Climate change strongly disrupts the water cycle by accelerating it. Some regions record higher rainfall over the year but experience droughts during the summer and a risk of flooding at other times. Clearly, everything is going wrong…
When we look at climate change, we realize that the long-term consequences revolve a lot around the water cycle.
We will see today how everything we have seen in previous episodes of Café Climat disrupts the water cycle. Increase in precipitation, risk of flooding, threat of drought: nothing is going right…
What is the water cycle?
In nature, when heated by the sun, water in rivers, lakes and oceans evaporates. The water vapour which is thus formed joins the atmosphere. The higher it rises, the colder it gets.
It eventually condenses to form clouds of rain water droplets liquid or ice crystals. This water, which has become liquid (or solid) again, then falls back towards the surface of the Earth and the cycle resumes.
On the continents, two options are available to water that falls as 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 infiltrated water then evaporateswhen the sun shines or the wind blows:
- Directly when the water is on the surface or on bare ground.
- By using the network of roots and leaves when the soil is planted with vegetation.
The water thus returns to the atmosphere through a phenomenon called ” evapotranspiration “.
Another part of the infiltrated water is used to feed the groundwater. During the cold period of the year in particular, the soil is loaded with water and this infiltrates deeply until it is beyond the reach of the evapotranspiration phenomenon.
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 evenly distributed over the surface of the planet.
Less rainfall makes cultivation difficult and heavier rainfall in some areas may cause flooding. The risk of flooding will then be all the higher.
But that’s not all…
On the maps below, you get an idea of the predicted change in annual precipitation (left) and summer (right) for the period 2071-2100 compared to the reference period 1971-2000 (for the IPCC RCP 8.5 forcing scenario).
At constant precipitation, if the temperature is higher, this means more evaporation and therefore the soil dries out.
Greater evaporation of water before it can infiltrate the soil is not good for crops but also for us to replenish fresh water supplies.
An increased risk of flooding
Finally, it is the same problem for the 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 rivers when they melt and then, when there will be no more glaciers, a decrease in fresh water reserves available in these regions in the summer.
What must be remembered is that the water cycle is already disturbed. This means higher 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.