World's deltas subsiding, says study
Two-thirds of the world's major deltas, home to nearly half a billion
people, are caught in the scissors of sinking land and rising seas,
according to a study published Sunday.
The new findings, based on satellite images, show that 85 percent of
the 33 largest delta regions experienced severe flooding over the past
decade, affecting 260,000 square kilometres (100,000 square miles).
Delta land vulnerable to serious flooding could expand by 50 percent
this century if ocean levels increase as expected under moderate climate
change scenarios, the study projects.
Delta land vulnerable to serious flooding could expand by 50
percent this century |
Worst hit will be Asia, but heavily populated and farmed deltas on
every continent except Australia and Antarctica are in peril, it says.
On a five-tier scale, three of the eleven deltas in the highest-risk
category are in China: the Yellow River delta in the north, the Yangtze
River delta near Shanghai, and the Pearl River Delta next to Guangzhou.
The Nile in Egypt, the Chao Phraya in Thailand and the Rhone River
delta in France are also in the top tier of danger.
Just below these in vulnerability are seven other highly-populated
deltas, including the Ganges in Bangladesh, the Irrawaddy in Myanmar
(Burma), the Mekong in Vietnam and the Mississippi in the United States.
These flood plains and others all face a double-barrelled threat,
reports the study, published in the journal Nature Geoscience.
On the one side, a range of human activity - especially over the last
half-century - has caused many delta regions to subside. Without human
interference, deltas naturally accumulate sediment as rivers swell and
spread over vast areas of land.
But upstream damming and river diversions have held back the layers
that would normally build up.
Intensive subsurface mining has also contributed mightily to the
problem, notes the study, led by James Syvitski of the Institute of
Arctic and Alpine Research at the University of Colorado. The Chao
Phraya delta, for example, has sunk 50 to 150 millimetres (two to six
inches) per year as a result of groundwater withdrawal, while a
3.7-metre (12-foot) subsidence of the Po Delta in Italy during the 20th
century was due to methane mining.
Indeed, oil and gas mining contribute to so-called "accelerated
compaction" in many of the most vulnerable deltas, according to the
study, the first to analyse a decade's worth of global daily satellite
images.
The other major threat is rising sea levels driven by global warming.
In a landmark report in 2007, the UN's Intergovernmental Panel on
Climate Change (IPCC) predicted oceans would rise by 18-59 centimetres
(7.2 and 23.6 inches) by 2100.
More recent studies that take into account the impact of melting
icesheets in Greenland and Antarctica have revised that estimate upwards
to at least a metre (39 inches) by century's end. The already
devastating impact of such increases will be amplified by more intense
storms and hurricanes, along with the loss of natural barriers such as
mangroves.
In the Irrawaddy delta the coastal surge caused by Cyclone Nargis
last year flooded an area up to six metres (20 feet) above sea level,
leaving 138,000 people dead or missing. AFP |