“Global Warming Or Not? The Relative Roles Of Externally Forced Versus Internally Generated Decadal Climate ” By Gerald A. Meehl, NCAR

Seminar Announcement

I am pleased that the role of internal climate dynamics in climate system heat changes (and other aspects of the climate system) is finally becoming better appreciated. This movement towards a broader view is ot be presented in the seminar below [h/t to Jason English]. Note that the talk will be live webcast at http://www.fin.ucar.edu/it/mms/ml-live.htm. This abstract is an important new contribution to our understanding of how the climate system works [highlight added]

Global warming or not? The relative roles of externally forced versus internally generated decadal climate “

Tuesday, 15 May, 2012
3:30 p.m. Mesa Lab, Main Seminar Room
NCAR, 1850 Table Mesa Drive

The last decade has been marked by very little globally averaged warming trend. This has led some to conclude that global warming has stopped for good. However, there have been previous decades when there was little or no global temperature increase set against the background of a longer term warming trend. This raises the issue of the relative roles of internally generated decadal climate variability and externally forced climate system response. The mid-1970s climate shift is given as an example of globally averaged temperatures dramatically rising after a period of little warming. Results show that this shift was part internally generated, related to a transition from a negative to positive phase of the Interdecadal Pacific Oscillation (IPO), and part externally forced by increasing greenhouse gases (GHGs) with a possible contribution from solar forcing. To better understand the recent period of little warming, 21st century simulations with CCSM4 are analyzed to show that during decades of slightly negative global temperature trend, the IPO is in a negative phase, along with reductions of Antarctic Bottom Water (AABW) formation and a weaker Atlantic Meridional Overturning Circulation (AMOC). Thus, during hiatus decades, the excess heat being trapped in the system due to increasing GHGs goes into the deep ocean. Conversely, decades in the model when there is an unusually large positive global temperature trend show the opposite response. This highlights the importance of understanding relative contributions of external forcing and internally generated variability for the decadal climate prediction problem.

My Comments:

  • First, if  “excess heat [is] being trapped in the system due to increasing GHGs…. into the deep ocean” this heat may be sequestered there indefinitely. It is not clear how such heat could be quickly transferred back into the atmosphere.
  • Moreover, what is the magnitude of the flux of heat through the upper 700m of the ocean in the model, and can this be seen in the real world observations?
  • Finally, this is yet another example of why the use of the global average surface temperature trend (which does not sample this heat when it is used to diagnose global warming) is a very inadequate metric for this purpose.

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