Half of global methane emissions come from aquatic ecosystems, much of this is human-made

Between 2000 and 2006, global methane emissions stabilised but emissions began steadily rising again in 2007.

from http://www.firstpost.com/tech/science/half-of-global-methane-emissions-come-from-aquatic-ecosystems-much-of-this-is-human-made-9527221.html