during the 20th century which, in turn, was an order of
magnitude larger than the rate of rise over the two mil- Projections of 21st century sea-level rise
lennia prior to the 18th century.
The Intergovernmental Panel on Climate Change (IPCC) pro-
vides the most authoritative information on projected sea-
Contributions to sea-level change
level change. The IPCC Third Assessment Report (TAR) of
2001
23
projected a global averaged sea-level rise of between
20 and 70 cm (the limits of the model projections) between
The two main reasons for sea-level rise (Figure 6C.4) are
1990 and 2100 using the full range of IPCC greenhouse gas
thermal expansion of ocean waters as they warm, and scenarios and a range of climate models. When an addi-
increase in the ocean mass, principally from land-based
tional uncertainty for land-ice changes was included, the full
sources of ice (glaciers and ice caps, and the ice sheets
range of projected sea-level rise was 9 to 88 cm
7
. For the
of Greenland and Antarctica). Global warming from in-
IPCC’s Fourth Assessment Report (AR4), 2007, the range of
sea-level projections, using a much larger range of models,
creasing greenhouse gas concentrations is a significant
is 18 to 59 cm (with 90 per cent confidence limits) over the
driver of both contributions to sea-level rise.
period from 1980-2000 to 2090-2100
15
. To allow a margin for
the ice sheet uncertainties discussed above, the IPCC AR4
From 1955 to 1995, ocean thermal expansion is estimat- increased the upper limit of the projected sea-level rise by
ed to have contributed about 0.4 mm per year to sea-
10 to 20 cm above that projected by the models, but stated
level rise
16
, less than 25 per cent of the observed rise over
that “larger values cannot be excluded, but understanding of
the same period. For the 1993 to 2003 decade, when the
these effects is too limited to assess their likelihood or pro-
vide a best estimate or an upper bound for sea-level rise.”
best data are available, thermal expansion is estimated
to be significantly larger, at about 1.6 mm per year for
While the 2001 and 2007 IPCC projections are somewhat
the upper 750 m of the ocean alone
17
, about 50 per cent
different in how they treat ice sheet uncertainties and the
of the observed sea-level rise of 3.1 mm per year. Kaser confidence limits quoted, a comparison of the projections
and others
18
estimate the melting of glaciers and ice
(Figure 6C.5) shows the end results are similar, except that
caps (excluding the glaciers surrounding Greenland and
the lower limit of the 2001 projections has been raised from
Antarctica) contributed to sea-level rise by about 0.3 mm
9 to 18 cm.
per year from 1961 to 1990 increasing to about 0.8 mm
From the start of the IPCC projections in 1990 to 2006, ob-
per year from 2001–2004.
served sea level has been rising more rapidly than the central
range of the IPCC (2001 and 2007) model projections and
The ice sheets of Greenland and Antarctica have the po-
is nearer to the upper end of the total range of the projec-
tential to make the largest contribution to sea-level rise, tions shown in Figure 6C.5
24
, indicating that one or more of
but they are also the greatest source of uncertainty (see
the model contributions to sea-level rise is underestimated.
also Section 6A). Since 1990 there has been increased
Rahmstorf
25
developed a simple statistical model that re-
snow accumulation at high elevation on the Greenland
lated 20th century surface temperature change to 20th cen-
tury sea-level change. Using this relationship and projected
ice sheet, while at lower elevation there has been more
surface temperature increases, estimated 21st century sea-
widespread surface melting and a significant increase
level rise might exceed the IPCC projections and be as large
in the flow of outlet glaciers
19
. The net result is a de-
as 1.4 m.
crease in the mass of the Greenland ice sheet – a posi-
158 GLOBAL OUTLOOK FOR ICE AND SNOW