Concerns about global climate change and the greenhouse effect center on rising levels of greenhouse gases, including atmospheric carbon dioxide. Plants use CO₂ for photosynthesis, and laboratory studies suggest that increased CO₂ levels could cause a rise in photosynthetic activity. However, as one environmentalist noted, “What plants do in environmental chambers may not happen in nature, where there are many other interacting variables.” Strictly from the standpoint of physiological effects, what are some possible ramifications of a rapid doubling of atmospheric CO₂ on plants in temperate environments? In arid environments?