Future prospects - Ice Ages and Interglacials: Measurements, Interpretation, and Models - page 326

The first diculty is in defining the average temperature of the Earth. 5 Most
estimates of the Earth's average temperature are based on land and ocean surface
measurements around the globe and the application of some sort of averaging
scheme. However, as Rapp (2008) showed in some detail, the global network of
temperature measurement stations that have amassed 100 years of data is not only
sparse but also mainly centered in the U. S. and Western Europe. Furthermore,
the number of stations has been decreasing at an alarming rate late in the 20th
century. In addition to the lack of long-term temperature data from around the
globe, there are numerous problems associated with many of the measurement
stations. These include poor siting because of local obstructions or reflectors;
urban heating effects; changes in the environment of the station, the instrument
used, or its location over the years; changes in observing practices and the method
used to calculate mean temperature; and lack of maximum-minimum temperature
capability at most sites. The problem of urban heat islands is particularly impor-
tant. Hansen et al. (2010) carried out an extensive review and analysis of the
adequacy of the global temperature measurement network. They claimed that the
network was adequate and when they made corrections for urban areas these
corrections were small. However, Imhoff et al. (2010) and Zhang et al. (2010)
reported that urban heat effects are far greater than had previously been supposed,
as much as 7 C higher in many cases. In addition, there are uncertainties in the
calibration of long-term sea temperature records.
Rapp (2008) quoted some leading experts in the field of Earth temperature
monitoring who said (amongst other things):

''Climate researchers have used existing, operational networks because they
have been the best, and sometimes only, source of data available. They have
succeeded in establishing basic trends of several aspects of climate on regional
and global scales. Deficiencies in the accuracy, quality, and continuity of the
records, however, still place serious limitations on the confidence that can be
placed in the research results.''
''It's very clear we do not have a climate observing system This may come

as a shock to many people who assume that we do know adequately what's going
on with the climate but we don't.''

...

An alternative to surface measurement stations is the use of satellite
observations to assess global temperatures. Douglass and Christy (2009) presented
evidence that past problems with calibration have been solved and they presented
the latest tropospheric temperature measurements. They argued that satellite
measurements of temperature are superior to ground-based measurements because
they provide a global view and do not suffer from boundary layer effects or

5 The author's book Assessing Climate Change, Praxis Publishing, Second Edition, 2010
provides an in-depth discussion of the procedures used to estimate global temperatures and the
results obtained so far.