Klotzbach, P.J., Pielke Sr., R.A., Pielke Jr., R.A., Christy, J.R. and McNider, R.T. 2009. An alternative explanation for differential temperature trends at the surface and in the lower troposphere. Journal of Geophysical Research 114: 10.1029/2009JD011841.
What was done
The authors calculated global linear temperature trends over the 1979-2008 time period based on data provided them by (1) the National Climate Data Center (NCDC, near-surface thermometer data), (2) the Hadley Center (HadCRUT3v, near-surface thermometer data), (3) the University of Alabama in Huntsville (UAH, lower-troposphere satellite data), and (4) the Remote Sensing Systems (RSS, lower-troposphere satellite data), after which they compared the four global trends and then subdivided each of them into land and ocean components for further analyses.
What was learned
Klotzbach et al., as they describe it, "find that there have, in general, been larger linear trends in surface temperature data sets such as the NCDC and HadCRUT3v surface data sets when compared with the UAH and RSS lower-tropospheric data sets," which variation in warming trends, as they continue, "is also confirmed by the larger temperature anomalies that have been reported for near surface air temperatures (e.g., Zorita et al., 2008; Chase et al., 2006, 2008; Connolley, 2008)." In addition, they report that "the differences between surface and satellite data sets tend to be largest over land areas."
What it means
With respect to the observation that "differences between surface and satellite data sets tend to be largest over land areas," the five researchers say that this fact indicates "there may still be some contamination because of various aspects of land surface change [such as growing urban heat islands], atmospheric aerosols and the tendency of shallow boundary layers to warm at a greater rate," which ultimately leads them to "consider the possible existence of a warm bias in the surface temperature trend analyses."
We would only add that this possibility is looking ever more likely, especially in light of the ClimateGate email revelations of potential tampering with (i.e., inflation of) near-surface air temperature data over earth's land area, as well as the fact that Santer et al. (2005) have noted that "a non-significant trend differential [between the surface and satellite warming trends] would also occur if the surface warming had been over-estimated by [only] 0.05°C per decade in the IPCC data," and that this seemingly small differential could be responsible for a full half-degree Centigrade upward trend of near-surface air temperature over the planet's land area over the course of the 20th century.
Chase, T.N., Wolter, K., Pielke Sr., R.A. and Rasool, I. 2006. Was the 2003 European summer heat wave unusual in a global context? Geophysical Research Letters 33: 10.1029/2006GL027470.
Chase, T.N., Wolter, K., Pielke Sr., R.A. and Rasool, I. 2008. Reply to comment by W.M. Connolley on "Was the 2003 European summer heat wave unusual in a global context?" Geophysical Research Letters 35: 10.1029/2007GL031574.
Connolley, W.M. 2008. Comment on "Was the 2003 European summer heat wave unusual in a global context?" by Thomas N. Chase et al. Geophysical Research Letters 35: 10.1029/2007GL031171.
Santer, B.D., Wigley, T.M.L., Mears, C., Wentz, F.J., Klein, S.A., Seidel, D.J., Taylor, K.E., Thorne, P.W., Wehner, M.F., Gleckler, P.J., Boyle, J.S., Collins, W.D., Dixon, K.W., Doutriaux, C., Free, M., Fu, Q., Hansen, J.E., Jones, G.S., Ruedy, R., Karl, T.R., Lanzante, J.R., Meehl, G.A., Ramaswamy, V., Russell, G. and Schmidt, G.A. 2005. Amplification of surface temperature trends and variability in the tropical atmosphere. Science 309: 1551-1556.
Zorita, E., Stocker, T.F. and von Storch, H. 2008. How unusual is the recent series of warm years? Geophysical Research Letters 35: 10.1029/2008GL036228.Reviewed 9 December 2009