You can just point them at https://skepticalscience.com/oneliners.php for a lengthy list of the arguments they're going to use and the trivial reasons why they're wrong, but it doesn't get at the core of the problem: you can't reason somebody out of a place they didn't reason themselves into.
>Studies have shown that increased concentrations of carbon dioxide increase photosynthesis, spurring plant growth.
>Results showed that carbon dioxide fertilization explains 70 percent of the greening effect, said co-author Ranga Myneni, a professor in the Department of Earth and Environment at Boston University. “The second most important driver is nitrogen, at 9 percent. So we see what an outsized role CO2 plays in this process.”
https://climate.nasa.gov/news/2436/co2-is-making-earth-green...