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The ocean influences climate by storing and transporting large amounts of heat, freshwater, and carbon, and by exchanging these properties with the atmosphere. About 93% of the excess heat energy stored by the Earth over the last 50 years is found in the ocean (Church et al., 2011; Levitus et al., 2012). The ability of the ocean to store vast amounts of heat reflects the large mass and heat capacity of seawater relative to air and the fact that ocean circulation connects the surface and interior ocean. More than three quarters of the total exchange of water between the atmosphere and the Earth’s surface through evaporation and precipitation takes place over the oceans (Schmitt, 2008). The ocean contains 50 times more carbon than the atmosphere (Sabine et al., 2004) and is at present acting to slow the rate of climate change by absorbing about 30% of human emissions of carbon dioxide (CO2) from fossil fuel burning, cement production, deforestation and other land use change (Mikaloff-Fletcher et al., 2006; Le Quéré et al., 2010). Changes in the ocean may result in climate feedbacks that either increase or reduce the rate of climate change. Climate variability and change on time scales from seasons to millennia is therefore closely linked to the ocean and its interactions with the atmosphere and cryosphere. The large inertia of the oceans means that they naturally integrate over short-term variability and often provide a clearer signal of longer-term change than other components of the climate system. Observations of ocean change therefore provide a means to track the evolution of climate change, and a relevant benchmark for climate models.

The lack of long-term measurements of the global ocean and changes in the observing system over time makes documenting and understanding change in the oceans a difficult challenge (Appendix 3.A). Many of the issues raised in Box 2.1 regarding uncertainty in atmospheric climate records are common to oceanographic data. Despite the limitations of historical records, AR4 identified significant trends in a number of ocean variables relevant to climate change, including ocean heat content, sea level, regional patterns of salinity, and biogeochemical parameters (Bindoff et al., 2007). Since AR4, substantial progress has been made in improving the quality and coverage of ocean observations. Biases in historical measurements have been identified and reduced, providing a clearer record of past change. The Argo array of profiling floats has provided near-global, year-round measurements of temperature and salinity in the upper 2000 m since 2005. The satellite altimetry record is now more than 20 years in length. Longer continuous time series of important components of the meridional overturning circulation and tropical oceans have been obtained. The spatial and temporal coverage of biogeochemical measurements in the ocean has expanded. As a result of these advances, there is now stronger evidence of change in the ocean, and our understanding of the causes of ocean change is improved.

This chapter summarizes the observational evidence of change in the ocean, with an emphasis on basin- and global-scale changes relevant to climate, with a focus on studies published since the AR4. As in Chapter 2, the robustness of observed changes is assessed relative to sources of observational uncertainty. The attribution of ocean change, including the degree to which observed changes are consistent with anthropogenic climate change, is addressed in Chapter 10. The evidence for changes in subsurface ocean temperature and heat content is assessed in Section 3.2; changes in sea surface temperature (SST) are covered in Chapter 2. Changes in ocean heat content dominate changes in the global energy inventory (Box 3.1). Recent studies have strengthened the evidence for regional changes in ocean salinity and their link to changes in evaporation and precipitation over the oceans (Section 3.3), a connection already identified in AR4. Evidence for changes in the fluxes of heat, water and momentum (wind stress) across the air–sea interface is assessed in Section 3.4. Considering ocean changes from a water-mass perspective adds additional insight into the nature and causes of ocean change (Section 3.5). Although direct observations of ocean circulation are more limited than those of temperature and salinity, there is growing evidence of variability and change of ocean current patterns relevant to climate (Section 3.6). Observations of sea level change are summarized in Section 3.7; Chapter 13 builds on the evidence presented in this and other chapters to provide an overall synthesis of past and future sea level change. Biogeochemical changes in the ocean, including ocean acidification, are covered in Section 3.8. Chapter 6 combines observations with models to discuss past and present changes in the carbon cycle. Section 3.9 provides an overall synthesis of changes observed in the ocean during the instrumental period and highlights key uncertainties. Unless otherwise noted, uncertainties (in square brackets) represent 5 to 95% confidence intervals.

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