Bruce Mapstone, chief, CSIRO Marine and Atmospheric Research, Hobart, Tasmania (note
Hobart, there's more) wrote to
The Australian "Sea-level observations". February 9th, 2012
BOB Carter's references to sea level ("Scientific research drowning in a sea of alarmism", 7/2) require clarification.
Sea level observations come from measurement by satellites and, as Carter notes, a network of tide gauges around the Australian coastline operated by the Bureau of Meteorology and similar gauges internationally.
These observations show that rates of sea-level rise around Australia range from 2.6mm a year to 9.5mm a year since the early 1990s, compared with the global average rate over a similar period of about 3.2mm a year.
Variation in sea levels from region to region is to be expected, as Carter also notes, but continuing analysis indicates the dominant trend around Australia for the past 20 years is upward and above the global average.
The global average of observations is near the high end of the Inter-governmental Panel on Climate Change model projections over the same period, indicating that the range of model projections encompass changes that have occurred, and indeed that the models may be underestimating observed changes.
Lets' jump right in and have a look at the Hobart tide gauge data - right on Bruce's doorstep at CSIRO Mansions on Castray Esplanade, overlooking the harbour, and probably within sight of the gauge -
View Larger Map
(click charts to enlarge)
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| Hobart, Tasmania - Sea Level 1987-2010 |
Oh dear, things are not looking too good for Bruce's "clarification", the rate from 1987 is
-0.438 mm/year (0.67 from 1962). The odd 1987 start date is due to the fact that there are large gaps and anomalous readings prior to then. The gauge was displaced several times by ship's masters who couldn't park neatly, and had to be replaced (the gauge, not the masters, but who knows?).
He was referring to an opinion piece by Bob Carter two days previously, in which he attacked a "a science-fiction prediction of a 91cm rise in sea level in the district by 2100". Carter ridiculed Lake Macquarie City Council's new coastal planning regulations, which control new developments around the lake, mandating higher floor levels to "account for sea level rise and wave action". Yet the NSW Department of Planning has said there's "uncertainty surrounding the manner in which sea level will rise", which means whatever floor levels they enforce are likely to be either inadequate or excessive. The "91" figure (notice it's not a round 90 or "almost a metre") implies an estimate accuracy of +/- 1cm. The range is given as LOW: 18 cm. MEDIUM: 55 cm. HIGH: 91 cm. So the overall spread is 1:5, yet the figures are quoted to a single cm accuracy? Over a period of 90 years? That really is "science fiction". I could stick my neck out and say 50 +/-50 centimetres, which should cover most eventualities. Of course, I could be wrong. In fact I know I'll be wrong, which is why my neck will stay
in.
The regulations are based on that 91cm
maximum predicted rise by 2100, and Carter says
This prediction comes from the NSW government, which in turn sourced it from a UN political body, the Intergovernmental Panel on Climate Change. As with its unnecessarily alarmist projections of global warming, the IPCC's estimate of future sea-level rise derives from speculative computer models.
He also lists what he terms "four facts"
First, that rates of sea-level change vary around the Australian coast. This means any new coastal planning regulations (if and where they are needed) should be based on the appropriate local sea-level measurements rather than a hypothetical global average.
Second, the longest east coast tide-gauge record, from Fort Denison (Sydney), records an average rate of rise over the past 100 years of about 1mm a year (10cm a century).
Third, that other tide gauges, scattered around Australia as part of the national tidal network, mostly record rates of long-term rise between about 0.5mm and 2.5mm a year with no change in behaviour in the late 20th century that might reflect a human (global warming) influence.
And, fourth, that the Sydney tide gauge, as well as other long tidal records from nearby (Fremantle, Auckland) and overseas, exhibits a slowing rate of sea-level rise over the past 40 years.
How do Carter's "facts" stack up? His first is common sense. A health service bases its policies on local , not world, nor even regional demographic statistics and forecasts. It's logical to expect local and state authorities to do the equivalent as regards local planning.
His second is indisputable (except by Bruce Mapstone and others, who just ignore it. "Post-normal" science doesn't need facts).
His third is correct if you accept his qualification of
mostly, and I accept it, having analysed data from most of the 95 National Tidal Centre tide gauges (plus two privately owned) around Australia. How does it square with what Bruce Mapstone says? First note that Brucie (I feel I know him well already) doesn't actually
refute what Carter wrote. In short, he can't - it's fact. Note also what Mapstone actually said - "from 2.6mm a year to 9.5mm a year since the early 1990s". "Early 1990s" is a bit vague, isn't it? Well no, it isn't, because the CSIRO have chosen to use a subset of 14 of those 97 gauges, installed and monitored by the
ABSLMP project at the
National Tidal Centre of the
Bureau of Meteorology. In other words, they've chosen to use just
one seventh of the data available to them. Cherry anyone?
The fourth is indeed fact. Data from most of the long-term records show a slowing rather than an acceleration, and most sites show a flat or declining trend since 1999. I've posted on the CSIRO's data selection bias and more recent, lower rates
here.
Back to Bruce's last paragraph
The global average of observations is near the high end of the Inter-governmental Panel on Climate Change model projections over the same period, indicating that the range of model projections encompass changes that have occurred, and indeed that the models may be underestimating observed changes.
Beat that for gobbledygook - if the model "projections" didn't "encompass changes that have occurred" then they can't be much use. If the models
may be underestimating "observed changes" it doesn't automatically follow that they are also underestimating future "projections". I don't know, and
he doesn't know what assumptions are made in those models.
Then there's a (partial but flawed) rebuttal of Mapstone's "non-rebuttal" the following day. "
Levels all at sea" from one William Kininmonth:
THE clarification on Australian sea-level observations by Bruce Mapstone (Letters, 9/2) fails to reveal all of the uncertainties relating to the data.
Australian tide gauges accurately measure the relative movements of sea level with respect to the gauges. However, the vertical movement of individual tide gauges is uncertain.
The vertical movement
isn't uncertain, at least not for some of the gauges. A
global network of Continuous Global Positioning System (CGPS) stations is already established, designed to monitor land movement and/or gauge movement. There are 12 around Australia, near the ABSLMP stations. Here's the current plot for
Sydney - appropriately the station's in Macquarie Park!
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| Latest ULR GPS position time series for Sydney |
A note for those who've heard of the CGPS stations used by the NTC - the stations are remote from the gauges (up to several km inland), they monitor land movement, and do not "automatically adjust" the vertical position of the gauge. That's both impractical (the gauges are of necessity firmly fixed to their supports), and undesirable. It's not only the sea that rises and falls with the pull of the sun and moon, but the Earth's crust too (crustal tide), up to about 10cm range, depending on latitude, greatest near the point nearest to the sun and/or moon. If the gauge position was kept constant relative to the GPS satellites, the gauge would be dropping relative to the land as the tide and land rose, and would register a higher reading than it should, and a lower reading as tide and land fell.
And for the last word, from a body which even Bruce might take note of, The Tasmanian Planning Commission, in its sober but a trifle "consensus science" "
State of the Environment (SoE) Tasmania 2009" report is precise and factual (my bold):
Sea-level rise for Australia is only slightly less than the global average (NTC 2007; Church and White 2006) and coastal observations confirm that sea-levels have been rising since at least 1920 by approximately 1.2 mm/yr (Church et al. 2006; IPCC 2001; see also Pittock and Wratt 2001; Lambeck 2002). A common feature in many model projections is a higher than the global average sea-level rise off the southeast coast of Australia and in a band stretching across the Indian and southern Pacific oceans at about 30°–45°S (CSIRO 2009). Sea-level rise is greatest (about 3 mm/yr) in the eastern equatorial Pacific and western equatorial Indian Ocean. Observed rates of rise are smallest (about 1 mm/yr) in the western equatorial Pacific and eastern Indian Ocean, particularly the northwest coast of Australia. Two of the longest continuous Australian tide gauge records are from Fremantle in Western Australia (92 years) and Fort Denison in New South Wales (83 years) indicate that the observed rate of sea-level rise relative to the land has been 1.38 mm/yr and 0.86 mm/yr respectively (DPIWE 2004). Regional variations in the rate of sea-level rise are weaker for much of the rest of the global oceans (AGO 2003).
They haven't got the last word, though, I have. If the CSIRO chooses to use the state-of-the-art NTC SEAFRAME gauge data for (some of) Australia, why didn't they use data from similar NTC gauges across the Pacific islands for their "Climate Change in the Pacific" reports instead of spatially coarse satellite data? Why did they choose to use (coarse) gridded HADSST2 sea surface temperature data rather than localised in-situ measurements recorded by those same gauges? You answer the question, I already know the answer.
![Click on a red dot to jump to the relevant sea level graph](https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEgnkfe-KIX-AhM5XPeFklgJ12RcZgwCjtT6Me6wOzK1RJ1plzEZbHLLfyJgrHhqqqPtOHs73BiHGtYvlMtA715A1p_PJ8n0jjg1gVzbqi7Wrd_eoqvOWdKav943FBmTnuYYHcM28zgWsOg/s1600/pacific.gif" "Click on a red dot to jump to the relevant sea level graph")
