Although it is clear that warming has occured over the past 100 years, many uncertainties crop up in the real time and historical measurement of climate change that are related to instrumentation,(and changing instrumentation), various definitions as to what constitutes a high and low of a day, and changes in instrument exposure due to changes in site, or changing landuse around a site. Questions can be raised as to the choice of the most appropriate metric with which to use in measuring such changes (Is the 2 meter temperature the best choice for diagnosing a warming of the climate system?). Many more questions come to the fore when considering the projection of future climates. They include the impact of parameterization vs explicit handling of physical processes in global climate models due to resolution of models, computational capabilities, and an incomplete knowledge that can be found at all scales and in all components of the climate system. Examples of areas that suggest a current lack of understanding in the climate system (even as regards "natural change) include the 10C to 20C "flickering" of annual temperatures on the scale of several years or decades that is seen in the icecore record of mid and high latitude locales before the recovery from the last ice age, and the rapid desertification that took place over the Sahara in a period of a century about 5,000 years ago. Questions also abound in areas that can never be known in advance, and treated as a constant in the models. This includes the continuing manner in which the where, when, the nature of, and on what geographical scale that land use changes occur on.
While the governmental and economic planning communities cannot expect that uncertainties in the projection of future climates will be narrowed in the near future, it may unwise to suggest that future plans should be directed to a range of scenerios as narrow as what is being suggested by the IPCC.
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