I dont have the exact study, but Christy has served as a Contributor and Lead Author for the U.N. reports by the Intergovernmental Panel on Climate Change.
He has also served on five National Research Council panels or committees and has performed research funded by NASA, NOAA, DOE, DOT and the State of Alabama
and has published many articles including studies appearing in Science, Nature, Journal of Climate and The Journal of Geophysical Research. Dr. Christy has provided testimony to several congressional committees.
Link:
http://www.nsstc.uah.edu/users/john.christy/about.html
Christy also has a Ph.D. degree in Atmospheric Sciences from the University of Illinois.
I would think he knows what he's talking about