Mediterranean climate

What is a Mediterranean climate?

A Mediterranean climate is a climate with mild, wet winters and warm to hot, dry summers. Its defining feature is that warmth and water arrive at different times of the year. When the soil is warm enough for strong growth, it is dry; when it is moist, the days are short and cool. Every garden and every planting in these regions has to answer this offset.

The name comes from the Mediterranean Basin, but the same climate occurs on five continents. In German it is also called Etesienklima, in Italian clima mediterraneo.

The Köppen-Geiger definition: Cs, Csa, Csb

The criteria are set out precisely in the present and future Köppen-Geiger maps that Beck and colleagues published in 2018 at a resolution of one kilometre. A climate counts as temperate (C) if the warmest month averages more than 10 degrees and the coldest month lies between 0 and 18 degrees.

A temperate climate becomes a Mediterranean climate (Cs) when the summer is dry: the driest summer month receives less than 40 millimetres of precipitation and less than a third of what falls in the wettest winter month. Summer and winter are the warmer and the colder half of the year, April to September and October to March.

Three subtypes follow from the summer temperature:

• Csa, hot summer: the warmest month averages at least 22 degrees.
• Csb, warm summer: the warmest month stays below 22 degrees, but at least four months average more than 10 degrees.
• Csc, cold summer: only one to three months average more than 10 degrees.

Five regions of the world

Mediterranean climates are found in five regions: the Mediterranean Basin, California, central Chile, the Cape region of South Africa and southern and south-western Australia. They lie on the western and south-western margins of continents, roughly between 30 and 40 degrees of latitude. According to a review by Rundel and colleagues (2018), together they cover only about 2.2 percent of the world's land area but are home to roughly one sixth of the world's vascular plant flora. Outside the wet tropics, no other biome is as rich in plant species.

The climate is younger than much of its vegetation. Hard-leaved shrublands already grew on poor soils in the wetter Cretaceous; the summer drought only emerged in the middle Miocene.

Why the summer is dry

The IPCC places the Mediterranean Basin in a transition zone between the circulation of the mid-latitudes and that of the subtropics. For all five regions, Seager and colleagues (2019) name the same two forces: the storm tracks of winter and the subtropical highs of summer.

In summer the subtropical high moves towards the pole and settles over these coasts. Under a high, air sinks, warms and dries out, and rain stays away for months. In winter the high retreats towards the equator, and the westerlies with their low-pressure systems bring most of the year's rain. The year therefore has two seasons rather than four: a wet one, roughly from October to April, and a dry one, roughly from May to September.

Winter rain and autumn downpours

Rain in a Mediterranean climate is not only seasonal but often intense. After months of drought, the first storms fall on dry, hard soil. The MedECC report on the Mediterranean Basin calls this the Mediterranean paradox: stronger and more frequent floods in autumn and winter, and more drought in summer. On bare, crusted ground much of the water runs off instead of soaking in and takes soil with it.

How the rain falls depends strongly on the terrain. Mountains force moist air upwards and catch more rain on their windward side, while the leeward side stays drier.

Fire as a shaping force

A wet season that grows plant mass, followed by months of drought, turns vegetation into fuel. Rundel and colleagues (2016) see the ability to persist through recurring fire as a main driver of the emergence of new plant species in four of the five regions. Central Chile is the exception: there, according to the same authors (2018), fire has played only a minor role since the uplift of the Andes.

Some plants resprout after fire from the root stock, others die but leave seeds that germinate afterwards. Geophytes, plants with bulbs, corms or other storage organs below ground, are an important part of the flora after fire in all five regions.

How plants adapt

Hard leaves. Evergreen shrublands with small, tough, often leathery leaves dominate the five regions. Such sclerophyll leaves lose little water and last through the summer. Examples from the Mediterranean are holm oak, cork oak, mastic, myrtle and strawberry tree, the plants of the macchia.

Deep roots. A global compilation by Canadell and colleagues (1996) gives an average maximum rooting depth of about 5 metres for sclerophyllous shrublands and forests. Such roots reach water in deeper layers long after the surface has dried out.

Summer rest. Many plants grow from autumn to spring and pause in summer. Annuals complete their life before the drought and survive it as seed. Geophytes withdraw into bulbs and tubers, let their leaves wither and sprout again with the first autumn rain.

Sardinia as an example

Sardinia, in the middle of the western Mediterranean, has rain in the winter half of the year and a long dry summer; a forestry study by Bianchi and colleagues (2003) assigns all the climate stations it examined in dry areas of the island to the Mediterranean bioclimate. Within the island the terrain makes large differences. A study by Bodini and Cossu (2010) shows that high, steep mountains close to the sea along the central and south-eastern coast cause an east-west gradient in precipitation, above all in autumn, when warm, moist air flows in from Africa. Extreme rainfall events are most frequent in the central east of the island, the area where the Ogliastra lies. Two stations there show how large the differences are over short distances: Genna Cresia, at 272 metres in the hills, receives an average of 818 millimetres a year according to the ISPRA climate normals, the coastal station of Capo Bellavista near Arbatax only 420 according to the climate atlas of the Air Force weather service. How rain and evaporation are spread over the year there is shown in the water balance.

Sardinia also shows the pressure on this climate. A large share of the island is considered at risk of desertification, and the macchia on the hills is both a sign of past losses and a protection for the soil that remains.

A hotspot of climate change

The IPCC concludes for the Mediterranean Basin that temperatures are about 1.5 degrees above the pre-industrial level, with more heat extremes, and that warming has exceeded the global average since the 1980s (high confidence). According to MedECC, the basin, land and sea together, warmed by 1.5 degrees while the globe warmed by 1 degree, and in summer by well over 2 degrees.

The comparison needs care. Land surfaces generally warm faster than the air over the oceans, and Guiot and Cramer, two of the MedECC authors, point out that the Mediterranean is not unusual in that respect. It is a hotspot because of the combination of risks: heat, drought, sea level rise, fire and a densely populated coast.

For the 21st century, annual warming is projected to be about 20 percent and summer warming about 50 percent larger than the global average. Precipitation is likely to decrease in most areas by 4 to 22 percent, depending on the emission scenario (medium confidence); in the central and southern basin by about 4 percent for each degree of global warming. Droughts have already become more frequent and more intense, especially in the north (high confidence), and heavy rainfall there is likely to increase as well. Seager and colleagues find that all five Mediterranean-type regions except the one in North America have become drier in winter since 1901, and models project further drying.

What this means for permaculture

A Mediterranean climate does not lack water so much as it lacks water at the right time. The gardener's task is to carry the winter rain into the summer.

Hold water. Rainwater harvesting, swales along the contour and cisterns slow the water down and keep it on the land. This also softens the autumn downpours and keeps the local water cycle going.

Cover the soil. Bare soil crusts, heats up and loses water quickly. Mulch, living ground cover and fallen leaves shade it, break the impact of heavy rain and slow evaporation.

Plant in autumn. A tree planted with the first autumn rain has the whole wet season to root into moist soil before its first summer. Research on plantations in Spain links the survival of young plants above all to how well they grow during the wet season, while summer drought causes most losses. The Mediterranean garden year follows this rhythm.

Create shade. Shade lowers soil temperature and evaporation and protects young plants. Trees, support plants and hedges create a microclimate in which more can grow than the regional climate alone would allow.

Choose plants for the future. Climate-adapted trees, xeriscaping and, with care, assisted migration plan for hotter summers.

How Terra Humana understands the Mediterranean climate

For Terra Humana the Mediterranean climate is above all a question of water and timing: rain falls in one half of the year, growth is needed in the other. We plant with the rain, prune in the rainy season and keep the shade in summer, which keeps the burning sun off leaves, bark and soil. The climate figures describe the region; what the land shows in a given year corrects them. Because the climate is changing, we watch which plants cope with the summers as they are now.