Discontinuités

J'ai découvert récemment un article intitulé A closer look at United States and global surface temperature change , il est signé : Hansen, J.E., R. Ruedy, Mki. Sato, M. Imhoff, W. Lawrence, D. Easterling, T. Peterson, T. Karl et est paru en 2001 dans Journal of Geophysical Research.

Quelques extraits :

However, caution is required in making adjustments, as it is possible to make the long-term change less realistic even in the process of eliminating an unrealistic short-term discontinuity. Indeed, if the objective is to obtain the best estimate of long-term change, it might be argued that in the absence of metadata defining all changes, it is better not to adjust discontinuities.

The measured temperature includes an urban warming effect (exaggerated relative to year-to-year variability for clarity), a discontinuity due to the station moving to the city outskirts, and a lesser continued human effect in the suburbs. In this case, if the record is adjusted for the discontinuity (Figure 1b), the adjusted long-term temperature trend is less realistic than the trend of the unadjusted data, as urban warming at two locations has been incorporated into the adjusted record.

It follows that a necessary concomitant of discontinuity adjustments is an adequate correction for urban warming. Otherwise, if the discontinuities in the temperature record have a predominance of downward jumps over upward jumps, the adjustments may introduce a false warming, as in Figure 1. This might happen, for example, if it is more common for stations to move from population centers toward the suburbs, rather than vice versa.

La figure dont il est question :

J'ai évidemment une réserve à propos de la courbe dite "undisturbed". Comme nous l'avons vu dans les billets précédents, cette courbe est encore fortement perturbée.

Cette découverte a été une assez grande surprise pour moi car le phénomène du biais des discontinuités dont j'ai déjà parlé (notamment ici, ici et ici) y est traité à peu près correctement. Cela signifie que la communauté des scientifiques qui s'occupent des données de températures est depuis dix ans parfaitement au courant du problème mais qu'à part Hansen pour GISSTemp, personne n'en a tenu compte. Pourquoi ?