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. 2008 Sep 19;117(2):155–158. doi: 10.1289/ehp.11501

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Changes in morphine production and concentration (mean ± SE) from wild poppy (Papaver setigerum) as a function of rising levels of atmospheric CO₂ (Ziska et al. 2008b), corresponding roughly to atmospheric concentrations from 1950, today, and those projected for the years 2050 and 2090, respectively. Different letters indicate significant differences as a function of CO₂ concentration using Fisher’s protected least significant difference.