tions and differences in data-assimilation approaches for estimating the state of the ocean (e.g., Church et al., 2010). The Simple Ocean Data Assimilation model (Carton et al., 2005; Carton and Giese, 2008) uses a multivariate sequential approach to force the ocean model toward observed temperature and salinity data. Ocean dynamics and other properties are not preserved. Using this approach, the estimated thermosteric sea-level trend from 1968 to 2001 is similar to the observed estimates (Figure 3.1). Kohl and Stammer (2008) used a more sophisticated approach, which synthesizes the observed data into a dynamically consistent model using the adjoint assimilation technique. To ensure dynamical consistency, the model forcing fields are modified. The estimated thermosteric sea-level trend using this method shows a large decrease until 1975 and then a larger rise afterward (Figure 3.1).

Ocean data assimilation has been an active research topic only since the 1990s. Over time, it may become a more reliable source for studies of decadal sea-level variability and change (Church et al., 2010).

Satellites

A few investigators have inferred global steric sea-level rise from the Gravity Recovery and Climate Experiment (GRACE) and altimeter data (e.g., Lombard et al., 2007; Cazenave et al., 2009). Satellite altimetry measures the total sea-level change (steric plus ocean mass) and GRACE measures ocean mass change. The difference between the two measurements provides an independent estimate of the steric sea-level change. However, estimates made this way vary significantly.

Summary

The thermal expansion estimates in the IPCC Fourth Assessment Report were made before temperature biases due to the XBT and MBT depth errors were discovered. Efforts to improve the IPCC (2007) estimates have focused on using new temperature data, correcting instrument bias, and improving data processing methods. New estimates of thermosteric sea-level rise are generally higher than those estimated by the IPCC (2007) for the past four or five decades and generally lower than those estimated by the IPCC (2007) for the past 10–15 years (Figure 3.3). However, the new estimates overlap significantly with the IPCC (2007) estimates, within errors.

Estimates of thermosteric sea-level rise for the upper 700 m of the ocean have lower uncertainties than

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FIGURE 3.3 Comparison of thermosteric sea-level estimates for the full ocean depth from IPCC (2007; blue) and subsequent estimates (red). The bars represent the highest and lowest estimates. Long-term trends are for 1961–2003 (IPCC) and 1951–2005 (new estimates); short-term trends are for 1993–2003 (IPCC) and 1993–2008 (new estimates). SOURCE: IPCC estimates from Bindoff et al. (2007); new estimates are from Tables 3.1 and 3.2 based on data from Domingues et al. (2008), Ishii and Kimoto (2009), and Church et al. (2011).



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