Lake Victoria, in East Africa, is the world's largest tropical freshwater lake. At 68,800km², it's also the second largest freshwater lake in the world after Lake Superior in North America. On a clear day you cannot see the other side of Lake Victoria, yet this vast body of water has dried up several times in the past – and it could happen again.
Over the past 100,000 years, the lake has completely dried up at least three times. Each time it was probably replaced by a vast grassland.
My colleagues and I found that the lake could dry up again in as little as 500 years because of changes in temperature, rainfall and orbital forcing – the effect on climate of slow changes in the tilt of the Earth's axis. Our predictions are based on historical and geologic data from the last 100,000 years.
Inadequate and conflicting data on long term weather trends make it hard to be conclusive. And we can't be sure of how climate will change in the future due to human actions without more data. Over the past few decades, the frequency of drought in East Africa has increased but climate models project an overall increase in rainfall over the next century for this area.
Previous studies on Lake Victoria's future water levels have been done, but didn't have evidence for past changes in rainfall or include orbital forcing.
Based on historical and geologic observations, our findings show that Lake Victoria can dry up very quickly with small decreases in annual rainfall. Knowing whether rainfall is going to increase or decrease over the next 100 years becomes very important.
Today about 30 million people in Rwanda, Burundi, Uganda, Kenya and Tanzania rely on the lake for fishing, irrigation, drinking water and, in Uganda, electricity. Lake Victoria is also the source of one of the River Nile's major tributaries, the White Nile. About 250 million people rely on the Nile in Ethiopia, Uganda, South Sudan, Sudan and Egypt.
Factors that affect the lake level
For our research, we needed to examine all the factors that could affect the size and level of Lake Victoria, including rainfall, temperature, evaporation and rivers flowing into and out of the lake.
The main inflow is the Kagera River, which drains the highlands of Rwanda and Burundi. The White Nile flows out of the lake. It also loses a lot of water to evaporation which, at the equator, is very high due to intense sunlight – almost equal to the amount of rainfall falling on the lake.
From previous research we knew that the lake dried up twice, at 17,000 and 15,000 years ago.
To reconstruct lake levels as far back as 18,000 years, scientists examined diatoms (a type of algae) collected from the bottom of the lake to see how fresh or brackish the water used to be. Brackish diatoms indicate lower lake levels because the water becomes saltier as the water evaporates.
Our new research examined the chemistry of fossil soils along the edge of the lake to understand past changes in the amount of rainfall. Because the orbit of the Earth around the sun has varied, we also looked at how sunlight reaching the Earth has changed, and will change.
Using all this information we were able to create a model of the lake's past, present, and future.
Our findings show that the amount of annual rainfall in the Lake Victoria Basin must continue to be at least 75% of current rainfall amounts (105 cm each year) or the lake will disappear.
Once the lake has dried up rainfall needs to be at least 131 cm every year to refill the lake. Depending on other changes in the weather, this could take between hundreds and thousands of years. If there was less than 94% of today's rainfall, it would take at least 10,000 years to refill.
Climate models all predict an increase in temperature over the next 100 years, which will affect evaporation for this region.
Previous drops in lake levels over the last 60 years were caused by a natural decrease in annual rainfall and by Uganda releasing more than the agreed upon water to generate electricity. We found that if previously observed natural rates of lake level fall are projected into the future, the White Nile could stop flowing out of Lake Victoria in as little as 10 years at the fastest rates. But we need more research to know just how likely this is.
Our research shows just how delicate the balance is between rainfall and evaporation for Lake Victoria. The lake could dry up in as little as 500 years. But the consequences of falling levels for people living around it would begin much sooner.
In as little as 100 years, the major port cities around Lake Victoria could lose access because the lake has shrunk. Currently these cities depend on the income generated from the fishing industry and fresh water.
Kenya could lose all access to the lake in 400 years. This sets up a potentially dangerous dynamic between Kenya and Uganda, which already fight over fishing rights – very lucrative due to the 1 million pounds of fish harvested from the lake every year.
There is an urgent need for a greater understanding of how rainfall will change in this region. Lake Victoria's continued existence is directly related to rain and evaporation.
We need to increase monitoring of temperatures, precipitation and other weather data, such as humidity, in the region. We must also measure the water flowing into and out of Lake Victoria via all the rivers.
A greater understanding of the lake's history would also improve our ability to understand any patterns in the lake drying up. Deeper drilling for sediment samples would provide information to help us predict and prepare for the potential future of Lake Victoria.
Emily J. Beverly receives funding from the National Science Foundation, National Geographic, and the Leakey Foundation.
By Emily J. Beverly, Assistant Professor, University of Houston