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By Root 721 0

high-yielding crop varieties, fertilizers, and pesticides to farmers in the developing world—meant that food prices actually fell in real terms throughout the period of rapid population growth between 1960 and 2000. Irrigation allows food to be grown in areas without sufficient natural rainfall, and as a result can be highly productive as levels of sunshine and temperatures in arid tropical areas also tend to be high. Today irrigated agriculture covers 275 million hectares and produces 40 percent of humanity’s food on just 20 percent of our land.9 Many activists I know have devoted years of their lives to campaigning against big dams because of the local people displaced or dire ecological impacts, but the truth is that much of the water we need for farmland irrigation is captured behind those dams—according to one estimate they support 12–16 percent of current global food production.10

DAMMED ECOLOGY

But water, like all the planet’s major renewable resources—fisheries, soils, and forests—is fundamentally limited in terms of how much of it can be sustainably used at any one time by humans. Water differs from fisheries or forests in that it can never be destroyed and will go on circulating forever with or without humans. But the amount of water flowing through the Earth’s rivers at any one time is also a fixed quantity, which we can do little to alter. Fresh water is the lifeblood of the biosphere, and rivers the blue arteries through which this life force circulates. Use too much of it, and we risk causing severe and irreversible ecological changes, just as in the other planetary boundary areas. All living organisms need water just as they need food; as our hold on the planet’s land surface has deprived other species of food, so our dominion over the hydrological cycle has increasingly deprived them of water.

Rivers are vital ecological zones: They provide habitat for plants and animals within myriad different ecosystems. Wetland vegetation purifies water, for example, and large wetland areas can even help generate rainfall much farther afield by increasing water evaporation.11 Rivers deliver sediments and nutrients to delta areas and coastal fisheries, while also absorbing pollutants like nitrogen and phosphorus, breaking down waste, and delivering clean water over a far wider scale than any human sewage works. Freshwater ecosystems also harbor biodiversity far out of proportion to their area: River, lake and wetland habitats cover less than 1 percent of the world’s surface, yet provide a home for over a quarter of all known vertebrates—more than 126,000 known animal species in total, and about 2,600 aquatic plants.12

In Botswana’s Okavango Delta, for instance, the world’s largest inland delta supports an incredible diversity of animal and plant life where waters draining from the Angolan highlands spill out into the otherwise arid Kalahari Desert. These seasonal floodplains are home to buffalos, elephants, hippos, lions, wild dogs, and the beautiful sitatunga antelope, which together share the fertile marshes with 1,300 types of plant and more than 400 species of bird.13 Many animals are only occasional users of freshwater habitats, but are highly dependent on them nevertheless: In Asia, land animals using swamp or riparian areas include the orangutan, Javan rhinoceros, Asian elephants, tapirs, and proboscis monkeys. Sadly, all these animals are today endangered, in large part because of habitat loss.

Globally the extinction rate for freshwater species is five times higher than for terrestrial species, and according to the latest IUCN Red List survey, 37 percent of assessed species are counted as threatened.14 In general, because of dam-building and other human alterations to water flows, freshwater ecosystems are some of the most threatened on Earth. In the United States, originally blessed with some of the highest levels of freshwater biodiversity in the world, a shocking 123 species have gone extinct since 1900. The decline continues: 69 percent of the 300 species of freshwater mussels are endangered or extinct,