The world's largest hot desert, the Sahara, is around 9 million square kilometers. In an attempt to block the expansion of the Gobi Desert, China is planting 9 million acres or 36,400 square kilometers of forest. This initiative will cost around $8 billion. At this price, the entire Sahara desert could be turned into a forest for $1.98 trillion, which is which is $1.02 trillion less than Joseph Stiglitz and Linda Bilmes' estimate of $3 trillion for the total cost of the Iraq War.
Thanks to Hope Rider for research assistance. Image used under a Creative Commons license from alex lichtenberger.