Space Colonization may be necessary to meet Earth's agricultural needs.

Here's a long but "interesting" read.

http://www.fromthewilderness.com/free/ww3/100303_eating_oil.html

Contains stats & arguments for:

"Modern intensive agriculture is unsustainable. It is damaging the
land, draining water supplies and polluting the environment. And all
of this requires more and more fossil fuel input to pump irrigation
water, to replace nutrients, to provide pest protection, to remediate
the environment and simply to hold crop production at a constant. Yet
this necessary fossil fuel input is going to crash headlong into
declining fossil fuel production."

[See: Spacesettlers msg 7035 by Xenophile]