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warming. The gains in East Antarctic ice between 1992 and 2003 north of 81.6 S
(below which the satellite never orbited) was about 45
7 billion t. This is enough
to offset Antarctica's net contribution to sea-level rise by 0.12
±
0.02 mm year 1 .
The IPCC's 2007 report put Antarctica's net average annual contribution to sea level
rise for the period 1993-2003 as being 0.21
±
0.35 mm year 1 . However, whereas a
far more detailed assessment of Antarctica's contribution to sea-level rise is required,
the picture that emerges is that the continent, being the fastest-warming continent
on the planet, may well be on the cusp. If so then ongoing detailed monitoring will
be required. We will return to this and the related research in the next chapter's
subsection on sea-level rise (section 6.6.3).
With regards to Greenland, in 2009 work by Michiel van den Broeke, Jonathan
Bamber, Janneke Ettema and colleagues used satellite gravity observations from the
Gravity Recovery and Climate Experiment (known as GRACE). They found that
the total 2000-8 mass loss from Greenland was approximately 1500 Gt, which is
equivalent to 0.46 mm year 1 of global sea-level rise (and note that this is excluding
thermal expansion). Now, Greenland (as well as Antarctica) also accrues ice from
snowfall. Taking this into account (that is, without the moderating effects of increased
snowfall and refreezing) post-1996 Greenland ice-sheet mass losses would have been
100% higher than this approximate 1500 Gt. However, since 2006, high summer melt
rates have increased Greenland ice-sheet mass loss to 269 Gt year 1 (0.75 mm year 1
of equivalent sea-level rise). This compares to the 2007 IPCC assessment (which
noted that Greenland melt has risen very considerably since the 1960s) reporting that
Greenland's contribution to sea-level rise rose from 0.05
±
0.12 mm year 1
±
during
0.35 mm year 1 for the period 1993 to 2003. This is broadly in
line with the IPCC's A2 scenario estimate for Greenland's contribution to sea-level
rise from its 1980-99 mean to the mean for the final decade of the 21st century,
which is about a centimetre. This may seem trivial, and it is compared to other factors
that are thought to dominate 21st-century sea-level-rise estimates (especially thermal
expansion), but the 2007 IPCC assessment also notes that 'models do not yet exist
that address key processes that could contribute to large rapid dynamical changes in
the Antarctic and Greenland Ice Sheets that could increase the discharge of ice into
the ocean'. What the GRACE results show is that the melting that is occurring even
now is already pushing at the upper end of IPCC estimates.
In 2012 new estimates from the GRACE satellite were published (Jacob et al.,
2012). This gave a global estimate of glacier and ice sheet melt for the period 2003-
10 of 536
1961-2003 to 0.21
±
0.26 mm year 1 to global
sea-level rise (again note that this excludes thermal expansion factors). The estimate
for annual mass loss of just the Greenland/Antarctic and surrounding glaciers and ice
sheets between 2003 and 2010 of 384
±
93 Gt year 1 , which contributed 1.48
±
71 Gt year 1
±
contributed 1.06
±
0.19 mm
year 1 to sea-level rise.
To get a good estimate of likely sea-level rise by the end of the 21st century
we essentially need three things: good corroboration of mass-loss estimates (this is
beginning to come through); a good run of mass-loss data covering two decades to
see how things change (this is a work in progress and will last some years); and a
good estimate of the likely sea-level rise from thermal expansion (we do not yet have
this). In short, we may have to wait for a decade or so before the IPCC can reduce
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