“The city is our baseline for what might happen in future decades, and with all the negative effects global warming may have, there may be a bit of a silver lining,” said Stephanie Searle, a plant physiologist who led a Columbia University research project on tree growth, and now works as a biofuels researcher at the nonprofit International Council on Clean Transportation. “Higher nighttime temperatures, at least, may boost plant growth.” Robust growth takes more carbon dioxide out of the atmosphere. Still, some emissions are not helpful to plants. There are also plenty of modern pollutants, like ozone and heavy metals, which are toxic to plants, to humans or to both. And so far, the long-term effects on plant life on a heated planet are unclear. “I try to avoid words like ‘good,’ ‘bad,’ ‘detrimental’ or ‘beneficial,’ ” said Kevin L. Griffin, an ecophysiologist at Columbia University who participated in a study about the “heat island effect” on the red oak trees in New York. The effects of higher, mostly urban emissions are what prompted Dr. Ziska to reappraise global warming as a potential benefit to humanity. In an essay last summer in the journal Proceedings of the Royal Society B, Dr. Ziska and a group of colleagues from across the world argued that an expected increase in world population to 9 billion people from 7 billion by 2050 necessitated a “green revolution” to enhance yields of basic grains. Carbon dioxide, the group suggested, could be the answer.
