Sunday, March 18, 2012

2659.txt

cc: Phil Jones <P.JonesatXYZxyz.ac.uk>, mannatXYZxyz.edu
date: Sat, 31 May 2008 19:24:29 -0400 (EDT)
from: Gavin Schmidt <gschmidtatXYZxyzs.nasa.gov>
subject: RE: [Fwd: of buckets and blogs...]

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Phil - here's the text minus figures and links... It's subject to a little
revision, but let me know if there are any factual or emphasis issues that
are perhaps misplaced.

Thanks

Gavin

========


Of buckets and blogs

This last week has been an interesting one for observers of how climate
change is covered in the media and online. On Wednesday an interesting
paper (Thompson et al) was published in Nature, pointing to a clear
artifact in the sea surface temperatures in 1945 and associating it with
the changing mix of fleets and measurement techniques at the end of World
War II. The mainstream media by and large got the story right - puzzling
anomaly tracked down, corrections in progress after a little scientific
detective work, consequences minor - even though a few headline writers
got a little carried away in equating a specific dip in 1945 ocean
temperatures with the more gentle 1940s-1970s cooling that is seen in the
land measurements. However, some blog commentaries have gone completely
overboard on the implications of this study in ways that are very
revealing of their underlying biases.

The best commentary came from John Nielsen-Gammon's new blog where he
described very clearly how the uncertainties in data - both the known
unknowns and unknown unknowns - get handled in practice (read this and
then come back). Stoat, quite sensibly, suggested that it's a bit early to
be expressing an opinion on what it all means. But patience is not one of
the blogosphere's virtues and so there was no shortage of people
extrapolating wildly to support their pet hobbyhorses. This in itself is
not so unusual; despite much advice to the contrary, people (the media and
bloggers) tend to weight individual papers that make the news far more
highly than the balance of evidence that really underlies assessments like
the IPCC. But in this case, the addition of a little knowledge made the
usual extravagances a little more scientific-looking and has given it some
extra steam.

Like almost all historical climate data, ship-board sea surface
temperatures (SST) were not collected with long term climate trends in
mind. Thus practices varied enormously among ships and fleets and over
time. In the 19th Century, simple wooden buckets would be thrown over the
side to collect the water (a non-trivial exercise when a ship is moving,
as many novice ocean-going researchers will painfully recall). Later on,
special canvas buckets were used, and after WWII, insulated 'buckets'
became more standard - though these aren't really buckets in the
colloquial sense of the word as the photo shows (pay attention to this
because it comes up later).

The thermodynamic properties of each of these buckets are different and so
when blending data sources together to get an estimate of the true
anomaly, corrections for these biases are needed. For instance, the canvas
buckets give a temperature up to 1C cooler in some circumstances (that
depend on season and location) than the modern insulated buckets.
Insulated buckets have a slight cool bias compared to temperature
measurements that are taken at the inlet for water in the engine room
which is the most used method at present. Automated buoys which became
more common in recent decades tend to be cooler than the engine intake
measures as well. The recent IPCC report had a thorough description of
these issues (section 3.B.3) fully acknowledging that these corrections
were a work in progress.

And that is indeed the case. The collection and digitisation of the ship
logbooks is a huge undertaking and continues to add significant amounts of
20th Century and earlier data to the records. This dataset (ICOADS) is
continually growing, and the impacts of the bias adjustments are
continually being assessed. The biggest transitions in measurements
occurred at the beginning of WWII between 1939 and 1941 when the sources
of data switched from European fleets to almost exclusively US fleets (and
who tended to use engine inlet temperatures rather than canvas buckets).
This offset was large and dramatic and was identified more than ten years
ago from comparisons of simultaneous measurements of night-time marine air
temperatures (NMAT) which did not show such a shift. The experimentally
based adjustment to account for the canvas bucket cooling brought the sea
surface temperatures much more into line with the NMAT series (Folland and
Parker, 1995). (Note that this reduced the 20th Century trends in SST).

More recent work (for instance, at this workshop in 2005), has focussed on
refining the estimates and incorporating new sources of data. For
instance, the 1941 shift in the original corrections, was reduced and
pushed back to 1939 with the addition of substantial and dominant amounts
of US Merchant Marine data (which mostly used engine inlets temperatures).

The version of the data that is currently used in most temperature
reconstructions is based on the work of Rayner and colleagues (reported in
2006). In their discussion of remaining issues they state:

Using metadata in the ICOADS it is possible to compare the
contributions made by different countries to the marine component of the
global temperature curve. Different countries give different advice to
their observing fleets concerning how best to measure SST. Breaking the
data up into separate countries' contributions shows that the assumption
made in deriving the original bucket correctionsthat is, that the use of
uninsulated buckets ended in January 1942is incorrect. In particular, data
gathered by ships recruited by Japan and the Netherlands (not shown) are
biased in a way that suggests that these nations were still using
uninsulated buckets to obtain SST measurements as late as the 1960s. By
contrast, it appears that the United States started the switch to using
engine room intake measurements as early as 1920.

They go on to mention the modern buoy problems and the continued need to
work out bias corrections for changing engine inlet data as well as minor
issues related to the modern insulated buckets. For example, the
differences in co-located modern bucket and inlet temperatures are around
0.1 deg C:


(from John Kennedy).

However it is one thing to suspect that biases might remain in a dataset
(a sentiment shared by everyone), it is quite another to show that they
are really there. The Thompson et al paper does the latter quite
effectively by removing variability associated with some known climate
modes (including ENSO) and seeing the 1945 anomaly pop out clearly. In
doing this in fact, they show that the previous adjustments in the pre-war
period were probably ok (though there is substantial additional evidence
of that in any case - see the references in Rayner et al, 2006). The
Thompson anomaly seems to coincide strongly with the post-war shift back
to a mix of US, UK and Dutch ships, implying that post-war bias
corrections are indeed required and significant. This conclusion is not
much of a surprise to any of the people working on this since they have
been saying it in publications and meetings for years. The issue is of
course quantifying and validating the corrections, for which the Thompson
analysis might prove useful. The use of canvas buckets by the Dutch,
Japanese and some UK ships is most likely to blame, and given the mix of
national fleets shown above, this will make a noticeable difference in
1945 up to the early 1960s maybe - the details will depend on the seasonal
and areal coverage of those sources compared to the dominant US
information. The schematic in the Independent is probably a good first
guess at what the change will look like (remember that the ocean changes
are constrained by the NMAT record shown above).

So far, so good. The fun for the blog-watchers is what happened next. What
could one do to get the story all wrong? First, you could incorrectly
assume that scientists working on this must somehow be unaware of the
problems (that is belied by the frequent mention of post WWII issues in
workshops and papers since at least 2005, but never mind). Next, you could
conflate the 'buckets' used in recent decades (as seen in the graphs in
Kent et al 2007's discussion of the ICOADS meta-data) with the buckets in
the pre-war period (see photo above). If you do make that mistake however,
you can extrapolate to get some rather dramatic (if erroneous)
conclusions. For instance, that the effect of the 'corrections' would be
to halve the SST trend from the 1970s. Gosh! (The mismatch this would
create with the independent NMAT data series should not be mentioned). But
there is more! You could take the (incorrect) prescription based on the
bucket confusion, apply it to the full global temperatures (land included,
hmm) and think that this merits a discussion on whether the whole IPCC
edifice had been completely undermined (Answer: no). And it goes on - the
bucket confusion was pointed out but the complaint switches to the scandal
that it wasn't properly explained.

All this shows is wishful thinking overcoming logic. However many times
there is a similar rush to judgment that is subsequently showed to be
based on nothing, it still adds to the vast array of similar 'evidence'
that keeps getting trotted out by by the ill-informed. The excuse that
these are just exploratory exercises in what-if thinking wears a little
thin when the 'what if' always leads to the same (desired) conclusion.
This week's play-by-play was quite revealing on that score.

*--------------------------------------------------------------------*
| Gavin Schmidt NASA/Goddard Institute for Space Studies |
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| Tel: (212) 678 5627 New York, NY 10025 |
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| gschmidtatXYZxyzs.nasa.gov http://www.giss.nasa.gov/~gavin |
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