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 user 2006-08-30 at 11:57:00 am Views: 79
  • #16345

    Melting ice dilutes northern seas
    pouring into northern oceans is slowly turning high-latitude waters
    less salty.Shrinking ice sheets and melting glaciers are partly
    responsible for the freshening effect, a review in the journal Science
    has confirmed.If salinity levels continue to drop, dramatic changes to
    the North Atlantic currents could occur.But more work is needed to be
    sure that rising global temperatures are to blame, say the authors.”For
    the last 50 years, oceanographers have been cruising seas at northern
    latitudes taking vertical profiles of salinity, and they have observed
    gradual declines,” said lead author Bruce Peterson, of the Marine
    Biological Laboratory, Woods Hole, US.”The salt water, although still
    very salty, is getting fresher.”

    Warm shallows, cold depths
    volume of fresh water is a good match for the amount which rivers,
    precipitation, sea-ice melt and glacier melt are producing. Run-off
    from these sources must be creating the dilution effect, the
    researchers conclude.The measurements are taken from the Nordic seas
    and Atlantic Sub-polar Basins.Cold water from the Arctic is usually
    exchanged for warm water from the tropics in a self-propelling cycle.In
    the north, the warm water arriving via surface currents sinks and flows
    back to warmer climes through the deep ocean. Because fresher water is
    less dense, it does not sink so far as salty water would at the same
    temperature.If the trend continues, the changes to this current system
    may be significant. “It is expected that the North Atlantic circulation
    will slow down,” said Professor Peterson.

    Global warming or not?
    how likely is the trend to continue? It is not yet possible to be sure
    to what extent global warming can be blamed for the changes, say the
    authors.Fluctuating salinity could potentially result from a normal
    periodical weather pattern known as the North Atlantic Oscillation
    index. If this index is high, seas in the north are less salty than
    average Until 1995, the changes were in line with what would be
    expected under the climatic conditions; but when the index changed
    recently, becoming low or neutral, the proportion of fresh water did
    not go down accordingly in some places.So the greater bulk of fresh
    water running into the sea is probably – at least in part – due to
    rising global temperatures. “I suspect parts of it are due to global
    warming. It’s a difficult quandary,” said Professor Peterson.If salt
    levels continue to decline, and currents change, the implications for
    aquatic ecosystems would be dramatic.”The organisms in the oceans are
    affected by the distribution of sea ice, and by temperatures and
    salinity fields, and all of these would change,” the author
    explained.”Changes in these currents would have tremendous impact for
    fisheries and other species important to man.”