The oceans are warming, with global ocean temperature rising by 0.10°C from the surface to 700 m over 1961–2003. Ocean heat content has increased, absorbing energy at 0.21 ± 0.04 W m⁻² globally. Two-thirds of this energy is absorbed in the upper 700 m. Global ocean heat content shows interannual and inter-decadal variability, with higher warming rates from 1993–2003 and some cooling since 2003. Salinity trends show global freshening in subpolar regions and salinification in tropical and subtropical areas, consistent with changes in precipitation and atmospheric water transport. Observations do not allow reliable global salinity estimates. Key oceanic water masses are changing, but there is no clear evidence for ocean circulation changes. Southern Ocean mode waters and Upper Circumpolar Deep Waters have warmed since the 1960s. Similar but weaker warming patterns are observed in the Gulf Stream and Kuroshio mode waters. Long-term cooling is observed in the North Atlantic subpolar gyre and central North Pacific. Since 1995, the upper North Atlantic subpolar gyre has been warming and becoming more saline. It is very likely that the Atlantic meridional overturning circulation (MOC) has changed significantly at interannual to decadal time scales. No coherent evidence for a trend in MOC strength has been found over the last 50 years. Ocean biogeochemistry is changing. Total inorganic carbon has increased by 118 ± 19 GtC since the pre-industrial period, with a decrease in the fraction of emitted CO₂ taken up by the oceans. This is consistent with the expected rate of carbon absorption, but uncertainties prevent firm conclusions. The increase in total inorganic carbon has decreased the depth at which calcium carbonate dissolves and reduced surface ocean pH by 0.1 units since 1750. Observations show a trend of decreasing pH at 0.02 units per decade. Global mean sea level has been rising at 1.8 ± 0.5 mm yr⁻¹ from 1961–2003. For the 20th century, the average rate was 1.7 ± 0.5 mm yr⁻¹, consistent with the TAR estimate of 1–2 mm yr⁻¹. Sea level change is non-uniform spatially, with some regions showing rates up to several times the global mean. There is evidence for an increase in extreme high water worldwide related to storm surges and regional climate variations. The rise in global mean sea level is accompanied by considerable decadal variability. From 1993–2003, the rate of sea level rise was 3.1 ± 0.7The oceans are warming, with global ocean temperature rising by 0.10°C from the surface to 700 m over 1961–2003. Ocean heat content has increased, absorbing energy at 0.21 ± 0.04 W m⁻² globally. Two-thirds of this energy is absorbed in the upper 700 m. Global ocean heat content shows interannual and inter-decadal variability, with higher warming rates from 1993–2003 and some cooling since 2003. Salinity trends show global freshening in subpolar regions and salinification in tropical and subtropical areas, consistent with changes in precipitation and atmospheric water transport. Observations do not allow reliable global salinity estimates. Key oceanic water masses are changing, but there is no clear evidence for ocean circulation changes. Southern Ocean mode waters and Upper Circumpolar Deep Waters have warmed since the 1960s. Similar but weaker warming patterns are observed in the Gulf Stream and Kuroshio mode waters. Long-term cooling is observed in the North Atlantic subpolar gyre and central North Pacific. Since 1995, the upper North Atlantic subpolar gyre has been warming and becoming more saline. It is very likely that the Atlantic meridional overturning circulation (MOC) has changed significantly at interannual to decadal time scales. No coherent evidence for a trend in MOC strength has been found over the last 50 years. Ocean biogeochemistry is changing. Total inorganic carbon has increased by 118 ± 19 GtC since the pre-industrial period, with a decrease in the fraction of emitted CO₂ taken up by the oceans. This is consistent with the expected rate of carbon absorption, but uncertainties prevent firm conclusions. The increase in total inorganic carbon has decreased the depth at which calcium carbonate dissolves and reduced surface ocean pH by 0.1 units since 1750. Observations show a trend of decreasing pH at 0.02 units per decade. Global mean sea level has been rising at 1.8 ± 0.5 mm yr⁻¹ from 1961–2003. For the 20th century, the average rate was 1.7 ± 0.5 mm yr⁻¹, consistent with the TAR estimate of 1–2 mm yr⁻¹. Sea level change is non-uniform spatially, with some regions showing rates up to several times the global mean. There is evidence for an increase in extreme high water worldwide related to storm surges and regional climate variations. The rise in global mean sea level is accompanied by considerable decadal variability. From 1993–2003, the rate of sea level rise was 3.1 ± 0.7