Marcel Leroux is not a hack, from what I can tell. He was a Professor of Climatology at Jean Moulin University in France and the director of the Laboratory of
Climatology, Risk, and Environment before his death in 2008.
As far as I can tell, his main scholarship is a book entitled Global Warming: Myth or Reality? where he is critical of the prevailing scientific opinion.
This book was published without the peer-review process, which is always a red alert for me; however I have been unable to find on the web any critical analysis of the book, only glowing praise from the denier crowd who hang their hats on a lot of discredited figures.
However, I was unable to find anything discrediting the ideas in this book. The only critique I found Leroux address was from a French farmer (link is a .PDF), which is somewhat less than robust.
The book costs several hundred dollars, so I am reluctant to shell out that kind of money to find out for myself. I do wonder if there are critiques of the book or his work that I can examine first.