What do the Clinton Foundation, Tony Blair, and Coldplay’s latest album have in common? They’re all “carbon neutral,” meaning they don’t cause any net increase in the amount of carbon dioxide in the atmosphere. The science behind the slogan is this: You make up for whatever carbon dioxide your work produces by investing in schemes that absorb the gas or reduce the amount of fossil fuels used, thereby diminishing the amount of carbon dioxide emitted. The simplest and most popular path to carbon neutrality is planting trees, as they naturally absorb carbon dioxide.

A cottage industry has already sprung up to cater to would-be Johnny Appleseeds, and it is booming. Sadly, this effort may be fruitless. Writing in the journal Nature, University of Minnesota Professor Peter Reich points out that because nitrogen levels are not rising as fast as carbon dioxide levels, trees and plants simply can’t keep up.

Despite the appeal of the idea, it is “90 percent flawed” and “not sustainable at the global level,” says Reich. Worse, he worries that people will think such efforts are sufficient. The real solutions to global warming are “not going to be pain free,” predicts Reich. In other words, it’s time to stop hugging those...