Thursday, May 17, 2012


date: Tue, 31 Aug 2004 08:35:47 +0100
from: Jonathan Gregory <>
subject: [Wg1-ar4-ch06] sea level, land ice, attribution of sea level change
to: IPCC-WG1 <>,,,,,

Dear colleagues

I'm writing this to chapters 4 (obs of cryospheric changes), 5 (obs of ocean
changes), 6 (palaeoclimate), 9 (understanding and attributing climate change)
and 10 (projections). The subjects of this email are (a) ice and sea level,
and (b) attribution of 20th-century sea-level change (not just the ice
contribution). I apologise for disturbing those who are not concerned with
these subjects.

I see on the agenda for Trieste there's a cross-chapter meeting of 4 and 6
about ice sheets and sea level. That's good. It notes possible participation
by chapter 5. I would suggest that there is a certainly a need for a
discussion including chapter 5, and also the sea-level interest of chapter 10
(i.e. me), either at this or another meeting.

In the TAR, the land-ice contribution to sea level was dealt with entirely in
the sea-level chapter, for the palaeorecord, 20th century, and future. This
made it easy to coordinate its treatment. In the AR4 sea-level is dispersed
into several chapters. I agree with this being done, but it means we have to
take care to be consistent. It's very welcome that we have lots of experts in
in cryospheric topics this time. I'd particularly like to discuss:

* Observational and model-based estimates of C20 land-ice contributions (both
ice sheets and glaciers) to sea level. For the projections, we obviously need
to use models. As far as possible, we should try to use the same models for
both projections and model estimates of the C20 contributions. In the TAR we
used mass-balance sensitivities, an area inventory and volume-area scaling
(papers of Zuo and Oerlemans, Gregory and Oerlemans, Van de Wal and Wild) for
glaciers, and and ice sheet model with degree-day ablation, precipitation and
dynamics (Huybrechts) for ice sheets. The input was AOGCM temperature and
precipitation for the C20 and the future. I would find it very useful to talk
about these approaches and alternatives.

* The future of the Greenland and West Antarctic ice-sheets, especially the
difficult question of dynamic instability. It would be helpful to talk about
where these issues will be covered, and what the current spread of views is in
the glaciological community. Discussion of what changes have occurred in the
ice sheets in the past clearly belongs in chapters 4 or 6, while the question
of how they might behave in the future goes in chapter 10, where it is likely
to be a controversial topic. Yet the past and future are obviously related, so
much so that in the TAR they were covered in the same section of the sea-level

* Ongoing adjustment of the ice sheets to past climate change. This background
term persists into the future so is relevant to projections.

My other issue is the explanation of C20 sea-level change i.e. whether we can
reconcile observed rates (both tide-gauge and altimeter) with the
contributions we have identified (thermal expansion, glaciers, ice-sheets,
etc.). Chapter 4 is going to assess C20 land-ice contributions to sea level,
and chapter 5 the thermal expansion. For thermal expansion, as for land ice,
it would be good to use similar model methods for past and future changes
e.g. they can be done from AOGCMs in both cases.

In the extended outline, 5.5 is a mixture of observations of sea level change
(mean and extremes), and explanations of it. I would like respectfully to
suggest that the "explanations" doesn't really belong in chapter 5, which is
an observations chapter. Instead, I would propose that the explanation of past
sea-level rise should be discussed in chapter 9, the detection and attribution
chapter. The reasons are:

* It then comes after *all* the relevant material, including palaeoclimate,
which might have something to say about long-term ice-sheet contributions, and
climate models, which are needed for quantitative modelling of the past.

* Chapter 9 is where explanations of changes in other quantities are discussed
(temperature, precipitation, ocean heat content, etc.) so it makes sense.

* It's really quite interdisciplinary, certainly no more to do with ocean obs
than with ice obs, and also involves discussion of other points which
naturally arise in chapter 9, such as the relative contribution of natural and
anthropogenic factors. In fact, as you know, sea level can have a direct
anthropogenic contribution (reservoirs and groundwater mining in particular),
not climate-related at all. This is mentioned in the outline of 5.5, but
isn't a subject of ocean or sea level observation.

* We might be able and wish to make a detection/attribution kind of statement,
such as sea level change is beyond natural variability, can be attributed
partially to anthropogenic forcing, etc. We did have a weak statement of this
kind in the TAR. "It is very likely that the 20th century warming has
contributed significantly to the observed sea level rise, through thermal
expansion of sea water and widespread loss of land ice." The basis of this
statement was in the sea-level chapter, not the D&A chapter, but in the
technical summary and the summary for policymakers we put it along with the
(much stronger) temperature-based D&A statements, where it logically belongs,
I think.

* Section 5.5 would be a more focussed discussion of observed changes in
sea-level, which in themselves are quite a complicated issue. The discussion
of the interpretation of these changes deserves a section of its own, as it
had in the TAR (11.4), and that would raise its profile. It's something
people often ask about.

I wasn't at any of the scoping meetings, where maybe this question was
discussed. If so, please bear with my raising it again, but I'd like to ask
that we talk about the treatment of this subject in Trieste. Perhaps it could
be a matter for a another cross-chapter meeting. Of course this is not
directly to do with projections - I'm writing about it because I'm interested
(!) and because of my concern for consistent model treatment and terms in past
explanations and future projections.

Best wishes

Jonathan Gregory
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