World’s water cycle getting unpredictable, says UN study

Sub: Geo

Sec: Oceanography

Context:

Increasingly intense floods and droughts are a “distress signal” of what is to come as climate change makes the planet’s water cycle ever more unpredictable, the United Nations said.

Details:

Last year, the world’s rivers were their driest for more than 30 years, glaciers suffered their largest loss of mass in half a century, and there were also a “significant” number of floods.
Last year marked the hottest year on record, characterized by extreme weather events such as prolonged droughts and floods worldwide. These extremes were driven by both naturally occurring phenomena like La Niña and El Niño, as well as human-induced climate change.
A warmer atmosphere holds more moisture, leading to more intense rainfall, while faster evaporation worsens drought conditions.

Water resources are either excessive or insufficient, creating severe challenges:

Africa was the most impacted continent in terms of human casualties.
In Libya, catastrophic floods caused by dam collapses in September 2023 resulted in over 11,000 deaths and affected 22% of the population.
Floods also devastated regions like the Greater Horn of Africa, Democratic Republic of Congo, Rwanda, Mozambique, and Malawi.
Currently, 3.6 billion people experience insufficient access to fresh water at least once a month, with this figure projected to rise to over five billion by 2050.

Massive Glacier Melt:
- From September 2022 to August 2023, glaciers lost over 600 billion tonnes of water, the largest melt observed in the past 50 years.
- This rapid melt threatens long-term water security for millions, but urgent action remains lacking.

Water cycle:

The hydrologic cycle, also known as the water cycle, explains the journey taken by water molecules as they go from the surface of the Earth to the atmosphere and back again, occasionally going below the surface.
At its core, the water cycle is the motion of the water from the ground to the atmosphere and back again.
Water travels through many different layers of the earth’s surface, including the oceans, glaciers, and lakes, as well as simultaneously (or more slowly) moving through the soil and rock layers below the surface. These are complex paths.
The water is afterwards released back into the atmosphere.
The fact that the hydrologic cycle has no beginning and no end is a basic aspect of it.

Key Processes in the Water Cycle:

Evaporation:
- Liquid water turns into gas (water vapour).
- Requires energy from the sun, atmosphere, or other sources (e.g., body heat through sweating).
- Cooling effect, as seen after swimming or showering.
Transpiration:
- Water evaporates from plants via small openings called stomata, mostly on leaf undersides.
- Depends on atmospheric humidity and soil moisture.
- 99.9% of water from transpiration is released into the atmosphere; only 1% aids plant growth.
Condensation:
- Water vapour turns back into liquid, forming clouds or dew.
- Occurs when the air temperature cools to the dew point.
- Releases heat, contributing to weather phenomena like hurricanes.
Precipitation:
- When condensation particles collide and grow too large, they fall as rain, snow, sleet, or hail.
- Provides Earth’s main source of fresh water, with an average of 38.5 inches (980 mm) of precipitation annually.
Runoff:
- Excess water from precipitation flows over land, forming rivers and lakes.
- Some runoff evaporates; the rest returns to oceans.
- In closed lakes (with no outflow), evaporation leads to increased salinity (e.g., Dead Sea, Great Salt Lake).