According to Xu et al. (2012), “levels of atmospheric CO2 and O3 have increased rapidly in the last five decades,” and they say that “it is predicted that at the end of this century, the average levels of CO2 and O3 in the Earth’s atmosphere are going to reach 700 ppm and 80 ppb, respectively (IPCC, 2007).”
In an experiment designed to evaluate the opposing effects of these two atmospheric trace gases on Chinese pine (Pinus tabulaeformis) trees at the year AD 2100, Xu et al. grew four-year-old trees in loamy soil with no extra fertilizer within twelve open-top chambers in May 2006 within the populated central area of Shenyang city in northeastern China, where the trees were exposed to either current ambient air of about 400 ppm CO2 and 40 ppb O3or 700 ppm CO2 and 80 ppm O3, plus all combinations thereof.
