concentrations (colour bar and white isolines; in nmol kg¹) for preindustrial times (colour bar and white isolines) (a), the present day (b; 2016), and in a scenario involving doubling of atmospheric Cant (c). The black line shows the isopycnal delimiting the upper and lower limbs of the AMOC. Dots represent samples. Dark shaded area denotes the sea bed.
Evolution of ocean acidification from preindustrial to the doubling of atmospheric Cant scenario a–c, Along the Ovide section (Fig. 1b) are shown H⁺ concentrations (colour bar and white isolines; in nmol kg¹) for preindustrial times (colour bar and white isolines) (a), the present day (b; 2016), and in a scenario involving doubling of atmospheric Cant (c). The black line shows the isopycnal delimiting the upper and lower limbs of the AMOC. Dots represent samples. Dark shaded area denotes the sea bed.
… 
Progression of anthropogenic CO2 in surface waters versus excess of atmospheric CO2 at different temperatures and CO2 disequilibriums a–d, The concentration of anthropogenic CO2 ([Cant], in μmol kg⁻¹; blue dots), the Cant ratio (navy encircled dots) and the Hant ratio (red dots) are plotted against the excess of atmospheric CO2 that has resulted from anthropogenic activity during 1959–2016 for: a, a 2 °C warming and 5% of CO2 disequilibrium; b, a 2 °C warming and 15% of CO2 disequilibrium; c, a 20 °C warming and 5% of CO2 disequilibrium; and d, a 20 °C warming and 15% of CO2 disequilibrium. The Cant ratio (or Hant ratio) is the ratio between the [Cant] (or [H⁺]ant) at any time and the average [Cant] ([H⁺]ant) for the period covered by the OVIDE cruises (2002–2016). The values computed for the doubling of atmospheric Cant scenario (that is, for an excess of atmospheric CO2 of 220 ± 20 p.p.m.) are also shown (blue arrowheads). The progression of the Hant ratio fits to a straight line, while the Cant ratio decreases when increasing the atmospheric excess CO2 owing to the buffer capacity of the marine CO2 system (the Revelle factor). An increase of 72 ± 3% in Cant above the average [Cant] for the Ovide period (2002–2016) is computed for the doubling of atmospheric Cant scenario. This is practically independent of the seawater temperature and the degree of CO2 disequilibrium⁴¹.
Progression of anthropogenic CO2 in surface waters versus excess of atmospheric CO2 at different temperatures and CO2 disequilibriums a–d, The concentration of anthropogenic CO2 ([Cant], in μmol kg⁻¹; blue dots), the Cant ratio (navy encircled dots) and the Hant ratio (red dots) are plotted against the excess of atmospheric CO2 that has resulted from anthropogenic activity during 1959–2016 for: a, a 2 °C warming and 5% of CO2 disequilibrium; b, a 2 °C warming and 15% of CO2 disequilibrium; c, a 20 °C warming and 5% of CO2 disequilibrium; and d, a 20 °C warming and 15% of CO2 disequilibrium. The Cant ratio (or Hant ratio) is the ratio between the [Cant] (or [H⁺]ant) at any time and the average [Cant] ([H⁺]ant) for the period covered by the OVIDE cruises (2002–2016). The values computed for the doubling of atmospheric Cant scenario (that is, for an excess of atmospheric CO2 of 220 ± 20 p.p.m.) are also shown (blue arrowheads). The progression of the Hant ratio fits to a straight line, while the Cant ratio decreases when increasing the atmospheric excess CO2 owing to the buffer capacity of the marine CO2 system (the Revelle factor). An increase of 72 ± 3% in Cant above the average [Cant] for the Ovide period (2002–2016) is computed for the doubling of atmospheric Cant scenario. This is practically independent of the seawater temperature and the degree of CO2 disequilibrium⁴¹.
… 
Mean velocities orthogonal to Ovide Mean velocities (in m s⁻¹) orthogonal to the Ovide section for the period 2002–2016. The Ekman velocities—estimated from wind stress averaged over the months of the cruises and equally distributed over the first 30 m—were added to the geostrophic velocities derived from a box inverse model17,51,52,53. Positive (or negative) velocities indicate northeastward (or southwestward) currents. The horizontal grey lines are the isopycnals marking σ0 = 27.80 kg m⁻³, σ1 = 32.15 kg m⁻³, σ2 = 36.94 kg m⁻³ and σ4 = 45.85 kg m⁻³. At the bottom are shown distances from Greenland. Grey shaded area denotes the sea bed. The acronyms indicate topographic features: the Reykjanes Ridge (RR), the Eriador Seamount (ESM), and the Azores–Biscay Ridge (ABR).
+4
Mean velocities orthogonal to Ovide Mean velocities (in m s⁻¹) orthogonal to the Ovide section for the period 2002–2016. The Ekman velocities—estimated from wind stress averaged over the months of the cruises and equally distributed over the first 30 m—were added to the geostrophic velocities derived from a box inverse model17,51,52,53. Positive (or negative) velocities indicate northeastward (or southwestward) currents. The horizontal grey lines are the isopycnals marking σ0 = 27.80 kg m⁻³, σ1 = 32.15 kg m⁻³, σ2 = 36.94 kg m⁻³ and σ4 = 45.85 kg m⁻³. At the bottom are shown distances from Greenland. Grey shaded area denotes the sea bed. The acronyms indicate topographic features: the Reykjanes Ridge (RR), the Eriador Seamount (ESM), and the Azores–Biscay Ridge (ABR).
… 
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22 FEBRUARY 2018 | VOL 554 | NATURE | 515
LETTER doi:10.1038/nature25493
Meridional overturning circulation conveys fast
acidification to the deep Atlantic Ocean
Fiz F. Perez1*, Marcos Fontela1*, Maribel I. García-Ibáñez1*, Herlé Mercier2*, Anton Velo1, Pascale Lherminier2, Patricia Zunino2,
Mercedes de la Paz1, Fernando Alonso-Pérez1, Elisa F. Guallart1 & Xose A. Padin1
Since the Industrial Revolution, the North Atlantic Ocean has been
accumulating anthropogenic carbon dioxide (CO2) and experiencing
ocean acidification1, that is, an increase in the concentration
of hydrogen ions (a reduction in pH) and a reduction in the
concentration of carbonate ions. The latter causes the ‘aragonite
saturation horizon’—below which waters are undersaturated with
respect to a particular calcium carbonate, aragonite—to move to
shallower depths (to shoal), exposing corals to corrosive waters2,3.
Here we use a database analysis to show that the present rate of
supply of acidified waters to the deep Atlantic could cause the
aragonite saturation horizon to shoal by 1,000–1,700metres in the
subpolar North Atlantic within the next three decades. We find that,
during 1991–2016, a decrease in the concentration of carbonate
ions in the Irminger Sea caused the aragonite saturation horizon to
shoal by about 10–15metres per year, and the volume of aragonite-
saturated waters to reduce concomitantly. Our determination of
the transport of the excess of carbonate over aragonite saturation
(xc[CO32−])—an indicator of the availability of aragonite to
organisms—by the Atlantic meridional overturning circulation
shows that the present-day transport of carbonate ions towards the
deep ocean is about 44 per cent lower than it was in preindustrial
times. We infer that a doubling of atmospheric anthropogenic CO2
levels—which could occur within three decades according to a
‘business-as-usual scenario’ for climate change
4
—could reduce the
transport of xc[CO32−] by 64–79 per cent of that in preindustrial
times, which could severely endanger cold-water coral habitats. The
Atlantic meridional overturning circulation would also export this
acidified deep water southwards, spreading corrosive waters to the
world ocean.
Atmospheric CO2 levels have increased from 280 to 400 parts per
million (p.p.m.) since the Industrial Revolution. The global ocean has
captured some 30% of this anthropogenic CO
2
(C
ant
), thereby acting as
a climate regulator
1
. This CO
2
absorption has led to a decrease in sea-
water pH (of about 0.12 units) and in the supersaturation of calcium
carbonate (CaCO3)—effects known collectively as ocean acidification.
Ocean acidification can particularly affect marine calcifiers
1
by favour-
ing the dissolution of CaCO3-based shells and skeletons2. Notably,
deep cold-water coral (CWC) reefs formed by scleractinian corals—
such as the ecosystem engineer Lophelia pertusa—are highly vulner-
able to ocean acidification3. The global distribution of CWC seems
to be partly limited by the depth of the aragonite saturation horizon
(ASH), which marks the boundary between aragonite-stable waters
above and dissolution-prone waters below
5
. In preindustrial times,
more than 95% of CWC locations were found above the ASH6, pro-
viding evidence that environments located below the ASH are hostile
to CWC growth. In the North Pacific Ocean (Fig. 1a), where the ASH
is only 500metres deep
7
, the distribution of aragonitic CWC is patchy
and CWCs do not develop to form the large, deep reef frameworks
that are abundant in the North Atlantic
8
, where the ASH has tended
to occur at depths of more than 2,000metres. However, ocean acifidi-
cation is causing the ASH to shoal, thus exposing CWCs to CaCO
3
undersaturation. Although laboratory experiments suggest that adult
L. pertusa can acclimatize to CaCO
3
undersaturation
9,10
, the long-term
survival of CWC reefs in undersaturated water is questioned, because
L. pertusa skeletons become weaker when exposed yearlong to levels
of ocean acidification that are predicted to occur in the future
10
, and
the dead skeletal framework that supports the reef itself is likely to
dissolve in undersaturated waters11.
To determine the degree of aragonite saturation of the world ocean
waters, we calculated xc[CO32−] (in μmol kg−1) by using quality-
controlled global data sets of marine CO
2
system measurements
12,13
.
A positive (or negative) xc[CO32−] indicates waters that are supersaturated
(or undersaturated) with aragonite (see Methods). We find that high
positive
xc
[CO
32−
] values occur in the North Atlantic while negative
values occur in the North Pacific; this is consistent with the known
distribution of CWC14 below 1,000metres (Fig. 1a). About 61% of
CWCs found below 1,000metres are located in the North Atlantic (this
number rises to 78% at depths below 1,500metres), where
xc
[CO
32−
]
is greater than 0, with an average xc[CO32−] of 24.5 μmol kg−1 below
1,000metres (and 15 μmol kg−1 below 1,500metres). These are half the
natural (preindustrial) xc[CO32−] values (see Extended Data Table 1).
The Atlantic meridional overturning circulation (AMOC) created the
favourable conditions for CWC growth in the North Atlantic by con-
veying ventilated waters loaded with relatively high pH and positive
xc[CO32−] to the deep Atlantic Ocean.
In the centre of the Irminger Sea (Fig. 1b), winter deep convection
recorded during 1991–2016 explains a persistent increase in Cant from
30 to 50 μmol kg
−1
and the associated deep injection of ocean acidi-
fication in the ventilated subpolar mode water (Fig. 2a). During the
same period, the atmospheric C
ant
grew from 85 p.p.m. to 123 p.p.m.
A thick layer of low salinity (less than 34.91) traces the strong con-
vection events that occurred during the first half of the 1990s and
during 2014–2016. During those strong convection events, subpolar
mode water was ventilated down to 1,500metres, showing high
temporal variability and no indication of a slowing-down of deep
convection15,16. The present-day surface ocean has about a 30% higher
concentration of H+ ions ([H+]) than the natural (preindustrial)
surface ocean
1
. At the centre of the Irminger Sea, the [H
+
] increase
affects a layer about 1,500metres thick (Fig. 2c)—the deepest signal
yet seen of the direct injection of ocean acidification. Since 2002,
anthropogenic perturbations have caused the isolines that mark 25%
and 30% anthropogenic [H+] to deepen from the surface down to
1,500metres. The isolines that mark xc[CO32−] progressively ascend
at about 10–15metres per year (Fig. 2b), with some slightly faster
ascension periods that are related to deep convection events (arrows
in Fig. 2b). The effect of these deep convection events is buffered by
1Instituto Investigaciones Marinas (IIM, CSIC), calle Eduardo Cabello, 6, 36208, Vigo, Spain. 2Centre National de la Recherche Scientifique (CNRS), Ifremer, Université de Brest, Institut de
Recherche pour le Développement, Laboratoire d’Océanographie Physique et Spatiale (LOPS), Centre Ifremer de Bretagne, 29280, Plouzané, France.
*These authors contributed equally to this work.
© 2018 Macmillan Publishers Limited, part of Springer Nature. All rights reserved.
Content courtesy of Springer Nature, terms of use apply. Rights reserved
... The North Atlantic ocean contains more anthropogenic CO2 than any other ocean basin, due to deep-water formation that occurs there, which effectively transfers CO2 from the surface into the ocean interior (Sabine et al., 2004;Khatiwala et al., 2013). While North Atlantic deep-water formation is climatically beneficial in terms of exporting CO2 from the surface into the interior ocean, Pérez et al. (2018) showed that it also causes ocean acidification at depth. In addition to this physical pumping of anthropogenic carbon to the deep water, there are a number of other key processes that result in intermediate and deep waters having elevated carbon and lower pH compared to the surface waters. ...
... Numerous observations across the North Atlantic, using repeat transects or time-series stations, are now showing changes in ocean chemistry in the interior ocean that reflect increasing ocean acidification (OSPAR, 2022). In the Irminger Sea and Iceland Sea there is particular rapid acidification in the interior ocean, and this has been previously highlighted (Olafsson et al., 2009;García-Ibáñez et al., 2016;Pérez et al., 2018Pérez et al., , 2021Vázquez-Rodríguez et al., 2012), while data from the UK's Extended Ellett Line transect between Scotland and Iceland also shows increases in inorganic carbon at depth (Humphreys et al., 2016). The consequences of ocean acidification are therefore not restricted to the surface ocean, but based on experimental work, have the potential to impact marine species and ecosystems at greater depths, such as cold-water coral reefs, which occur around the UK shelf and northeast Atlantic at depths from 200 m to 1500 m (Hoegh-Guldberg et al., 2017;Fontela et al., 2020). ...
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... These correlations reflect that the ensemble members with the weakest reduction in AMOC having the greatest warming locally, and also that the ensemble members with the strongest reduction in AMOC actually have local cooling. The tongue of highly correlated SSTs in the Icelandic Basin is a branch of the North Atlantic Current (Perez et al., 2018), while the pattern of the highly correlated NBTs reflect the complex 660 deep water pathways and bathymetry of this region. There is a competition between the warming associated with a rising global mean temperature, and a cooling associate with a reduced AMOC (Drijfhout, 2015) -over most of the domain, the warming dominates, whereas in this small region, the cooling dominates, resulting in a region of negligible projected SST warming. ...
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The change in anthropogenic CO2 (Canth) in the Atlantic Ocean is linked to the Atlantic Meridional Overturning Circulation (AMOC), that redistributes Canth meridionally and in depth. We have employed direct biogeochemical measurements and hydrographic data from the last 30 years, adjusted using inverse models for each decade with both physical and biogeochemical constraints. We then have computed the meridional transports and the vertical transports between two sections at the interphases by advection and diffusion. We have focused on the repeated sections at three latitudes—30°S, 24, and 55°N, dividing the Atlantic into two boxes. We have divided the net transport into upper, deep and abyssal layers, with an upper and abyssal northward transport of Canth and a southward component in deep layers. The change in time in the net transports of Canth appears to be mainly due to modifications in the transport of upper layers. The lower layer of the AMOC, a combination of deep and abyssal waters, maintain more consistent transports in time. Vertical advection plays an important role in the North Atlantic, exporting Canth from upper to deep layers. In the South Atlantic, the newly formed Antarctic Bottom Water exports Canth from abyssal to deep layers. The strong gradient in Canth concentration at the interphase of upper and deep layers results in a strong vertical diffusion.
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Multiscale mechanical consequences of ocean acidification for cold-water corals
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  • May 2022
Ocean acidification is a threat to deep-sea corals and could lead to dramatic and rapid loss of the reef framework habitat they build. Weakening of structurally critical parts of the coral reef framework can lead to physical habitat collapse on an ecosystem scale, reducing the potential for biodiversity support. The mechanism underpinning crumbling and collapse of corals can be described via a combination of laboratory-scale experiments and mathematical and computational models. We synthesise data from electron back-scatter diffraction, micro-computed tomography, and micromechanical experiments, supplemented by molecular dynamics and continuum micromechanics simulations to predict failure of coral structures under increasing porosity and dissolution. Results reveal remarkable mechanical properties of the building material of cold-water coral skeletons of 462 MPa compressive strength and 45–67 GPa stiffness. This is 10 times stronger than concrete, twice as strong as ultrahigh performance fibre reinforced concrete, or nacre. Contrary to what would be expected, CWCs retain the strength of their skeletal building material despite a loss of its stiffness even when synthesised under future oceanic conditions. As this is on the material length-scale, it is independent of increasing porosity from exposure to corrosive water or bioerosion. Our models then illustrate how small increases in porosity lead to significantly increased risk of crumbling coral habitat. This new understanding, combined with projections of how seawater chemistry will change over the coming decades, will help support future conservation and management efforts of these vulnerable marine ecosystems by identifying which ecosystems are at risk and when they will be at risk, allowing assessment of the impact upon associated biodiversity.
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Ocean Ventilation Controls the Contrasting Anthropogenic CO2 Uptake Rates Between the Western and Eastern South Atlantic Ocean Basins
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The South Atlantic Ocean is an important region for anthropogenic CO2 (Canth) uptake and storage in the world ocean, yet is less studied. Here, after an extensive sensitivity test and method comparison, we applied an extended multiple linear regression method with six characteristic water masses to estimate Canth change or increase (ΔCanth) between 1980s and 2010s in the South Atlantic Ocean using two meridional transects (A16S and A13.5) and one zonal transect (A10). Over a period of about 25 years, the basin‐wide ΔCanth was 3.86 ± 0.14 Pg C decade⁻¹. The two basins flanking the Mid‐Atlantic Ridge had different meridional patterns of ΔCanth, yielding an average depth‐integrated ΔCanth in the top 2000 m of 0.91 ± 0.25 mol m⁻² yr⁻¹ along A16S on the west and 0.57 ± 0.22 mol m⁻² yr⁻¹ along A13.5 on the east. The west‐east basin ΔCanth contrasts were most prominent in the tropical region (0–20°S) in the Surface Water (SW), approximately from equator to 35°S in the Subantarctic Mode Water (Subantarctic Mode Water (SAMW)), and all latitudes in the Antarctic Intermediate Water (AAIW). Less ΔCanth in the eastern basin than the western basin was caused by weaker ventilation driven by SAMW and AAIW formation and subduction and stronger Antarctic Bottom Water (AABW) formation in the former than the latter. In addition to the spatial heterogeneity, Canth increase rates accelerated from the 1990s to the 2000s, consistent with the overall increase in air‐sea CO2 exchange in the South Atlantic Ocean.
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The implications of COP21 for our future climate
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  • Dec 2016
Rising CO2 in the atmosphere is the main cause of anthropogenic climate change, and the data shows a clear increase in global temperature of about 1 °C since pre-industrial levels. Changes in climate extremes are also occurring, with observed increases in the frequency of heat waves, in intense precipitation (rainfall and snowfall) in many places, and in sea level and storm surges. A changing climate with rising extremes has associated risks for food production and other health-related impacts. In order to limit climate change well below 2 °C, our carbon emissions must rapidly follow a decreasing trajectory to near zero.
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Fossil-fueled development (SSP5): An energy and resource intensive scenario for the 21st century
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This paper presents a set of energy and resource intensive scenarios based on the concept of Shared Socio-Economic Pathways (SSPs). The scenario family is characterized by rapid and fossil-fueled development with high socio-economic challenges to mitigation and low socio-economic challenges to adaptation (SSP5). A special focus is placed on the SSP5 marker scenario developed by the REMIND-MAgPIE integrated assessment modeling framework. The SSP5 baseline scenarios exhibit very high levels of fossil fuel use, up to a doubling of global food demand, and up to a tripling of energy demand and greenhouse gas emissions over the course of the century, marking the upper end of the scenario literature in several dimensions. These scenarios are currently the only SSP scenarios that result in a radiative forcing pathway as high as the highest Representative Concentration Pathway (RCP8.5). This paper further investigates the direct impact of mitigation policies on the SSP5 energy, land and emissions dynamics confirming high socio-economic challenges to mitigation in SSP5. Nonetheless, mitigation policies reaching climate forcing levels as low as in the lowest Representative Concentration Pathway (RCP2.6) are accessible in SSP5. The SSP5 scenarios presented in this paper aim to provide useful reference points for future climate change, climate impact, adaption and mitigation analysis, and broader questions of sustainable development.
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The Global Ocean Data Analysis Project version 2 (GLODAPv2) – an internally consistent data product for the world ocean
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  • Aug 2016
Version 2 of the Global Ocean Data Analysis Project (GLODAPv2) data product is composed of data from 724 scientific cruises covering the global ocean. It includes data assembled during the previous efforts GLODAPv1.1 (Global Ocean Data Analysis Project version 1.1) in 2004, CARINA (CARbon IN the Atlantic) in 2009/2010, and PACIFICA (PACIFic ocean Interior CArbon) in 2013, as well as data from an additional 168 cruises. Data for 12 core variables (salinity, oxygen, nitrate, silicate, phosphate, dissolved inorganic carbon, total alkalinity, pH, CFC-11, CFC-12, CFC-113, and CCl4) have been subjected to extensive quality control, including systematic evaluation of bias. The data are available in two formats: (i) as submitted but updated to WOCE exchange format and (ii) as a merged and internally consistent data product. In the latter, adjustments have been applied to remove significant biases, respecting occurrences of any known or likely time trends or variations. Adjustments applied by previous efforts were re-evaluated. Hence, GLODAPv2 is not a simple merging of previous products with some new data added but a unique, internally consistent data product. This compiled and adjusted data product is believed to be consistent to better than 0.005 in salinity, 1ĝ€ % in oxygen, 2ĝ€% in nitrate, 2ĝ€% in silicate, 2ĝ€% in phosphate, 4ĝ€μmolĝ€kgĝ '1 in dissolved inorganic carbon, 6ĝ€μmolĝ€kgĝ '1 in total alkalinity, 0.005 in pH, and 5ĝ€% for the halogenated transient tracers. The original data and their documentation and doi codes are available at the Carbon Dioxide Information Analysis Center (<a hrefCombining double low line"http://cdiac.ornl.gov/oceans/GLODAPv2/" targetCombining double low line" blank">http://cdiac.ornl.gov/oceans/GLODAPv2/ ). This site also provides access to the calibrated data product, which is provided as a single global file or four regional ones - the Arctic, Atlantic, Indian, and Pacific oceans - under the <a hrefCombining double low line"http://dx.doi.org/10.3334/CDIAC/OTG.NDP093GLODAPv2" targetCombining double low line"blank">doi:10.3334/CDIAC/OTG.NDP093-GLODAPv2 . The product files also include significant ancillary and approximated data. These were obtained by interpolation of, or calculation from, measured data. This paper documents the GLODAPv2 methods and products and includes a broad overview of the secondary quality control results. The magnitude of and reasoning behind each adjustment is available on a per-cruise and per-variable basis in the online Adjustment Table.
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Dissolved Organic Carbon in the North Atlantic Meridional Overturning Circulation
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  • May 2016
The quantitative role of the Atlantic Meridional Overturning Circulation (AMOC) in dissolved organic carbon (DOC) export is evaluated by combining DOC measurements with observed water mass transports. In the eastern subpolar North Atlantic, both upper and lower limbs of the AMOC transport high-DOC waters. Deep water formation that connects the two limbs of the AMOC results in a high downward export of non-refractory DOC (197 Tg-C·yr−1). Subsequent remineralization in the lower limb of the AMOC, between subpolar and subtropical latitudes, consumes 72% of the DOC exported by the whole Atlantic Ocean. The contribution of DOC to the carbon sequestration in the North Atlantic Ocean (62 Tg-C·yr−1) is considerable and represents almost a third of the atmospheric CO2 uptake in the region.
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Physically driven patchy O 2 changes in the North Atlantic Ocean simulated by the CMIP5 Earth system models: CMIP5: Modeled O 2 Trends in the NA
Article
  • Jul 2017
The subpolar North Atlantic is a key region for the oceanic uptake of heat, oxygen, and carbon dioxide. Centennial oxygen (O2) changes are investigated in the upper 700 m of the North Atlantic Ocean using a subset of Earth system models (ESMs) included in the Coupled Model Intercomparison Project phase 5. The climatological distributions of dissolved O2 averaged for the recent past period (1975–2005) are generally well captured, although the convective activity differs among the models in space and strength, and most models show a cold bias south of Greenland. By the end of the twenty-first century, all models predict an increase in depth-integrated temperature of 2–3°C, resultant solubility decrease, weakened vertical mass transport, decreased nutrient supply into the euphotic layer, and weakened export production. Despite an overall tendency of the North Atlantic to lose oxygen, patchy regions of O2 increase are observed due to the weakening of the North Atlantic Current (NAC) causing a regional solubility increase (the warming hole effect) and a decrease in the advection of subtropical, low-O2 waters into the subpolar regions (the nutrient stream effect). Additionally, a shift in the NAC position contributes to localized O2 changes near the boundaries of water masses. The net O2 change reflects the combination of multiple factors leading to highly heterogeneous and model-dependent patterns. Our results imply that changes in the strength and position of the NAC will likely play crucial roles in setting the pattern of O2 changes in future projections.
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Further Intensification of Deep Convection in the Labrador Sea in 2016: Intensified 2016 Labrador Sea Convection
Article
  • Dec 2016
There has been a progressive deepening of winter convection in the Labrador Sea since 2012, with the individual-profile maximum depth exceeding 1800 m since 2014 and reaching 2100 m in 2016. This increase, during repeated positive phases of the winter North Atlantic Oscillation (NAO), resembles that during the formation of the record-depth (2500 m) Labrador Sea Water (LSW) class in 1987-1994, attributed to repeated positive NAO forcing having provided critical preconditioning. The 2012-2016 LSW class is one of the deepest and most persistent ever observed (back to 1938). Year-round observations from profiling Argo floats since 2002 complemented by annual surveys are providing novel information on the seasonal-to-decadal evolution of LSW, such as its variable density, the recent multi-year preconditioning and its 2016 density being the highest since the mid-1990s. These findings should help international observation programs and numerical model studies investigating LSW influences on the subpolar North Atlantic and Atlantic Meridional Overturning Circulation.
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The Shared Socioeconomic Pathways and their energy, land use, and greenhouse gas emissions implications: An overview
Article
  • Sep 2016
This paper presents the overview of the Shared Socioeconomic Pathways (SSPs) and their energy, land use, and emissions implications. The SSPs are part of a new scenario framework, established by the climate change research community in order to facilitate the integrated analysis of future climate impacts, vulnerabilities, adaptation, and mitigation. The pathways were developed over the last years as a joint community effort and describe plausible major global developments that together would lead in the future to different challenges for mitigation and adaptation to climate change. The SSPs are based on five narratives describing alternative socio-economic developments, including sustainable development, regional rivalry, inequality, fossil-fueled development, and middle-of-the-road development. The longterm demographic and economic projections of the SSPs depict a wide uncertainty range consistent with the scenario literature. A multi-model approach was used for the elaboration of the energy, land-use and the emissions trajectories of SSP-based scenarios. The baseline scenarios lead to global energy consumption of 400–1200 EJ in 2100, and feature vastly different land-use dynamics, ranging from a possible reduction in cropland area up to a massive expansion by more than 700 million hectares by 2100. The associated annual CO2 emissions of the baseline scenarios range from about 25 GtCO2 to more than 120 GtCO2 per year by 2100. With respect to mitigation, we find that associated costs strongly depend on three factors: (1) the policy assumptions, (2) the socio-economic narrative, and (3) the stringency of the target. The carbon price for reaching the target of 2.6 W/m2 that is consistent with a temperature change limit of 2 �C, differs in our analysis thus by about a factor of three across the SSP marker scenarios. Moreover, many models could not reach this target from the SSPs with high mitigation challenges. While the SSPs were designed to represent different mitigation and adaptation challenges, the resulting narratives and quantifications span a wide range of different futures broadly representative of the current literature. This allows their subsequent use and development in new assessments and research projects. Critical next steps for the community scenario process will, among others, involve regional and sectoral extensions, further elaboration of the adaptation and impacts dimension, as well as employing the SSP scenarios with the new generation of earth system models as part of the 6th climate model intercomparison project (CMIP6).
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