Here are Dr. Curry’s summarizing comments:
After reading all of these papers, I would have to conclude that if the CMIP5 historical simulations are matching the ‘observations’ of ocean heat content, then I would say that they are getting the ‘right’ answer for the wrong reasons. Not withstanding the Cheng et al. paper, the ‘right’ answer (in terms of magnitude of the OHC increase) is still highly uncertain.
The most striking findings from these papers are:
- the oceans appear to have absorbed as much heat in the early 20th century as in recent decades (stay tuned for a forthcoming blog post on the early 20th century warming)
- historical model simulations are biased toward overestimating ocean heat uptake when initialized at equilibrium during the Little Ice Age
- the implied heat loss in the deep ocean since 1750 offsets one-fourth of the global heat gain in the upper ocean.
- cooling below 2000 m offsets more than one-third of the heat gain above 2000 m.
- the deep Pacific cooling trend leads to a downward revision of heat absorbed over the 20th century by about 30 percent.
- an estimated 20% contribution by geothermal forcing to overall global ocean warming over the past two decades.
- we do not properly understand the centennial to millennia ocean warming patterns, mainly due to a limited understanding of circulation and mixing changes
These findings have implications for:
- the steric component of sea level rise
- ocean heat uptake in energy balance estimates of equilibrium climate sensitivity
- how we initialize global climate models for historical simulations
While each of these papers mentions error bars or uncertainty, in all but the Cheng et al. paper, significant structural uncertainties in the method are discussed. In terms of uncertainties, these papers illustrate numerous different methods of estimating of 20th century ocean heat content. A much more careful assessment needs to be done than was done by Cheng et al., that includes these new estimates and for a longer period of time (back to 1900), to understand the early 20th century warming.
In an article about the Cheng et al. paper at Inside Climate News, Gavin Schmidt made the following statement:
“The biggest takeaway is that these are things that we predicted as a community 30 years ago,” Schmidt said. “And as we’ve understood the system more and as our data has become more refined and our methodologies more complete, what we’re finding is that, yes, we did know what we were talking about 30 years ago, and we still know what we’re talking about now.”
Sometimes I think we knew more of what we were talking about 30 years ago (circa the time of the IPCC FAR, in 1990) than we do now: “it aint what you don’t know that gets you in trouble. It’s what you know for sure that just aint so.”
The NASA GISS crowd (including Gavin) is addicted to the ‘CO2 as climate control knob’ idea. I have argued that CO2 is NOT a climate control knob on sub millennial time scales, owing to the long time scales of the deep ocean circulations.
A talking point for ‘skeptics’ has been ‘the warming is caused by coming out of the Little Ice Age.’ The control knob afficionadoes then respond ‘but what’s the forcing.’ No forcing necessary; just the deep ocean circulation doing its job. Yes, additional CO2 will result in warmer surface temperatures, but arguing that 100% (or more) of the warming since 1950 is caused by AGW completely neglects what is going on in the oceans.
[Hifast Note: The comment thread at Dr. Curry’s Climate Etc. is essential reading as well.]