Jos de Laat has alerted us to an interesting new paper on the use of the ocean heat content changes as the metric to diagnose global warming and cooling. it is
Palmer, M. D., D. J. McNeall, and N. J. Dunstone (2011), Importance of the deep ocean for estimating decadal changes in Earth’s radiation balance, Geophys. Res. Lett., 38, L13707, doi:10.1029/2011GL047835.
The abstract reads [highlight added]
“We use control run data from three Met Office Hadley Centre climate models to investigate the relationship between: net top-of-atmosphere radiation balance (TOA), globally averaged sea surface temperature (SST); and globally averaged ocean heat content (OHC) on decadal timescales. All three models show substantial decadal variability in SST, which could easily mask the long-term warming associated with anthropogenic climate change over a decade. Regression analyses are used to estimate the uncertainty of TOA, given the trend in SST or OHC over the same period. We show that decadal trends in SST are only weakly indicative of changes in TOA. Trends in total OHC strongly constrain TOA, since the ocean is the primary heat store in the Earth System. Integrating OHC over increasing model levels, provides an increasingly good indication of TOA changes. To achieve a given accuracy in TOA estimated from OHC we find that there is a trade-off between measuring for longer or deeper. Our model results suggest that there is potential for substantial improvement in our ability to monitor Earth’s radiation balance by more comprehensive observation of the global ocean.”
The summary includes
“We have carried out analysis of the relationship between decadal trends in time‐integrated TOA, SST, and OHC using annual control simulation data from three generations of Met Office Hadley Centre climate models. At decadal time scales, SST trends place only a weak constraint on TOA over the same period. Conversely, full‐depth OHC strongly constrains TOA, since the ocean is the dominant heat reservoir in the Earth System. Surprisingly, we find that one must integrate OHC to depths in excess of 4000 m before the gain in information with depth becomes saturated. We note that the upper 4000 m in these models represents about 90% of the total ocean volume.”
The relationship between the uncertainty in TOA change estimated by OHC has been investigated as a function of time and depth. For a given uncertainty, there is a trade‐off between measuring longer and deeper. Our results suggest that the potential pay off of a deep ocean observing array [e.g., Garzoli et al., 2010] is the ability to better resolve, and therefore monitor, changes in Earth’s radiation balance.In terms of monitoring anthropogenic climate change, we would expect to see a more monotonic signal in total OHC than SST, which is highly subject to oceanic heat re‐distribution on multi annual to decadal time scales. Since the models appear to re‐arrange heat at all depths on decadal time scales, there may also be additional benefit to decadal forecasts of a more complete observing system through improved initialisation. These findings highlight the need to sustain the Argo observations to 2000 m and provide strong motivation for the development of a deep ocean observing array.”
Here are my comments on this paper:
1. The study is model-based. As discussed in my post
models are being used to claim real-world effects, when they actually are just hypotheses that need to be examined with quantitative comparison to real data.
2. In this context, if the models are correct on the transfer of heat deep into the oceans, this should be seen in the Argo network. As heat is moved downward, we should see these warm pulses moving downward in the vertical profiles in the layer above about 700m where the data coverage is quite good. To my knowledge we do not see this heat transfer.
Nonetheless, I am pleased to see ocean heat content changes being elevated in importance in climate science with respect to global warming and cooling as discussed and analyzed, for example, in
Ellis et al. 1978: The annual variation in the global heat balance of the Earth. J. Geophys. Res., 83, 1958-1962
Pielke Sr., R.A., 2003: Heat storage within the Earth system. Bull. Amer. Meteor. Soc., 84, 331-335.
R. S. Knox, David H. Douglass 2010: Recent energy balance of Earth International Journal of Geosciences, 2010, vol. 1, no. 3 (November) doi:10.4236/ijg2010.00000.
C. A. Katsman and G. J. van Oldenborgh, 2011: Tracing the upper ocean’s ‘missing heat’. Geophysical Research Letters (in press).
3. If there is such transport of warm water to deep depths in the oceans, this heat is unlikely to be available on even decadal time scales for much transfer back into the atmosphere, where it could affect weather circulation patterns and atmospheric global warming. Of course, sea level rise would still occur from this heat, and, presumably, on some longer time scale this heat will reemerge. Nonetheless, such a long-term sink of heat, if it is occurring, may be an unrealized negative feedback on atmospheric warming.