19 January 2022

1.1 Introduction to the water and climate nexus

Author/Compiled by
Sophie Gonthier (ISW)
Reviewed by
Elysa Vaillancourt (ISW)

Executive Summary

Freshwater resources are under pressure due to the unsustainable use of resources and the constant increase in demand. Climate change is creating additional pressure by negatively impacting the demand, quantity, quality, and accessibility of freshwater which will gradually intensify as global warming increases. In this factsheet, you will understand the connection between fresh water and climate change, the direct and indirect impacts of climate change on freshwater resources and the link with water stress.


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Before we start, ask yourself a few questions:

  • What does climate change mean to you?
  • How is climate change affecting your region? How is it affecting the fresh water resources in your region?  
  • Why are climate change and water stress closely linked together?

Characteristics, terms & definitions

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Freshwater resources on Earth

Fresh water is essential to human life. It is not only the human body that need fresh water to survive, fresh water is also needed for food production. Without water, there is no growth, no food and no life on earth (UNICEF, 2017).

The unsustainable use of water resources is the main cause of water stress. Certain human activities have a significant impact on freshwater resources, such as land use, changes in the morphology of rivers and groundwater, the construction and operation of hydroelectric power plants, the construction of dams and the drainage of wetlands and irrigation. Climate change is putting additional pressure on water resources by impacting the water cycle. (IPCC, 2019)

Water cycle

The water cycle is the system by which water passes between different forms including gaseous form, liquid form and solid form. It plays a key role in the renewable nature of freshwater resources since it controls the movement of water on land and thus the amount of water that will ultimately end up in freshwater reservoirs. (USGS, s.d.a; UNESCO, 2012)

See here for more details on the water cycle and fresh water resources on earth


Climate change, global warming and water 

Natural greenhouse emissions (GHGs) capturing mechanisms such as plants, soil and oceans are unable to maintain the Earth's climate balanced due to the increase in the concentration of GHGs in the atmosphere. At the same time data is pointing to the warming of the planet  as well as an increase in the occurrence of extreme weather events. The IPCC confirms the influence of anthropogenic activities on climate variations. (IPCC, 2018)

Global warming is reflected in rising average air and ocean temperatures, melting snow and ice, and rising sea levels (IPCC, 2019). These climate variations increase the frequency and intensity of extreme weather events, leading to natural disasters. No less than 90 per cent of natural disasters are water-related, 43 per cent of which are floods and 5 per cent are droughts. Both phenomena have an impact on freshwater resources which leads to significant economic and social costs. (UNESCO, 2019a)

Every year, the consequences of global warming push more than 26 million people into poverty. Sadly, developing countries, which emit low GHGs, are the first to feel the impacts of climate change due to their geographic location, economic fragility, and dependence on ecosystem services. (World Bank [WBG], 2019a; IPCC, 2014a) Ecosystem services are the benefits that humans derive from nature, such as fresh water, pollination, and food (World Economic Forum [WEF], 2019). Their economic value is estimated at US$125 trillion per year (Costanza et al., 2014).

See here for more details on climate change and global warming


Impacts of global warming on the water cycle

The IPCC states that the relationship between global warming and the impact on freshwater resources is direct (IPCC, 2018). The climatic factors with the greatest influence on freshwater resources are precipitation, temperature, and evapotranspiration. Their variation accelerates the water cycle in several regions of the world which has an impact on, among other things, the frequency and intensity of precipitation, the melting of snow and ice, the increase in evapotranspiration and water vapour in the atmosphere, changes in soil moisture and water infiltration, and runoff. (IPCC, 2013)

Note that other factors such as natural variations and the consequence of human activities also affect freshwater resources. (IPCC, 2013)


  • Precipitation and evapotranspiration 

The increase in atmospheric temperature results in an increase on the atmosphere’s water-holding capacity, however with little effect on the relative humidity level. This increases the rate of evaporation on a large part of the aquatic surfaces. For its part, there is a very high chance that evapotranspiration on the earth's surface will increase in a warmer climate. Evapotranspiration affects soil moisture, runoff, the amount of water in reservoirs, and the salinity level of aquifers. (IPCC, 2014b)

This can have a significant impact on the variation in precipitation observed globally which plays an important role in the availability of freshwater resources. Furthermore, the occurrence and intensity of heavy precipitation events are very likely to increase as temperatures increase, mainly in mid-latitude and humid tropical regions such as in Canada, Russia, East Africa, northern Central Asia and the Amazon region. These phenomena are characterized by extreme precipitation causing high runoff interspersed with dry periods with a high rate of evapotranspiration. However, the increase in extreme precipitation does not mean a greater total annual amount of precipitation. (IPCC, 2014a)

The increase in droughts is due to decreased rainfall and increased evapotranspiration that change soil characteristics (IPCC, 2018). A major consequence of the change in rainfall is desertification mainly in arid, semi-arid and dry sub-humid regions. Desertification is caused by the lack of water in some regions, which causes the soils to dry up and lose their properties. (Water Coalition, 2014) Ironically, the increase in extreme rainfall and flooding is linked to the increased risk of droughts (IPCC, 2014b).


  • The cryosphere


The cryosphere represents a large reserve of fresh water on Earth, representing almost 70% of the total amount of fresh water in the reserves on the land surface. It encompasses freshwater reserves in permafrost, snow, land ice, glaciers and ice caps. More than 15% of the population depends on freshwater basins fed by melting snow and land ice as their main freshwater resource. (Water Coalition, 2014) The cryosphere is sensitive to climatic variations such as temperature and precipitation.

The available data shows that the volume of the cryosphere has decreased overall over the last decade. The majority of climate scenarios very likely predict that the rate of sea level rise will accelerate during the 21st century, and unevenly across the world due to the accelerated melting of the cryosphere. This rise in sea level will affect both the loss of coastlines and flooded areas mainly in lowlands, deltas and low-lying islands. (IPCC, 2018; IPCC, 2019)


  • Runoff and river flow

Variations in the amount and form of precipitation, the amount of water in solid form in the cryosphere, and in the rate of evaporation have an effect on runoff and river flow. In general, changes in precipitation are consistent with changes in river flow. This predicts a variation in the average flow of a large number of rivers around the world due to climate change. (IPCC, 2019)

At first, a temporary increase in runoff is predicted. This increase in runoff will only be beneficial if regions are adequately equipped to capture and store this additional fresh water. Otherwise, the additional water will ultimately join the saltwater bodies. Another challenge will be to adapt to temporal variations in flows due to changes in the seasons. Studies show that this increase in runoff will gradually decrease over the next century as glaciers melt. Evidence suggests that glacier-fed rivers in several regions have already experienced peak flows. (IPCC, 2019)


Difference between Climate Change and Global Warming?

Climate Change 

“Climate change is a long-term change in the average weather patterns that have come to define Earth’s local, regional and global climates. (…) It refers to both human and naturally produced warming and the effects it has on our planet.” (NASA, 2021)

Global warming

“Global warming is the long-term heating of Earth’s climate system observed since the pre-industrial period (between 1850 and 1900) due to human activities, primarily fossil fuel burning, which increases heat-trapping greenhouse gas levels in Earth’s atmosphere.” (NASA, 2021) It is measured as an increase in the average temperature at the earth's global surface (IPCC, 2018).

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The previous sections have shown the vulnerability of freshwater resources in the face of climate change due to its impact on the water cycle. However, despite the different projections, the extent of the impacts of climate change on freshwater resources remains uncertain, as it depends on several variables, which are themselves uncertain, such as the hydrological model, precipitation, GHGs emissions, the vulnerability of a climate or population, and the extent and success of the adaptation measures put in place. The United Nations Educational, Scientific and Cultural Organization (UNESCO) considers the effects of climate change such as the quantity of fresh water available, the quality of fresh water as well as on the accessibility of resources, can accentuate the already existing water crisis and social inequalities (UNESCO, 2019a).

Now, let’s have a concrete look on how the variation of the water cycle impact freshwater resources:


Freshwater demand

The variation in temperature and precipitation is contributing to worsening the water crisis. These factors influence the demand for water and increase competition between different consumer sectors for access to freshwater resources. (IPCC, 2014b)


Freshwater quality


The overall increase in water temperature caused by global warming results in a reduction of the overall quality of available fresh water, in particular by causing an increase in the number of pathogens present in the water and by encouraging the spread of bacteria and other contaminants at a higher speed. This has disastrous impacts on human health, mainly in areas were access to water and sanitation is not available or adequate. On top of this, both droughts and floods often cause contamination of freshwater supplies. (IPCC, 2018; UNICEF, 2015b)

Increased heavy rainfall events will also play an important role in the rapid spread of pathogens, faecal coliforms and pollutants, both to surface and groundwater supplies. Increased runoff intensifies the load of nutrients, salts, heavy metals, and pathogens in reservoirs, and increasing river flow worsens the rate of transport of these pollutants. Studies also show an increase in the presence of pollutants and faecal coliforms in groundwater after episodes of heavy rain. The decrease in river flow will also increase the salinity of lakes, rivers and estuaries. Water salinity will also tend to increase in shallow groundwater reserves, particularly in semi-arid and arid regions where evapotranspiration is higher. (IPCC, 2014b) Saltwater flooding caused by sea-level rise or storms will also contaminate groundwater in coastal areas if these areas are not adequately equipped to drain and evacuate water from flooded areas effectively. Fresh water will therefore become too salty for the use for which it is intended. (UNICEF, 2015a; WEF, 2019)


Freshwater quantity

The amount of fresh water available is influenced by several factors: temperature, precipitation, runoff, infiltration and the condition of underground reserves. Precipitations supplies freshwater sources through runoff and water infiltration into the soil. Changes in the water cycle that lead to changes in precipitation have a direct effect on the capacity for renewal and the availability of freshwater resources. (IPCC, 2014b) River flow and runoff are affected by the amount and form of precipitation as well as melting ice and snow (IPCC, 2014b). Variations in the volume of water in solid form in the cryosphere have a negative impact on the water supply of the populations normally benefiting from this resource. The change in precipitation shape and the accelerated melting of snow and ice increase river flow and runoff in winters and decrease it in summer (IPCC, 2019). On the one hand, the decrease in river flow in summer has the consequence of accentuating droughts (IPCC, 2014b). On the other hand, increased river flow can cause flooding since the infrastructure is not adequate to harvest and store fresh water from the accelerated melting of glaciers. This results in the decrease in fresh water reserves since a portion of this fresh water ends up in the oceans and in the seas and thus turns into salt water, which also has the effect of raising the sea level. In the long run, once the river flow has reached its highest point, the amount of water available downstream of the glacier will decrease. Changes in the state of the cryosphere also affect runoff. (IPCC, 2019) Variations in runoff will affect the level of freshwater reserves. In particular, an increase in the surface waters of lakes in Mongolia and China is observed. This is attributed to the accelerated melting of the cryosphere as well as changes in the shape of precipitation. On the other hand, many surface freshwater basins are seeing their reserves decrease due to rising temperatures, droughts and human activities. The change in river flow will have an impact not only on human consumption, but also on hydropower production. (IPCC, 2014a)

Extreme rainfall often does not help increase freshwater supplies. In fact, as a result of heavy rainfall, the soil often reaches its infiltration capacity because it is saturated. The same problem applies to tanks that do not have the capacity to store such large amounts of water. Heavy rainfall increases the risk of flooding and also destroys ecosystems, endangers human life and health, and damages and destroys infrastructure, including infrastructure that provides access to fresh water. (UNESCO, 2012)

The global trend is to reduce groundwater reserves. In one of these most recent reports, the IPCC concluded, with a high degree of confidence, that climate change will have a significant impact on the depletion of freshwater resources in almost all dry subtropical regions. (IPCC, 2019)

In some cases, this decrease is related to changes in the water cycle that cause a decrease in the supply of groundwater by surface water and infiltration. However, in most cases, the decline in freshwater reserves is largely due to human use beyond the natural recharge capacity of the source. (IPCC, 2014b) Freshwater resources are under initial pressure to over-exploit freshwater supplies to meet the growing demand for freshwater. Groundwater is often considered a source of emergency water in the event of a shortage of fresh surface water. Climate change adds additional pressure to freshwater supplies by increasing average temperatures, which increases the demand for fresh water and thus the overexploitation of groundwater resources. (Wada et al., 2017) Normally, groundwater reservoirs fill with precipitation water that seeps into the soil and feeds surface water and groundwater reserves. However, with decreased precipitation and less available surface fresh water, the renewable nature of groundwater reservoirs is at risk. (UNESCO, 2012) A study of four aquifers in Spain shows the interaction between decreasing groundwater reserves, decreasing precipitation and increasing evapotranspiration (Aguilera and Murillo, 2009).


Freshwater accessibility 

In many situations, the problem becomes evident when accessibility to quality fresh water is affected. The decrease in the quantity and quality of available fresh water makes it more difficult to access quality fresh water because of the expensive infrastructure required to access it.

In addition, variation in the frequency and intensity of floods and droughts will make existing infrastructure inadequate for the new challenges of water management. Natural disasters contribute to the problem of accessibility to water by damaging or destroying local infrastructure. For example, flooding caused by heavy rainfall (amongst other things) damages infrastructure and jeopardizes the population's access to clean fresh water. (UNICEF, 2015a) Integrated management of water management and land use would make it possible to adapt to the new situation of freshwater resources. Such changes will require a change in behaviour and significant investments. (FAO, 2018b)

What is water stress?

Water stress is the concept used to assess the level of exposure of a territory to a risk of freshwater scarcity. According to the United Nations, "[it] is defined as the proportion of water withdrawals by all sectors of activity, in relation to the available water resources". (FAO, 2018a) Water stress can be calculated in terms of economic water stress or in terms of physical water stress:


Economic water stress refers to the amount of water available as well as the socio-economic growth of a region. This type of measure is more useful in developing regions where the demand for water is likely to increase significantly due to the country's growth. Generally, there is a situation of economic water stress when the infrastructure needed to exploit freshwater resources is not available or when there is poor management of resources. This is the case in sub-Saharan Africa, a region with freshwater resources that are not accessible for economic reasons. (UNESCO, 2012)


Physical water stress refers to the total amount of fresh water abstracted, including the amount of water needed to meet the water needs of the environment, relative to the total amount of renewable fresh water available. Some calculation methods include the amount of water from alternative sources such as recycled water and desalinated water. (FAO, 2018a)

The Water Cycle

To learn more about the water cycle click here! 

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