If the 50 biggest nations did this twice per year, that is over 35 billion trees planted a year, and reforestation with deciduous, carbon-capturing trees could put a dent in the carbon dioxide/climate crisis.
Smithsonian magazine article. Jason Daley in the Smithsonian writes:
Reforestation has been in the news a lot lately, mainly because a paper published in Science earlier this month mapped out the millions of square miles on Earth that could be reforested. If all that land was filled with trees, the researchers estimated, it could drop carbon dioxide levels in the atmosphere by 25 percent. Other scientists pushed back, saying the estimates were overly generous, and that the climate benefits of reforestation are little studied, highly variable and restoring so much land would be politically and technically difficult.
The Ethiopian government says this is only part of a wider plan to plant over 4 billion trees and reforest some of their land used by development. Ethiopia used to be 30% forested; now forests only cover 5% of the nation’s territory, and they want to change that for the better.