Sudden rains and a changing climate: what is really happening

In recent years, many weather events seem to have become more extreme. Prolonged periods of drought are often followed by very intense rainfall concentrated in just a few hours. This pattern, increasingly common in Italy and the Mediterranean, is one of the clearest signs of climate change.

It is not the total amount of rain falling in a year that is changing, but the way it falls. Shorter but much more intense rainfall events are becoming more frequent, with significant consequences for both the land and ecosystems. Understanding these changes is also essential for those who design and manage gardens, because the changing climate is profoundly altering the conditions in which plants live.

• It rains on fewer days, but more intensely
• Why rainfall is becoming more intense
• What this means for gardens and the landscape
• Resilient gardens in a changing climate

It rains on fewer days but more intensely

A study conducted by the University of Milan together with the National Research Council and other international institutes analyzed the evolution of intense rainfall in Italy between 1986 and 2022. The results show that very intense rainfall events in an hour have almost doubled in the last 35 years.

According to the data, the increase in these events is not uniform across the peninsula. In summer, for example, the increase is particularly evident in the Alpine and pre-Alpine areas between Piedmont, Lombardy, and Alto Adige. In autumn, however, extreme events have increased mainly along some coastal areas, such as Liguria, Sardinia, and the Ionian regions.

In some areas, the average number of extreme rainfall events has gone from about ten per year in the 1990s to more than twenty in recent years.

Why rainfall is becoming more intense

The main reason for this change is linked to global warming. A warmer atmosphere is able to hold a greater amount of water vapor. When this vapor condenses, it can generate more intense rainfall concentrated in very short periods.

In other words, the energy available in the climate system is increasing. This favors the formation of more violent thunderstorms, often accompanied by very rapid convective phenomena. For this reason, many regions of the world are observing an apparently contradictory phenomenon: longer periods of drought alternating with sudden and very intense rainfall.

Rainfall concentrated in just a few hours poses a major challenge for the land. Natural systems and urban infrastructure are often designed to manage rainfall distributed over time, not short-duration extreme events.

When large amounts of water fall in a short period, the following can occur:

• urban flooding
• surface landslides
• river flooding
• soil erosion

Many urban drainage systems, designed decades ago, are unable to handle these peaks in intensity. For this reason, extreme weather events are now one of the main causes of environmental and infrastructural damage.

What this means for gardens and the landscape

Even in gardens, the changing rainfall pattern can have significant consequences. Violent rainfall tends to run off quickly on the surface without soaking into the ground. The result is paradoxical: even after a severe thunderstorm, the soil can remain relatively dry in its deeper layers, because the water did not have time to slowly penetrate the ground.

This phenomenon is particularly evident in compacted or poorly structured soils. For this reason, it is becoming increasingly important design green spaces capable of better managing rainwater and it is essential to design gardens capable of retaining and slowing down rainwater, encouraging its infiltration into the ground instead of rapid runoff. Techniques such as soil modeling, the creation of absorption areas, and the use of plants suited to the Mediterranean climate can help turn even intense rainfall into a useful resource for the garden.

. Some useful strategies include:

• improving soil structure
• using natural mulch
• encouraging water infiltration into the ground
• choosing plants adapted to variable climatic conditions

Resilient gardens in a changing climate

In an increasingly unstable climate, garden design also takes on an ecological dimension. Choosing species suited to the local climate and capable of tolerating both periods of drought and sudden rainfall makes it possible to create greener spaces that are more resilient and sustainable. Mediterranean and xerophytic plants, often used in low-water-use gardens, represent an interesting example of natural adaptation to variable climatic conditions.

A garden designed with these principles is not only easier to manage, but also helps improve the soil's ability to retain water, reduce erosion, and promote biodiversity. In a rapidly changing climate, rethinking the relationship between plants, water, and the landscape therefore becomes one of the most important challenges for contemporary gardening.