> Climate-ecosystem interactions are even more iffy.
>
> For instance, I've read in several locations that the reason the
> Sahara is a desert, now, rather than a prairie, is because of
> centuries of sheep grazing.
I think it is a slight bit more complex than that. During the ice age
the convection cells were somewhat different, and instead of just
moving the humid air of the tropics above the Sahara they dropped the
moisture there, creating a thriving ecosystem. When the climate
shifted, the area dried up. Here is the part I'm uncertain of: did the
Sahara immediately become a desert or did it do so due to grazing? It
is not inconceivable that a large prarie could remain stable,
especially since it affects the climate in turn, but sooner or later
it may tunnel to the even more stable desert state. Whether this push
was through grazing or a natural fluctuation I don't know (I lean
towards the natural side, since there are plenty of deserts at that
latitude around the world that seems to have appeared independently).
It should be noted that the main reason for desert spreading today are
grazing and mismanaged agriculture, but this means spreading the
desert outside its "natural" borders.
> Then ask where you're going to put all the displaced people you've
> created by flooding coastlines.
Exactly. Personally I think we ought to terraform Mars first so that
we learn the ropes before tackling the Earth (although I consider
planets a waste of matter, but that is just personal opinion).
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