IPCC Assessment in depth, Part 4: How is climate change affecting the world’s water?

The threat that climate change poses to human well-being and the health of the planet is “unequivocal”, says the latest report from the United Nations’ Intergovernmental Panel on Climate Change (IPCC). In this in-depth Q&A series, Carbon Brief unpacks the key findings of the report and the developments since the IPCC’s last assessment. (Read previous section here)

By Carbon Brief

The fourth chapter of the report is dedicated to how climate change is affecting water.

At present, roughly half of the world’s human population experiences severe water scarcity for at least one month per year, the report estimates with medium confidence.

And, since the 1970s, 44% of all “disaster events” on Earth have been flood-related, the report says. (The IPCC says “disasters” involve extreme weather events “coupled with high vulnerability and exposure” in human populations.)

Nearly half a billion people live in “unfamiliarly wet areas” – where long-term average rainfall is high as previously seen in only one in six years – while 163 million people live in “unfamiliarly dry areas”, the report estimates.

Heavy rainfall intensity has increased in “many regions” since the 1950s. Around 700 million people live in regions where maximum daily rainfall has increased, the report estimates.

In the northern hemisphere, snow cover across all months of the year is decreasing, the report says. Snow cover extent peaked in the northern hemisphere from the 1950s to 1970s, the report says, and has been in decline ever since.

Over the past two decades, the loss of ice from glaciers – frozen freshwater rivers found in high latitudes – has exceeded 0.5 metres of water equivalent each year. (Water equivalent represents the volume of water created from melting the ice.) This is higher than at any other time since records began.

The map below shows changes to glaciers across the world from 2000 to 2019.

The figure shows changes to glacier elevation (green bars; in metres per year); “mass balance”, a measure of the total amount of ice held by glacier once summer melting and winter recovery have been taken into account (blue bars; in metres of water equivalent per year); and mass change (in billions of tonnes per year).

The figure shows that glacier mass change rate has been highest in the western US and Canada, Iceland and Svalbard, a Norwegian archipelago located between mainland Norway and the North Pole.

The melting of glaciers is impacting “humans and ecosystems”, including “vulnerable high mountain and polar communities”, the report says.

It cites a study finding changing ice conditions in the Canadian Arctic have affected access to trails for Inuit communities. It also cites a US federal report finding the melting of ice has affected Alaskan Native populations’ access to culturally-significant species.

The report also says there is a “knowledge gap” in how ice-related floods, including glacier-related and ice-jam floods, are responding to ongoing climate change.

There is a “clear trend” of increases to streamflow in rivers in the northern high latitudes, the report says with high confidence.

Meanwhile changes to groundwater stores, which are relied upon by billions of people for water, are less straightforward, according to the findings.

For example, the report notes that groundwater storage has declined in many parts of the world since the start of the 21st century, largely due to a growing demand for water for crop irrigation.

In higher altitudes, warming has altered groundwater levels, which may have played a role in reduced spring-time recharge, it adds.

It is very likely that human-caused climate change affected global patterns of soil moisture over the 20th century, the report says, with consequences for agriculture, ecosystems and the severity of extreme weather events.

Human-caused climate change has also made floods and droughts more likely and more severe.

Between 1970 and 2019, 7% of all “disaster events” were drought-related. However, drought disasters accounted for 34% of disaster deaths, particularly in Africa.

Meanwhile, severe rains made more likely by climate change have caused catastrophic flooding in areas such as western Europe, China, Japan, the US, Peru and Brazil since 2014.

Changes in these water-related hazards “disproportionately impact vulnerable populations such as the poor, women, children, Indigenous peoples and the elderly in all locations”, the report warns.

Changes to the water cycle are impacting agriculture, the report explains. Between 1983 and 2009, around three-quarters of the global harvested area experienced yield losses because of drought, costing the world an estimated US$166bn, the report says.

In the future, “water-related risks are projected to increase with every degree of global warming”, the report says.

At 2C of global warming, 3 billion people could face water scarcity, the report says with medium confidence. At 4C, this figure rises to 4 billion.

Under a scenario where greenhouse gas emissions do not peak until 2060 (“RCP6.0”), roughly 1.5 billion people who “critically depend” on run-off from mountains for freshwater could face negative impacts.

Every fraction of temperature rise will also worsen drought and flood risk, the report says.

Under RCP6.0 and SSP2, the global population exposed to “extreme-to-exceptional” drought could increase from 3% to 8% over the 21st century.

Extreme agricultural drought across large areas of North and South America, the Mediterranean and Eurasia is projected to be twice as likely at 1.5C, 150-200% more likely at 2C and over 200% more likely at 4C, when compared to current conditions.

Risks to agricultural yields from temperature and water changes could be three times higher at 3C rather than 2C, the report says with medium confidence.

Projected increases in rainfall intensity will cause more flooding, the report says.

Direct flood damages could be four to five times higher if global warming reaches 4C rather than 1.5C, the report says. In addition, GDP losses related to flooding could be 1.2-1.8 times higher if warming reaches 2C rather than 1.5C, the authors say, with medium confidence.

The maps below show projected changes to global river flooding by the 2080s under a low (SSP1-2.6), medium (SSP2-4.5) and high (SSP5-8.5) scenario. On the chart, blue shows increases in flood frequency while red indicates decreases. Dots illustrate results where there is high agreement between climate models.

At 4C of global warming, around 10% of the global land area, supporting 2.1 billion people, could face increases in extreme streamflow, the report says.

Meanwhile, some parts of Europe, including the Mediterranean, could see the potential for hydropower decrease by 40% at 3C, compared to 10% at 2C and 5% at 1.5C, the report adds.

Next: What does the IPCC report say about impacts on food and agriculture?

All sections:

1. What is the IPCC report?
2. How is climate change affecting land ecosystems?
3. What impact is global warming having on marine life?
4. How is climate change affecting the world’s water?
5. What does the IPCC report say about impacts on food and agriculture?
6. What risks does climate change pose to the world’s cities?
7. What does the report say about public health, conflict and migration?
8. How does climate change affect poverty and progress towards the sustainable development goals?
9. How is the world adapting to climate change?
10. What the latest IPCC report says about carbon fee and dividend

Authors:

Aruna Chandrasekhar holds an MSc in environmental change and management from the University of Oxford. She previously worked as a freelance journalist for a variety of publications, including the New York Times, Guardian, Scroll and Caravan.

Daisy Dunne holds a BSc in biology from the University of Bristol and a science journalism MA from City, University of London. She was The Independent’s climate correspondent from November 2020 to 2021. Prior to this, she was Carbon Brief’s science writer from 2017 to 2020.

Josh Gabbatiss covers climate and energy policy. Josh holds a BSc in zoology from the University of Bristol. He previously worked as science correspondent at The Independent and prior to that was a freelance journalist for a variety of scientific publications including New Scientist and BBC Earth.