Second Deepest Lake Now Warmer Than in Past 1,500 Years

livescience.com Yahoo News 16 May 10;

Lake Tanganyika, the second oldest and the second-deepest lake in the world, is warmer than it has been in more than 1,500 years, a new analysis released today finds.

The east African rift lake has experienced unprecedented warming during the last century, and its surface waters are the warmest on record. The warmer waters are linked to a decrease in the lake's productivity, likely affecting fish stocks upon which millions of people in the region depend, the study found.

Rift lakes are created when two pieces of continental crust expand apart and eventually become ocean basins over millions of years. Lake Tanganyika is 13-million years old and nearly a mile deep (1.5 kilometers). The world's deepest lake is Lake Baikal in Siberia at 1,642 meters (5,387 feet).

Researchers took core samples from the lakebed that laid out a 1,500-year history of the lake's surface temperature, the first record of temperature variability for the lake over this time span. The rift is part of a giant crack in Africa that will eventually create a new ocean.

A high average temperature of 78.8 degrees Fahrenheit (26 degrees Celsius), measured in 2003, is the warmest the lake has been in that millennium and a half. Lake Tanganyika also experienced its biggest temperature change in the 20th century, which has affected its unique ecosystem that relies upon the natural conveyance of nutrients from the depths to jumpstart the food chain upon which the fish survive.

"Our data show a consistent relationship between lake surface temperature and productivity (such as fish stocks)," said geologist Jessica Tierney of Brown University."As the lake gets warmer we expect productivity to decline, and we expect that it will affect the fishing industry."

Warming causes productivity in the lake to decline because it creates stark differences in water density - the water at the surface become much warmer than the water at depth, and cold water is denser than warm water. The warmer, less dense surface waters are less likely to overturn and mix with the cooler waters below, so it becomes more difficult for winds to churn the water and cycle nutrients and oxygen between the lake's surface and depths.

An estimated 10 million people live near the lake, and fishing is a crucial component for the region's diet and livelihood: Up to 200,000 tons of sardines and four other fish species are harvested annually from Lake Tanganyika, a haul that makes up a significant portion of local residents' diets, according to a 2001 report by the Lake Tanganyika Biodiversity Project.

Lake Tanganyika is bordered by Burundi, the Democratic Republic of Congo, Tanzania, and Zambia - four of the poorest countries in the world, according to the United Nations Human Development Index.

Warming in Lake Tanganyika highest in 1,500 years
Yahoo News 16 May 10;

PARIS (AFP) – Lake Tanganyika, the second oldest and second deepest lake in the world, is now at its warmest in 1,500 years, threatening the fishing industry on which millions of lives depend, scientists said on Sunday.

The evidence comes from cores drilled into sedimentary layers in the lake bottom that point to climate changes over many centuries.

Tanganyika's surface waters, at 26 degrees Celsius (78.8 degrees Fahrenheit), are now at temperatures that are "unprecedented since AD 500," they reported in the journal Nature Geoscience.

The warming accelerated in the late 20th century, tallying with abundant data from other sites pointing the finger at man-made, heat-trapping greenhouse gases, they said.

As it has warmed, the lake has also suffered a fall in biological activity, they said.

Surface layers that warm become harder to penetrate by cool currents, welling up from the lake's depths, which bring vital nutrients that feed the first links in the food chain. Ultimately, commercial fish species become affected.

"The people throughout south-central Africa depend on the fish from Lake Tanganyika as a crucial source of protein," said Andrew Cohen, a professor of geological sciences at the University of Arizona, who took part in coring expeditions in 2001 and 2004.

"This resource is likely threatened by the lake's unprecedented warming since the late 19th century and the associated loss of lake productivity."

An estimated 10 million people in Burundi, Tanzania, Zambia and the Democratic Republic of Congo depend on the lake, using it for drinking water and for fish, of which up to 200,000 tonnes, mainly sardines, are harvested each year.

The paper, led by Jessica Tierney, a geologist at Brown University, appears in the journal Nature Geoscience.

In a separate study, a pair of Swiss scientists spelt out detailed warnings about the peril facing Europe's Mediterranean rim from global warming.

Previous work has already established the Iberian peninsular and European countries on the Mediterranean as badly exposed to heatwaves and water stress, based on current warming trends.

Erich Fischer and Christoph Schaer of the ETH Zurich technical university predicted the frequency of heatwave days will rise from an average of about two days per summer for the period 1961-1990 to around 13 days for 2021-2050.

For the period 2071-2100, there will be 40 heatwave days per summer.

"In terms of health impacts, our projects are most severe for low-altitude river basins in southern Europe and for the Mediterranean coasts, affecting many densely populated urban centres," the pair say.

In 2003, around 40,000 people were killed by a devastating heatwave that gripped Europe.

Africa's Lake Tanganyika Warming Fast, Life Dying
Tim Cocks, PlanetArk 17 May 10;

The lake, which straddles the border between Tanzania in East Africa and the Democratic Republic of the Congo, is the world's second largest by volume and its second deepest, the paper says.

Lead scientist on the project Jessica Tierney told Reuters the sharp rise in temperature coincided with rises in human emissions of greenhouse gases seen in the past century, so the study added to evidence that emissions are warming the planet.

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.

Geologists at Rhode Island's Brown University used carbon dating to measure the age of sediments on the lake floor. They then tested fossilized micro-organisms whose membranes differ at various temperatures to gauge how hot it was at times past.

The results were published in Nature Geoscience on Sunday.

"Lake Tanganyika has experienced unprecedented warming in the last century," a press release accompanying the paper said. "The warming likely is affecting valuable fish stocks upon which millions of people depend."

"INTENSE WARMING"

Most climate change studies have focused on the atmosphere, but increasingly scientists are studying the effects on the oceans, seas and lakes, which all absorb a huge amount of heat.

The paper argues that recent rises in temperature are correlated with a loss of biological productivity in the lake, suggesting higher temperatures may be killing life.

"Lake Tanganyika has become warmer, increasingly stratified and less productive over the past 90 years," the paper says.

"Unprecedented temperatures and a ... decrease in productivity can be attributed to (human) ... global warming."

The rise in temperature over the past 90 years was about 0.9 degrees Celsius and was accompanied by a drop in algae volumes.

"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," Tierney said by telephone from the United States. "We've seen intense warming in recent times ... not down to natural variations in climate."

She said the lake life had been harmed because in a lake as deep as Tanganyika, the nutrients form at the bottom but the algae needed to make use of them live at the top.

Higher surface temperatures mean less mixing of waters at the top and bottom." That's why a warmer lake means less life."

But the paper admits that other factors, like overfishing, may be doing more harm than any warming.

(Editing by Paul Casciato)