level rates for the full ocean depth using measured rates for the upper 700 m by other investigators (Table 3.1) and the Domingues et al. (2008) value for the deep ocean below 700 m (Table 3.2). For the longer observational period (approximately five decades), the committee calculated rates ranging from 0.5 ± 0.12 mm yr-1 (based on Ishii and Kimoto, 2009) to 0.59 ± 0.11 mm yr-1 (based on Ishii, personal communication, 2011). These rates are lower than the Domingues et al. (2008) rates, but they are comparable within errors. For the post-1993 observational period, the committee’s calculated rates are 1.0 ± 0.19 mm yr-1 and 1.43 ± 0.31 mm yr-1 (Table 3.2). The most recent estimate of Ishii (personal communication, 2011) is comparable to estimates of Domingues et al. (2008) and Church et al. (2011), within their reported errors.

The above estimates of the global thermosteric sea-level trend and its variability on interannual and decadal timescales differ, sometimes substantially. For example, Domingues et al. (2008) shows a continued thermosteric sea-level rise after 2004, whereas Levitus et al. (2009) and Ishii and Kimoto (2009) show a plateau (top panel of Figure 3.1). These differences result from uncertainties in the data and the choice of instrument bias corrections, processing approach, baseline mean climatology, mapping technique, and treatment of unsampled or undersampled areas. Correcting for XBT fall-rate bias reduced the errors in the thermosteric sea-level trend (S. Levitus, personal communication, 2011). However, uncertainties in the bias corrections remain the dominant source of error, especially for recent decades (Ishii and Kimoto 2009; Willis et al., 2009; Gouretski and Reseghetti, 2010; Lyman et al., 2010).

Different data processing approaches also may account for some differences among thermosteric sea-level estimates, such as the relatively high estimates of Domingues et al. (2008) for 1961–2003 and the relatively low estimates of Ishii (personal communication, 2011) for 1961–2008 for the upper 700 m. The treatment of data in unsampled and undersampled regions of the world’s oceans also can introduce uncertainties (Purkey and Johnson, 2010). Sampling problems are particularly acute in the Southern Ocean and likely result in estimates of thermosteric sea-level rise that are biased low (Gille, 2008; Church et al., 2010).

Models

The warming observed in the upper ocean also has been inferred from ocean-atmosphere climate models. For example, Pierce et al. (2006) found general consistency between models and observations for ocean warming, with the signal disappearing around 600 m depth. Climate model simulations also suggest heat uptake by the deep ocean (Katsman and van Oldenborgh, 2011; Meehl et al., 2011). Song and Colberg (2011), using an ocean general circulation model constrained by sea-surface temperature and atmospheric radiation measurements, found a strong warming signal of 1.1 mm yr-1 below 700 m for the 1993–2008 period. This value is much higher than observational estimates (Purkey and Johnson, 2010; Kouketsu et al., 2011; Loeb et al., 2012), for reasons that are currently under debate.

Data Assimilation

Ocean data assimilation techniques can be used to obtain estimates of deep-ocean warming and the resulting thermosteric sea-level rise by constraining the numerical models with available data. There are, however, significant differences between the various data assimilation products and direct observations, arising in part from uncertainties in direct observa-

TABLE 3.2 Committee Estimates of Thermosteric Sea-Level Rise for the Full Ocean Depth

Data Source Used in the Estimate

Period

Thermosteric Sea-Level Rise Estimates, This Report (mm yr-1)a

Ishii and Kimoto (2009)

1951–2005

0.5 ± 0.12

Kuo and Shum (personal communication, 2011)

1955–2009

0.53 ± 0.14

Ishii (personal communication, 2011)

1961–2008

0.59 ± 0.11

Ishii and Kimoto (2009)

1993–2005

1.43 ± 0.31

Ishii (personal communication, 2011)

1993–2009

1.0 ± 0.19

a Calculated from estimates of the upper 700 m of the ocean by various investigators and the Domingues et al. (2008) rate of 0.2 ± 0.1 mm yr-1 for the deep ocean below 700 m.



The National Academies | 500 Fifth St. N.W. | Washington, D.C. 20001
Copyright © National Academy of Sciences. All rights reserved.
Terms of Use and Privacy Statement