Ocean heat storage changes should be where the focus is with respect to diagnosing the magnitude of global warming, as summarized in the paper
Pielke Sr., R.A., 2003: Heat storage within the Earth system. Bull. Amer. Meteor. Soc., 84, 331-335.
One of the issues, however, is whether heat is being transferred deep into the ocean, and thus sequestered there for a long time, perhaps to reappear at the surface unexpectedly. A paper in 2007 looked at this issue (thanks to Fergus Brown for alerting us to it). This paper is
Gregory C. Johnson, Sabine Mecking Bernadette M. Sloyan and Susan E. Wijffels 2007: Recent bottom water warming in the Pacific Ocean. J. of Climate. Volume 20. November 2007.
and has the abstract
“Decadal changes of abyssal temperature in the Pacific Ocean are analyzed using high-quality, full-depth hydrographic sections each occupied at least twice between 1984 and 2006. The deep warming found over this time period agrees with previous analyses (Fukasawa et al. 2004; Kawano et al. 2006b). The analysis presented here suggests it may have occurred after 1991, at least in the North Pacific. Mean temperature changes for the three zonal and three meridional hydrographic sections analyzed here exhibit abyssal warming often significantly different from zero at 95% confidence limits for this time period. Warming rates are generally larger to the south, and smaller to the north. This pattern is consistent with changes being attenuated with distance from the source of bottom water for the Pacific Ocean, which enters the main deep basins of this ocean southeast of New Zealand. Rough estimates of the change in ocean heat content suggest that the abyssal warming may amount to a significant fraction of upper world ocean heat gain over the past few decades.”
The text includes
“Between 3000 m (or 4000 m in the case of P06) and the bottom these estimates of heat flux range from 0.01 W m–2 along 47°N (P01) to 0.06 W m–2 along 170°W south of the equator (P15S). These values are between 5 and 30% of the heating trend of 0.2 W m–2 estimated for the 0–3000 m world ocean heat content change between 1955 and 1998 (Levitus et al. 2005) and between 2 and 10% of the heating trend of 0.6 W m–2 (per unit area of the Earth’s surface) estimated for the 0–750 m world ocean heat content change between 1993 and 2003 (Willis et al. 2004). Thus, abyssal Pacific Ocean heat content variations may contribute a small but significant fraction to the Earth’s heat budget…… The data from these repeat sections suggest that abyssal variations may contribute significantly to global heat, and hence sea-level, budgets. To close ocean heat, sea level, and likely freshwater budgets on interannual timescales, the ocean below 2000 m must be much better sampled in space and time than it has been, or is likely to be, relying on repeat hydrography alone.“
This is an important paper with respect to diagnosing the radiative imbalance of the climate system (i.e. global warming and cooling). Moreover, if heat is being stored in deep depths, this would help explain why sea level continues to rise yet the upper ocean has not been warming in recent year. It also means that the feedback of this heat into the atmosphere is delayed, or even lost for a very long time in terms of how this heat affects the rest of the climate system.