While we’re often confronted with images of tall-canopied rainforests being felled or waters plagued with plastic, the slow shrinkage and disappearance of low-lying grasses and shrubs in the world’s drier regions frequently fails to capture emotion, attention, funding and concern.
The Earth’s drylands are extremely complex, delicate and ever-evolving structures that provide vast amounts of the world’s food; are home to a giant chunk of the global population; and contain a diverse range of tough, resilient species that have valuable secrets to share about both our collective past and our common future – including our responses to the climate crisis.
Take a moment to school up on these unsung biomes, and see why they deserve so much more than a second glance.
Drylands are biomes in which, on average, water transfer from the land to the atmosphere through evaporation and transpiration outweighs rainfall by at least 150 percent – a calculation that’s known as the aridity index. They include ecosystems such as rangelands, grasslands, woodlands, savannahs and scrublands, comprising more than 40 percent of the Earth’s land surface. Drylands are situated in both tropical and temperate areas, often with high mean temperatures. Water is usually scarce, rainfall tends to be unpredictable and seasonal climate fluctuations are often extreme.
There are four sub-categories of drylands, which are classified according to their aridity indices: dry and sub-humid; semi-arid; arid; and hyper-arid.
These are the least-arid of all drylands, with an aridity index of 0.5–0.65. They support the densest human populations of all dryland types and include biomes like grasslands and savannahs. They tend to have unpredictable rainfall patterns, but their biodiversity is well-adapted for this, with multi-layered, structurally-complex vegetation that facilitates high water infiltration and storage and protects against erosion. While dry and sub-humid lands make up only 8.7 percent of the Earth’s land surface, the majority of the world’s crops and livestock are farmed within these ecosystems.
Semi-arid lands, which stretch across 15.2 percent of the Earth’s land surface, have an aridity index of 0.2–0.5. Annual rainfall in these areas tends to be around 500-800 millimeters, and the presence of dry gale-force winds and very high air temperatures keeps evaporation rates high. Because there is not enough vegetation for stationary farming, migratory pastoralism is the main livelihood activity here. This is the sub-category that’s most vulnerable to degradation because it’s considerably sensitive to human impacts while also supporting a relatively high population.
Arid lands have an aridity index of 0.05–0.2, and make up 10.6 percent of the Earth’s land surface. Dispersed plant clumps are the main form of vegetation here, and these develop slowly and are sensitive to trampling by livestock. Arid and hyper-arid lands are also often classified as deserts, though there is considerable debate about how and when this label should be applied.
Hyper-arid lands have an aridity index of under 0.05 and make up 6.6 percent of the Earth’s surface. They receive under 100 millimeters of rainfall per year – and sometimes no rain for several years. They are largely unvegetated, with most cultivation and plant growth concentrated in oases and croplands where the plants are irrigated by local groundwater sources. Traditionally, these water sources were carefully managed, but they have come under increasing pressure from growing populations in the last few decades, which has led to issues such as soil salinization, vegetation loss and erosion.
Drylands stretch across 41.3 percent of the land surface on Earth, and are present on each of the world’s inhabited continents. They make up 77 percent of Australia’s land area, 66 percent of Africa, 40 percent of Asia, 24 percent of Europe, and 15 percent of Latin America.
Drylands are home to more than 38 percent of the Earth’s population – that’s around 3 billion people. They also support around 44 percent of the world’s cultivated systems and half of its livestock.
Dryland peoples have developed unique strategies to cope with low water and climatic variability, such as a huge range of unique systems of water harvesting, cultivation practices, climate forecasting and medicinal plant use. Their cultural diversity mirrors the diversity of the ecosystems that they inhabit, with 24 percent of global languages associated with the grassland, savanna and shrubland biomes alone.
However, despite their outsize contributions to the global food system, dryland dwellers are among the world’s poorest: 72 percent of drylands are in developing countries, and dryland populations, on average, lag behind the rest of the world in human well-being and development indicators.
When in balance, dryland pastoral and farming systems can play a mutually-beneficial role for the ecosystems in which they inhabit: grazing animals break down plant matter in their stomachs and then excrete it as dung, which enriches the soil and encourages further plant growth.
These ecosystems, which might not look particularly action-packed at first glance, also support an incredibly wide range of biodiversity – both wild and cultivated. They harbor numerous species that are found nowhere else in the world, such as the Saiga antelope (Saiga tatarica) on the Asian steppe, and the American bison (Bison bison) in North America’s grasslands. They’re also the origin sites for at least 30 percent of the world’s cultivated plants, as well as numerous livestock breeds.
That biodiversity will likely prove critical for climate change adaptation. Dryland species have adapted to extreme and variable conditions over millennia, and as such offer a valuable genetic reservoir for new cultivated species of plants and animals that might thrive better in the altered environments of the future.
As you now know, dryland plants, animals and human residents survive by carving out delicate niches in tough conditions. But with climate change is making these conditions even more extreme and unpredictable, putting drylands in a particularly vulnerable position.
Changes in rainfall patterns could lead to severe vegetation loss, resulting in soil surface hardening and increased water runoff, meaning dryland ecosystems will struggle to retain any water they do receive. Meanwhile, the heightened frequency and intensity of droughts will be of particular concern to dryland species and communities, which are already stretched by lengthy dry periods.
This vulnerability is heightened by a lack of knowledge and understanding of dryland biomes, which often leads to unsustainable management practices such as overgrazing, deforestation and conversion to cropland. Such practices are already responsible for a high level of land degradation in these regions – around 20 to 35 percent of drylands are already degraded to some extent.
The livelihood implications are extreme: over 250 million people are currently directly affected by dryland degradation, and around a billion people’s livelihoods in over 100 countries are considered to be at risk. According to the UN, water scarcity could displace between 24 and 700 million people in arid and semi-arid areas by 2030.
The degradation of drylands is also stuck in a vicious feedback loop with climate change. Dryland biomes store over half of global soil carbon, while their plants put away 14 percent of all biotic carbon (carbon that originates from living organisms). Degradation, however, releases that carbon into the atmosphere.
Ambitious emission cuts are a critical component of saving Earth’s drylands – and the species and communities that call them home. Scaling up sustainable and regenerative land management techniques across dryland biomes, while cutting down on damaging practices like intensive mono-cropping and irrigation, will also be important to protecting drylands’ ecosystem functions and services.
Conserving and restoring soil biodiversity is a critical component of this quest, as it ensures that vegetation for agriculture and farming is maintained all year round, including in the critical dry seasons. Many traditional practices address this challenge, such as the ‘zaï’ pits – seed or seedling holes filled with compost – that western Sahelian farmers use to concentrate nutrients and moisture.
Agroforestry (planting trees and agricultural crops together in a way that mimics the structure of a natural forest) and low/no-till agriculture (which avoids ploughing) both help to protect and restore soil structure, and enhance its water absorption and storage capacity. Making the most of existing water resources, such as by applying specialized water harvesting techniques and using treated wastewater for irrigation, are also key.
Managing livestock mobility and stocking rates also plays an important role. Traditional practices like Al Hima in Jordan – under which livestock is moved around seasonally to take account of grass life cycles – can provide useful starting points. In recent years, many migratory pastoralists have had such traditional practices limited or outlawed: governments can contribute to positive change by updating policies to grant pastoralist communities the rights and enabling factors to continue these practices.
In the years to come, raising the profile of these oft-ignored ecosystems will be critical to channeling the action and resources required to ensure they’re able to feed and support a huge swathe of the world’s human, animal and plant inhabitants for generations to come.
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