THE water supplies of dozens of major cities around the world are at risk from a previously ignored aspect of global warming. Within the next few decades rising sea levels will pollute underground water reserves with salt.
Long before the rising tides flood coastal cities, salt water will invade the porous rocks that hold fresh water. “Even if we can fix the coastline, this saline incursion will increase,” says Vincent Post, a hydrogeologist at the Free University, Amsterdam. The problem will be compounded by sinking water tables due to low rainfall, also caused by climate change, and rising water usage by the world’s growing and increasingly urbanised population.
Underground water is the largest reserve of fresh water on the planet. Most cities use groundwater where possible because it is less prone to pollution than river water. Many people have no alternative: more than 2 billion depend on it, including most of the world’s rural poor and many of the world’s megacities, including Mumbai, Bangkok, Jakarta, and Karachi (see map).
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Salt water is already invading these supplies. It has penetrated 5 kilometres beneath Manila, in the Philippines, while the water table is sinking. As sea levels rise, the invasion will accelerate, leaving some cities “at risk of abandonment”, according to the International Association of Hydrogeologists, which organised last week’s meeting at the University of East Anglia in Norwich, UK.
The conference highlighted the fact that the fate of underground water was one of the least-studied effects of climate change. “I don’t think the scale of saline inundation facing these communities has been realised by governments,” says Andrew Skinner, former director of environmental protection at the UK government’s Environment Agency.
Africa is the continent most dependent on underground water. It also has the largest water supply problem, and its groundwater reserves are probably the most vulnerable to climate change. Yet the last impact assessment in 2001 by the UN’s Intergovernmental Panel on Climate Change (IPCC) had just one brief mention of groundwater in Africa. Four years on, little has been done. Delegates last week said scientists had let Africa down.
The threat to underground water reserves is a problem in the industrialised world too. Large areas of south and east England, for example, rely on underground water that is already over-extracted. Water use may be restricted this summer because poor winter rains meant there was minimal recharge to underground water reserves.
Studies presented at the conference predicted a further 20 to 40 per cent decline in recharge to the UK’s aquifers over the next 20 years. At the same time demand for water for gardens and crop irrigation in the UK is likely to rise by a third or more.
Amarenda Sinha, a geologist at the University of Rajasthan in Jaipur, India, says the expected loss of most Himalayan glaciers by the 2030s will drastically cut recharge from rivers to groundwater reserves on the plains of northern India, which provide food for half a billion people. He thinks the Indian government’s $200 billion Rivers Interlinking Project, intended to divert water from the great northern rivers to the parched south and west, will fail. “The water will not be available,” he says.
