Farming is the largest way humans have reshaped the planet. Cities get the attention, but they cover a small share of land. Fields and pastures cover far more. Every choice about what to grow and how to grow it is also a choice about forests, rivers, soils and the other species that share them.

Land

By common estimates, about half of the world's habitable land is used for agriculture. Most of that is not cropland but pasture and land for animal feed. Livestock use roughly three quarters of farmland yet provide less than a fifth of the world's calories, though they supply a larger share of protein. Clearing land for farming, especially for cattle and soybeans in South America and oil palm in Southeast Asia, is the leading cause of tropical deforestation. Clearing also releases carbon, but here the concern is the land itself.

Land also raises questions of justice. Who owns farmland, and who works it, shapes whether farming lifts people out of poverty. Large purchases of farmland in poor countries by foreign investors, often called land grabs, have displaced communities that held land by custom without formal title. Secure rights to land, especially for women and Indigenous peoples, are now widely seen as part of food security.

Soil

Soil forms very slowly, often over centuries, but can be lost in a single season. Plowing bare land exposes it to wind and rain. The American Dust Bowl of the 1930s showed what happens when grassland is broken and drought follows. Farmers had plowed up the prairie grass that held the soil, and then drought struck; storms carried topsoil from the southern plains as far as the Atlantic coast. The response, including the Soil Conservation Service founded in 1935, spread practices such as contour plowing, windbreaks and cover crops. Today many farmers use no-till methods that leave crop residue on the surface to protect the ground.

Water

Agriculture is by far the largest user of fresh water. Farming accounts for about 70 percent of the fresh water people withdraw from rivers, lakes and aquifers. Much of the world's food now comes from irrigated land, and many irrigated regions draw on groundwater faster than rain replaces it. Fertilizer that washes off fields feeds algae downstream. Each summer, nutrients carried by the Mississippi help create a low-oxygen dead zone in the Gulf of Mexico, where fish and shrimp cannot survive.

Biodiversity

Farming affects wildlife directly, by replacing habitat, and indirectly, through pesticides and runoff. Farmers also depend on wild species, such as bees and other pollinators, which most of the world's crop types benefit from. And farming faces a diversity problem of its own. Farmers once grew thousands of local varieties, each adapted to its valley or climate. Many have vanished as a few high-yielding varieties spread. Seed banks preserve crop diversity for future breeding. The best known is the Svalbard Global Seed Vault, opened in 2008 inside an Arctic mountain in Norway, which holds backup copies of seeds from collections around the world.

A pioneer of this idea was the Russian botanist Nikolai Vavilov, who collected seeds on five continents in the 1920s and 1930s. He died in a Soviet prison in 1943, a victim of Stalin's campaign against genetics. During the siege of Leningrad, staff at his institute guarded its seed collection, and several starved to death rather than eat it.

Sparing or sharing?

Ecologists divide over how best to protect nature while feeding people. Land sparing means farming intensively on less land so that more can stay wild. Land sharing means farming more gently, with hedges, trees and wildlife on the farm itself, even if yields are lower. Sparing tends to suit species that need large untouched habitats; sharing suits species that thrive in mixed landscapes. Most researchers now favor a mix that depends on place. In 1864 the American scholar George Perkins Marsh, an early voice on these questions, wrote that "man is everywhere a disturbing agent." The real question is not whether farming disturbs nature, but how much and where.

Worked example: the Aral Sea

In 1960 the Aral Sea, on the border of today's Kazakhstan and Uzbekistan, was one of the four largest lakes in the world. Soviet planners then diverted its two feeder rivers, the Amu Darya and the Syr Darya, to irrigate vast cotton fields in the desert. Cotton output soared. The sea began to shrink.

By the 2010s most of the sea had dried, leaving a salty desert dotted with stranded fishing boats. The fishing industry collapsed. Winds lifted dust laced with salt and farm chemicals from the old seabed, and nearby communities suffered high rates of respiratory illness. Summers grew hotter and winters colder without the sea to soften them. Since 2005 a dam has helped the small northern part of the sea, in Kazakhstan, partly recover, but the larger southern basin has not returned.

The Aral Sea shows how land, water and biodiversity connect. A decision about one crop, made far away, changed a local climate, an economy and the health of thousands of people. It also shows that recovery is sometimes possible when water is given back.