Natural and anthropogenic variations in methane sources during the past two millennia

C. J. Sapart, G. Monteil, M. Prokopiou, R.S.W. van de Wal, J. Kaplan, K.M. Krumhardt, C. van der Veen, S. Houweling, M.C. Krol, Thomas Blunier, Dorthe Dahl-Jensen