Scientists have issued a new warning to the world’s coastal megacities that the threat from subsiding land is a more immediate problem than rising sea levels caused by global warming.
A new paper from the Deltares Research Institute in the Netherlands published in April identified regions of the globe where the ground level is falling 10 times faster than water levels are rising - with human activity often to blame.
In Jakarta, Indonesia’s largest city, the population has grown from around half a million in the 1930s to just under 10 million today, with heavily populated areas dropping by as much as six and a half feet as groundwater is pumped up from the Earth to drink.
The same practice led to Tokyo’s ground level falling by two meters before new restrictions were introduced, and in Venice, this sort of extraction has only compounded the effects of natural subsidence caused by long-term geological processes.
"The Sahel’s ability to produce food is not keeping pace with its growing population, and global warming will only exacerbate the imbalance, according to a new study. Among the 22 countries making up the arid region in northern Africa, the population grew to 471 million in 2010 from 367 million in 2000, a jump of nearly 30%. As the population grew rapidly, the production of crops remained essentially unchanged. Using satellite images to calculate annual crop production in the conflict-ridden Sahel belt, south of the Sahara desert, the researchers then compared output with population growth and food and fuel consumption."
Forests worldwide are at "equally high risk" to die-off from drought conditions, warns a new study published this week in the journal Nature. The study, conducted by an international team of scientists, assessed the specific physiological effects of drought on 226 tree species at 81 sites in different biomes around the world. It found that 70 percent of the species sampled are particularly vulnerable to reduction in water availability. With drought conditions increasing around the globe due to climate change and deforestation, the research suggests large swathes of the world's forests — and the services they afford — may be approaching a tipping point.
Water is critical to trees, transporting nutrients, providing stabilizing, and serving as a medium for the metabolic processes that generate the energy needed for a tree to survive. Mechanically, water moves through plants via their xylem, a tissue that can be compared to a system of tubes. Transpiration or release of water from a plant's leaves keeps the system moving. But when water availability is insufficient, the process begins to break down, having substantial impacts on the health of a tree. While this has long been observed, until recently the exact mechanism that triggers drought stress in forests was poorly understood. The new study argues that "hydraulic failure" may be a key factor. Effectively, insufficient water availably leads a tree to start pulling air bubbles — called gas emboli — into its xylem impeding the flow of water. Hydraulic failure is akin to attempting to drink through a broken straw — air bubbles significantly reduce the amount of liquid that reaches the top of the straw.
The researchers found that a wide range of trees are susceptible to "hydraulic failure". "We show that 70% of 226 forest species from 81 sites worldwide operate with narrow... hydraulic safety margins against injurious levels of drought stress and therefore potentially face long-term reductions in productivity and survival if temperature and aridity increase as predicted for many regions across the globe," the authors write. "Safety margins are largely independent of mean annual precipitation, showing that there is global convergence in the vulnerability of forests to drought, with all forest biomes equally vulnerable to hydraulic failure regardless of their current rainfall environment."
The results provide insight on why drought-induced forest die-off is occurring in a range of forest types, including tropical rainforests which are not typically considered at risk of drought. Over the past 15 years, forests in Borneo and the Amazon have suffered from widespread drought-induced decline. Drought stress if often accompanied by increased incidence of fires, either from natural sources like lightning or human activities like burning for cattle pasture of plantation establishment.
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