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Don’t estimate acceleration by fitting a quadratic…

Filed under: — stefan @ 20 November 2012

… if your data do not look like a quadratic!

This is a post about global sea-level rise, but I put that message up front so that you’ve got it even if you don’t read any further.

The reputable climate-statistics blogger Tamino, who is a professional statistician in real life and has published a couple of posts on this topic, puts it bluntly:

Fitting a quadratic to test for change in the rate of sea-level rise is a fool’s errand.

I’d like to explain why, with the help of a simple example. Imagine your rate of sea-level rise changes over 100 years in the following way:
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A sea level Golden Horseshoe nominee*

Filed under: — gavin @ 14 October 2012

I was reading a sign high on the wall behind the bar:

‘Only genuine pre-war British and American whiskeys served here’

I was trying to count how many lies could be found in those nine words, and had reached four, with promise of more …”

Dashiell Hammett, “The Golden Horseshoe”

Google News occasionally throws up some obscure postings that I would never otherwise come across. A recent example was a letter to an editor of a Scottish newspaper (not my usual reading material) declaring that “Climate change is not man-made”. The letter itself is uninteresting – a basic confusion between weather and climate seguing into a NIMBY-ish rant about windmills. Ho hum.

However, in one of the comments from a “Dr John Cameron, St Andrews” (posted 9/Oct/2012), there was this unrelated pseudo-factoid:

As regards the catastrophic sea level rise in the Pacific, it became obvious some 20 years ago that results from island tide gauges did not support computer predictions. Scientists from Flinders University in Adelaide set up new, modern, tide-gauges in 12 Pacific islands to test whether there was in fact any evidence of sinking. Recently the whole project was abandoned as there had been no sign whatsoever of a change in sea level at any of the 12 islands for the past 16 years.

Now this is specific enough to probably actually refer to something real, but doesn’t pass the sniff test for something that might actually be true. Scientists don’t set up monitoring stations only to get the answer they want and then stop monitoring if it doesn’t happen. This only happens in the fevered imaginations of conspiracy theorists. So I was intrigued enough to investigate what this actually referred to…
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My oh Miocene!

Guest commentary by Sarah Feakins

Our recent study in Nature Geoscience reconstructed conditions at the Antarctic coast during a warm period of Earth’s history. Today the Ross Sea has an ice shelf and the continent is ice covered; but we found the Antarctic coast was covered with tundra vegetation for some periods between 20 million and 15.5 million years ago. These findings are based on the isotopic composition of plant leaf waxes in marine sediments.

That temperatures were warm at that time was not a huge surprise; surprising, was how much warmer things were – up to 11ºC (20ºF) warmer at the Antarctic coast! We expected to see polar amplification, i.e. greater changes towards the poles as the planet warms. This study found those coastal temperatures to be as warm as 7ºC or 45ºF during the summer months. This is a surprise because conventional wisdom has tended to think of Antarctica being getting progressively colder since ice sheets first appeared on Antarctica 34 million years ago (but see Ruddiman (2010) for a good discussion of some of the puzzles).
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References

  1. S.J. Feakins, S. Warny, and J. Lee, "Hydrologic cycling over Antarctica during the middle Miocene warming", Nature Geosci, vol. 5, pp. 557-560, 2012. http://dx.doi.org/10.1038/NGEO1498
  2. W.F. Ruddiman, "A Paleoclimatic Enigma?", Science, vol. 328, pp. 838-839, 2010. http://dx.doi.org/10.1126/science.1188292

Far out in North Carolina

Filed under: — stefan @ 24 June 2012

The extensive salt marshes on the Outer Banks of Carolina offer ideal conditions for unravelling the mysteries of sea level change during past centuries. Here is a short report from our field work there – plus some comments on strange North Carolina politics as well as two related new papers published today in Nature Climate Change.


The Outer Banks of Carolina are particularly vulnerable to coastal erosion and sea-level rise, partly because the land is subsiding and the banks are naturally moving landward. On the ocean front, land is continually being lost.
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OHC Model/Obs Comparison Errata

Filed under: — gavin @ 22 May 2012

This is just a brief note to point out that a few graphs that I have put together showing Ocean Heat Content changes in recent decades had an incorrect scaling for the GISS model data. My error was in assuming that the model output (which were in units W yr/m2) were scaled for the ocean area only, when in fact they were scaled for the entire global surface area (see fig. 2 in Hansen et al, 2005). Therefore, in converting to units of 1022 Joules for the absolute ocean heat content change, I had used a factor of 1.1 (0.7 x 5.1 x 365 x 3600 x 24 x 10-8), instead of the correct value of 1.61 (5.1 x 365 x 3600 x 24 x 10-8). This problem came to light while we were redoing this analysis for the CMIP5 models and from conversations with dana1981 at skepticalscience.com.

These graphs appeared in Dec 2009, May 2010, Jan 2011 and Feb 2012. In each case, I have replaced the graph with a corrected version while leaving a link to the incorrect version. Links to the figures will return the corrected image (and this is noted on the image itself). Where possible I used the data that were current at the time of the original post. Fortunately this only affects the figures used in these blog postings and not in any publications. Apologies for any confusion.

This figure shows the comparison using the most up-to-date observational products (NODC, PMEL):

The basic picture is unchanged – model simulations were able to capture the historical variance in OHC (as best we know it now – there remains significant structural uncertainty in those estimates). There are clear dips related volcanic eruptions (Agung, El Chichon, Pinatubo), and an sharp increase in the 1990s. Note that in GISS-EH (same AGCM but with a different ocean model) OHC increases at a slightly slower rate than seen with GISS-ER above. Looking at the last decade, it is clear that the observed rate of change of upper ocean heat content is a little slower than previously (and below linear extrapolations of the pre-2003 model output), and it remains unclear to what extent that is related to a reduction in net radiative forcing growth (due to the solar cycle, or perhaps larger than expected aerosol forcing growth), or internal variability, model errors, or data processing – arguments have been made for all four, singly and together.

Analyses of the CMIP5 models will provide some insight here since the historical simulations have been extended to 2012 (including the last solar minimum), and have updated aerosol emissions. Watch this space.

References

  1. J. Hansen, "Earth's Energy Imbalance: Confirmation and Implications", Science, vol. 308, pp. 1431-1435, 2005. http://dx.doi.org/10.1126/science.1110252

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