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\"From New York Times bestselling author Susan Casey, an awe-inspiring portrait of the mysterious world beneath the waves, and the men and women who seek to uncover its secrets For all of human history, the deep ocean has been a source of wonder and terror, an unknown realm that evoked a singular, compelling question: What's down there? Unable to answer this for centuries, people believed the deep was a sinister realm of fiendish creatures and deadly peril. But now, cutting-edge technologies allow scientists and explorers to dive miles beneath the surface, and we are beginning to understand this strange and exotic underworld: A place of soaring mountains, smoldering volcanoes, and valleys 7,000 feet deeper than Everest is high, where tectonic plates collide and separate, and extraordinary life forms operate under different rules. Far from a dark void, the deep is a vibrant realm that's home to pink gelatinous predators and shimmering creatures a hundred feet long and ancient animals with glass skeletons and sharks that live for half a millennium-among countless other marvels. Susan Casey is our premiere chronicler of the aquatic world. For The Underworld she traversed the globe, joining scientists and explorers on dives to the deepest places on the planet, interviewing the marine geologists, marine biologists, and oceanographers who are searching for knowledge in this vast unseen realm. She takes us on a fascinating journey through the history of deep-sea exploration, from the myths and legends of the ancient world to storied shipwrecks we can now reach on the bottom, to the first intrepid bathysphere pilots, to the scientists who are just beginning to understand the mind-blowing complexity and ecological importance of the quadrillions of creatures who live in realms long thought to be devoid of life. Throughout this journey, she learned how vital the deep is to the future of the planet, and how urgent it is that we understand it in a time of increasing threats from climate change, industrial fishing, pollution, and the mining companies that are also exploring its depths. The Underworld is Susan Casey's most beautiful and thrilling book yet, a gorgeous evocation of the natural world and a powerful call to arms\"-- Provided by publisher.

The concentration of dissolved oxygen in aquatic systems helps to regulate biodiversity(1,2), nutrient biogeochemistry(3), greenhouse gas emissions(4), and the quality of drinking water(5). The long-term declines in dissolved oxygen concentrations in coastal and ocean waters have been linked to climate warming and human activity(6,7), but little is known about the changes in dissolved oxygen concentrations in lakes. Although the solubility of dissolved oxygen decreases with increasing water temperatures, long-term lake trajectories are difficult to predict. Oxygen losses in warming lakes may be amplified by enhanced decomposition and stronger thermal stratification(8,9) or oxygen may increase as a result of enhanced primary production(10). Here we analyse a combined total of 45,148 dissolved oxygen and temperature profiles and calculate trends for 393 temperate lakes that span 1941 to 2017. We find that a decline in dissolved oxygen is widespread in surface and deep-water habitats. The decline in surface waters is primarily associated with reduced solubility under warmer water temperatures, although dissolved oxygen in surface waters increased in a subset of highly productive warming lakes, probably owing to increasing production of phytoplankton. By contrast, the decline in deep waters is associated with stronger thermal stratification and loss of water clarity, but not with changes in gas solubility. Our results suggest that climate change and declining water clarity have altered the physical and chemical environment of lakes. Declines in dissolved oxygen in freshwater are 2.75 to 9.3 times greater than observed in the world's oceans(6,7) and could threaten essential lake ecosystem services(2,3,5,11).

Twenty-first century projections for the Mediterranean water properties have been analyzed using the largest ensemble of regional climate models (RCMs) available up to now, the Med-CORDEX ensemble. It is comprised by 25 simulations, 10 historical and 15 scenario projections, from which 11 are ocean–atmosphere coupled runs and 4 are ocean forced simulations. Three different emissions scenarios are considered: RCP8.5, RCP4.5 and RCP2.6. All the simulations agree in projecting a warming across the entire Mediterranean basin by the end of the century as a result of the decrease of heat losses to the atmosphere through the sea surface and an increase in the net heat input through the Strait of Gibraltar. The warming will affect the whole water column with higher anomalies in the upper layer. The temperature change projected by the end of the century ranges between 0.81 and 3.71 °C in the upper layer (0–150 m), between 0.82 and 2.97 °C in the intermediate layer (150–600 m) and between 0.15 and 0.18 °C in the deep layer (600 m—bottom). The intensity of the warming is strongly dependent on the choice of emission scenario and, in second order, on the choice of Global Circulation Model (GCM) used to force the RCM. On the other hand, the local structures reproduced by each simulation are mainly determined by the regional model and not by the scenario or the global model. The salinity also increases in all the simulation due to the increase of the freshwater deficit (i.e. the excess of evaporation over precipitation and river runoff) and the related increase in the net salt transport at the Gibraltar Strait. However, in the upper layer this process can be damped or enhanced depending upon the characteristics of the inflowing waters from the Atlantic. This, in turn, depends on the evolution of salinity in the Northeast Atlantic projected by the GCM. Thus a clear zonal gradient is found in most simulations with large positive salinity anomalies in the eastern basin and a freshening of the upper layer of the western basin in most simulations. The salinity changes projected for the whole basin range between 0 and 0.34 psu in the upper layer, between 0.08 and 0.37 psu in the intermediate layer and between − 0.05 and 0.33 in the deep layer. These changes in the temperature and salinity modify in turn the characteristics of the main water masses as the new waters become saltier, warmer and less dense along the twenty-first century. There is a model consensus that the intensity of the deep water formation in the Gulf of Lions is expected to decrease in the future. The rate of decrease remains however very uncertain depending on the scenario and model chosen. At the contrary, there is no model consensus concerning the change in the intensity of the deep water formation in the Adriatic Sea and in the Aegean Sea, although most models also point to a reduction.

Changes in deep-ocean circulation and stratification have been argued to contribute to climatic shifts between glacial and interglacial climates by affecting the atmospheric carbon dioxide concentrations. It has been recently proposed that such changes are associated with variations in Antarctic sea ice through two possible mechanisms: an increased latitudinal extent of Antarctic sea ice and an increased rate of Antarctic sea ice formation. Both mechanisms lead to an upward shift of the Atlantic meridional overturning circulation (AMOC) above depths where diapycnal mixing is strong (above 2000 m), thus decoupling the AMOC from the abyssal overturning circulation. Here, these two hypotheses are tested using a series of idealized two-basin ocean simulations. To investigate independently the effect of an increased latitudinal ice extent from the effect of an increased ice formation rate, sea ice is parameterized as a latitude strip over which the buoyancy flux is negative. The results suggest that both mechanisms can effectively decouple the two cells of the meridional overturning circulation (MOC), and that their effects are additive. To illustrate the role of Antarctic sea ice in decoupling the AMOC and the abyssal overturning cell, the age of deep-water masses is estimated. An increase in both the sea ice extent and its formation rate yields a dramatic “aging” of deep-water masses if the sea ice is thick and acts as a lid, suppressing air–sea fluxes. The key role of vertical mixing is highlighted by comparing results using different profiles of vertical diffusivity. The implications of an increase in water mass ages for storing carbon in the deep ocean are discussed.

The overturning pathways for the surface-ventilated North Atlantic Deep Water (NADW) and Antarctic Bottom Water (AABW) and the diffusively formed Indian Deep Water (IDW) and Pacific Deep Water (PDW) are intertwined. The global overturning circulation (GOC) includes both large wind-driven upwelling in the Southern Ocean and important internal diapycnal transformation in the deep Indian and Pacific Oceans. All three northern-source Deep Waters (NADW, IDW, PDW) move southward and upwell in the Southern Ocean. AABW is produced from the denser, salty NADW and a portion of the lighter, low oxygen IDW/PDW that upwells above and north of NADW. The remaining upwelled IDW/PDW stays near the surface, moving into the subtropical thermoclines, and ultimately sources about one-third of the NADW. Another third of the NADW comes from AABW upwelling in the Atlantic. The remaining third comes from AABW upwelling to the thermocline in the Indian-Pacific. Atlantic cooling associated with NADW formation (0.3 PW north of 32°S; 1 PW = 1015W) and Southern Ocean cooling associated with AABW formation (0.4 PW south of 32°S) are balanced mostly by 0.6 PW of deep diffusive heating in the Indian and Pacific Oceans; only 0.1 PW is gained at the surface in the Southern Ocean. Thus, while an adiabatic model of NADW global overturning driven by winds in the Southern Ocean, with buoyancy added only at the surface in the Southern Ocean, is a useful dynamical idealization, the associated heat changes require full participation of the diffusive Indian and Pacific Oceans, with a basinaveraged diffusivity on the order of the Munk value of 10⁻⁴ m² s⁻¹.

While a rapid sea ice retreat in the Arctic has become ubiquitous, the potential weakening of the Atlantic meridional overturning circulation (AMOC) in response to global warming is still under debate. As deep mixing occurs in the open ocean close to the sea ice edge, the strength and vertical extent of the AMOC is likely to respond to ongoing and future sea ice retreat. Here, we investigate the link between changes in Arctic sea ice cover and AMOC strength in a long simulation with the EC-Earth–Parallel Ice Sheet Model (PISM) climate model under the emission scenario RCP8.5. The extended duration of the experiment (years 1850–2300) captures the disappearance of summer sea ice in 2060 and the removal of winter sea ice in 2165. By introducing a new metric, the Arctic meridional overturning circulation (ArMOC), we document changes beyond the Greenland–Scotland ridge and into the central Arctic. We find an ArMOC strengthening as the areas of deep mixing move north, following the retreating winter sea ice edge into the Nansen Basin. At the same time, mixing in the Labrador and Greenland Seas reduces and the AMOC weakens. As the winter sea ice edge retreats farther into the regions with high surface freshwater content in the central Arctic Basin, the mixing becomes shallower and the ArMOC weakens. Our results suggest that the location of deep-water formation plays a decisive role in the structure and strength of the ArMOC; however, the intermittent strengthening of the ArMOC and convection north of the Greenland–Scotland ridge cannot compensate for the progressive weakening of the AMOC.

Deep-water access is arguably the most effective, but under-studied, mechanism that plants employ to survive during drought. Vulnerability to embolism and hydraulic safety margins can predict mortality risk at given levels of dehydration, but deep-water access may delay plant dehydration. Here, we tested the role of deep-water access in enabling survival within a diverse tropical forest community in Panama using a novel data-model approach.We inversely estimated the effective rooting depth (ERD, as the average depth of water extraction), for 29 canopy species by linking diameter growth dynamics (1990-2015) to vapor pressure deficit, water potentials in the whole-soil column, and leaf hydraulic vulnerability curves. We validated ERD estimates against existing isotopic data of potential water-access depths.Across species, deeper ERD was associated with higher maximum stem hydraulic conductivity, greater vulnerability to xylem embolism, narrower safety margins, and lower mortality rates during extreme droughts over 35 years (1981-2015) among evergreen species. Species exposure to water stress declined with deeper ERD indicating that trees compensate for water stress-related mortality risk through deep-water access.The role of deep-water access in mitigating mortality of hydraulically-vulnerable trees has important implications for our predictive understanding of forest dynamics under current and future climates.

Water masses are bodies of water with distinctive physical and biogeochemical properties. They impart vertical structure to the deep ocean, participate in circulation, and can be traced over great distances, potentially influencing the distributions of deep-sea fauna. The classic potential temperature-salinity (θ-s) diagram was used to investigate deep-sea sponge (demosponge genus Geodia) association with water masses over the North Atlantic Ocean and Nordic Seas. A novel analysis was conducted, based on sampling the curvature of climatological mean θ-s curves at sponge locations. Sponges were particularly associated with turning points in the θ-s curves, indicative of intermediate and deep water masses. Arctic geodiid species (G. hentscheli and G. parva) were associated with Arctic intermediate and deep waters in the Nordic Seas, and with dense overflows into the northern North Atlantic. Boreal species (G. atlantica, G. barretti, G. macandrewii, and G. phlegraei) were associated with upper and intermediate water masses in the Northeast Atlantic and with upper, Atlantic-derived waters in the Nordic Seas. Taken together with distributional patterns, a link with thermohaline currents was also inferred. We conclude that water masses and major current pathways structure the distribution of a key deep-sea benthic faunal group on an ocean basin scale. This is most likely because of a combination of the physical constraints they place on the dispersal of early life-history stages, ecophysiological adaptation (evolved tolerances) to specific water masses, and the benefits to filter-feeders of certain phenomena linked to water column structure (e.g. nepheloid layers, internal waves/tides, density-driven currents).

For decades oceanographers have understood the Atlantic meridional overturning circulation (AMOC) to be primarily driven by changes in the production of deep-water formation in the subpolar and subarctic North Atlantic. Indeed, current Intergovernmental Panel on Climate Change (IPCC) projections of an AMOC slowdown in the twenty-first century based on climate models are attributed to the inhibition of deep convection in the North Atlantic. However, observational evidence for this linkage has been elusive: there has been no clear demonstration of AMOC variability in response to changes in deep-water formation. The motivation for understanding this linkage is compelling, since the overturning circulation has been shown to sequester heat and anthropogenic carbon in the deep ocean. Furthermore, AMOC variability is expected to impact this sequestration as well as have consequences for regional and global climates through its effect on the poleward transport of warm water. Motivated by the need for a mechanistic understanding of the AMOC, an international community has assembled an observing system, Overturning in the Subpolar North Atlantic Program (OSNAP), to provide a continuous record of the transbasin fluxes of heat, mass, and freshwater, and to link that record to convective activity and water mass transformation at high latitudes. OSNAP, in conjunction with the Rapid Climate Change–Meridional Overturning Circulation and Heatflux Array (RAPID–MOCHA) at 26°N and other observational elements, will provide a comprehensive measure of the three-dimensional AMOC and an understanding of what drives its variability. The OSNAP observing system was fully deployed in the summer of 2014, and the first OSNAP data products are expected in the fall of 2017.

Deep water formation in the northern North Atlantic has been of long-standing interest because the resultant water masses, along with those that flow over the Greenland–Scotland Ridge, constitute the lower limb of the Atlantic meridional overturning circulation (AMOC), which carries these cold, deep waters southward to the subtropical region and beyond. It has long been assumed that an increase in deep water formation would result in a larger southward export of newly formed deep water masses. However, recent observations of Lagrangian floats have raised questions about this linkage. Motivated by these observations, the relationship between convective activity in the Labrador Sea and the export of newly formed Labrador Sea Water (LSW), the shallowest component of the deep AMOC, to the subtropics is explored. This study uses simulated Lagrangian pathways of synthetic floats produced with output from a global ocean–sea ice model. It is shown that substantial recirculation of newly formed LSW in the subpolar gyre leads to a relatively small fraction of this water exported to the subtropical gyre: 40 years after release, only 46% of the floats are able to reach the subtropics. Furthermore, waters produced from any one particular convection event are not collectively and contemporaneously exported to the subtropical gyre, such that the waters that are exported to the subtropical gyre have a wide distribution in age.