The first thing anyone who wishes to do a literature review needs to do is to decide what question they wish to answer. Many people seem to assume that my question was, “What percentage of scientists accept anthropogenic global warming [AGW]?” But that was not my question. Rather it was, “What fraction of peer-reviewed scientific papers reject AGW and what evidence do they present?” In other words, is there a scientific case against anthropogenic global warming? To answer that question, I needed to find peer-reviewed papers about global warming and review them sufficiently to judge whether they rejected anthropogenic global warming, or offered another alternative. One could go to the library and start reading articles, but that way would take a lifetime. Instead, I turned to the online Web of Science, a compendium of the peer-reviewed literature in all subjects. The WoS allows you to search articles by title, topic, author, date, journal, etc. in any combination.
To find articles about global warming, one naturally uses the search term “global warming.” Some articles might be under the topic “global climate change,” so I also used that as a search term. This search produced 13,950 articles for the period 1991 through mid-November 2012. This number does not capture every article on global warming, nor every article that rejects AGW, since some might be under other keywords. What it does capture is the number and proportion of articles with topics “global warming” or “global climate change” that reject AGW as I define reject. It turns out to be a very small number and a very small proportion.
[Note: most recently, as explained on the home page, I searched for articles from 2013 with keywords "climate change." See 2014 Update II below.]
Search the Web of Science [university access required] as follows:
Topic = "global climate change" (quotation marks essential)
Enhanced Science Index only
On November 10, 2012 this gave 40 results.
Repeat this search but substitute Topic="global warming". This gives 131 results.
But some articles will have used both "global warming" and "global climate change" and these must be subtracted otherwise they are counted twice.
Search with Topic="global warming" AND Topic=" global climate change" in the second field. Use Publication Year=1991 and add a field for Articles or refine the search for articles only. This gives 11 results.
Therefore the total number of unique records for 1991 is 40 + 131 - 11 = 160
Repeat for each year. Or, an easier way is to do three searches for the entire period, 1991-2012, then combine the searches using the OR operator to remove the double counting. Still another way would be to export the records to Excel, then use Excel to identify and remove duplicates.
Read some combination of titles, abstracts, and entire papers as necessary to judge whether a paper rejects human-caused global warming or professes to have a better explanation of observations.
The Web of Science also lists the number of times each article has been cited, and much more. At the bottom of the search page, you can export the results to an Excel file.
Note that some papers that one might expect to find listed were classified as "Review" or "Editorial Material" by WoS. I did not count those.
2014 Update I
I followed the same procedure, first comparing my original database, which was completed on Nov. 12, 2012, with all the records for 2012, to identify the new records that were not in the original set. There were 347 new ones, which I then added to the 1,911 records for 2013. This gave a total of 2,258 articles answering to "global warming" or "global climate change" from Nov. 12, 2012 through Dec. 31, 2013. Only one article rejected AGW. See here. Download the Excel database here.
2014 Update II
Finally, in order to test the proposition that articles rejecting AGW might be under other keywords, I decided to search for articles with the keyword phrase "climate change" but neither "global warming" nor "global climate change," since I had already reviewed those. From 1991-2013, there have been 54,785 such articles, too many to deal with. Therefore I restricted the search for articles with keywords "climate change" to 2013. That produced 8,974 articles, which I reviewed. In my judgment, only one rejected AGW. See that article here. Certainly there may be articles published prior to 2013 with the keyword phrase "climate change" that rejected AGW, but I see no reason why the proportion of those articles should be higher.
Combining the three searches, I have reviewed 13,950+2,258+8,974=25,182 articles. Of those, 26 reject AGW. That is 0.1 percent, or 1 in 1,000. For 2013, there are 1,911articles [search terms global warming and global climate change] + 8,974 [climate change] = 10,885, 2 of which are rejections, about 1 in 5,440.