Simulated And Observed Variability In Ocean Temperature And Heat Content by AchutaRao Et Al

There is a new paper on comparing ocean heat content changes with model simulations of this change, as well as an effective analysis of the role of the observation system on diagnosing this climate metric (thanks to Willie Soon and Inkstain for alerting me to the article). The paper is

AchutaRao, K. M., M. Ishii, B. D. Santer, P. J. Gleckler, K. E. Taylor, T. P. Barnett, D. W. Pierce, R. J. Stouffer, and T. M. L. Wigley, 2007: Simulated and observed variability temperature and heat content. 10768–10773 PNAS June 26, 2007 vol. 104 no. 26.

The abstract reads,

“Observations show both a pronounced increase in ocean heat content (OHC) over the second half of the 20th century and substantial OHC variability on interannual-to-decadal time scales. Although climate models are able to simulate overall changes in OHC, they are generally thought to underestimate the amplitude of OHC variability. Using simulations of 20th century climate performed with 13 numerical models, we demonstrate that the apparent discrepancy between modeled and observed variability is largely explained by accounting for changes in observational coverage and instrumentation and by including the effects of volcanic eruptions. Our work does not support the recent claim that the 0- to 700-m layer of the global ocean experienced a substantial OHC decrease over the 2003 to 2005 time period. We show that the 2003–2005 cooling is largely an artifact of a systematic change in the observing system, with the deployment of Argo floats reducing a warm bias in the original observing system.”

The paper emphasizes that ocean heat content is the appropriate climate diagnostic to monitor global warming or cooling; e.g. see their statement at the beginning of their paper,

“Observations suggest that the world’s oceans were responsible for most of the heat content increase in the earth’s climate system between 1955 and 1998″,

and they extend their analysis through 2006 in the new paper. This need to focus on ocean heat content changes has been a major recommendation of Climate Science (e.g. see # 4).

Their paper also identified a problem with the diagnosis of ocean cooling which was independently found by Lyman et al (see).

However, there are issues in this paper that the authors are silent about.

They are:

1. In Figure 5 in their paper, there appears to be a lack of significant continued warming in recent years. Since the ocean heat content changes can be used to diagnose the radiative imbalance of the climate system (see and see), this indicates at most only a small radiative warming recently. This lack of warming is at variance to their claim that the model results and the observations are consistent. The paper spent time (appropriately) questioning the Lyman et al 2006 study, but did not pursue the significance of this mismatch between the observations and the predicted more-or-less monotonic warming by the models.

2. The authors’ confidence in the models provides a hypothesis that can be tested over the next few years to confirm or refute the predictive skill of their models. The challenge is for them to show they can skillfully predict the recent (last few years) lack of global -average ocean warming, as well as its change over the next few years. As posted on Climate Science in the weblog

A Litmus Test For Global Warming – A Much Overdue Requirement,

this real time comparison between models and observations is an essential test that is required to accept their model predictions as skillful.

In the Climate Science weblog, I posed the following question,

“What is the magnitude of ocean heat storage changes each year?”

and added

“For global warming to occur, the heat, as measured in Joules, needs to increase each year.

The heat accumulation for the period from 2002 to the present and into the future needs to be a high priority. For example to sustain a global warming rate of 1 Watt per meter squared since 2002 for the following ten years requires the accumulation of 1.6 *10**23 Joules within the climate system.”

3. The authors should have presented both their model results and observations in units of Watts per meter squared (as recommended in Pielke 2003). The model forcings should have been separated into terms of radiative forcing and radiative feedback.

The reason to separate is so they can compare with the 2007 IPCC Statement for Policymakers which gives a mean value of 1.78 Watts per meter squared (see Figure SPM.2) for the “Global average radiative forcing (RF) estimates and ranges in 2005″. The radiative forcing diagnosed from the observed ocean heat content changes includes both the radiative forcing and the radiative feedbacks (e.g. the water vapor feedback).

Unfortunately, the AchutaRao et al paper is silent on this issue. See also

The Net Climate Feedbacks Must Be A Negative Effect On The Global Average Radiative Imbalance If The IPCC Conclusion Of Net Anthropogenic Radiative Forcings Is Correct.

Thus while this peer reviewed paper is a constructive contribution to the debate, it did not address other key questions for which it has the results to accomplish. Climate Science recommends that the authors take this next step in their assessment.

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