New Paper “On Potential Causes For An Under-Estimated Global Ocean Heat Content Trend In CMIP3 Models” By Cai Et Al 2010

There is a new paper on the comparison of model and observed global ocean heat content changes for the period 1951 to 1999 (thanks to Josh Willis for alerting us to it!). The paper is

Cai, W., T. Cowan, J. M. Arblaster, and S. Wijffels (2010), On potential causes for an under-estimated global ocean heat content trend in CMIP3 models, Geophys. Res. Lett., 37, L17709, doi:10.1029/2010GL044399.

“Trends in global oceanic heat content (OHC) over the late 20th century as simulated by climate models that incorporate all radiative forcing factors are smaller than the observed, but the causes are not clear. Given the cooling effect associated with increasing anthropogenic aerosols and natural forcing (i.e., volcanic aerosols), we examine their respective roles in the simulated global OHC trend and the associated ocean temperature structure, using targeted experiments from two models, designed to separate the individual impacts of these forcing components. We show that it is more likely that the indirect effect of aerosols, not volcanic aerosols alone, is the reason for the bulk of weaker  modelled OHC trends. Further, anthropogenic aerosols are essential for simulating the structure of the observed temperature changes, including a concentrated cooling in the Southern Hemisphere subtropical latitudes, consistent with a more stable global Conveyer, a greater strengthening of the subtropical gyre circulation, and a stronger Southern Annular Mode trend in targeted experiments with anthropogenic aerosol forcing.”

The introduction starts with the text

“An ensemble mean of 20th century model experiments (submitted as part of the Coupled Model Intercomparison Project Phase 3 (CMIP3)) forced with all forcing factors
(ALL) such as time‐varying well‐mixed greenhouse gases, anthropogenic and volcanic aerosols, ozone depletion, and solar irradiance, produces a linear trend in the upper 300m OHC over 1961–1999 that is 28% smaller than the observed [Domingues et al., 2008].”

An excerpt from the conclusion reads

“A subgroup of CMIP3 20th century experiments with all forcing factors produces a global OHC trend since 1951– 1999 that is too weak when compared with the observed,
whereas another subgroup without natural forcings overestimates the global OHC trend.”

This study is the type we need to test the IPCC model projections. This study, of course, is for the period 1951 to 1999. There is a need to include the last 11 years in this assessment, since the last few years have seen little, if any warming, as diagnosed by the upper ocean heat content changes; i.e. see

R. S. Knox, David H. Douglass 2010: Recent energy balance of Earth  International Journal of Geosciences, 2010, vol. 1, no. 3 (November) – In press doi:10.4236/ijg2010.00000.

In addition,  the Cai et al 2010 paper is not only puzzling because it examined only up through 1999, they also could only look at modeled natural variability, not the real world variability, the later of which is clearly larger than the models have so far been able to replicate. Moreover, despite their claim that the “Coupled Model Intercomparison Project Phase 3 (CMIP3)) [is] forced with all forcing factors“, it is not, as we document in the 2005 NRC report

National Research Council, 2005: Radiative forcing of climate change: Expanding the concept and addressing uncertainties. Committee on Radiative Forcing Effects on Climate Change, Climate Research Committee, Board on Atmospheric Sciences and Climate, Division on Earth and Life Studies, The National Academies Press, Washington, D.C., 208 pp.
http://www.nap.edu/openbook/0309095069/html/

An update of the Cai et al  study to 2010 would be of considerable value, as well as the identification of what climate forcings and feedbacks are incompletely modelled and/or are left out of the models.

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