Securing adequate supplies of clean water in a changing climate is one of the world’s most urgent social, political, economic, and environmental challenges. World Water Day, an annual UN-sponsored event, calls attention to these problems and the many efforts focused on solutions. Here are five things you need to know about water:

1. Most water we use goes to agriculture

Agriculture places significant pressure on the world's fresh water, accounting for nearly 70% of all water withdrawals. That number can rise to more than 90% in countries like Pakistan where farming is most intensive. Unless substantial efforts are made to reduce food waste and increase the water-use productivity of farming – to get more “crop per drop” – water demands in the agricultural sector are projected to increase in the coming years to keep up with population growth.

Image: FAO

Growing food and fibre and raising livestock endangers the world’s ecosystems, which are at risk from degradation, pollution, and extraction of water. In estuaries, rivers, and lakes, harmful algal blooms, fuelled by the growing use of fertilizers, are a global menace. Mats of toxic algae – in a rainbow swirl of greens, reds, and browns – kill fish, turn away tourists, contaminate drinking water, and depress property values.

Large lakes and river deltas have dwindled after decades of withdrawals that exceed deposits. The shrinking of the Aral Sea, at one point the world’s fourth-largest lake, is due to cotton irrigation in Central Asia. Important marsh ecosystems are declining. Perhaps half of the world’s wetlands have been filled in and the rate of loss has accelerated in recent decades.

2. Climate change adaptation means responding to changes in water distribution and quality

Climate change influences the availability and quality of water resources. On a hotter planet, extreme and irregular weather events such as floods and droughts are becoming more frequent. One reason why is that a warmer atmosphere holds more moisture. Existing precipitation patterns are expected to become entrenched. Dry areas will become drier, and wet areas wetter.

Water quality is changing, too. Warmer river and lake temperatures reduce dissolved oxygen and make habitats more lethal to fish. Warming waters are also more prolific incubators of harmful algae, which are toxic to aquatic life and humans.

The man-made systems that capture, store, move, and treat water were not designed with this variability in mind. Snowpack will shrink and melt earlier. Adaptation to a changing climate means investing in more resilient water infrastructure, from urban drainage to water storage.

Explore our Transformation Map showing the interconnected issues around water.

3. Water is a growing source of conflict and a target in war

From conflicts in the Middle East to protests in Africa and Asia, water is playing an increasing role in civic unrest and armed conflict. More often than not, nations and regions compromise to solve difficult water management disputes. The Indus Waters Treaty, which divides the Indus River tributaries between India and Pakistan, is one lauded example that has endured for nearly six decades.

But these old norms of co-operation are being tested by the unpredictable nature of climate change, population growth, and subnational conflicts. Dramatic swings in seasonal water supplies, a problem often ignored until a crisis emerges, threaten regional, local, and global stability by influencing agricultural output, population migration, and human wellbeing.

Water supplies and water infrastructure can be wielded as weapons in war, and be targeted in military action. Islamic State militants took control of the Mosul Dam in Iraq, in 2014. The next year they bombed water pipelines and closed dam outlets in Syria that choked the flow of the Euphrates for six months.

Image: J. Carl Ganter

4. Billions of people do not have safe, accessible water and sanitation

Some 2.1 billion people do not have safe, affordable, and accessible drinking water, and more than 4.5 billion lack sanitary toilet facilities, according to the UN. This dirty water sickens and kills millions of people annually from diarrhoea and other waterborne diseases.

Because substances readily dissolve into it, water, which is called the universal solvent, is often where pollutants end up. Aquifers, rivers, and tap water can carry the chemical and bacterial markers of their surroundings – lead from pipes, industrial solvents from manufacturing facilities, mercury from unlicensed gold mines, viruses from animal waste, and nitrates and pesticides from farm fields.

5. Groundwater is the world’s largest source of liquid freshwater. But it is poorly understood and dramatically overexploited

The amount of water in aquifers, also called groundwater, is more than 25 times that of the water in the entire planet’s rivers and lakes.

Roughly 2 billion people rely on groundwater as a primary source of drinking water and nearly half the water used to irrigate crops comes from underground.

Despite this dependence, too little is known about the quality and quantity of groundwater that is available. That ignorance has led, in many cases, to overuse, and many aquifers in the world’s breadbasket countries, which produce large quantities of wheat and grain, are being depleted. Indian officials, for example, say that country faces its worse ever water crisis, in large part due to shrinking water tables that have dropped hundreds of metres below ground.

What’s next?

The theme of World Water Day 2019 is “Leaving No One Behind”. Creating a water-secure world for the planet’s 7.5 billion people will be a herculean effort, made more gruelling by the trials of climate change. But it is possible, and there are already steps in that direction.

China is investing in urban green spaces that will make its cities “spongier”, to soak up rainfall and minimize flooding. Philadelphia, Singapore, and others are also investing billions of dollars to soften the city hardscape. America’s Clean Water Act, over more than four decades, has purged pollutants from rivers and helped revitalize waterfronts. Internationally, more and more business leaders and government officials are talking about the value of water.

Those conversations, and the funding and policies that flow from them, are essential. Billions of people are counting on them.

This article is written by Brett Walton, co-curator of the Transformation Map on Water.