A lot has been made of a paper (Lyman et al, 2006) that appeared last year
that claimed that the oceans had, contrary to expectation, cooled over the
period 2003-2005. At the time, we (correctly) pointed out that this result
was going to be hard to reconcile with continued increases in sea level rise
(driven in large part by thermal expansion effects), and that there may
still be issues with way that the new ARGO floats were being incorporated
into the ocean measurement network. Now it seems as if there is a problem in
the data and in the latest analysis, the cooling has disappeared.
Ocean heat content changes are potentially a great way to evaluate climate
model results that suggest that the planet is currently significantly out of
equilibrium (i.e. it is absorbing more energy than it is emitting). However,
the ocean is a very big place and the historical measurement networks are
plagued with sampling issues in space and time. Large scale, long term
compilations globally (such as by Levitus et al, 2001; Willis et al, 2004)
and regionally (i.e. North Atlantic) have indicated that the oceans have
warmed in recent decades at pretty much the rate the models expected.
Since 2000, though, ARGO - which is a network of floats that move up and
down in the ocean and follow the currents - has offered the potential to
dramatically increase the sampling density in the ocean and provide, pretty
much for the first time, continuous, well spaced data from the least
visited, but important parts of the world (such as the Southern Oceans).
Data on ocean heat content from these floats had been therefore eagerly
anticipated.
Initial ARGO measurements were incorporated into the Willis et al, 2004
analysis, but as the ARGO data started to dominate the data sources from
around 2003, Lyman et al reported that the ocean seemed to be cooling. These
were only short term changes, and while few would confuse one or two
anomalous years with a long term trend, they were a little surprising, even
if they didn't change the long term picture very much.
The news this week though is that all of that 'cooling' was actually due to
combination of a faulty pressure reading on a subset of the floats and a
switch between differently-biased observing systems (Update: slight change
in wording to better reflect the paper). The pressure error meant that the
temperatures were being associated with a point higher in the ocean column
than they should have been, and this (given that the ocean cools with depth)
introduced a spurious cooling trend when compared to earlier data. This
error may be fixable in some cases, but for the time being the suspect data
has simply been removed from the analysis. The new results don't show any
cooling at all.
Are we done then? Unfortunately no. Because of the paucity of measurements,
assessments of ocean heat content need to use a wide variety of sensors,
each with their own quirks and problems. Combined with switches in data
sources over the years, there is a significant potential for non-climatic
trends to creep in. In particular, the eXpendable BathyThermographs (XBTs -
sensors that are essentially just thrown off the side of the ship) have a
known problem in that they didn't fall as quickly as they were originally
assumed to. This gives a warm bias (see this summary from Ingleby and Palmer
or the paper by Gouretski and Koltermann) , particularly in data from the
1970s before corrections were fully implemented. We are still going to have
to wait for the 'definitive' ocean heat content numbers, however, it is
important to note that all analyses give long term increases in ocean heat
content - particularly in the 1990s - whether they include the good ARGO
data or exclude the XBTs or not).
From realclimate more he
http://www.realclimate.org/index.php...-not/#more-436