Review on Ocean Heat Content and Ocean Warming Lei Huang Abstract Since the mid-20th century, there is substantial increase in the average measured temperature of the Earth's near-surface air and oceans, which is now known as "global warming". Global mean surface temperature increased 0.74 ± 0.18 °C during the 100 years ending in 2005. The Intergovernmental Panel on Climate Change (IPCC) concludes that most of the increase since the mid-twentieth century is "very likely" due to the increase in anthropogenic greenhouse gas concentrations. Natural phenomena such as solar variation combined with volcanoes probably had a small warming effect from pre-industrial times to 1950 and a small cooling effect from 1950 onward. Climate model projections summarized by the IPCC indicate that average global surface temperature will likely rise a further 1.1 to 6.4 °C during the twenty-first century. This range of values results from the use of differing scenarios of future greenhouse gas emissions as well as models with different climate sensitivity. Although most studies focus on the period up to 2100, warming and sea level rise are expected to continue for more than a thousand years even if greenhouse gas levels are stabilized. The delay in reaching equilibrium is a result of the large heat capacity of the oceans. In this review, the temporal and spatial variability of the heat content in the world ocean are estimated, and temperature changes in different layers of the ocean are compared with model results. Besides, linear trends in salinity for the world ocean from 1955 to 1998 are calculated and analyzed. 1. Introduction Oceans cover approximately 71% of the Earth's surface and have great impact on the biosphere. The evaporation of these oceans is how we get most of our rainfall, and their temperature determines our climate and wind pattern. The Intergovernmental Panel on Climate Change (IPCC) (1995), the World Climate Research Program (WCRP) (1995) and the U.S. National Research Council (NRC) (1999) have identified the role of oceans as being critical to understanding the variability of Earth’s climate system. Physically we expect this to be so because of the high density and specific heat of seawater. Oceans also have the thermal inertia and heat capacity to help maintain and ameliorate climate variability. Studies using instrumental data to evaluate a warming trend of Earth’s climate system due to increasing concentrations of greenhouse gases (GHGs) have focused on surface air temperature and sea surface temperature (IPCC 1995). These variables have shown an average warming of approximately 0.6°C at Earth’s surface during the past 100 years. Recent comparisons (Mann et al. 1998; Mann 1999; Briffa et al. 1995) with paleoclimatic proxy data indicate that the observed increase in surface temperature during the past century is unprecedented during the past 1000 years. The results of these studies, along with similar results found using general circulation model (GCM) and energy balance model (Tett et al. 1999; Crowley 2000; Delworth et al. 2000; Stott et al. 2000), which include forcing by the observed time-dependent increase in GHGs and sulfate aerosols, as well as changes in solar irradiance and volcanic aerosols, provide evidence that the warming of Earth’s surface during the past few decades is of anthropogenic origin. Despite the agreement between models and observations, it is conceivable that some of the surface warming might be compensated by a cooling of other parts in Earth’s climate system. Conversely, additional warming may be occurring in other parts of the climate system, such as the recently observed warming of the world ocean (Levitus et al. 2000). In this review, we first looked at the interannual-to-decadal variability of the heat content (mean temperature) of the world ocean from the surface through 3000 m depth for the period 1948-1998 (Levitus et al. 2000), then we compared the temporal variability of the heat content of the world ocean, of the global atmosphere, and of components of Earth's cryosphere during the latter half of the 20th century (Levitus et al. 2001). The results of two different models forced by observed and estimated concentrations of greenhouse gases and the direct effect of sulfate aerosols on the atmosphere are compared with observations (Barnett et al. 2001; Levitus et al. 2001). Interannual variability in upper ocean heat content is studied using satellite altimetric height combined with in situ temperature profiles (Willis et al. 2004). At last, salinity changes during the period 1955-1998 for the world ocean are estimated to study the large-scale trends in salinity over this period (Boyer et al. 2005). 2. Temporal Variability of Upper Ocean Heat Content Figure 1 shows the variability of yearly heat content anomalies in the upper 300 m for 1948 to 1998 for individual ocean basins defined using the Equator as a boundary. The anomaly fields for the Atlantic and Indian oceans, for both the entire basins and Northern and Southern Hemisphere basins of each ocean, show a positive correlation. In each basin before the mid-1970s, temperatures were relatively cool, whereas after the mid-1970s these oceans are in a warm state. The year of largest yearly mean temperature and heat content for the North Atlantic is 1998. Both Pacific Ocean basins exhibit quasi-bidecadal changes in upper ocean heat content, with the two basins positively correlated. During 1997 the Pacific achieved its maximum heat content. A decadal-scale oscillation in North Pacific sea surface temperature (Pacific Decadal Oscillation) has been identified (Nitta et al. 1989; Trenberth 1991), but it is not clear if the variability we observe in Pacific Ocean heat content is correlated with this phenomenon or whether there are additional phenomena that contribute to the observed heat content variability. Figure 2 shows the heat content for 5-year running composites by individual basins integrated through 3000-m depth. The distributions are presented through 3000-m depth for consistency. There is a consistent warming signal in each ocean basin, although the signals are not monotonic. The signals between the Northern and Southern Hemisphere basins of the Pacific and Indian oceans are positively correlated, suggesting the same basin-scale forcings. The temporal variability of the South Atlantic differs significantly from the North Atlantic, which is due to the deep convective processes that occur in the North Atlantic. Before the 1970s, heat content was generally negative. The Pacific and Atlantic oceans have been warming since the 1950s, and the Indian Ocean has warmed since the 1960s. The delayed warming of the Indian Ocean with respect to the other two oceans may be due to the sparsity of data in the Indian Ocean before 1960. The range of heat content for this series is on the order of 20 ×1022 J for the world ocean. Fig. 1. Time series for the period 1948-1998 of ocean heat content (1022 J) in the upper 300 m for the Atlantic, Indian, Pacific, and world oceans. Fig. 2. Time series of 5-year running composites of heat content (1022 J) in the upper 3000 m for each major ocean basin. The linear trend is estimated for each time series for the period 1955-1996, which corresponds to the period of best data coverage. The trend is plotted as a red line. The percent variance accounted for by this trend is given in the upper left corner of each panel. 3. GCM Model simulation results compared with observations Figure 3 shows the estimated temporal variability of global ocean and atmospheric heat content, based on instrumental data for the 1955-1996 period. The atmospheric sensible heat content is based on the National Centers for Environmental Protection/National Center for Atmospheric Research (NCEP/NCAR) reanalysis fields (Kalnay et al. 1996) and is shown as anomalies averaged for 1-year period. The ocean heat content curve is based on analyses of 5-year running composites of historical ocean data. Other components of atmospheric energy change, associated with changes in the latent heat of evaporation and geopotential height field, are an order of magnitude smaller than the change in sensible heat in the NCEP/NCAR analyses. The increase in observed ocean heat content is 18.2×1022 J (based on the linear trend for the 1957-1994 period but prorated to 1955-1996), whereas the increase in atmospheric heat content is more than an order of magnitude smaller. Fig. 3. Time series of various components of the observed and simulated global heat content. The observed global ocean heat content is shown in black; the open circles denote the observed global mean atmospheric heat content. The red curve denotes the ensemble mean global ocean heat content from a set of three simulations (experiment GSSV) using a coupled ocean-atmosphere model. The blue curve denotes the same for an additional set of three simulations (experiment GS), which is similar to experiment GSSV except that the radiative effects of changes in solar irradiance and volcanic aerosols are omitted. The observed increase in oceanic heat content is compared to the simulation results of a coupled model of Earth’s climate system. The coupled ocean-atmosphere-ice model is developed at the Geophysical Fluid Dynamics Laboratory (GFDL), and is higher in spatial resolution than an earlier version used in many previous studies of climate variability and change (Manabe et al. 1991; Manabe et al. 1994) but employs similar physics. The coupled model is global in domain and consists of GCMs of the atmosphere (spectral model with rhomboidal 30 truncations, corresponding to an approximate resolution of 3.75° longitude by 2.25° latitude, with 14 vertical levels) and ocean (1.875° longitude by 2.25° latitude, with 18 vertical levels).