And even though on average more warmth
will mean more evaporation, and therefore more water vapour in the
atmosphere and more precipitation in some of those zones that already
have ample rainfall, the pattern could be different in the arid lands.
All the calculations so far indicate that
these drylands will increase in area, and become drier with time.
Already 250 million people are trying to scrape an increasingly meagre
living from lands which are degrading swiftly, either because they are
turning to desert, or because they are overgrazed.
Hard on microbes
But to make things worse, climate
scientists predict that between 2080 and 2099, soil moisture will
decrease by between 5% and 15% worldwide. And that in turn could have a
profound effect on the levels of carbon and nitrogen nutrients naturally
in the topsoils.
What keeps soils alive, and productive, is
the compost or humus of leaf litter, animal dung, withered roots and
other decaying vegetation in the first metre or so of topsoil: this in
turn feeds an invisible army of tiny creatures that recycle the nutrient
elements for the next generation of plant life.
But these microbes also need water to
thrive. The consortium of researchers predicted that as the soils got
drier, biological activity would decrease, but geochemical processes
would accelerate. That is, nutrients that depended on little living
things in the soil would drain away, but other elements – phosphorus
among them – would increase, because they would be winnowed from the
rock by mechanical weathering or erosion.
The research team tested this argument
with samples from 16 countries, including the Negev desert in Israel,
the woodlands of New South Wales in Australia, the Altiplano of Peru,
and the Pampas lowlands of Argentina.
These regions could all expect from 100mm
of rainfall a year to 800 mm; all soil samples were analysed in the same
laboratory in Spain.
And as predicted, they revealed an
increasing imbalance: more phosphorus, less carbon and nitrogen as they
became drier. Such a trend would actually feed back into global warming:
ideally, more vigorous plant growth would absorb more carbon dioxide.
But if vegetation wilts, and soils turn to
dust over large areas of already parched land, then the carbon dioxide
levels in the atmosphere will increase even more.
“Plants need all of these elements, in the
correct amounts, and at the right times, but increasing aridity will
upset this balance, leading to a breakdown in essential soil processes,”
said David Etheridge, of the University of New South Wales, one of the
“As the world’s population grows, people
will increasingly rely on marginal lands – particularly drylands – for
production of food, wood and biofuels. But these ecosystems will be
severely affected by imbalances in the cycle of carbon, nitrogen and
Climate News Network]