I last commented about Antarctica and SLR when I “reviewed” the AR5 cryosphere chapter. As I noted there, things have come on quite a way since I were a lad (2005, 2004) and the major advance looks to be GRACE, even if they sometimes recalibrate their isostatic rebound; see-also this from 2009. And now, via Sou, I find Mass gains of the Antarctic ice sheet exceed losses by Zwally et al. Which says
Mass changes of the Antarctic ice sheet impact sea-level rise as climate changes, but recent rates have been uncertain. Ice, Cloud and land Elevation Satellite (ICESat) data (2003–08) show mass gains from snow accumulation exceeded discharge losses by 82 ± 25 Gt a–1, reducing global sea-level rise by 0.23 mm a–1…
This contradicts the aforementioned AR5, which says
Overall, there is high confidence that the Antarctic ice sheet is currently losing mass. The average ice mass change to Antarctica from the present assessment has been –97 [–135 to –58] Gt yr–1 (a sea level equivalent of 0.27 mm yr–1 [0.37 to 0.16] mm yr–1) over the period 1993–2010, and –147 [–221 to –74] Gt yr–1 (0.41 [0.61 to 0.20] mm yr–1) over the period 2005–2010. These assessments include the Antarctic peripheral glaciers
And you can if you wish see what that is based on:
And you can’t really fix the discrepancy by quibbling about the exact time period used, either. Again from AR5:
Or see-also figure 4.16. So, how do Zwally et al. resolve the discrepancy, which is almost entirely over East Antartica? Because it really is up to them to do the resolution. Not entirely convincingly I’d say:
A likely cause for the lower GR estimate is the sensitivity of the GR estimates to the glacial isostatic adjustment (GIA) correction, as discussed in the Appendix where we note that a –1.6 mm a–1 change in the modeled dB/dt would bring the GR and our dM/dt into agreement…
Which seems to amount to, if we change some numbers we could get a different answer. But if we change some of Zwally’s numbers, we could also get a different answer. FWIW, I’d trust GRACE more than anything else; it just feels intrinsically more reliable.
However, in at least one sense, none of this matters. What we care about above all else is the change in the mass budget of Antarctica, because that feeds into changes in the rate of SLR. After all, we already know the current rate of sea level rise. If Z is right about East Antarctic and that is actually gaining mass, all it means is that somewhere else is losing more mass than we thought. East Antarctic is probably least interesting for changes in mass balance because, whilst huge (~50 m of SLR in total, as against ~5 for West Antarctica of Greenland; those numbers are all for total melt, which won’t happen soon for any of them) its very slow; Greenland, the Antarctic Peninsula or West Antarctica are more interesting for change.
from ScienceBlogs http://ift.tt/1kndAvG
I last commented about Antarctica and SLR when I “reviewed” the AR5 cryosphere chapter. As I noted there, things have come on quite a way since I were a lad (2005, 2004) and the major advance looks to be GRACE, even if they sometimes recalibrate their isostatic rebound; see-also this from 2009. And now, via Sou, I find Mass gains of the Antarctic ice sheet exceed losses by Zwally et al. Which says
Mass changes of the Antarctic ice sheet impact sea-level rise as climate changes, but recent rates have been uncertain. Ice, Cloud and land Elevation Satellite (ICESat) data (2003–08) show mass gains from snow accumulation exceeded discharge losses by 82 ± 25 Gt a–1, reducing global sea-level rise by 0.23 mm a–1…
This contradicts the aforementioned AR5, which says
Overall, there is high confidence that the Antarctic ice sheet is currently losing mass. The average ice mass change to Antarctica from the present assessment has been –97 [–135 to –58] Gt yr–1 (a sea level equivalent of 0.27 mm yr–1 [0.37 to 0.16] mm yr–1) over the period 1993–2010, and –147 [–221 to –74] Gt yr–1 (0.41 [0.61 to 0.20] mm yr–1) over the period 2005–2010. These assessments include the Antarctic peripheral glaciers
And you can if you wish see what that is based on:
And you can’t really fix the discrepancy by quibbling about the exact time period used, either. Again from AR5:
Or see-also figure 4.16. So, how do Zwally et al. resolve the discrepancy, which is almost entirely over East Antartica? Because it really is up to them to do the resolution. Not entirely convincingly I’d say:
A likely cause for the lower GR estimate is the sensitivity of the GR estimates to the glacial isostatic adjustment (GIA) correction, as discussed in the Appendix where we note that a –1.6 mm a–1 change in the modeled dB/dt would bring the GR and our dM/dt into agreement…
Which seems to amount to, if we change some numbers we could get a different answer. But if we change some of Zwally’s numbers, we could also get a different answer. FWIW, I’d trust GRACE more than anything else; it just feels intrinsically more reliable.
However, in at least one sense, none of this matters. What we care about above all else is the change in the mass budget of Antarctica, because that feeds into changes in the rate of SLR. After all, we already know the current rate of sea level rise. If Z is right about East Antarctic and that is actually gaining mass, all it means is that somewhere else is losing more mass than we thought. East Antarctic is probably least interesting for changes in mass balance because, whilst huge (~50 m of SLR in total, as against ~5 for West Antarctica of Greenland; those numbers are all for total melt, which won’t happen soon for any of them) its very slow; Greenland, the Antarctic Peninsula or West Antarctica are more interesting for change.
from ScienceBlogs http://ift.tt/1kndAvG
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