It would hold up if the average temperature 1000-1300 wouldn't be above todays temperature. They hardly burned any fossil fuels at all back then. There's no correlation between carbon emissions and mean temperatures. All we've got is one Swedish researchers theory from the 70'ies that still has never been confirmed.
Since none of the scientists are certain about anything regarding temperatures, the field is open for any loud-mouthed moron to make a stand unoposed. Nobody can say they're wrong. Enviromentalists arguments tend to be so emotional. It's like, "if you don't recycle you hate nature". Which is just bollocks, but everybody seems to buy into the rhetoric.
edit: sorry about that. I checked. The temperatures are higher today than they where 1000-1300 ago. oops. I need to read a bit more.
You're forgetting that we can put soil on top of a landfill and make it into nature again. And in a couple of hundred years it'll be all recycled naturally. That is what is being done today with landfills. It's the whole idea and it's not a problem. The available landfill space is in fact infinate. Because we'll never run out of new ground to fill up.
Finding a way to dump less crap is always good, since energy efficiency is always in everybodies best interest.