New York Times: Hey, you know what's actually good for plants? Warmth and CO2

Studying Cities to Find Global Warming’s Benefits - NYTimes.com

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.

Since 1960, world atmospheric carbon dioxide concentrations have risen by 24 percent to 392 parts per million and could reach 1,000 parts per million by the end of this century.
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The Columbia team’s first red oak experiments ended in 2006, and average minimum temperatures in August were 71.6 degrees at the city site, but 63.5 degrees in the Catskills. Researchers also noticed that the city oaks had elevated levels of leaf nitrogen, a plant nutrient.

The team did two more rounds of experiments, then in 2008 made a final outdoor test using fertilized rural soil everywhere so all the seedlings got plenty of nitrogen. The urban oaks, harvested in August 2008, weighed eight times as much as their rural cousins, mostly because of increased foliage.

Flashback: 5 Politicians Who Got the Science "Wrong" | LiveScience

"The dangers of carbon dioxide?" Santorum said on March 12, 2012. "Tell that to a plant, how dangerous carbon dioxide is."

Santorum is not alone in his use of the "carbon is not dangerous" talking point. Rep. Michele Bachmann (R-Minn.) has downplayed human carbon emissions arguing that carbon dioxide is "natural," and saying, "life on planet Earth can't even exist without carbon dioxide."