In reply to Postmanpat:
> Yup, if Trever's characterisation of tony's link is correct. Just as I said in my OP!
> Having said that, if I'm understanding who is saying what in tony's link, the Gregory report itself is saying that "The reconstructions account for the approximate constancy of the rate of GMSLR during the 20th century, which shows small or no acceleration, despite the increasing anthropogenic forcing."
> So at least one reputable report would appear to question whether there has been acceleration. I think......
I'm guessing the actual paper itself is behind a paywall for most people here, so I'll lay out what it's saying for people. I'll admit out that I've only read the abstract and conclusions, the detail of methods and results is rather involved and not within my area of expertise.
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What the paper ISN'T about:
The study doesn't concern itself with warming or GMSLR trends post ~1990. There is little doubt that this is accelerating due to increasing contributions from the ice sheets. It finds no evidence that this isn't the case, agreeing with previous studies, and quoting a study by Church et al (2011) which is widely cited.
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What the paper IS about:
The paper sets out to reconcile differences between the observations of GMSLR (i.e. checking sea level against an actual height gauge) and the estimates of the three main contributions (loss of ice from glaciers and ice sheets, loss of water from land based storage, and thermal expansion of water already in the oceans). This is set out clearly by the authors:
> "The title of this paper refers to the difficulty of accounting for the magnitude of twentieth-century global mean sea level rise (GMSLR) estimated from the tide gauge records. Previous authors have found observed GMSLR to exceed the sum of the quantified contributions, especially in the early decades of the century, when the influence of anthropogenic climate change was small"
The problem being that calculating estimates for such processes on such a vast scale means taking data from a variety of disparate sources and making a bunch of assumptions. That's already fraught with difficulty and uncertainty, and becomes harder as you go further back in history because technology was less advanced, computerised data storage didn't exist, international standards may not have been in place, and the understanding of climate change as a serious issue was lacking.
For example, ocean measurements were non-existent in the first half of the 20th Century, so atmosphere–ocean general circulation models (AOGCMs) are used to estimate thermal expansion of the oceans. The estimate of this study is higher than previous studies because the authors make an adjustment based on the long term effects of volcanism on ocean heat storage. So the assumptions are updated and the estimates should be more accurate.
The study also finds that early 20th Century GMSLR from glaciers and ice sheets was roughly equivalent to that in the latter half. The authors lay out some possible reasons for this. There's a lot of uncertainty in the contribution from ice sheets (i.e. Antarctica and Greenland), but the contribution from these is almost certainly accelerating today. Due to their continental scale, the ice sheets take longer to respond to climate forcings so there is a lag time.
The overall picture from the paper is that the climate was cooler than it's long term average in the early part of the century, due to volcanic activity and reduced solar irradiance. Therefore part of the trend of warming and increasing GMSLR was due to a rebound from this cooler state, which led into the anthropogenic warming trend we now see.
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GWPF interpretation:
The GWPF (Global Warming Policy Forum) interpretation is to paint this paper as an estimate of current acceleration trends, which is a highly dishonest reading of it, because it's explicitly not about that, and the authors state so.
The phrase ‘about as likely as not’ was at no point used by either the IPCC or the authors of the study to refer to acceleration trends of the GMSLR. The IPCC report states it is 'likely' that GMSLR rates have accelerated since 1900, which is not at odds with the results of the studies mentioned.
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I hope that's useful in clearing things up!