David, I suspect PA has been influenced by the following study: Randall J. Donohue, Tim R. McVicar, Michael L. Roderick, Graham D. Farquhar, «Impact of CO2fertilization on
maximum foliage cover across the globe's warm, arid environments», Geophysical Research Letters Volume 40, Issue 12, pages 3031 — 3035, 28 June 2013 DOI: 10.1002 / grl.50563