The Book of Genesis describes the pharaoh of Egypt having two dreams: one in which he is standing by the Nile and sees seven fat cows devoured by seven thin cows, and one in which he sees seven healthy ears of grain swallowed up by seven thin and blighted ones. When neither Pharaoh nor his advisers are able to interpret the dream, they call in a prisoner, Joseph, who tells him that dream means Egypt will experience “seven years of great plenty” followed by “seven years of famine” and that his government should “gather all the food of these good years that are coming” and keep it for the coming famine. Pharaoh is impressed, and Joseph is named vizier to oversee the stockpiling.

The story is often interpreted as an environmental parable about the importance of preparing for climate change as well as a reminder that, for those whose lives depend on the Nile, long periods of drought have always been inevitable and expected. This centuries-old awareness explains what makes the massive new Grand Ethiopian Renaissance Dam so controversial. The dam has pitted the countries that share the Nile against one another in a dispute over who will have the rights to the river in years to come, in both fat years and lean ones.

Climate change is making it even more likely that the familiar cycles of these years will soon be disrupted. This unpredictability has only raised the stakes of a conflict that could be a preview of other water tensions in years to come around the world.

Ethiopia is the source of the Blue Nile, the river’s largest tributary, and the source of 86 percent of the water reaching Egypt after traveling through Sudan. Ethiopia’s government has dreamed of building a dam on the river for decades, but was consistently opposed by Egypt, the most powerful country in the region. In 2011, with Egypt still reeling and distracted from the recent overthrow of President Hosni Mubarak, Ethiopia began work on the GERD, near the border with Sudan, with a budget of $4.8 billion. When fully filled, likely after another four to six years, it will contain up to 74 billion cubic meters of water—the largest hydroelectric plant in Africa.

For Ethiopia, the benefits of the dam project are obvious. Currently, 70 percent of Ethiopians lack access to electricity. It’s also a major statement from a rising economic power and could provide a political boost for the government of Nobel Peace Prize–winning Prime Minister Abiy Ahmed at a time of rising unrest. “The economic benefit is clear, but politically, the dam is one of the very few issues that mobilizes Ethiopians across the board. It provides a very clear opportunity for unifying the people,” says Addisu Lashitew, a fellow at the Brookings Institution.

But Egypt sees the dam as a potentially existential threat that jeopardizes the water supply for its 98 million people. Egypt has threatened military action to prevent the construction of the dam in the past. While war seems unlikely now that the dam is more than 70 percent complete, leaders are now pushing for reassurance that the country’s water supply will be maintained. Sudan, the third country involved, has generally been more supportive of the dam—the project could reduce flooding on the Blue Nile and give the country access to its hydropower—but also wants guarantees about safety issues.

In theory, and if everything goes well, these downstream countries have nothing to worry about. Ethiopia isn’t diverting the river for irrigation; it’s just storing it to generate electricity. The water will still be released. The risky period will come in the next few years while the dam is being filled. “It could be managed in a way that is not harming Sudan or Egypt,” says Kenneth Strzepek, a research scientist at MIT. “However, the size of the dam is about 1½ times the annual flow of the river. So if they were to fill it in one year, that could stop 50 percent of the water that goes to Egypt.” Even if Ethiopia fills the dam slowly, a natural drought during the filling of the dam or in its early years of operation could prevent water from reaching Egypt. It’s possible these droughts could become more common or last longer as climate change intensifies. Egypt seeks a binding commitment from Ethiopia that it will allow a set amount of water to flow downstream. Ethiopia has thus far resisted making this commitment.

“This isn’t a water issue. It’s a trust issue,” says Strzepek.

Trust has been hard to come by lately. Talks over the dam broke down in February when Ethiopia refused to sign a U.S.-drafted statement that Egypt had agreed to, saying it would require draining the dam to unacceptably low levels in the event of a drought.

Egypt had also demanded that the dam not be filled until a binding agreement over management of the river was reached. But this summer, seasonal rains began filling the dam, angering Egypt. It didn’t exactly help when Ethiopia’s foreign minister tweeted a triumphalist message: “The river became a lake. … The Nile is ours.”

Egypt and Sudan are far from the only countries that share a water source, and similar versions of this conflict have played out elsewhere. Rival jurisdictions fighting over water rights is hardly a new political dynamic: A dispute between California and Arizona over use of the Colorado River is the basis of one of the longest-running Supreme Court cases in U.S. history. But climate change could complicate matters further.

The dispute has led to fears of a brewing “water war,” but for now, the two sides appear to be keeping their cool. Talks have since resumed, mediated by the African Union and its current chair, South Africa, though Egypt and Sudan threatened to walk out. The three countries are now preparing to present proposals for the management of the dam later this week.

Nonetheless, Ashok Swain, a professor at Uppsala University in Sweden and chair of international water cooperation at UNESCO, argues that despite the tensions, the countries involved are likely to reach an agreement eventually: Given the high costs of war, and the importance of the Nile, they don’t have much other choice. “It is now forcing these three countries to get into some kind of agreement. That will translate into larger cooperation on other issues eventually,” says Swain.

Strzepek, a member of the Intergovernmental Panel on Climate Change, says that in the Nile region, “we know temperatures are going up, but precipitation is much harder to predict. The models are all over the place. We could get higher floods and lower droughts. So, any future agreement has to be a dynamic agreement that is updated as we learn what is happening with climate change.”

From the Himalayas to the U.S.-Mexico border, the sharing of water resources on an ever-warmer, evermore-crowded planet is likely to be among the most complex issues of international diplomacy in the coming century.

This doesn’t mean that “water wars” are inevitable. As Swain’s research encouragingly finds, while it’s not hard to find example of international river-sharing disputes in recent history, none of them have resulted in violence. But sharing these resources equitably in a harsher and more unpredictable environment is going to require a level of trust that’s been hard to find lately.

Joseph’s advice to prepare for the future is as pertinent as ever, but it’s harder to know what that future will hold.