Ever planted a tree to feel better about your environmental footprint? It's a good idea, but it might not have as much of an effect as you'd hope. A simulation-based study has determined that it would be utterly unrealistic to plant enough trees to offset humanity's CO2 emissions as they are -- the plantations would need to be so big that they'd "eliminate most natural ecosystems" or cut into food production. Even under the reductions from the Paris Climate Agreement, you'd still have to replace natural ecosystems on an area more than a third the size of the world's forests. The most viable option involving trees would require both "ambitious" emissions reductions and improvements to both nurturing the plants as well as capturing their CO2.
This doesn't mean humanity is doomed, provided the predictive model is reasonably accurate. However, it does reinforce the idea that any solution to emissions reduction is likely to be complex, and will usually involve a lot more than planting greenery.
For the researchers, this includes both reducing fossil fuel consumption and implementing multiple techniques for lowering CO2 levels beyond planting trees. You could make more efficient use of available land by eliminating food waste, as an example. Reforestation is still wise, but it's just one piece in a larger puzzle that can include everything from smarter farming to solar power.