Archive for rivers

Ocean’s heat cycle shows that atmospheric carbon may be headed elsewhere

Posted by hbenway 
· Thursday, August 16th, 2018 

Studies over the past 25 years have supported the existence of a large net land biosphere CO2 sink of 0.5–2 PgC yr-1. Significant uncertainties remain, however, regarding the long-term partitioning between northern, tropical, and southern land sinks, in part connected to the uncertain ocean carbon sink. These uncertainties limit our capacity to predict earth system response to anthropogenic changes and design effective mitigation strategies.

Land sinks from atmospheric inversion (1990-2010 average) with two different ocean/river fluxes: (top) previous ocean inversion-based carbon fluxes; and (bottom) updated pCO2-based air-sea flux with a scaled-up river flux of 0.78 PgC /yr.

In a recent study published in Nature Geoscience, Resplandy et al. (2018) used models and field observations to demonstrate that the world’s oceans transport heat between the northern and southern hemispheres in the same way that carbon is transported. The transport of heat, however, is easier to observe. By tracking this heat, they showed that the Southern Ocean — while still a substantial carbon sink —may not take up as much carbon as previously thought, and that ocean currents might transport 20 to 100% more carbon from the northern to the southern hemisphere. To maintain this additional transport of carbon, they showed that the amount of carbon entering the ocean from rivers may be as much as 70% higher than estimated in previous global carbon budget studies. These changes in the ocean and river carbon transport imply that up to 40% of the world’s atmospheric carbon absorbed by land ecosystems needs to be reallocated from existing estimates.

Authors
L. Resplandy, Princeton University
Ralph Keeling, Scripps Institution of Oceanography/ UCSD
Christian Rödenbeck, Max Planck Institute
Briton Stephens, NCAR
Matthew Long, NCAR
Samar Khatiwala, University of Oxford
Keith Rodgers, Princeton University
Laurent Bopp, ENS Paris
Pieter Tans, NOAA’s ESRL

Do rivers supply nutrients to the open ocean?

Posted by mmaheigan 
· Wednesday, May 24th, 2017 

Rivers carry large amounts of nutrients (e.g., nitrogen and phosphorus) to the sea, but we do not know how much of that riverine nutrient supply escapes biological and chemical processing in shallow coastal waters to reach the open ocean. Most global ocean biogeochemical models, which are typically unable to resolve coastal processes, assume that either all or none of the riverine nutrients entering coastal waters actually contribute to open ocean processes.

While we know a good deal about the dynamics of individual rivers entering the coastal ocean, studies to date have been limited to a few major river systems, mainly in in developed countries. Globally, there are over 6,000 rivers entering the coastal ocean. In a recent study, Sharples et al (2017) devised a simple approach to obtain a global-scale estimate of riverine nutrient inputs based on the knowledge that low-salinity waters entering the coastal ocean tend to form buoyant plumes that turn under the influence of Earth’s daily rotation to flow along the coastline. Using published data on such flows and incorporating the effect of Earth’s rotation, they obtained estimates of typical cross-shore plume width and compared them to the local width of the continental shelf. This was used to calculate the residence time of riverine nutrients on the shelf, which is the key to estimating how much of a given nutrient is consumed in shelf waters vs. how much is exported to the open ocean.

Global distribution of the amount of riverine dissolved inorganic nitrogen that escapes the continental shelf to reach the open ocean.

The results indicate that, on a global scale, 75% (80%) of the nitrogen (phosphorus) supplied by rivers reaches the open ocean, whereas 25% (20%) of the nitrogen (phosphorus) is consumed on the shelf (e.g., fueling coastal productivity). Limited knowledge of nutrient cycling and consumption in shelf waters represents the primary source of uncertainty in this study. However, well-defined global patterns related to human land use (e.g., agricultural fertilizer use in developed nations) emerged from this analysis, underscoring the need to understand how land-use changes and other human activities will alter nutrient delivery to the coastal ocean in the future.

 

Authors:
Jonathan Sharples (School of Environmental Sciences, University of Liverpool, UK)
Jack Middelburg (Department of Earth Sciences, Utrecht University, Netherlands)
Katja Fennel (Department of Oceanography, Dalhousie University, Canada)
Tim Jickells (School of Environmental Sciences, University of East Anglia, UK)

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