• Adjustments are complicated, sometimes as large as the trend itself, involve expert judgments, and
cannot be stringently evaluated because of lack of traceable standards.
• Unlike surface trends, reported upper-air trends vary considerably between research teams
beginning with the same raw data owing to their different decisions on how to remove
non-climatic factors.
• The diurnal cycle, which must be factored into some adjustments for satellite data, is well
observed only by surface observing systems.
• No available observing system has reference stations or multi-sensor instrumentation that
would provide stable calibration over time.
• Most observing systems have not retained complete metadata describing changes in observing
practices which could be used to identify and characterize non-climatic influences.
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