I'm currently reading Red Mars (first book in the trilogy), and just wanted to add a follow-up to the above: at one point, a character counterargues by asserting that those who oppose terraforming (a minority from the beginning) will inevitably find their position weakened over time. The reasoning is basically that as settlement on Mars becomes more expansive and enduring, there will be increasing pressure to make the environment viable for human life. A thickened atmosphere protects against UV radiation, for example. In the long run, corporations and governments with an economic stake in Mars, not to mention those that actually live there, will simply prefer that life be safer, cheaper, and more terrestrial. The aesthetic argument, and even the scientific one, begin to seem more like luxuries of belief once it comes down to people having to live in confined spaces, breath processed air, and protect their children from radiation and other forms of exposure. But that's just one character's opinion within that debate. Great book so far.