The monitoring of the climate attained a great importance and a public attention in discussions of a possible global warming in the past years.
The aim of climate monitoring is to describe the situation of the climatic system and compare it with a reference period. Results of the climate monitoring are maps of average values, their deviation from normal or long time series from average values or other indices. In addition, the comparison with well-known extreme values (weather records) is helpful, in order to be able to make a classification.
Reports on special events are another tool to describe the climatic situation. In these reports interesting meteorological situations are described in more detail. The annual Climate Status Reports each year are dedicated to new main topics on the one hand, but also contain contributions, which describe the climate of the past year in Germany. The Climate Status Reports are available in German only, although some English contributions may be included.
A pre-requisite for climate monitoring is the availability of long time series of meteorological variables, which represent natural changes only and are as unimpaired as possible. This is not trivial. A well-known effect is e.g. the urban heat island. If an observing station during the course of the years was more and more surrounded by buildings, the temperature may have increased due to this fact. This artificial increase has to be separated from a possibly existing real trend. Therefore simply measuring is not enough. One must also put substantial efforts in securing the optimum quality of the measurements. This includes carefully selecting and maintaining the measuring site itself, controlling of the measuring instruments, as well as the long-term and safe archiving of the data. Meta data are just as important. This information is needed to describe the data. E.g. if it is unknown, where exactly a measurement was made and which boundary conditions prevailed, serious statements are impossible.
Besides traditional observing methods, remote sensing becomes more and more important for gathering information. Data from remote sensing instruments such as satellites today are available for long enough periods to also derive climatological information. But for remote sensing data to be used for climatological purposes, quality control is essential, too.
Therefore at the 5th Conference of the Parties to the UNFCCC GCOS/GOOS/GTOS Climate Monitoring Principles have been formulated (FCCC/CP/1999/7, p. 108). These principles should guide operators and managers of observing systems which are already or might be used for climate monitoring. The 13th Conference agreed upon a new version of those principles which are now also valid for providers of satellite data (resolution 11/CP.13 in FCCC/CP/2007/6/Add.2).