Planting trees to sequester fossil carbon
If an unlimited area of the Earth were available and suitable for planting trees on, and if they could be planted fast enough and continuously, the fossil carbon we burn could be stored in the wood and thereby prevented from entering the atmosphere (provided the ever increasing number of trees were replaced when they eventually died).
However, the Earth is not of infinite surface area. The rate at which we burn fossil carbon at present is so great that it would fill up the unplanted land area on Earth in about 10 years, and would fill up the soil beneath it in another 10 years. (Forestation increases the carbon storage of soil.)
Also, food, fuel and materials for the Earth's human population have to be grown somewhere, and various non-forest habitats have to be preserved, so suitable spare land is in fairly short supply. The result is that, although trees can provide vast amounts of fossil-free fuel and materials, and provide various ecological services, they have almost no potential for fossil carbon offsetting. It would be better if people stopped pretending otherwise.