In the discussion on the Levitus et al 2012 paper
Levitus, S., et al. (2012), World ocean heat content and thermosteric sea level change (0-2000), 1955-2010, Geophys. Res. Lett.,doi:10.1029/2012GL051106, in press
which I have posted on several times; e.g. see
the level of uncertainty in the ocean data has not been emphasized. This post is to highlight this issue with respect to modeled uncertainty of ocean heat content changes.
As shown in the figure below from Levitus et al 2012, they claim that the uncertainty range of the observations (the vertical lines on the red line) narrows to very small levels in recent years.
However, with respect to the uncertainty of the multi-decadal global model predictions, when run in a hindcast mode, the paper
provides valuable insight into this issue. I posted on this paper in
With respect to the ocean heat data specifically, Sen Gupta et al write that [highlight added]
Even in the absence of external forcing, climate models often exhibit long-term trends that cannot be attributed to natural variability. This so called ‘climate drift’ arises for various reasons including: perturbations to the climate system on coupling component models together and deficiencies in model physics and numerics…….To illustrate this we have focussed on simulated trends over the second half of the 20th century……Below depths of 1000 to 2000m, drift dominates over any forced trend in most regions. As such steric sea-level is strongly affected and for some models and regions the sea-level trend direction is reversed….”
Clearly, even if the observed uncertainty was as small as concluded by Levitus et al 2012 [which given the areal coverage they report at 2000m is unrealistically small] any attempt to use the multi-decadal climate model predictions to provide an explantion for this warming at depth (even if real) is not robust scientifically.