Vanishing waters in a warming world

This roundup of The Conversation’s climate coverage was first published in our award-winning weekly climate action newsletter, Imagine.

Around the world, rivers and lakes that sustained civilisations for millennia are vanishing before our eyes. The Caspian Sea – the world’s largest inland body of water – has shrunk dramatically in just a few decades. The Ganges nourishes hundreds of millions of people across India and Bangladesh, yet is drying at a rate scientists say is unprecedented in the past thousand years.

Climate change isn’t solely to blame for the woes of the Caspian or the Ganges, of course. In nearly all cases, what’s going on is some combination of human and climate factors. But there is a trend.

Let’s start with rivers.

Writing in 2022, Catherine E. Russell, then of the University of Leicester, notes that:

“The Loire in France broke records in mid-August for its low water levels, while photos circulating online show the mighty Danube, Rhine, Yangtze and Colorado rivers all but reduced to trickles.”

In her analysis of why rivers worldwide are running dry, she points out that:

“climate change is altering where freshwater is found: such that, in general, places with plenty are getting more while places with little are getting less.”

She says this is making rivers more “flashy”: prone to breaking records for both high and low water levels. The flashiness is exacerbated by humans extracting water and putting rivers in concrete straitjackets.

So what we’re seeing isn’t just a series of droughts. These drying rivers represent a structural change in how water is moving through the land, driven by climate change but also decades of overuse and engineering decisions.

Read more: Rivers worldwide are running dry – here's why and what we can do about it

This is particularly apparent in the Ganges, India’s largest and longest river. There, “stretches of river that once supported year-round navigation are now impassable in summer. Large boats that once travelled the Ganges from Bengal through Varanasi now run aground where water once flowed freely.”

That’s according to Mehebub Sahana, a rivers expert at the University of Manchester, who has written about a new study that puts the current drying in historical context. Scientists in India, writes Sahana, gathered 1,300 years of flow data and say the river and its wider system of tributaries has never faced dry spells as severe as it has in the past decade.

As the world warms, Sahana notes, “the monsoon which feeds the Ganges has grown increasingly erratic”. But there are other factors at play: “Water has been diverted into irrigation canals, groundwater has been pumped for agriculture, and industries have proliferated along the river’s banks. More than a thousand dams and barrages have radically altered the river itself.”

In Sahana’s words, this results in “a river system increasingly unable to replenish itself”.

To save the Ganges, India will have to extract less groundwater and irrigation water. Upstream India and downstream Bangladesh will have to better coordinate their efforts. And major funding and political agreements “must treat rivers like the Ganges as global priorities”.

Read more: The Ganges River is drying faster than ever – here's what it means for the region and the world

‘A relatively new phenomenon’

Something similar is happening with lakes.

While at Keele University, the geographer Antonia Law looked at the climate-related threat to lake wildlife.

She notes there has already been a “staggering decline” in freshwater species diversity since the 1970s, but that “climate change [now] threatens to drive even deeper losses”.

“Lake heatwaves – when surface water temperatures rise above their average for longer than five days – are a relatively new phenomenon. But by the end of this century, heatwaves could last between three and 12 times longer and become 0.3°C to 1.7°C hotter. In some places, particularly near the equator, lakes may enter a permanent heatwave state. Smaller lakes may shrink or disappear entirely, along with the wildlife they contain, while deeper lakes will face less intense but longer heatwaves.”

Needless to say, this is not great news for any person or animal that relies on those lakes. That’s particularly the case as “unlike those living elsewhere, most lake animals cannot simply move to another habitat once their lake becomes uninhabitable”. Many lakes, says Law, are on course for “a sweltering, breathless and lifeless future”.

Read more: Climate change: world's lakes are in hot water – threatening rare wildlife

That’s the case even for the biggest lake (sort of) of all: the Caspian Sea.

Here’s Simon Goodman, an ecologist at the University of Leeds who has tracked the seals in the Caspian for more than two decades:

“Once a haven for flamingos, sturgeon and thousands of seals, fast-receding waters are turning the northern coast of the Caspian Sea into barren stretches of dry sand. In some places, the sea has retreated more than 50km. Wetlands are becoming deserts, fishing ports are being left high and dry, and oil companies are dredging ever-longer channels to reach their offshore installations.”

Goodman says variations in the Caspian Sea level were once linked to agricultural irrigation (the same thing that caused the Aral Sea to disappear a few hundred miles to the east), but “now global warming is the main driver of decline”.

That’s because rising temperatures are disrupting the water cycle. Rivers and rainfall are bringing less water, while the hotter sun is evaporating more water than ever. With no link to the wider oceans (aside from a single canal, which is also drying up), the Caspian just can’t keep up.

As things stand, Goodman says, the decline could eventually reach 18 metres, “which is about the height of a six-storey building”. “Even an optimistic ten-metre decline would uncover 112,000 square kilometres of seabed – an area larger than Iceland.”

The five countries around the Caspian Sea have recognised the danger. The world does not need another Aral Sea. But Goodman fears “the rate of decline may outstrip the pace of political cooperation”.

Read more: Climate change is fast shrinking the world's largest inland sea

There are many more stories like these. We’ve looked at the Ganges and the Caspian Sea, but this could easily have been a newsletter about Lake Victoria, the world’s second largest freshwater lake, or about drying rivers in Europe making it harder to generate nuclear power (pushing up energy prices in the UK), or about the complete disappearance of Bolivia’s second largest lake.

In all these cases, it’s worth remembering that once a river runs dry or a lake shrivels up, it’s not just water that disappears: it’s entire ecosystems and ways of life.