Deforestation is one of the reasons some deserts have spread so rapidly. trees behave as a shield to that encroachment

I recently saw a video about doing this - apparently the desert is actually a really bad option if we want to combat climate change - the core problem is that the desert currently reflects a lot of light from the sun. If we cover it with forest or whatever, then suddenly we adjust the albedo of the earth and absorb more heat, negating some of the impact. Combined with the energy need to pump (and desalinate for trees - your suggestion of kelp would presumably help there) water, etc... make it not really work.

Source: https://www.youtube.com/watch?v=lfo8XHGFAIQ

I recommend watching the video - it explains it better and goes into detail - it cites its own sources.

I don't think planting trees is considered to be the unquestionably most practical solution. But it's a simple solution that people can do on their own, and it has a lot of other side benefits like habitat, shade, & proven health benefits in humans.

Even if we had to replant the trees every decade, that would only be 5 billion dollars a year, forever. Trivially affordable at "world power" scale.

Lack of water is a much more real blocker, with space not far behind.