Graphic representation of 97–98%
The figure is based on Doran 2009 and Anderegg 2010 studies.
Doran &amp; Zimmerman 2009 study:
http://tigger.uic.edu/~pdoran/012009_Doran_final.pdf
Anderegg et al. 2010 study:
http://www.pnas.org/content/early/2010/06/04/1003187107.full.pdf en:Proceedings of the National Academy of Sciences of the United States of America
http://www.pnas.org/content/suppl/2010/06/07/1003187107.DCSupplemental/pnas.201003187SI.pdf
The first study (Doran &amp; Zimmerman 2009) is a survey with 2 questions.  In the first question, 96.4% of "climatologists who are active publishers on climate change" agree that mean global temperatures have risen "compared with pre-1800s levels" (compared with before the 1550-1850 A.D. Little Ice Age ended).  In the second question, 97.4% (75 of 77) agree that human activity "is a significant contributing factor" in temperature change.  The study concludes the distribution of answers to its survey questions implies that debate on the "role played by human activity is largely nonexistent" amongst climate experts.
According to the second study (Anderegg et al. 2010), there are only 2% to 3% UE ("unconvinced by the evidence") disagreeing scientists amongst a group of 50 to 200 climate researchers ranked as top.  Expertise was defined as determined by the number of articles published by climate journals.  The top 50 scientists considered CE ("convinced by the evidence") wrote an average of 408 articles each which were successfully published.
Scientists were counted as UE if having signed a public "statement strongly dissenting from the views of the IPCC."  That resulted in a list of 472 UE scientists, of whom 1 was in the top 50 ("2% of the top 50 climate researchers") as defined by the publication-number ranking system.

Scientists were counted as CE ("convinced by the evidence") when in the list of those credited by the IPCC as having done research utilized by AR4 Working Group I.  That resulted in a list of 619 names, which, after adjusting for duplication, became a total of 903 when also adding in those who signed one of several statements supporting the IPCC.
Own work. Original source: "Skeptical Science" blog, 11 May 2011 by John Cook
Sagredo