We recently published our 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.
This paper raises serious issues with respect to the use of observed land surface air temperatures to diagnose multi-decadal global temperature trends and to report regional and local temperature anomalies and extremes. A major key finding from our study is that the magnitude of global warming is significantly overstated using surface air temperature as a metric.
In part I of this set of weblogs, the first section of our JGR paper is discussed. This section of our paper is titled
Definition of a Global Average Surface Temperature
The definition of the global average surface temperature used by the IPCC and others can be expressed as
dH/dt = f -T’/λ
where H is the heat content of the land-ocean-atmosphere system, f is the radiative forcing (i.e. the radiative imbalance), T’ is the change global average surface temperature in response to the change in H, and λ is called the “climate feedback” parameter which defines the rate at which the climate system returns forcing to space as infrared radiation and/or as changes in reflected solar radiation (such as from changes in clouds, sea ice, snow, vegetation, etc).
There is a fundamental problems, however, with the use of this equation for the description for global warming.
T is defined in the above equation as a global proxy for the thermodynamic state of the climate system. As such, it must be tightly coupled to that thermodynamic state of the climate system. Specifically, in this context, T is the global average radiative temperature of the Earth’s surface since the outgoing radiative flux at the top of the atmosphere is determined to a large extent by the surface radiative temperature. However, this outgoing longwave radiation is proportional to the fourth power of T. T’ = +1 C in the polar latitudes in the winter, for example, would have much less of an effect on the change of longwave emission than T’=+1°C increase in the tropics. The spatial distribution of T’ matters, whereas the equation given above ignores the consequences of spatially varying values of T’.
T’ at any location is also not a constant with height even near the Earth’s surface. Variations of T’ over just a few meters of height, particularly on light wind nights, can be significant, as we document in our paper (which will be weblogged on further on a subsequent weblog)
Lin, X., R.A. Pielke Sr., K.G. Hubbard, K.C. Crawford, M. A. Shafer, and T. Matsui, 2007: An examination of 1997-2007 surface layer temperature trends at two heights in Oklahoma. Geophys. Res. Letts., 34, L24705, doi:10.1029/2007GL031652.
Therefore, the use of the equation dH/dt = f -T’/λ to define global warming is flawed.
Rather than use this formulation, the obvious preference should be to directly diagnose (predict) dH/dt, since, as shown in
Pielke Sr., R.A., 2003: Heat storage within the Earth system. Bull. Amer. Meteor. Soc., 84, 331-335.
dH/dt permits the assessment of climate system heat changes in units of heat (Joules) which allows the diagnosis of the magnitude of the global radiative imbalance of the climate system in Watts per meter squared.
Summary: There are three main conclusions from Part I of our JGR paper. They are:
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To diagnose the magnitude of global warming using the global average surface temperature anomaly, T’ must be tightly coupled to that thermodynamic state of the climate system; however, this is not an accurate charaterization of the Earth’s climate;
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In constructing a global average of T’, its spatial distribution matters since T’ in regions with a baseline colder temperature have a significantly smaller effect on the return of heat energy to space (through infrared emission) than regions with a warmer baseline temperature
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The height that T’ is measured matters, since T’ at the actual surface is not the same as T’ even slightly higher; i.e., T’ is not, in general, height invariant near the surface.
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The use of dH/dt to diagnose global warming is a much more scientifically robust approach than using a global average surface temperature anomaly.