Doug Freyburger wrote:
....
A question for climate geologists - As climate has changed across the
last several tens of millions of years, how much has the amount of
arable land changed? As the glaciers receded towards the poles the
deserts near the equator grew. How close to parity was that change?
Right now the USDA zones keep north in the northern hemisphere. How
much of that is a reduction of total arable land and how much of that is
a change of where the arable land is? And how much of the change in
amount of arable land is from other causes of desertification like the
human caused ones of deforrestation and irrigation causing gradual salt
build up in the soil?

The discussion never does seem to address the net change in arable land
as the glaciers recede and the deserts grow. Until you start reading
Billy's material about building up new soil and that's an indirect
reference.

a large portion of desertification is
from human activities like overgrazing
cows/sheep/goats and removing covering
forests for crops and firewood. some
areas the moisture in the forrests is
part of the local weather cycle. remove
the forrest, change the weather...

some desertland can be reclaimed by
doing simple things like lining up rocks
on the ground (which stops water from
flowing away quickly). soon these lines
trap seeds and the plants sprout and
that sets up a small windbreak which
further protects tree seedlings and
gives them a chance to grow.

as long as these are not grazed by goats
it can go a long ways towards getting some
growth going even in very harsh climates.

in China they are trying to reforrest
some areas, but i'm not sure how much
success they've had. i don't think they
have enough moisture or organic stuff
planted along with the saplings so they
bake before they can grow. instead they
probably need an approach like the one
above that starts small and works up
to supporting trees one step at a time.


songbird