1. Introduction
With over 1000 times the heat capacity of the atmosphere, the World Ocean is thelargest repository for changes in global heat content [Levitus et al., 2005]. Monitoring ocean heat content is therefore fundamental to detecting and understanding changes in the Earth’s heat balance. Past estimates of the global integral of ocean heat content anomaly (OHCA) indicate an increase of 14.5 × 1022 J from 1955 to 1998 from the surface to 3000m [Levitus et al., 2005] and 9.2 (± 1.3) × 1022 J from 1993 to 2003 in the upper (0 – 750m) ocean [Willis et al. 2004].
These increases provide strong evidence of global warming. Climate models exhibit similar rates of ocean warming, but only when forced by anthropogenic influences [Gregory et al., 2004; Barnett et al., 2005; Church et al.,2005; Hansen et al., 2005].
While there has been a general increase in the global integral of OHCA during thelast half century, there have also been substantial decadal fluctuations, including a short period of rapid cooling (6 × 1022 J of heat lost in the 0–700 m layer) from 1980 to 1983 [Levitus et al., 2005].
Most climate models, however, do not contain unforced decadal variability of this magnitude [Gregory et al., 2004; Barnett et al., 2005, their Figure S1; Church et al., 2005; and Hansen et al., 2005] and it has been suggested that such fluctuations in the observational record may be due to inadequate sampling of ocean temperatures [Gregory et al., 2004]. We have detected a new cooling event that began in 2003 and is comparable in magnitude to the one in the early 1980s. Using high resolution satellite data to estimate sampling error, we find that both the recent event and the cooling of the early 1980s are significant with respect to these errors.
5. Discussion
This work has several implications. First, the updated time series of ocean heat content presented here (Figure 1) and the newly estimated confidence limits (Figure 3) support the significance of previously reported large interannual variability in globally integrated upper-ocean heat content [Levitus et al., 2005]. However, the physical causes for this type of variability are not yet well understood.
Furthermore, this variability is not adequately simulated in the current generation of coupled climate models used to study the impact of anthropogenic influences on climate [Gregory et al., 2004; Barnett et al. 2005; Church et al. 2005; and Hansen et al., 2005]. Although these models do simulate the long-term rates of ocean warming, this lack of interannual variability represents ashortcoming that may complicate detection and attribution of human-induced climate influences.
Changes in OHCA also affect sea level. Sea level rise has a broad range of
implications for climate science as well as considerable socioeconomic impacts
[Intergovernmental Panel on Climate Change (IPCC), Climate Change 2001: The Scientific Basis, 2004]. Diagnosing the causes of past and present sea level change and closure of the sea level budget is therefore a critical component of understanding past changes in sea level as well as projecting future changes. The recent cooling of the upper ocean implies a decrease in the thermosteric component of sea level. Estimates of total sea level [Leuliette et al., 2004;
http://sealevel.colorado.edu], however, show continued sea-level rise during the past 3 years. This suggests that other contributions to sea-level rise, such as melting of land-bound ice, have accelerated. This inference is consistent with recent estimates of ice mass loss in Antarctica [Velicogna and Wahr, 2006] and accelerating ice mass loss on Greenland [Rignot et al., 2006] but closure of the global sea level budget cannot yet be achieved. New satellite observations from the Gravity Recovery and Climate Experiment (GRACE; launched in March, 2002 and administered by NASA and Deutsches Zentrum für Luft-und Raumfahrt, GRACE will map Earth's gravity field approximately once every 30 days during its lifetime) should soon provide sufficient observations of the redistribution of water mass to more fully describe the
causes of recent sea-level change.
Finally,
the estimates presented here are made possible only by recent
improvements in the global ocean observing system. The sharp decrease in the error since 2002 is due to the dramatic improvement of in situ sampling provided by the Argo array of autonomous profiling CTD floats, and the real-time reporting of Argo data made it possible to extend the estimate through 2005. Characterization of the error budget, which is of paramount importance in the estimate of such globally averaged quan ies, was made feasible by the long-term maintenance of high quality altimeter missions such as TOPEX/Poseidon and Jason. The issues relating to sea level rise and the global water budget can only be addressed when the record of satellite gravity measurement from GRACE achieves adequate duration. GRACE, Argo, and satellite altimetry are core components of the global ocean observing system. Failure to maintain any one of these observing systems would seriously impair our ability to monitor the World Ocean and to unravel its importance to the climate system.