By James Lillywhite, producer
For centuries, wars have been fought over resources.
The need for land, oil, trade routes and other commodities have all led to deadly conflicts throughout history.
But soon we could be seeing more fighting over arguably the most precious resource of all - water.
As the effects of climate change get worse, and the planet gets hotter and drier, the fight for water is only going to intensify.
So - where could we see water wars? And how likely are they to actually happen?
Future Wars examines potential flashpoints and conflicts around the world. Watch the latest episode here:
Back in 1995, former World Bank vice president Dr Ismail Serageldin said: "If the wars of this century were fought over oil, the wars of the next century will be fought over water -- unless we change our approach to managing this precious and vital resource."
How much we have changed our approach to managing water is debatable.
By 2025, the UN predicts two-thirds of the global population will live in water scarce areas.
Five years after that, it is thought 700 million around the world could be at risk of displacement by water scarcity.
There is no grey area here. People cannot live without water.
So a lack of access to water essentially gives you two options - move to somewhere with more water, or fight for what is available.
In 2017, huge drought across Yemen, South Sudan, Somalia and parts of Nigeria put 20 million people at risk - and was described by the UN as the "worst humanitarian crisis since the Second World War".
That led to mass upheaval, with hundreds of thousands of people trying to find a safer place to live.
This was not a conflict, but it shows how a lack of water can lead to mass social upheaval - and crises on the level of wars.
Conflict over water is nothing new.
In fact, according to the Pacific Institute, throughout history there have been nearly 1,300 conflicts related to water.
These have not always been wars as such, but instead conflicts where water has either been the trigger for fighting, a weapon used in the war, or where water resources or systems have been a casualty.
One of the most notable conflicts related to water access started in the Darfur region of Sudan.
In the early 2000s, fighting broke out between pastoralists and farmers competing over water and land - the ensuing violence killed hundreds of thousands of people, as well as displacing many more.
It was a complex conflict, with sides divided by ethnic and religious lines. Water was not the sole reason for fighting there - but without doubt, it was one of the major reasons it started.
According to the UN, in the decades leading up to the conflict, northern Sudan saw the Sahara Desert advance southward by almost a mile each year, and a decrease in annual rainfall of 15% to 30%.
This drought and so-called 'desertification' led the farmers and pastoralists to compete for land and resources. This competition then boiled over into war in 2003 - with clashes still ongoing.
"Almost invariably, we discuss Darfur in a convenient military and political shorthand — an ethnic conflict pitting Arab militias against black rebels and farmers. Look to its roots, though, and you discover a more complex dynamic. Amid the diverse social and political causes, the Darfur conflict began as an ecological crisis, arising at least in part from climate change."
Near Sudan, another water-related conflict has threatened to escalate into war.
The Grand Ethiopian Renaissance Dam is a huge hydropower project on the Blue Nile, a tributary of the Nile that starts in Ethiopia.
It is estimated to have cost billions of dollars, and will eventually have a 6,000 megawatt capacity - the equivalent to six nuclear power stations.
Once fully active, it will be the biggest hydroelectric power plant in Africa - and will be a big boost to Ethiopia's economy.
The Grand Ethiopian Renaissance Dam in 2020
But it has also heightened political tension in the region with Sudan, and primarily Egypt.
Egypt relies heavily on the Nile - in fact, nearly all of its water comes from the river.
The Grand Ethiopian Renaissance Dam is upstream of Egypt, leading to fears it could disturb the water level.
Some in Egypt see this as an existential threat.
Since the first brick was laid back in 2011, Egypt has tried to stop the dam from being built.
At times, military conflict has seemed close.
In 2013, senior Egypt politicians were caught on camera discussing whether to destroy the dam - though an advisor to the then-president played that down to Sky News.
Former president Mohamed Morsi's foreign policy adviser, Essam El-Haddad, said: "It would not be totally acceptable to talk about the military option if you did not explore other options."
However, he went on to say: "It is a matter of life for Egypt. We cannot live without the Nile, it is a bloodline for us… there is no right for one society to develop something which harms another society."
The dam being built in 2017
Protests have taken place over the building of the dam. And violence has never seemed far away.
In 2018, the chief engineer and public face of the dam Simegnew Bekele was found dead in his vehicle in Addis Ababa.
Police ruled it was suicide - but it led to scores of people taking to the streets over fears he had been murdered.
Construction continued, with Ethiopian President Abiy Ahmed saying in 2019 "no force" could prevent the country from finishing the dam.
It started to produce electricity in early 2022 - but tension between countries continues.
The dam under construction in 2017
Anton Earle, the director of the Africa Regional Centre for the Stockholm International Water Institute told Sky News the next few years could be key.
"These dams are massive mega-projects that take anywhere between five and 10 years to fill the dam reservoir.
"You might have some dry years when the flow of the Nile is going to be naturally reduced and less water will be allowed to flow downstream, as Ethiopia's interest will be to hold as much back to fill the dam as fast as possible.
"So it's during the next few years where the greatest potential for conflict between Ethiopia and Egypt really comes to bear."
It isn't just in Africa where people fear water scarcity could lead to war.
In 2018 the Joint Research Centre listed these as likely places for so-called 'hydro-political interactions' - the Nile, Ganges-Brahmaputra, Indus, Tigris-Euphrates and Colorado rivers.
Violence over water has happened for centuries. Ancient Nubia and Egypt even battled over the Nile.
But there is a key reason the number of water-related conflicts are expected to rise - climate change.
Increasing global temperatures causes more water to evaporate, changing the water cycle.
That means more heavy rainfall in some places - and more drought in others.
So could we see climate change become more and more of a trigger for future wars? Sky News asked Charles Iceland, global director of water with the World Resources Institute's Food, Forests, Water, and the Ocean Program.
"We're seeing now, in real time, a lot of the things that climate scientists had been predicting, predicting for a couple of decades now or more.
"So we're seeing more floods and droughts, and it's wreaking havoc with all types of societies, from the most developed to the least developed. Of course, the least developed countries, the poorest ones, are the ones that can cope the least with these problems.
"[What happens is] there are too few resources for too many people, placing demands on those resources, and they end up fighting with one another.
"Of course, the same thing happens in the developed world. For example, there are fights all the time in the United States between states that take water from the same river, and they end up fighting over dwindling water resources, say in the US west, where water is becoming more and more scarce.
"But they don't end up fighting physical fights with one another. They don't end up in violence, but they do end up, for example, suing each other in court. So you do have conflicts, just not violent ones."
All of this does not mean water wars are inevitable.
In recent history there have been no wars solely about water. In fact, countries and communities have been more likely to co-operate to tackle any water crises.
Even at the Nile dam, despite years of tension and disputes, it is yet to boil over.
But the future could be different to the past.
There is no escaping the fact that the world is getting hotter and drier. And as it becomes more of a struggle for people globally to access such a vital resource like water, people could turn to more extreme measures to secure it.
