Does GISSTEMP overcount rural stations?
In a previous post, Eli quoted from Hansen, J.E., R. Ruedy, Mki. Sato, M. Imhoff, W. Lawrence, D. Easterling, T. Peterson, and T. Karl, 2001: A closer look at United States and global surface temperature change. J. Geophys. Res., 106, 23947-23963, doi:10.1029/2001JD000354 as to how the GISSTEMP team adjusts urban data
The urban adjustment in the current GISS analysis is a similar two-legged adjustment, but the date of the hinge point is no longer fixed at 1950, the maximum distance used for rural neighbors is 500 km provided that sufficient stations are available, and “small-town” (population 10,000 to 50,000) stations are also adjusted. The hinge date is now also chosen to minimize the difference between the adjusted urban record and the mean of its neighbors. In the United States (and nearby Canada and Mexico regions) the rural stations are now those that are “unlit” in satellite data, but in the rest of the world, rural stations are still defined to be places with a population less than 10,000. The added flexibility in the hinge point allows more realistic local adjustments, as the initiation of significant urban growth occurred at different times in different parts of the world.In the US, they used satellite observations of night lights to define what are rural, suburban and urban areas:
The percent of brightness refers to the fraction of the area-time at which light was detected, i.e., the percent of cloud-screened observations that triggered the sensor. These data are then summarized into three categories (0-8, 8-88, and 88-100%). From empirical studies in several regions of the United States, Imhoff et al. associate the brightest regions (which we designate as “bright” or “urban”) with population densities of about 10 persons/ha or greater and the darkest (“unlit” or “rural”) regions with population densities of about 0.1 persons/ha or less. As is apparent from Plate 1b, the intermediate brightness category (“dim” or “periurban”) may be a small town or the fringe of an urban area.after this classification the number of rural stations in the US is reduced to 214 USHCN stations and 256 of the GHCN stations (obviously a lot of duplication here)
As the contiguous United States covers only about 2% of the Earth’s area, the 250 stations are sufficient for an accurate estimate of national long-term temperature change, but the process inherently introduces a smoothing of the geographical pattern of temperature change.Outside of the US, they continue to use population data to define rural stations.
The bottom line is that the ONLY stations which contribute to the overall trend are the RURAL stations. Moreover, rural stations near heavily settled areas will be more strongly overcounted because the trends in the few rural stations in such an area will dominate all of the stations in the area and the nearby points on the grid to which the temperature data is fit.