This topic has been discussed several times on the Climate Science weblog (e.g. see ). With the current emphasis on this issue in the media (e.g. see Climate scientists issue dire warning ), it is worth stating what global warming, as a concept in physics, means.
Global warming is the addition of heat to the climate system. The unit of measure (its metric) is in Joules. Most of the reported global warming has been in the oceans in the mid-latitude Southern Hemisphere ocean (see Willis, J.K., D. Roemmich, and B. Cornuelle, 2004: Interannual variability in upper ocean heat content, temperature, and thermosteric expansion on global scales. J. Geophys. Res., 109, C12036, doi: 10.1029/2003JC002260. )
As stated in the Willis et al paper,
â”Maps of yearly heat content anomaly show patterns of warming commensurate with ENSO variability in the tropics, but also show that a large part of the trend in global, oceanic heat content is caused by regional warming at midlatitudes in the Southern Hemisphere.”
“…centered on 40S is spread more uniformly over the water column and warms steadily throughout the entire time series…”
This is for the period mid-1993 through mid-2003.
They further find that, with respect to the current rate of warming (as compared with the Levitus et al earlier data)
“….the warming rate in the early 1970s is comparable to the present rate…..With the present time series, it is therefore not possible to identify whether the recent increase in ocean warming is due to an acceleration of heat uptake by the ocean or is simply decadal variability”.
As I wrote in my September 9 2005 weblog
âA significant portion of the warming is at depth. The portion of this heat that is a depth below the thermocline is not readily available to heat the atmosphere above or to contribute to enhanced evaporation of water vapor from the ocean surface. This heat is ‘sequestered for an unknown period of time.'”
ââ¦.global warming has significant spatial variations. Global warming is not a more-or-less uniform warming spread across the oceans. Such a spatially complex warming pattern further supports the claim that a multiple set of climate forcings, in addition to the more homogeneous radiative forcing of the well-mixed greenhouse gases, is altering our climate. The reconstruction of the observed temporal evolution of the spatial pattern over the last several decades by the global climate models remains an unrealized goal.â?
To justify the claims of a catastrophic impact from global warming, one must show how this heat addition to the climate system results in such dramatic climate consequences. The use of a global average surface temperature clearly does not provide the rigor of scientific cause and effect that is promoted in the media.