Geophysica (1998), 34(3), 159-178 Observations Inhomogeneities and Detection of Climate Change The case of the Oulu (Finland) air temperature series R. Sneyers*, H. Tuomenvirta** and R. Heino** *Institut royal météorologique de Belgique **Finnish meteorological institute (Received: June 1998; Accepted: October 1998) Abstract It is now well known that inhomogeneities in observations and climate instability have the common statistical property of producing generally in climatological series abrupt changes in the distribution of their elements (change-points) characterized by shifts in the mean and (or) in the scatter around this mean (dispersion). It follows that elimination of inhomogeneities in the observations should allow a better appreciation of the evolution of local climates and of the derived regional and global ones. For undocumented inhomogeneities as for climate instability, among the different methods used for the detection of change-points, many have a questionable efficiency due to inappropriate testing or to incomplete account of the available information. The purpose of this paper is to give and to justify a logical approach of the problem and to illustrate its efficiency for the search of modifications in the statistical characterics of climatological series created by observations inhomogeneities. The interest of the Oulu (Finland) series consists in its length, the relative high latitude of its situation and the care with which the problem of inhomogeneities has been solved. In the conclusion, the importance of the statistical aspect of climate changes is emphasized. Key words: Climatological time series, homogenization, sequential statistical analysis, change-point search, joint significance levels, climate evolution 1. Introduction If the general causes of inhomogeneites in observations have always explained in an evident manner the abrupt character of changes in the distribution of elements in climatological time series, inversely, the conviction that climate instability is characterized by abrupt changes is still not universal. Actually, climate change has generally been made clear by the evolution of climatic features such as alteration of snow lines, extension or reduction of glaciers termini, lake level changes, etc. Moreover, the usual progressive way of these changes has made it generally unbelievable that climate itself could evolve abruptly. In particular, when the climate warming detected by Lewis (1947), Lysgaard (1948) and Vandenplas (1948) Published by the Geophysical Society of Finland, Helsinki