Lake Victoria in eastern central Africa supports over 40 million people, the industrial sectors of three large nations, and the largest freshwater tropical ecosystem in the world. New research, however, suggests that the lake might not be around to do all of this in the not-so-distant future.
“It’s kind of a pattern that it’s been drying up,” said lead researcher Emily Beverly. “Historically, the level of Lake Victoria has dropped pretty drastically,” and past research has shown that the lake twice dried out completely, 15,000 and 17,000 years ago. The team’s research showed that the lake also dried out at least once more in the past 100,000 years. Beverly is a sedimentary geologist at the University of Houston in Texas.
Beverly and her team modeled Lake Victoria’s water budget—how water gets into and out of the lake. They found that rainfall into the lake controlled its levels more than the evaporation rate did. With this budget, the team predicted what might happen to Lake Victoria in a future warmer climate.
Under the driest projected conditions—less than half the current amount of rainfall—Lake Victoria would stop supplying a major tributary of the Nile in about a decade. All major lakeside cities in Kenya, Uganda, and Tanzania could lose access to the lake in as little as 100 years, and the shoreline would retreat from Kenya altogether in 400 years.
“With much less rainfall, basically, you can’t support a lake,” she said. “With precipitation that low, the lake is going to dry up within 1,200 years.”
These results were published in Earth and Planetary Science Letters on 11 November.
The Lake’s Desiccated Past
In this new water budget of Lake Victoria, the researchers included rainfall amounts in the past roughly 100,000 years. They calculated this from paleosols collected from nearby outcrops. By going back that far, the researchers also needed to account for small changes in the amount of sunlight that fell on the basin over time as Earth’s orbit around the Sun wobbled and wavered.
Ultimately, they found that precipitation amounts overwhelmingly controlled when the lake dried out, how long it took, and how quickly it refilled.
Their water budget suggested that the lake dried out and filled up again quickly and often over the past 100,000 years. That cycle likely had a big impact on the evolution of the lake’s freshwater tropical fish and the development of early modern human society in the region.
The team then used this water budget model to try to peer into the future: What could Lake Victoria look like under future climate projections? If rainfall drops to less than 75% of the current mean annual rate, the lake will quickly dry out.
“With the worst-case scenario, you lose the Nile outlet, the Victoria Nile or White Nile which flows into the main Nile. It’s what supports the nonflood stage of the Nile, so it’s very important,” Beverly said. “And then, depending on which [rainfall] rate you use, you dry up the lake in 500–3,500 years. That’s not saying that this is going to happen, but this is saying how quickly this could happen.”
After drying out, the researchers explained, it would take rainfall close to or exceeding current amounts for the lake to refill enough to outflow to the White Nile again, a process that could take centuries.
“Catastrophic Impacts on the Human Population”
If you’re looking back millennia, taking mere centuries to refill a lake is pretty quick. But right now, there are an estimated 40 million people living in the Lake Victoria basin in Uganda, Kenya, and Tanzania in some of the most densely populated areas of Africa. These regions rely on Lake Victoria and its outlet rivers like the White Nile for food, water, and industrial transportation.
By those considerations, a century or more of a dry lake is alarming.
“The predicted imminent changes in the lake levels as proposed foreshadow catastrophic impacts on the human population and [on] animals that depend on this lake for sustenance and might adversely affect much of eastern Africa,” said Emma Mbua, principal research scientist for paleontology and paleoanthropology at the National Museums of Kenya. Mbua was not involved with this research. “Such changes could be nerve-racking and call for integrating scientific data into future regional policies for adequate planning.”
The trouble is, Beverly explained, that paleoclimate data and modern meteorological data from the region are severely lacking.
“The Lake Victoria Basin includes Kenya, Tanzania, Uganda, Rwanda, and Burundi,” she said, “and there [are] pretty good data from Kenya and Tanzania, but not from Uganda, Rwanda, or Burundi. Most of the data from those countries stop in the 60s. We really need to get back to collecting data from weather stations” and drilling sediment cores that look farther back into Lake Victoria’s past.
—Kimberly M. S. Cartier (@AstroKimCartier), Staff Writer