Anatomy of the AGN in NGC 5548 - VI. Long-term variability of the warm absorber J. Ebrero, J. S. Kaastra, G. A. Kriss, L. Di Gesu, E. Costantini, M. Mehdipour, S. Bianchi, M. Cappi, R. Boissay, G. Branduardi-Raymont et al. (6 more) A&A, 587 (2016) A129 Published online: 02 March 2016 DOI: 10.1051/0004-6361/201527808