This paper provides one of the first formal assessments of the link between anthropogenic climate change and the retreat of Pine Island Glacier, a major contributor to mass loss of the West Antarctic Ice Sheet. Using data assimilation methods, this study shows that the observed retreat over the industrial era is unlikely to have occurred without anthropogenic trends in forcing, and that anthropogenic forcing likely amplified 20th Century retreat of Pine Island Glacier by ~18%. The paper also highlights the uncertainties introduced by preconditioned ice mass loss in quantifying 20th Century forcing contributions and the possible role of longer-term changes in present-day retreat.