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
Page 1 |
Page 2 |
Page 3 |
Page 4 |
Page 5 |
Page 6 |
Page 7 |
Page 8 |
Page 9 |
Page 10 |
Page 11 |
Page 12 |
Page 13 |
Page 14 |
Page 15 |
Page 16 |
Page 17 |
Page 18 |
Page 19 |
Page 20 |
Page 21 |
Page 22 |
Page 23 |
Page 24 |
Page 25 |
Page 26 |
Page 27 |
Page 28 |
Page 29 |
Page 30 |
Page 31 |
Page 32 |
Page 33 |
Page 34 |
Page 35 |
Page 36 |
Page 37 |
Page 38 |
Page 39 |
Page 40 |
Page 41 |
Page 42 |
Page 43 |
Page 44 |
Page 45 |
Page 46 |
Page 47 |
Page 48 |
Page 49 |
Page 50 |
Page 51 |
Page 52 |
Page 53 |
Page 54 |
Page 55 |
Page 56 |
Page 57 |
Page 58 |
Page 59 |
Page 60 |
Page 61 |
Page 62 |
Page 63 |
Page 64 |
Page 65 |
Page 66 |
Page 67 |
Page 68 |
Page 69 |
Page 70 |
Page 71 |
Page 72 |
Page 73 |
Page 74 |
Page 75 |
Page 76 |
Page 77 |
Page 78 |
Page 79 |
Page 80 |
Page 81 |
Page 82 |
Page 83 |
Page 84 |
Page 85 |
Page 86 |
Page 87 |
Page 88 |
Page 89 |
Page 90 |
Page 91 |
Page 92 |
Page 93 |
Page 94 |
Page 95 |
Page 96 |
Page 97 |
Page 98 |
Page 99 |
Page 100 |
Page 101 |
Page 102 |
Page 103 |
Page 104 |
Page 105 |
Page 106 |
Page 107 |
Page 108 |
Page 109 |
Page 110 |
Page 111 |
Page 112 |
Page 113 |
Page 114 |
Page 115 |
Page 116 |
Page 117 |
Page 118 |
Page 119 |
Page 120 |
Page 121 |
Page 122 |
Page 123 |
Page 124 |
Page 125 |
Page 126 |
Page 127 |
Page 128 |
Page 129 |
Page 130 |
Page 131 |
Page 132 |
Page 133 |
Page 134 |
Page 135 |
Page 136 |
Page 137 |
Page 138 |
Page 139 |
Page 140 |
Page 141 |
Page 142 |
Page 143 |
Page 144 |
Page 145 |
Page 146 |
Page 147 |
Page 148 |
Page 149 |
Page 150 |
Page 151 |
Page 152 |
Page 153 |
Page 154 |
Page 155 |
Page 156 |
Page 157 |
Page 158 |
Page 159 |
Page 160 |
Page 161 |
Page 162 |
Page 163 |
Page 164 |
Page 165 |
Page 166 |
Page 167 |
Page 168 |
Page 169 |
Page 170 |
Page 171 |
Page 172 |
Page 173 |
Page 174 |
Page 175 |
Page 176 |
Page 177 |
Page 178 |
Page 179 |
Page 180 |
Page 181 |
Page 182 |
Page 183 |
Page 184 |
Page 185 |
Page 186 |
Page 187 |
Page 188 |
Page 189 |
Page 190 |
Page 191 |
Page 192 |
Page 193 |
Page 194 |
Page 195 |
Page 196 |
Page 197 |
Page 198 |
Page 199 |
Page 200 |
Page 201 |
Page 202 |
Page 203 |
Page 204 |
Page 205 |
Page 206 |
Page 207 |
Page 208 |
Page 209 |
Page 210 |
Page 211 |
Page 212 |
Page 213 |
Page 214 |
Page 215 |
Page 216 |
Page 217 |
Page 218 |
Page 219 |
Page 220 |
Page 221 |
Page 222 |
Page 223 |
Page 224 |
Page 225 |
Page 226 |
Page 227 |
Page 228 |
Page 229 |
Page 230 |
Page 231 |
Page 232 |
Page 233 |
Page 234 |
Page 235 |
Page 236 |
Page 237 |
Page 238