Misleading headline - "three quarters of global warming is manmade" implies 1/4 not.
By not prefacing the headline with "At least", the headline suggests that the remaining 1/4 of warming is caused by natural factors; this misleads readers.
The same article, with same "Three-quarters of climate change is man-made" headline, appeared in Nature News 2011-12-04, at http://www.nature.com/news/three-quarters-of-climate-change-is-man-made-1.9538
Joe Romm suggests a more informative headline would be: "...At Least 74% of Observed Warming Since 1950 Was Manmade; It’s Highly Likely All of It Was" (which is long, but just adding the "at least" would help a lot.) Link to Romm post: http://thinkprogress.org/romm/2011/12/05/382209/observed-warming-since-1950-was-manmade/