Desert greening is the conversion of deserts into moister environments with more vegetation. This could be used to combat desertification and (by sequestering CO2) climate change, and also to increase the amount of arable land.
This obviously requires a water source. One potential water source (mentioned in the previously linked page under the "Water" section) is seawater (or other saltwater, e.g. from saltwater aquifers), which can be used directly by halophytic plants. 97.5% of Earth's water is saltwater, making this more abundant by far than freshwater. Using saltwater for irrigation is already being done in saline agriculture.
This method would avoid conflicts with use of freshwater (an increasingly scarce resource) and would be cheaper and less energy-consuming than using desalinated water. Mangroves and saltmarshes are more effective at sequestering CO2 than other types of ecosystems. And this would also have economic benefits if the plant species used have useful products like food, fibre etc.
How feasible is this idea? What challenges would this face? One challenge I can think of is the limited number of plant species able to grow on saltwater.