Climate change strongly disrupts the water cycle by accelerating it. Some regions have more rainfall throughout the year but experience droughts during the summer and a risk of flooding at other times. Decidedly, everything is out of order …
When we look at climate change, we realize that the long-term consequences revolve a lot around the water cycle.
Today we will see how everything we have seen in previous episodes of Café Climat leads to disrupt the water cycle. Increased precipitation, risk of flooding, threat of drought: nothing is going well …
What is the water cycle?
In nature, when heated by the sun, water in rivers, lakes and oceans evaporates. The water vapour which is formed thus rejoins the atmosphere. The higher it goes, the cooler it gets.
It eventually condenses to form clouds of rain water droplets liquid or ice crystals. This water becomes liquid (or solid) again then falls back to the surface of the Earth and the cycle resumes.
On the continents, two options are available to the water that falls in the form of rain or snow:
- Stream on the ground, towards the lowest point of the surroundings to end up in the rivers and resume the water cycle mentioned above;
- Seep into the ground.
Much of the water infiltrated then evaporates, when the sun is shining or the wind is blowing:
- Directly when the water is on the surface or on bare ground.
- By borrowing the network of roots and leaves when the ground is planted with vegetation.
The water returns to the atmosphere through a phenomenon called ” evapotranspiration “.
Another part of the infiltrated water is used to supply the water tables. During the cold period of the year in particular, the soil becomes loaded with water and it infiltrates deeply until it is beyond the reach of the phenomenon of evapotranspiration.
From the water table, the water then joins the streams and then the rivers which themselves flow further into the seas and oceans. The water cycle mentioned above then resumes its course.
Climate change disrupts the water cycle
The first consequence of the increase in air temperature is the increase in evaporation and therefore theincreased precipitation. But this precipitation will not be distributed evenly over the surface of the planet.
Less rainfall makes it difficult to grow crops and more rainfall in some areas can cause flooding. The flood risk will then be all the higher.
But that’s not all…
On the maps below you get an idea of the expected evolution of annual precipitation (left) and summer (right) for the period 2071-2100 compared to the reference period 1971-2000 (for the RCP 8.5 forcing scenario of the IPCC).
At constant precipitation, if the temperature is higher, it means more evaporation and therefore the soils dry up.
Evaporation of more water before it can seep into the soil is not good for crops but also for us to replenish freshwater supplies.
An increased risk of flooding
Finally, it is the same problem for glaciers which serve as reservoirs of fresh water in solid form. They partly melt in summer, when it is hot, and this feeds liquid freshwater reserves downstream.
The melting of the glaciers will therefore have 2 phases: first a increase in streams when they melt and then, when there is no more glacier, a decrease in freshwater reserves available in these regions in summer.
The bottom line is that the water cycle is already disrupted. This means more rainfall in some places but paradoxically less available freshwater reserves.
In the next article, I put all the pieces of the puzzle together for the last episode of the series.
