Nairobi — East Africa's second biggest inland water, Lake Tanganyika has heated up sharply over the past 90 years and is now warmer than at any time for at least 1,500 years, according to a new scientific study published in Nature Geoscience.
The lake, which straddles the border between Tanzania, Burundi, Zambia and the Democratic Republic of the Congo, is a vital source of both protein for the thousands of people living on its shores and employment for the hundreds of fishermen who take their boats out each day.
But the rapidly warming lake's ecosystem is damaged meaning that in future, the 200,000 tonnes of fish caught annually - most of which are sardines - could be under threat.
The scientific journal says that there is already evidence that the lake has become less productive over the past 90 years. It says that "analyses of lake sediments show that this recent warming is unprecedented within the past 1,500 years," and that the trends have been attributed to climate change.
"Our records indicate that changes in the temperature of Lake Tanganyika in the past few decades exceed previous natural variability," the report says. "We conclude that these unprecedented temperatures and a corresponding decrease in productivity can be attributed to anthropogenic global warming, with potentially important implications for the Lake Tanganyika fishery."
The 'Great Lakes' such as Tanganyika, Malawi and Kenya's lake Turkana were formed millions of years ago by the tectonic plate movements that tore Africa's Great Rift Valley.
Some 10 million people live around Tanganyika and depend upon it for drinking water and food, mostly fish.Lake Tanganyika has 'worked' in the past because of the circulation of cold currents from the bottom of the lake to penetrate and recharge warmer layers at the top.
Now higher surface temperatures mean less mixing of waters at the top and bottom. "That's why a warmer lake means less life," the study says
"Our records indicate that changes in the temperature of Lake Tanganyika in the past few decades exceed previous natural variability," the paper found. "We conclude that these unprecedented temperatures and a corresponding decrease in productivity can be attributed to anthropogenic global warming.
The scientists know the lake is getting warmer because of carbon dating. By taking "lake cores" from depths of up 4,700 feet, and testing fossilised micro-organisms, they were able to create an accurate picture of temperature changes since AD500.
What they found was a gradual warming that accelerated alarmingly in recent decades as heat-trapping gases in the atmosphere have driven global climate change.
The scientists were also able for the first time to link the warming to lower productivity in the lake, concluding that higher temperatures were killing life. The study found that the warmer the surface of the lake has become, the harder it is for cold currents to rise from the bottom and penetrate and recharge warmer layers.
A less productive lake means fewer fish and therefore less food and income in one of the poorest regions on earth. "The people throughout south-central Africa depend on the fish from Lake Tanganyika as a crucial source of protein," Andrew Cohen, professor of geological sciences at the University of Arizona told the Independent newspaper.
"This resource is likely threatened by the lake's unprecedented warming since the late 19th century and the associated loss of lake productivity."
The surface temperature of the lake has risen by two degrees in the last 90 years to 26C, while the levels of algae that indicate the productivity of the lake have dropped.
The vast body of water, which at some points is more than 45 miles across is the region's main source of freshwater.
While scientists have previously focused on the Earth's atmosphere to understand the extent and consequences of climate change, they are increasingly looking to its lakes and seas, which absorb tremendous amounts of heat.
Jessica Tierney, from Brown University and the lead scientist on the paper, said: "We're showing that the trend of warming that we've seen is also affecting these remote places in the tropics in a very severe way."
Officials from the UN Environment Programme (Unep) in Nairobi, Kenya, have warned that Tanganyika is not alone among Africa's Great Lakes in being threatened by climate change.
Lake Victoria, the continent's largest by surface area but shallow in comparison with Tanganyika, has registered record temperature rises since the 1960s, which have added further stress to an ecosystem already under attack from human waste, agricultural run-off and invasive species.
But the scientific experts also acknowledge that other factors, like overfishing, may be doing more harm than any global warming at present.