Or we could kill all humans. That would solve global warming.
Mass genocide is often considered the best solution to many environmental problems.
Too bad mass genocide is "immoral" and "evil."
By what standard? A massive cull in he population will only result in a baby boom to replace them. The best ways to reduce the human population are education, birth control, giving women rights over their bodies, and a basic standad of living.
Well really, the best way to solve it is kill all humans.
But most people don't like that, so we'll go down to killing only certain humans.
So let's kill everyone in the developing and undeveloped worlds. First world countries are going to have low birth rates regardless of the size of the country. Not all land would be utilized (you'd see an expansion of large mechanized farms in areas that were formerly subsistence farms, though this wouldn't expand too much since supply and demand would be easily balanced with so little people and so much land) and, while commodities would likely be more expensive since super-low-wage workers would be eliminated, and likely replaced to some extent with machinery.
Though this wouldn't really solve problems like CO2 output too much, since the USA is still a major creator of CO2, unless the US made major steps towards safer energy production. It would also probably make oil cheaper since you wouldn't have to worry about the dangers coming from the native people in the area where oil is being drilled, which could exacerbate climate change.
On reducing the world population, giving everyone a decent standard of living would be enough to curtail population growth. This includes the poor. It's why I always find the idea of a guaranteed income interesting, like what was suggested in Switzerland a good bit ago.
But back to Ghost Spaghetti, yeah, those are the best ways to reduce human population, it's too bad so many people are unwilling to allow this.
P.S. Don't criticize my economic "theory", i'm not a student of economics