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Fwd: SEA LEVEL RISE - CAUSES

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  • Patrick Neuman
    ... wrote: Nature 428, 406 - 409 (25 March 2004) © 2004 Nature Publishing Group LAURY MILLER, BRUCE C. DOUGLAS: Mass and volume
    Message 1 of 1 , Apr 13, 2004
      --- In globalwarming@yahoogroups.com, "hombredelatierra"
      <hombredelatierra2@y...> wrote:
      Nature 428, 406 - 409 (25 March 2004) © 2004 Nature Publishing
      Group

      LAURY MILLER, BRUCE C. DOUGLAS: Mass and volume contributions to
      twentieth-century global sea level rise

      "The rate of twentieth-century global sea level rise and its causes
      are the subjects of intense controversy. Most direct estimates from
      tide gauges give 1.5–2.0 mm yr-1, whereas indirect estimates
      based on
      the two processes responsible for global sea level rise, namely mass
      and volume change, fall far below this range. Estimates of the volume
      increase due to ocean warming give a rate of about 0.5 mm yr-1 and
      the rate due to mass increase, primarily from the melting of
      continental ice, is thought to be even smaller. Therefore, either the
      tide gauge estimates are too high, as has been suggested recently, or
      one (or both) of the mass and volume estimates is too low. Here we
      present an analysis of sea level measurements at tide gauges combined
      with observations of temperature and salinity in the Pacific and
      Atlantic oceans close to the gauges. We find that gauge-determined
      rates of sea level rise, which encompass both mass and volume
      changes, are two to three times higher than the rates due to volume
      change derived from temperature and salinity data. Our analysis
      supports earlier studies that put the twentieth-century rate in the
      1.5–2.0 mm yr-1 range, but more importantly it suggests that mass
      increase plays a larger role than ocean warming in twentieth-century
      global sea level rise.


      At the time of the second IPCC assessment in 1999, there seemed to be
      little controversy regarding global sea level rise (GSLR). Most gauge
      estimates fell in the range 1.5–2.0 mm yr-1. Most of this rise
      was
      thought to result from ocean warming, with the rest due to melting of
      continental ice. However, by the third IPCC assessment in 2001, this
      consensus view had collapsed: new and better estimates of ocean
      warming had reduced the volume increase component to about 0.5 mm yr-
      1, and the mass component was thought to be even smaller. This left a
      large unexplained gap between direct and indirect estimates of GSLR,
      now known as the 'attribution problem'.

      Two recent studies offer opposing solutions to this dilemma. Cabanes
      et al argue that gauge rates are 2–3 times too high because the
      gauges happen to be located in areas of abnormally high ocean
      warming. They arrive at this result by comparing gauge-derived sea
      level trends with those obtained from objectively interpolated
      hydrographic measurements, concluding that the true rate of GSLR is
      actually 0.5–1.0 mm yr-1, mostly due to ocean warming. This
      solution
      provides a way out of the attribution problem, but implies a huge
      acceleration of GSLR in the 1990s if recent satellite altimetric
      estimates of 2.5 mm yr-1 (ref. 10) are to be believed. Alternatively,
      Antonov et al suggest that the problem may be solved by revising
      upward the mass component estimate. They show that the oceans are
      freshening at a rate equivalent to the addition of 1.4 mm yr-1 of
      fresh water, approximately the value needed to bring the mass plus
      volume rate close to the gauge rate. However, this solution assumes a
      continental ice source rather than floating ice, a key point that
      they are unable to demonstrate.

      Here we present a simple approach to the problem of distinguishing
      between mass and volume contributions to GSLR. We identify large
      areas in the Pacific and Atlantic oceans that are either bounded by,
      or adjacent to, several gauge sites exhibiting similar trends and
      variability. For those regions, we compare average gauge trends
      (which reflect both mass and volume change) with average dynamic
      height trends (which reflect only volume change), with the added
      distinction that we use raw rather than interpolated hydrographic
      data. In most areas, the difference between raw and interpolated data
      are unimportant. ...

      ... twentieth-century sea level rose at a rate several times higher
      than can be accounted for by volume (temperature and salinity)
      changes alone. ...

      <<Recall that volume increase is due to temp change - warm water
      expands - while mass change results from glacial melt. Frank>>

      Our results help to resolve two aspects of the present controversy
      over twentieth-century GSLR. Concerning the question of whether or
      not the gauge results are biased high owing to above-average local
      ocean warming, we conclude that they are not. It follows that the
      weight of direct evidence strongly suggests a rate of 1.5–2.0 mm
      yr-1
      for twentieth-century GSLR. This is in agreement with most
      traditional estimates from long gauge records, and is reasonably
      consistent with the 2.5 mm yr-1 rate obtained by ongoing satellite
      altimeters for the period 1993–2003. ...

      <<interesting aside: it is sometimes pointed out - incorrectly - that
      satellite temp trend data is not consistent with GW. In contrast,
      satellites give the strongest evidence for ocean level rise: 2.5
      millimeter per annum versus the more traditional 1.5-2.0 mm/yr. >>

      Concerning the causes of sea level rise, our results provide clear
      evidence that changes in ocean volume due to temperature and salinity
      account for only a fraction of sea level change, and that mass change
      plays a dominant role in twentieth-century GSLR. This aspect of our
      results is consistent with the results of Antonov et al., who show
      that the global oceans freshened during the latter half of the
      twentieth century by an amount equivalent to 1.4 mm yr-1 of fresh
      water, but goes further by indicating that the source must be
      continental. The only alternative to this interpretation is that what
      we identify as mass change is actually a mass redistribution within
      the global ocean rather than a mass increase due to the addition of
      fresh water. However, for this to be true, large areas of the global
      ocean would have to have falling sea levels for the entire twentieth
      century. Observations from the global tide gauge network, consisting
      mostly of gauge sites located along the margins of the ocean basins,
      do not support this viewpoint. Whether the mid-oceans are currently
      undergoing such changes can only be determined from long-term, high-
      precision satellite altimeter missions, such as the TOPEX/Poseidon
      and Jason missions, which are at present under way." © 2004 Nature
      Publishing Group
      --- End forwarded message ---
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