Agronomy professor Mary Beth Kirkham says experiments have shown that the elevated levels allow winter wheat and sorghum to use water more efficiently, which mitigates the effect drought has on those crops.
She’s written a book on the subject, using data going back to 1958, when the first accurate measurements of atmospheric carbon dioxide were made. Data shows that carbon dioxide concentrations have increased from 316 parts per million to 390 parts per million from 1958 to 2011, the last year with complete data available.
Kirkham says elevated carbon dioxide closes pores on the leaves through which water escapes. That means less water is used when carbon dioxide levels are elevated.