February 23, 2005

Making Weather

Bush and Schroeder are allegedly "in accord" over pollution reduction strategies. The German chancellor doubtless knows the track record of other people who've gotten Bush to sign on to their agendas, but here's hoping he can buck that trend.

Should human societies fail to make progress on managing pollution and the landscape better, the Australian outback stands as a grim reminder (thanks to Richard Schwartz for the link):

Once upon a time, Australia had a lush, green interior where grazing animals roamed, shrubs grew and the rain fell. Then, about 55,000 years ago, man arrived and started hunting the animals and burning the vegetation; ultimately, he drove the rain away and turned Australia's interior into the harsh, red, desert landscape that we see today.

...Using general circulation models (GCMs - climate simulators), Miller and his colleagues have been testing how sensitive the Australian monsoon is to changes in vegetation. They have found that plants appear to be the key to holding on to monsoon rainfall. When the model is run with vegetation covering the Australian interior, it gets twice the rainfall compared with a model run with no vegetation. "The GCM suggests that rainfall in the interior would be about 600mm per year when trees and plants cover the ground, compared with about 300mm per year when the ground is bare," Miller says.

Vegetation is likely to be important because it helps to recycle the rain via evaporation and transpiration. "Plants collect moisture and hold onto it. Without any vegetation the rain either evaporates, or sinks into the ground and disappears," explains Miller. Trees also add "surface roughness" to a landscape, which is thought to promote convection and to encourage rain-cloud formation. If Australia's earliest human inhabitants burnt enough vegetation, Miller believes that this could have tipped the balance and prevented the monsoon rains from reaching the interior. ...

Simple enough. If you want rain in the interior of a continent, keep the trees. Ditto if you want the water to stay around after it falls. Tree and shrub roots break up the ground so water can penetrate, while holding the soil in place so the best of it doesn't wash away. Grass roots perform some of these functions, but thick grass cover also encourages water to run off, with the typical suburban lawn being about as permeable as blacktop.

Rainforest land subjected to slash and burn treatment in preparation for farming tends to stop receiving the rainfall of neighboring forest cover, as though there was a boundary marker in the sky between the forest and the fields. And that's not entirely off as a description.

In Victoria B.C., the Butchart Gardens stand on the site of an abandoned quarry. They're lush and beautiful from one end to the other. It takes a lot of work to maintain them, but it's worth it.

Posted by natasha at February 23, 2005 08:26 PM | Environment | Technorati links |
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