Government Comments on the Final Draft of the SPM IPCC Working ...

<strong>Government</strong> <strong>Comments</strong> on the Final Draft of the SPM
IPCC Working Group I Fourth Assessment Report
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is more than double the (9 cm) low estimate from the TAR.
[Govt. of United States of America (Reviewer’s comment ID #: 2023-75)]
SPM-734 A 11:29 11:30 Sentence that reads “Smaller assessed uncertainties in glacier and ice cap changes also contribute to a reduced upper bound” is
awkwardly worded. Suggest instead something like “An improved knowledge base about glacier and ice cap changes have
enabled uncertainties in this area to be reduced, which has also contributed to a reduction in the upper bound for SLR
projections.”
[Govt. of Canada (Reviewer’s comment ID #: 2004-52)]
SPM-735 A 11:29 11:30 Suggest replacing the sentence "Smaller assessed uncertainties in glacier and ice cap7changes also contribute to a reduced
upper bound" as it is not clear that it is correct. A possible expanded reformulation would be: "Compared to the TAR a
smaller estimate of the contribution of glacier and ice caps has been made due to methodological improvements and changes,
however large uncertainties remain in relation to the global mass balance sensitivity, the total volume of ice and the
contribution of glacier and ice caps around Greenland and Antarctica, which account for about half of the total ice volume of
G&IC. The contribution of these latter glaciers has been limited to a fixed fraction (20%) of the calculated melting of G&IC
outside of the ice sheet peripheries, although it is likely that melt and discharge rate from these would increase with time
during the 21st century as is already evidenced from the Antarctic Peninsula and fringing glaciers of the Greenland ice sheet”.
Reason 1) Global mass balance sensitivity: - this has been tuned since the SOD to fit observations as the model based
estimates were far too low and remains static in the course of the 21st century. Model based estimates of this factor are 0.36 ±
0.14 mm yr–1 K–1, regression against data to the 1990s gives 0.65 ± 0.40 mm yr -1 K-1 and over recent decade ~0.8 mm yr–1
K–1 (Chapter 10 SOD pages 57-58) and final used is 0.80 ± 0.33 mm yr–1 K–1 .There is evidence of an increasing sensitivity
in recent decades. It is noted in Chapter 10 that there is a substantial difference between model based estimates and
observations and that "The current state of knowledge does not permit a satisfactory explanation of the difference" .
Reason 2) The total volume of ice in all G&IC appears to be more uncertain than in the TAR which estimated a total volume
of 0.5m SLE about half of which was in the periphery of the two ice sheets. The SLE volume for G&IC not immediately
associated with the ice sheets is 0.15-0.37m (Table 4.3) and around the ice sheets one estimate is given of 0.25m, giving a
total range of 0.4m-0.72m. The latter estimate does not include the ice volumes of the Antarctic Peninsula, nor the fringing
glaciers of East Antarctica which are dynamically disconnected from the ice sheet (4.5.1 page 17) and can be expected to, or
are already, respond(ing) on different timescale than the ice sheet itself.
Reason 3) The melting of ice from the peripheral ice sheets and glaciers of the ice sheets was not estimated in the TAR and
here is done by adding 20% to the non ice sheet related G&IC SLR estimates. There is little or no observational or physical
basis for applying this assumption throughout the 21st century. On one estimate the glaciers of the Antarctic Peninsula (4-25)
are presently discharging an ice volume equivalent to about 15% of the losses from G&IC not immediately associated with the
ice sheets in the 1993-2003 period. Even if 20% were an appropriate fraction now it cannot be assumed to remain constant
and most likely would increase as polar warming extends further and changes either the mass balance or the dynamics of the
G&IC immediately associated with the ice sheets. Hence this is very likely too low an estimate. (continuation comments
combined by TSU)
[Govt. of Germany (Reviewer’s comment ID #: 2011-34)]
SPM-736 A 11:29 11:29 Add “small” before “ice cap” and delete the callout for footnote 7.
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