Climate Science (and other weblogs) have posted detailed information on the issues associated with different methods to assess global climate system heat changes. Readers can access examples of these posts on Climate Audit, Watts Up With That, Hall of Record, ICECAP and The Blackboard.
This current Climate Science weblog is intended to just summarize the issue as there are still individuals who perpeturate the claim that measuring near surface air temperature at irregularly spaced observation sites around the globe can accurately diagnose global warming.
As repeatedly emphasized on Climate Science, the use of surface air temperature is not a measure of climate system heat since it has almost no mass associated with it. Heat of the climate system requires that temperature change (and heat associated with the phase of water) ocurr over mass. The relation of a change in global climate system heat change is dominated by ocean heat changes, as succincty summarized in Table 1 of
Levitus, S., J.I. Antonov, J. Wang, T.L. Delworth, K.W. Dixon, and A.J. Broccoli, 2001: Anthropogenic warming of Earth’s climate system. Science, 292, 267-269
where they write
“Our analysis of components of Earth’s heat balance quantitatively demonstrates that during the latter half of the 20th century, changes in ocean heat content dominate the changes in Earth’s heat balance.”
This heat change can be expressed as
ΔH = CΔT * Mass of the ocean where C is the specific heat capacity of sea water in units of Joules per degree per kilogram, T is temperature and M is the mass of the ocean in kilograms.
Thus if ΔH > 0, there is global warming.
No where is there a need to use a global average surface temperature trend to diagnose global warming, and, in fact, its use is misleading since there is an inconsequential amount of mass of the climate system involved. Over the oceans, the sea surface temperatures are correlated with the upper ocean heat content, but over land there are a variety of serious problems with its use as reported in the paper
Pielke Sr., R.A., C. Davey, D. Niyogi, S. Fall, J. Steinweg-Woods, K. Hubbard, X. Lin, M. Cai, Y.-K. Lim, H. Li, J. Nielsen-Gammon, K. Gallo, R. Hale, R. Mahmood, S. Foster, R.T. McNider, and P. Blanken, 2007: Unresolved issues with the assessment of multi-decadal global land surface temperature trends. J. Geophys. Res., 112, D24S08, doi:10.1029/2006JD008229.
Climate Science (and others; e.g. see) have been reporting on the shortcomings of the use of the global average surface average temperature trend to diagnose global warming, but this poor metric continues to be used by those advocating for particular policy actions on climate (e.g. see its inadequate and incomplete explanation in Wikipedia).
However, using the actual robust metric of global warming, ΔH, there has been none since 2004 (e.g. see), at least in the upper 700m of the ocean.