[Bernard Marx]

Agriculture often replaces natural vegetation. How can there be a significant shift in the balance then?

My question exactly. Of course the replacement of natural or agricultural vegetation with asphalt and concrete might have some long term effect (mainly on the global warmers' temperature gauges), but cities really represent a tiny percentage of the world's total area.

[ClearCase_guy]

Well, I think the whole GW thing is crap, but I think I can explain part of this thinking. Let me cover animals first. In a state of nature there are X number of cattle on the planet. They pass methane, which is a greenhouse gas. Now, with humans and modern farming methods, the number of cattle on the planet is probably "X times 10,000" and the amount of methane is increased "beyond what is natural".

Just so, with agriculture. If you clear an acre of forest, and plant an acre of corn and harvest and re-grow it annually, I would think that the impact on CO2 levels would be "beyond what is natural".

Let me be clear: I don't think any of this matters. The sun warms the plant. End of discussion. But I think it is still OK to say that human agriculture can have a greater impact than natural vegetation.

[-YYZ-]

Which keeps a larger amount of carbon locked up: an acre of forest or an acre of vegetable farm? Obviously the trees lock up more carbon than the plants that replaced them do. As long as the wood from those trees is kept intact, you're actually slightly ahead of the game. As soon as that wood decomposes, or is burned, you've released that carbon into the atmosphere. So yes, replacing woodlands with farms does put some carbon into the atmosphere, eventually. If you do slash and burn like in the Amazon, you immediately release the carbon into the atmosphere, and the land may be so poor when you're done farming it that it won't grow much of anything for a long, long time.

Of course, none of that really matters if global warming exists and is not caused by so-called greenhouse gasses.