We set up a general framework to study Tate cohomology groups of Galois modules along Z_p-extensions of number fields. Under suitable assumptions on the Galois modules, we establish the existence of a five-term exact sequence in a certain quotient category whose objects are simultaneously direct and inverse systems, subject to some compatibility. The exact sequence allows one, in particular, to control the behaviour of the Tate cohomology groups of the units along Z_p-extensions.

As an application, we study the growth of class numbers along what we call "fake Z_p-extensions of dihedral type". This study relies on a previous work, where we established a class number formula for dihedral extensions in terms of the cohomology groups of the units.

We are grateful to the anonymous referee for a thorough reading of our manuscript and for several remarks that improved the clarity and the readability of the text.