Problems Leading to Uncertainty

Problems Leading to Uncertainty

There are several problems that lead to uncertainty in constructing these hemispheric and global mean temperature records (Houghton et al, 1990):

  1. Spatial coverage of the data is incomplete and varies greatly.
  2. Changes have occurred in observing schedules and practices.
  3. Changes have occurred in the exposures of thermometers.
  4. Stations have changed their locations.
  5. Changes in the environment, especially urbanization, have taken place around many stations.
  6. Land use changes other than urbanization.
    7.
Problems due to item 1 are most pronounced before 1900. Studies on the data sets after 1900 where newer stations are successively included and excluded show that the spatial coverage beginning in 1900 is relatively representative of the entire globe.

Houghton (1990) has considerable discussion on each of these items except 6, which is a more recently described factor. They conclude that item 5, and urbanization in particular, is the largest source of possible error in the temperature record. As cities have expanded, formerly rural weather stations become surrounded by, or at least are influenced by, urban landscapes, which are known to have higher surface air temperatures. This source of error differs from the others primarily because, whereas the others may have some stations giving positive errors while others may give compensating negative errors, all stations subjected to urbanization will cause a monotonic warming of the record. However, analysis of temperature records from stations known to be continuously located in rural areas show the same general pattern as the complete data set of all stations.

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