A new paper is getting a lot of attention for its claim that the earth has about 2 billion acres of land that are currently unused and could support the planting of 500 billion trees. Once the trees were grown, they’d capture about 205 gigatonnes of carbon. That sounds great, but let’s give it some context:

We have emitted about 450 gigatonnes of carbon since the start of the Industrial Revolution. At our current rate, we’ll add another 205 gigatonnes in less than 20 years.

If we could really get all the countries of the world to plant trees on currently unused land that’s suitable for reforestation, that would be great. But even if we went all out and got 100 percent cooperation, it would take 50 or 60 years for these forests to grow to maturity. Unless we do something about actual emissions, we will have added at least 500 gigatonnes of additional carbon by then, bringing us to total emissions of about a billion gigatonnes of carbon. The trees would make only a small difference.

By all means, we should take this seriously and plant lots of trees. Even 50 or 100 gigatonnes of carbon capture would be a lot. But don’t fool yourself into thinking this is any kind of free ticket. We still have to figure out a way to reduce actual emissions by a huge amount if we want to avoid planetary catastrophe.