Ocean thermal expansion is one of the main contributors to long-term sea level change, as well as being part of regional and short-term changes. Water expands as it warms and shrinks as it cools.
From 1961 to 2003, the upper 700 metres of the global oceans absorbed about 3.6 x 1021 Joules per year, increasing global mean sea level (GMSL) by about 22 millimetres. This is equivalent to contributing about 0.52 mm/year to GMSL, and also to an air-sea flux of 0.36 Watts per square metre over the ocean area considered (65°S to 65°N). This contribution to GMSL is about one third of the total GMSL trend (1.6 mm/year) over this period.
From 1955 to 1995, earlier estimates of ocean thermal expansion is estimated to have contributed about 0.4 mm/year to sea level rise, less than 25 per cent of the observed rise over the same period. For the 1993 to 2003 decade, when the best data are available, thermal expansion was estimated to be significantly larger, at about 1.6 mm/year for the upper 750 m of the ocean alone, about 50 per cent of the observed sea level rise of 3.1 mm/year. (source: http://www.cmar.csiro.au/sealevel)
FIGURE
Figure 1: The top panel of the above plot show changes in the heat content of the top 700 metres of the ocean from 1960 to 2007. The bottom panel shows the change in thermosteric sea level. (http://www.cmar.csiro.au/sealevel)