[ph0rque]

Here's an up-to-date study and discussion of that question: https://news.ycombinator.com/item?id=19248716

The number of trees needed (1.2T) is pretty close to the estimation of the accepted answer in your link (1.5T). The difference is, the up-to-date study has found that there is enough space for the trees. Now we just need to plant them :)