Recent oxygen trends in the atmosphere and the oceans: what do we know?
Professor Ralph Keeling, Scripps Institute for Oceanography, USA
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.
Ocean impacts of climate change: the IPCC and beyond
Professor Monika Rhein, IUP-MARUM, University of Bremen, Germany
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.
Ecological consequences of ocean deoxygenation on continental margins
Professor Lisa A. Levin, Scripps Institution for Oceanography, USA
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.