> Thus, to reforest 3 billion acres at current prices will cost the world an investment of $3 trillion/year for 20 years. That sounds like a lot, but the world GDP of 2017 was $80 trillion. Therefore, this plan would require an investment of a little less than 4% of world GDP every year for 20 years.

$3T each year (4% of world GDP) for 20 years still sounds like a lot.