Ocean ventilation and deoxygenation in a warming world

09:05-09:40 Recent oxygen trends in the atmosphere and the oceans: what do we know?

Professor Ralph Keeling, Scripps Institute for Oceanography, USA

Abstract

Human activities are causing systematic decreases in the O₂ content of both the atmosphere and the oceans. The atmospheric loss is driven primarily by the burning of fossil-fuels while the oceanic loss is driven primarily by warming-induced reductions in O₂ solubility and slowing of ocean circulation, i.e. reduced ventilation. Oceanic deoxygenation could have potentially large environmental consequences, particularly if continued warming leads to an expansion of hypoxic or suboxic waters, as suggested by some models. Measurements of O₂ in both the ocean and atmosphere are recognized as having considerable diagnostic value, and this has fuelled an expansion of measurements and measurement capabilities in recent years. The oceanic O₂ measurements have helped to establish the magnitude and mechanisms of recent O₂ changes. The atmospheric O₂ measurements have helped to quantify global carbon sinks and to provide a window into sources of oceanic O₂ variability via the tracer atmospheric potential oxygen (APO ~ O₂+CO₂). APO measurements show strong signals related to ocean ventilation which vary from year to year, and the well-measured global APO trend can potentially be used to quantify the global oceanic deoxygenation rate. This talk will highlight results from oceanic and atmospheric O₂ measurements in the context of ongoing changes in ocean ventilation and deoxygenation.

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10:00-10:40 Ocean impacts of climate change: the IPCC and beyond

Professor Monika Rhein, IUP-MARUM, University of Bremen, Germany

Abstract

The ocean is a main player in the climate system. Owing to lack of historic observations, reliable estimates of, for instance, oceanic heat budgets were only possible after 1970. Since that time, ocean warming accounted for more than 90% of the increase in the Earth’s energy inventory, while only a small fraction heated the atmosphere, the continents, and was used for melting of sea ice, glaciers and ice sheets. The ocean also plays a major role in the long-term variability of the atmospheric temperature, and could for several years obscure the mean global temperature increase, mainly caused by anthropogenic emissions of CO2. The ocean warming also has consequences for the uptake of anthropogenic carbon by enhancing the vertical density stratification in the ocean und thus potentially reducing the ventilation of intermediate, deep, and bottom water. Changes in the intermediate water ventilation in the subpolar North Atlantic occurred from the 1990s to today, a period well covered with oceanic observations. Although caused by natural variability of atmospheric modes on multiannual time scales (i.e. the NAO), this region provides a test bed to study the processes and mechanisms to improve our understanding of the changes expected under global warming.

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11:30-12:10 Ecological consequences of ocean deoxygenation on continental margins

Professor Lisa A. Levin, Scripps Institution for Oceanography, USA

Abstract

Continental margins play fundamental roles in ocean biogeochemical cycling and are increasingly valued as a source of fisheries, energy, biodiversity and potentially mineral resources. Margin settings are highly sensitive to climate-induced changes in winds, upwelling, stratification, circulation, nutrient supply and freshwater input, all of which can affect oxygenation. Observations over the last half-century show major declines in ocean dissolved oxygen concentrations at intermediate depths, particularly on margins of the NE Pacific. Common consequences of margin deoxygenation include avoidance, range shifts, habitat compression, altered trophic structure, physiological and behavioural adaptation, with resulting changes in community composition and diversity.  This presentation will examine the biological consequences of margin deoxygenation and underlying mechanisms through use of (a) natural spatial gradients associated with oxygen minimum zones (OMZs), (b) long-term, time series observations, (c) historical records from sediment cores and (d) laboratory studies of physiological tolerances and behaviour.

Margins provide natural laboratories that offer a glimpse into ecosystems of the future and enable predictions regarding regions of high vulnerability to climate change. The fish and invertebrate communities on OMZ margins in the E. Pacific, N. Indian Ocean and off West Africa experience strong gradients in oxygen and other stressors that shape ecological pattern and evolutionary adaptation. Long-term data from southern California reveal habitat compression of ichthyoplankton and of benthic sea urchins consistent with deoxygenation over the last 25 years. For this same region historical sediment records document community response to oxygen fluctuations on margins over multiple time scales and laboratory experiments suggest a need to understand covariation of oxygen and CO2 on upwelling margins.

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13:30-14:10 Oxygen and deoxygenation in global ocean models

Dr Laurent Bopp, IPSL-LSCE, France

Abstract

In response to anthropogenic climate change, coupled climate-marine biogeochemical models used over the past 15 years all project a long-term decrease in the ocean O₂ inventory, referred to as ocean deoxygenation. This general trend is confirmed by the latest projections from the CMIP5 Earth System Models, with reductions in the ocean O₂ inventory from 1.5 to 4% in 2090s relative to 1990s for all future scenarios. Largest declines are concentrated at mid-depths in the North Atlantic, North Pacific and Southern Ocean. The processes at play are linked to surface ocean warming, which reduces both O₂ solubility and the rate at which the surface waters are transported downward. In the tropics however, projections of dissolved oxygen concentrations at mid-depths are not consistent across the suite of Earth System Models, leading to large uncertainties in the evolution of the oxygen minimum zones under anthropogenic global warming. There, changes in solubility and in the apparent oxygen utilization almost compensate so that projected changes in dissolved oxygen are small and inconsistent across models. In this talk, we will focus on how coupled climate models simulate the compensation between these two drivers of oxygen changes (O₂ saturation and apparent oxygen utilization) across a variety of time-scales, from the Last Glacial Maximum to interannual-variability, and on how these changes propagate into O₂ concentrations and the volume of oxygen minimum zones for these different time-scales.

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14:30-15:10 High latitude ocean ventilation and it's role in the Earth's climate transitions

Professor Alberto Naveira-Garabato, University of Southampton, UK

Abstract

The processes regulating ocean ventilation at high latitudes are re-examined based on a range of observations spanning all scales of ocean circulation – from the basin scales of gyres to the centimeter scales of turbulence. It is shown that ocean ventilation is controlled by mechanisms that are different in important ways from those that set the ocean's overturning circulation, contrary to the common assumption of broad equivalence between the two when interpreting the role of ventilation in Earth's major climate transitions. Illustrations of how recognizing this distinction may change our view of how ventilation changes shape climate transitions will be offered.

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16:00-16:40 Patterns of deoxygenation - natural and anthropogenic drivers

Professor Andreas Oschlies, GEOMAR and Kiel University, Germany

Abstract

Observational estimates and numerical models both indicate a significant decline in marine oxygen levels over the past decades. Spatial patterns, however, differ considerably between observed and modelled estimates, particularly in the tropical thermocline that hosts open-ocean oxygen minimum zones, where observations indicate a general oxygen decline, whereas most current models simulate increasing oxygen levels. Possible reasons for the apparent model-data discrepancies are examined. In order to attribute observed historical variations in oxygen levels, mechanisms of changes in oxygen supply are studied with sensitivity model simulations. Specifically, the role of equatorial jets, of lateral and diapycnal mixing processes, and of changes in the wind-driven circulation are investigated. Predominantly wind-driven changes in the low-latitude ventilation are identified as major factor contributing to oxygen changes in the low-latitude thermocline during the past decades. Possible implications for likely future climate change on the evolution of oxygen minimum zones are discussed.

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17:00-18:15 Poster session

09:00-09:40 The chemical physics of ocean oxygen losses under climate change

Dr Peter G. Brewer, Monterey Bay Aquarium Research Institute, USA

Abstract

For over 50 years ocean scientists have oddly represented ocean oxygen consumption rates as a function of depth but not temperature. This unique tradition or tactic now extends across such a wide range of oceanic biogeochemical processes that it inhibits useful discussion of climate change impacts where specific and fundamental temperature dependent terms are required. Depth, but not temperature, dependent functions as of now form the basis for the most widely used climate-biogeochemical models. Tracer based determinations of oxygen consumption rates, and thus CO2, PO4 etc production rates, in the deep sea are near universally reported as a function of depth in spite of their well-known microbial basis. In recent work we have shown that a carefully determined profile of oxygen consumption rates in the Sargasso Sea can be well represented by a classical Arrhenius function with and activation energy of 86.5 kJ mol-1 leading to a Q10 of 3.63. This indicates that for 2°C warming we will have a 29% increase in ocean oxygen consumption rates, and for 3°C warming a 50% increase, leading to large scale ocean hypoxia. Here we show that the same principles apply to a world-wide collation of tracer based oxygen consumption rate data and that some 95% of ocean oxygen consumption is driven by temperature, not depth, and thus has a strong climate dependence. The Arrhenius/Eyring equations are no simple panacea and they require a non-equilibrium steady state to exist. Where transient events are in progress this stricture is not obeyed and we show one such example. This rapid injection of fresh organic material and its associated microbial population is still clearly in non-steady state and revealed as such in the observed oxygen consumption rate data.

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10:00-10:40 Deoxygenation impacts on biogeochemical cycles, climatic gases, microbial activity and ecosystems.

Dr Carol Robinson, UEA, UK

Abstract

Dissolved oxygen concentration is a major determinant of the microbially mediated processes responsible for the production and turnover of climatically relevant gases in the ocean. Ocean models predict declines of 1 to 7% in the global ocean O2 inventory over the next century due to lower solubility of oxygen at warmer temperatures, and increased stratification preventing equilibration of the ocean interior with the atmosphere. Additionally, eutrophication continues to lead to hypoxic or anoxic conditions in the coastal zone.

Due to microbial respiration, water separated from equilibration with the atmosphere by increased stratification will contain both low O2 and high CO2 concentrations. Low oxygen environments enable the production of nitrous oxide as a by-product of nitrification and denitrification and methane through anaerobic methanogenesis, with most O2 deficient systems acting as net sources of CO2, N2O and CH4 to the atmosphere.

This presentation will provide an overview of the effect of ocean deoxygenation on microbial community structure and the cycling of climatically important gases including CO2, N2O and CH4.

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11:30-12:10 Biogeochemical regulation: anoxia in the past and present

Professor Andrew Watson FRS, University of Exeter, UK

Abstract

The major biogeochemical cycles which keep the present-day Earth habitable are linked by a network of feedbacks which has led to a broadly stable chemical composition of the oceans and atmosphere over hundreds of millions of years. This includes the processes which control both the atmospheric and oceanic concentrations of oxygen. However, one notable exception to the generally well-behaved dynamics of this system is the propensity for episodes of ocean anoxia to occur and to persist for 105 – 106 years, these OAEs (Ocean Anoxic Events) being particularly associated with warm “greenhouse” climates.  OAEs are, we believe, amplified by positive feedbacks on the nutrient content of the ocean: low oxygen promotes the release of phosphorus from ocean sediments, which increases ocean productivity and drives more anoxia in the subsurface water, leading to a potentially self-sustaining condition of deoxygenation.  The rapidly increasing degree of ocean deoxygenation occurring today as a result of the warming climate could conceivably result in such a very long-lasting and unpleasant consequence for the Earth’s biogeochemical cycles which underpin the “life support” system of the biosphere.

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13:30-14:10 Dynamic deoxygenation within intensified coastal upwelling circulations

Professor Andrew Bakun, University of Miami, USA

Abstract

Major deoxygenation commonly takes place in ocean regions that feature particularly intense coastal upwelling circulations. Prominent examples discussed herein include the seasonal Somali Current upwelling, which during the southwest monsoon becomes the most intense coastal upwelling cell existing in the world’s oceans, and the upwelling that occurs on a more continuous basis in the ocean off Lüderitz, Namibia, constituting the most intense of the world’s “classical” eastern ocean boundary upwelling systems. Concerns about enhanced ocean deoxygenation arise in view of the arguable likelihood that coastal upwelling systems around the world may further intensify as anthropogenic climate change proceeds.

Another suggestive example is the sudden seasonal appearance, just a decade and a half ago of an oxygen deficient “dead zone” that has since become quite a reliable annually recurring phenomena off the U.S. State of Oregon. This apparent “transience” in the causal dynamics is somewhat mirrored in an observed interannual–scale intermittence in the situation off Namibia.

A mechanistic scheme that may draw these examples into a common context is presented. The strength of horizontal surface flow divergence that drives the upwelling circulation, the pattern of horizontal flow vorticity and the potentially manageable abundance of strongly swimming small pelagic fish appear in this scheme as plausible controlling variables.

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14:30-15:10 Ecosystem modelling of oceanic oxygen and implications of emissions of nitrous oxide

Dr Oliver Andrews, UEA, UK

Abstract

Secular decreases in dissolved oxygen concentration have been observed within the tropical Oxygen Minimum Zones (OMZs) and at mid- to high latitudes over the last ~ 50 years.  Whilst a pervasive feature of recent Earth System Model (ESM) projections is a reduction in the ocean’s oxygen inventory in response to anthropogenic warming, current models are unable to consistently reproduce observed trends and variability, particularly within the OMZs.  Here we conduct a series of targeted hindcast experiments using a state-of-the-art global ecosystem model (PlankTOM10-NEMO) including idealised model experiments forced with observationally derived oxygen trends (1960 – 2010) in order to explore and review biases in model distributions of oceanic oxygen.  Using a model representation of marine nitrous oxide (N2O) cycling we investigate the implications of simulated changes in oxygen for historical emissions of N2O.  Despite occupying less than 10 % of the ocean by volume, hypoxic and suboxic waters account for ~ 25–50 % of open ocean N2O production as a consequence of both enhanced denitrification and improved N2O yields from nitrification under low-O2 conditions.  We show that the inability of models to reproduce historical shoaling and expansion of established tropical OMZs causes major discrepancies in estimates of oceanic N2O production.  Using historical oxygen fields from a range of Coupled Model Intercomparison Project Phase 5 (CMIP5) simulations we also assess recent past and projected changes in marine N2O from current generation ESMs.

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15:50-16:30 The ocean's changing role in the earth system in a warming world

Professor Katherine Richardson, University of Copenhagen, Denmark

Abstract

From an Earth system (ES) perspective, ocean feedbacks on the global carbon cycle (and, thereby, climate system) are of particular interest. While the physical ocean feedbacks in the climate system are relatively well constrained, the biologically mediated feedbacks remain poorly understood. Here, the current understanding of these biological feedbacks is examined with a focus on identifying areas in urgent need of further study. The temperature sensitivity of bacterial respiration in the surface layer of the ocean is reasonably well quantified and substantial changes in the distributions of oxygen and nutrient turnover in the ocean are projected in response to changes in bacterial activity in a warming world, leading to significant changes in the surface ocean-atmosphere CO2 flux. Biologically-mediated surface to deep ocean carbon flux in the form of particulate organic carbon (POC), i.e., the “biological pump”, is also critically important for the global carbon cycle. It is often assumed on the basis of model studies that the magnitude of POC produced in surface waters will decrease in a warmer ocean as increased stratification will reduce the delivery of nutrients to the surface layer and, thereby, reduce phytoplankton photosynthesis, i.e., the primary source of POC production. This assumption warrants re-examination for several reasons. 1) increased bacterial activity in the surface layer will increase the regeneration of inorganic nutrients in this layer; 2) our limited understanding of the vertical distribution of ocean photosynthesis and 3) limited understanding of the relationship between different plankton ecosystem types and the efficiency of the biological pump.

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16:45-17:00 Summary of discussions