Physical and chemical changes in the ocean over basin-wide zones and decadal or longer time-scales: perspectives on current and future conditions - Ocean Sustainability in the 21st Century - page 105

sea-level rise to 2100 published since 2007, and conclude that for a warming of more than
4 °C 'a pragmatic estimate of sea-level rise by 2100' is between 0.5 and 2 m. From their
risk analysis, region byregion, they state that there is a 'real risk ofthe forced displacement
of up to 187 million people over the century (up to 2.4% of global population)'. The most
recent comparison of data and models shows that the rate of sea-level rise of the past few
decades is greater than projected by the IPCC models. (Rahmstorf et al ., 2012 ) .
With respect to long-term global change, changes in ocean volume are good indicators
and can provide strong constraints on model simulations of the climate, while ocean
volume and density changes provide the same on global ice mass budgets. Significant in-
terannual and decadal variations of global sea level are, however, also found which are
not related to ocean volume redistributions. Differences in evaporation and precipitation in
the hydrological cycle, together with the global topography, generate sea-level differences
between the major oceans. The net effect of the Northern Hemisphere hydrological cycle
is a sea-level distribution with levels higher by 0.5-1 m in the North Pacific subtropical
gyre compared with the North Atlantic, and a sub-Arctic gyre in the North Pacific with
0.2-0.8 m higher water levels than in the North Atlantic Arctic seas. Furthermore, the sub-
tropical North Atlantic has a sea level higher by0.3-0.5m compared with the northern seas
and the Eurasian Arctic Basin (Carmack and McLaughlin 2011 ) . This difference supports
the northward flow of water from the warm North Atlantic gyre into the Arctic Ocean. No
pattern of the longer-term regional distribution of potential sea-level rise has emerged, al-
though model projections display common features. These include a maximum in sea-level
rise in the Arctic Ocean and a minimum in the Southern Ocean, south of the Antarctic Cir-
cumpolar Current (Church, 2006 and Gregory et al. , 2001 , cited by Church).
Land motion corrections to tide gauges from models of global isostatic adjustments
can contribute ± 2 mm/yr globally. Including global isostatic adjustments, the current best
estimate of sea-level rise is given as 3.2 ± 0.4 mm/yr over the period 1993-2009 (Church
andWhite, 2011 ) .Thebalanceofevidencesuggeststhattherehasbeenasignificantchange
in the long-term rate of sea-level rise, with roughly half due to ocean temperature change,
and other parts due to inflow of water from land, including ice melting.