It appears that the source of KD's view is this article by Nasif Nahle: http://www.biocab.org/carbon_dioxide_geological_timescale.ht...
In response to some of Nahle's research, I found this post here: http://hannahlab.org/climate-skeptics-nasif-nahles-shaky-mat...
As I commented in my first submission of this post, I also want to say:
It's always troubling when my brother, the skeptic, sends me such links. I'm not a scientist (neither is he) so I don't know what to make of such articles.
I always like to explore contrarian views, but this one doesn't sit well with me. It's as if 'skeptics' use such views to roll back all the progress we've made and to slow progress that's needed. It especially boils my blood when 'skeptics' use their argument to somehow tarnish Earth Day as Earth Day is so much more than global warming.
This opinion piece from the local paper sums up my feelings about what Earth Day does represent and what the skeptics ignore: http://planetjh.com/2017/04/18/the-new-west-earth-to-trump/