I find it fascinating how people gravitate toward madcap schemes while ignoring practical solutions. Humans are screwing up the Earth? Go to Mars! Humans have created anthropogenic climate change? Bring back mammoths!
In reality, as opposed to lurid fantasy-land, if we wanted to make significant inroads into reducing greenhouse gasses we would do the following:
Stop commuting to white-collar jobs, thus saving an estimated 12 million barrels of oil worldwide daily, and saving another 4 million barrels by shutting down hideously inefficient glass & steel skyscrapers containing those offices no white collar worker actually needs to commute to. 16 million barrels per day of foregone CO2 emissions would have a far greater effect on climate change than thousands of imaginary mammoths trampling down the Siberian permafrost. And unlike the mammoth fantasy, it's something we can do right now, easily. As for the mammoth fantasy - anyone remember how herbivores emit huge amounts of methane as a result of digesting all that plant material...?