The Ecosphere

The Ecosphere stands as a remarkable and complex web of interrelated elements on Earth, the only place in the universe where life is known to exist. It comprises the biosphere (all living things), hydrosphere (water), atmosphere (air), cryosphere (ice), lithosphere (rock), and pedosphere (the living soil where these layers meet) — and, increasingly, the anthroposphere, the human-made sphere now reshaping all the others. Together these envelopes maintain a fragile equilibrium, nurturing an abundant and diverse assortment of living organisms and ecosystems.

From Cosmos to Biosphere

01 · Two words for one living world

Biosphere and Ecosphere

Two words carry this essay. They are often used interchangeably, but they name different things, and the difference matters.

Biosphere. The totality of life on a planet: every living organism, and the diversity they embody at every level — of genes within species, of species, and of the ecosystems and functions they form together — along with the planetary-scale activity through which they capture energy, cycle matter and transform their surroundings. On Earth it has persisted without interruption for at least 3.5 billion years, holds roughly 550 billion tonnes of carbon in living matter, and is the only biosphere known in the universe.

Sources: diversity of genes, species and ecosystems after the Convention on Biological Diversity, Article 2 (United Nations, 1992); persistence from the fossil record (GLOBAÏA, A Geohistory of Life on Earth); living carbon (Bar-On et al., 2018).

Ecosphere. A planet’s biosphere together with the air, water, ice, rock and soil it depends on and continually reshapes, understood as one coevolving system. Its defining property is habitability — the capacity to sustain life over time, a capacity that life itself helps maintain. On Earth that habitability has been relatively stable: evidence for liquid water reaches back more than four billion years, through ice ages, Snowball Earths and five mass extinctions, held within liveable bounds by feedbacks between life, climate and rock, without design or purpose. Its resilience — the capacity to absorb shocks and reorganise without leaving a habitable state — is finite: pushed far enough, the system can shift into a different state, one that may no longer suit the life it carries now. The Ecosphere is bounded in space by the reach of life, and in time by the window in which star, galaxy and planet keep those conditions possible.

Sources: habitability (Cockell et al., 2016; Chopra & Lineweaver, 2016); early liquid water (Wilde et al., 2001); mass extinctions (Barnosky et al., 2011); regulation without purpose (Lenton et al., 2018; Kleidon, 2023); resilience (Holling, 1973; Walker et al., 2004).

02 · The only one we know

A crowded sky, a single biosphere

Before 1995, no planet was known around another Sun-like star (Mayor & Queloz, 1995). Today the NASA Exoplanet Archive lists more than 6,400 confirmed exoplanets. Most are unlike anything in our Solar System: worlds between Earth and Neptune in size — apparently the most common kind of planet around Sun-like stars, at least on close orbits, and one our own system lacks (Fressin et al., 2013) — hot giants grazing their stars, and a growing number of rocky planets. Most were found by the faint dimming of a star as a planet crosses its face; others by the wobble a planet’s gravity imposes on its star.

Among them are candidates for habitability. A 2026 catalogue (Bohl et al., 2026) counted 45 potentially rocky planets orbiting within the habitable zones of their stars, 24 of them within a narrower zone drawn from three-dimensional climate models. Only five have both a measured mass and a measured radius. None has yet shown a confirmed sign of life. Claims have been made — most prominently a possible sign of a gas linked to marine life on the planet K2-18 b (Madhusudhan et al., 2025) — but independent reanalyses have found insufficient evidence for it (Luque et al., 2025; Schmidt et al., 2025), and every such claim so far remains unconfirmed and contested.

So the planets are there, and the conditions for liquid water seem common. What we have not found, after decades of searching, is a second biosphere. Earth’s remains the only one we know: a single instance of something that, as far as we can yet tell, the universe does not do often — or does in ways we cannot yet see.

Why might that be? The papers do not agree, and the disagreement is itself instructive. One view holds that life starts easily: on Earth it appeared remarkably soon after the planet became habitable, and a statistical analysis by astronomer David Kipping finds a fast start at least 2.8 times more likely than a slow one (Kipping, 2020). Another holds that what is hard is not starting life but keeping it. In the Gaian bottleneck hypothesis, young planets drift toward freezing or boiling within their first billion years unless life evolves quickly enough to regulate its own climate — so extinction would be the cosmic default, and a long-lived biosphere the exception (Chopra & Lineweaver, 2016). Still others question whether Earth is even the best case: it may be only marginally habitable among possible worlds (Heller & Armstrong, 2014).

Each view leads to the same place. Earth’s biosphere is not a passenger on a habitable planet. It is one of the reasons the planet has stayed habitable for so long — and it is the only proof we have that this can happen at all.

03 · Habitable zones, from cosmos to soil

Nested windows

Habitability is not a property of a planet alone. It depends on the age of the universe, the kind of galaxy, the place within it, the star, the planet’s distance from that star, and the planet’s own interior and surface. Astronomers speak of habitable zones at each of these scales. They nest inside one another, and every scale has its own uncertainty.

04 · Cosmic time

When the universe became habitable

Life as we know it is made of elements forged inside stars — carbon, oxygen, nitrogen, phosphorus, iron. The first generations of stars had to live and die before rocky planets could form at all. The early universe was also violent: star formation, exploding stars and the blazing cores of galaxies all peaked some ten billion years ago (Madau & Dickinson, 2014; Mason & Biermann, 2018). Earth-like life therefore had a cosmic starting window, opening several billion years after the Big Bang. Estimates of when it opened differ by billions of years, depending on what each model takes to be the main hazard (Gonzalez, 2005; Mason & Biermann, 2018).

The window is still open, and will stay open long after the Sun is gone. Small red stars burn for trillions of years. If their planets can host life — still an open question, because such stars flare violently in their youth — then most of the life the universe will ever hold lies far in the future, and we are early (Loeb et al., 2016).

05 · Galaxies

Which galaxies?

Not all galaxies may be equally hospitable. The cores of dense galaxy clusters, where galaxies collide and central black holes flare, may be hostile to complex life — a proposed super-galactic habitable zone would exclude them (Mason & Biermann, 2018). Beyond that, there is little agreement. Large spirals like the Milky Way, rich in the heavy elements planets are made of, were long assumed to be the best homes. More recent studies have argued instead for quiet giant elliptical galaxies (Dayal et al., 2015), for galaxies whose central black holes stay calm (Mason & Biermann, 2018), and, tentatively, for metal-rich dwarf galaxies as a second favourable group (Stojković et al., 2019). None of these claims has yet been tested by observation. The honest answer is that we do not know which kinds of galaxies host most of the universe’s habitable planets.

06 · Within the Milky Way

A gradient, not a ring

Within our own galaxy, the Galactic Habitable Zone was first drawn as a ring. An influential 2004 model (Lineweaver et al., 2004) placed it between roughly 23,000 and 29,000 light-years from the galactic centre — far enough out to avoid the densest bombardment of exploding stars, close enough in for stars rich in planet-building elements. The Sun sits inside that ring, about 26,700 light-years from the centre (GRAVITY Collaboration, 2019).

Later work has blurred the ring. Some studies found that most of the galactic disk may be suitable for life (Prantzos, 2008). Simulations that let stars drift across the galaxy over billions of years place the best conditions further out (Vukotić et al., 2016). And a calculation of what it would take to sterilise a planet entirely — boiling its oceans — found such events vanishingly rare almost anywhere in the disk (Sloan et al., 2017). What survives is a gradient rather than a boundary: more hazards and more stars toward the centre, fewer of the heavy elements planets need toward the edge. The exact limits depend on each model’s assumptions, and remain debated.

07 · Around the Sun

The stellar habitable zone

Around every star lies a band of orbits where a planet like Earth could keep liquid water on its surface — the circumstellar habitable zone, or Goldilocks zone. Too close and water boils away in a runaway greenhouse, as it seems to have done on Venus; too far and even a thick blanket of carbon dioxide cannot keep it from freezing. Fittingly, the zone’s first name was ecosphere: the space physiologist Hubertus Strughold used it in the 1950s for the zone around the Sun warm enough for life (Strughold, 1953; 1955), before ecologists borrowed the word for the living Earth.

Where exactly the zone begins and ends depends on what one assumes. Drawn from the evidence of Venus and Mars, it reaches from about 0.75 to 1.77 times Earth’s distance from the Sun (Kopparapu et al., 2013; Bohl et al., 2026). Drawn for complex life, it shrinks sharply: animals cannot tolerate the high concentrations of carbon dioxide a planet would need to stay warm near the outer edge, and the zone for complex life may be only a fifth to half as wide as the conventional zone (Schwieterman et al., 2019).

The zone also moves. Stars brighten as they age: when Earth formed, the Sun shone about 30% less brightly than it does today (Gough, 1981). Its habitable zone is creeping outward, and in roughly two to three billion years Earth will be left on its hot inner edge (Rushby et al., 2013). The estimates differ — biology may run out of time sooner than the zone itself, as rising heat speeds the weathering of rock, which pulls carbon dioxide down below the level plants need (Mills et al., 2025) — but all agree that Earth is well past the middle of its habitable life. Smaller, dimmer stars change far more slowly, and their habitable zones last far longer.

Fuelling Earth’s climate, weather systems, and sustaining life itself, the Sun showers our planet with approximately 174 petawatts of energy — its measured brightness at Earth’s distance, about 1,361 watts per square metre, falling on the planet’s disk (Kopp & Lean, 2011). The heat escaping from Earth’s interior amounts to about 47 terawatts (Davies & Davies, 2010), so more than 99.9% of the energy Earth receives comes from the Sun, the power source behind almost every living process on our world.

The habitable zones of other stars are now being searched in earnest. The map below places the candidate worlds of the Planetary Habitability Laboratory’s Habitable Worlds Catalog (PHL, University of Puerto Rico at Arecibo), cross-referenced with the NASA Exoplanet Archive (NASA Exoplanet Science Institute), around the Sun. Its 70 candidates (March 2024 release) are drawn more generously than the 45 rocky planets counted above: they include planets up to about two and a half to three times Earth’s size that may not be rocky at all, so the two numbers differ by design. Some 31 of them lie within 50 light-years of the Sun; the most distant, found by the Kepler space telescope, lie thousands of light-years away.

08 · The planet itself

More than an orbit

Being in the habitable zone is not the same as being habitable. The Moon orbits within the Sun’s habitable zone and is barren (Schulze-Makuch & Guinan, 2016). A planet must be large enough to hold an atmosphere, active enough inside to recycle its crust and sustain a magnetic field, and endowed with water. Scientists weigh unfamiliar worlds in several ways, each measuring something different.

Temperature decides what water does.

Resemblance to Earth can be scored. The Earth Similarity Index blends a world’s size, density, escape velocity and surface temperature into one number between zero and one; because the four are multiplied, a single hostile ingredient drags the whole score down.

Habitable rooms need not lie on the surface. The region inside a world where temperature and pressure alone would let microbes survive — its eirenesphere — may be a thin skin, as on Earth, or a dark ocean beneath kilometres of ice, as on Europa.

Better than Earth? Earth is the only living world we know, so we treat it as the standard. But it may not be the best. A superhabitable world might be a little larger, a little older, and orbit a calmer, longer-lived orange star.

On Earth itself, the surface offers life an extraordinary range of rooms. Topography, the arrangement of Earth’s physical features, plays a critical role in shaping the distribution of life on our planet. The immense variation in elevation, terrain, and geological formations leads to a multitude of distinct habitats, each with its own unique set of conditions. Mountain ranges, valleys, deserts, and coastal areas, for example, exhibit strikingly different physical characteristics. These topographical features directly influence temperature and precipitation patterns, which in turn create the necessary conditions for diverse ecosystems and life forms to evolve and thrive.

Temperature is another crucial factor that contributes to the development of diverse habitats. Due to the Earth’s curvature, its axial tilt and the varying intensity of solar radiation received at different latitudes, there is a wide range of temperature zones across the planet. From the scorching heat of equatorial regions to the freezing cold of polar areas, these temperature variations foster the establishment of distinct ecosystems. Moreover, microclimates created by local topographical features, such as mountains or bodies of water, allow for pockets of unique habitats to exist. This diversity in temperature conditions ultimately supports a wide array of species adapted to live in their respective environments, further promoting the rich biodiversity found on Earth.

Precipitation is yet another essential component in the formation of diverse habitats. The global distribution of rainfall and snowfall is largely determined by atmospheric circulation patterns, which are influenced by factors such as topography, latitude, and proximity to bodies of water. As a result, precipitation levels can vary dramatically from one location to another, giving rise to diverse ecosystems such as rainforests, savannas, and deserts. The availability of water is critical for life, and different species have evolved to survive in habitats with varying degrees of moisture.

With an average temperature of approximately 15°C, Earth’s climate supports a variety of ecosystems, from the frozen tundra to the lush rainforests, and from the deepest ocean trenches to the highest mountain peaks.

The Earth’s atmosphere plays a crucial role in sustaining life by providing the right balance of gases such as nitrogen (78%), oxygen (21%), and trace amounts of other gases like carbon dioxide, argon, and water vapor. This composition helps regulate the planet’s temperature through the greenhouse effect and protects life from harmful solar radiation. Furthermore, Earth’s magnetic field shields the planet from dangerous solar wind and cosmic rays, making it a suitable environment for life to thrive.

09 · The living planet

Earth is a verb

Seen from far enough away, a biosphere stops being a collection of organisms and becomes a property of the planet. Life on Earth filled the air with oxygen, turned bare rock into soil, drew down carbon into limestone and coal, and shaped the colour of continents and the chemistry of the sea. From another star, the first sign of life on an exoplanet will not be a cell or a creature but the planet itself — an atmosphere held far from chemical balance, a surface that changes with the seasons. Some astrobiologists now describe an Earth-like planet less as a place than as a process: a world with the potential to drive the emergence of persistent systems that coevolve with one another (Wong et al., 2024).

The biosphere’s grip on the planet has deepened over time. Physicist Adam Frank and colleagues describe a sequence of planetary states: worlds without atmospheres; worlds with atmospheres but no life; worlds with a thin biosphere, like the early Earth, where microbes left only a faint mark; worlds with a thick biosphere, like Earth after the rise of oxygen, where life transforms the planet’s whole energy budget. Earth today, they argue, is becoming something new — a hybrid, in which one species’ activity has grown large enough to register alongside the biosphere’s own flows — and, for some elements such as phosphorus, to exceed the natural ones. That is the Anthropocene, seen from space (Frank et al., 2017). Explore the human end of this story in Biosphere Disruptors, and the mass of the living world compared with what we have built on Sfera.

Life itself may also be broader than the version we know. Astrobiologists Stuart Bartlett and Michael Wong proposed spelling it differently — lyfe — for any system that does four things at once: it feeds on a flow of energy, makes more of itself, holds itself steady against its surroundings, and learns. Earth’s life is one instance of lyfe. There may be others we would not recognise at first sight (Bartlett & Wong, 2020).

A single biosphere, then: the only one we know, nearly four billion years in the making, and somewhere past the middle of its time on this planet. What follows is a closer look at its parts — the water it moves, the habitats it builds, the diversity it holds — and at what it would take to keep it.

Water

Water on Earth exists in various forms, constantly cycling through different states and locations, shaping the planet’s diverse ecosystems and playing a critical role in its climate system. Oceans, rivers, and lakes make up the most visible components of the Earth’s hydrosphere, with oceans alone covering about 71% of the planet’s surface and holding roughly 97% of its water. These vast bodies of water are essential for supporting marine life and driving global weather patterns.

Rivers and lakes, though smaller in scale compared to oceans, are vital for sustaining terrestrial ecosystems and providing habitats for countless species of plants and animals. They act as pathways for the transport of water, nutrients, and sediments across the landscape.

Glaciers, massive bodies of ice formed from the accumulation and compaction of snow, store approximately 68.7% of the Earth’s freshwater. Found predominantly in polar regions and high-altitude mountains, they play a significant role in regulating sea levels and influencing regional climate conditions.

Water vapour, the gaseous state of water, is a key component of the Earth’s atmosphere. It contributes to cloud formation, precipitation, and the planet’s natural greenhouse effect. Water vapour also helps to distribute heat around the globe through atmospheric circulation, thereby maintaining the Earth’s overall temperature balance.

Groundwater, is found beneath the Earth’s surface within the pores and fractures of soil, sand, and rock formations. This hidden water reserve is an essential part of the hydrological cycle, as it feeds into rivers, lakes, and oceans, and sustains ecosystems during periods of drought.

Envision all the water on Earth coming together to form a colossal sphere with an impressive diameter of 1,385 kilometres, encapsulating the essence of our planet’s life-sustaining resource.

Next, picture fresh liquid water from the ground, lakes, swamps, and rivers converging into a mid-sized sphere with a diameter of 272.8 kilometres, representing the accessible sources that nourish countless ecosystems.

Finally, imagine the smallest sphere, with a diameter of 56.2 kilometres, symbolizing the lifeblood of Earth’s landscapes: fresh water from lakes and rivers that traverse and vitalize the planet’s surface.

Glaciers, known as the Third Pole, play a crucial role in local climates and freshwater supply. They provide a significant portion of water resources for nearly 20% of the global population, particularly in Asia where around 1.9 billion people depend on rivers originating from the Tibetan Plateau. In reality, glaciers and ice caps hold approximately 1.7% of the Earth’s total water and about 68.7% of its freshwater, with the vast majority of that glacial ice (roughly 99%) locked in the great ice sheets of Antarctica and Greenland. It is essential to preserve these frozen resources for the well-being of ecosystems, climate regulation, and the millions who rely on them for sustenance and livelihood.

Habitats

Earth’s diverse ecosystems, known as biomes, encompass various geographic regions with distinct climatic conditions, vegetation types, and species compositions. Biomes, such as tundras, grasslands, deserts, and wetlands, collectively contribute to the intricate tapestry of life on our planet. Among these, forests, coral reefs, and mangroves stand out as exceptional bastions of biodiversity, hosting an extraordinary array of species that play crucial roles in maintaining ecological balance.

Forests cover approximately 30% of the Earth’s land area, and they are home to more than 80% of terrestrial species. They play a crucial role in climate regulation by absorbing and storing large amounts of carbon dioxide, with tropical forests alone sequestering about 1.4 billion metric tons of carbon annually.

Coral reefs, often referred to as the “rainforests of the sea,” occupy less than 1% of the ocean floor but support nearly 25% of all marine species. The Great Barrier Reef, for example, spans over 2,300 kilometres and harbours 1,500 fish species and 600 types of coral. Coral reefs also provide essential ecosystem services, such as shoreline protection and supporting fisheries.

Mangroves, unique ecosystems found in coastal intertidal zones, cover around 150,000 km2 globally. They serve as vital nurseries for numerous fish and crustacean species, with over 200 fish species relying on these habitats for some part of their life cycle. Mangroves also play a critical role in carbon sequestration, storing up to four times more carbon per unit area than tropical rainforests.

Seamounts play a vital and significant role in supporting marine biodiversity. These underwater mountains act as ecological hotspots, attracting a diverse array of marine life, including various fish species, corals, and other invertebrates. Due to their unique geological structures, seamounts generate nutrient-rich upwellings that promote the growth of phytoplankton, forming the base of the marine food chain. These thriving ecosystems provide critical habitats, feeding grounds, and spawning sites for numerous species, some of which are commercially important. Moreover, seamounts often harbor distinct and endemic species, enhancing overall biodiversity and contributing to our understanding of marine life. By supporting rich, productive ecosystems, seamounts play a crucial role in maintaining the health and resilience of our oceans, which underscores the importance of protecting these unique habitats from human-induced threats.

Biological Diversity

The remarkable diversity of life on Earth is exemplified by the astounding variety of mammal and bird species that inhabit our planet. Mammal richness spans more than 6,000 described species, ranging from minuscule shrews to the colossal blue whale. Equally impressive is the bird richness, with roughly 11,000 described species gracing the skies and populating diverse ecosystems across the globe. These numbers showcase the incredible range of adaptation, ecological niche specialization, and evolutionary history that has shaped the mammalian and avian kingdoms, contributing to the intricate tapestry of life that blankets our world.

Preserving our Ecosphere

Protecting wild land and vast expanses of the planet is crucial for safeguarding Earth’s biodiversity, promoting healthy ecosystems, and bolstering human well-being. Expansive, undisturbed areas serve as essential habitats for innumerable species, including numerous threatened or endangered ones. These habitats contribute to the resilience and delicate equilibrium of ecosystems, where keystone species like large mammals play a pivotal role in shaping the composition of flora and fauna, affecting aspects ranging from water quality to carbon sequestration. Additionally, these untouched lands function as natural carbon sinks, counteracting the repercussions of climate change by absorbing and storing carbon dioxide. They also deliver indispensable ecosystem services such as water filtration, pollination, and erosion prevention, which yield direct benefits to human populations. As a result, the protection of wild lands represents not only an ethical obligation to preserve our planet’s natural legacy, but also a practical strategy for ensuring long-term ecological and economic sustainability.

An important 2022 paper reveals that by reintroducing just 20 large mammal species, such as brown bear, bison, lion, jaguar, and wild horse, over 8.5 million km² of land can regain intact large mammal communities. Large mammals are essential for maintaining healthy ecosystems, providing key ecosystem services that humans depend on. Currently, less than 15% of the world’s land retains complete large mammal assemblages, but rewilding efforts could restore more than half of the world’s ecoregions by reintroducing only 1-3 species in strategic locations.

Reintroducing species has been proven effective, as seen in the successful reintroduction of wolves in the US, which resulted in increased carbon storage, better water quality, and millions of dollars in economic benefit. By focusing on 20 key species, the study suggests that large mammal assemblages can be restored across 8.5 million km2 of land area, including 3.2 million km2 in North America, 2 million km2 in Latin America, 416,000 km2 in Africa, 944,000 km2 in Europe, and 1.9 million km2 in Asia.

Discover more by going to our Global Safety Net page.

By restoring degraded land, we can enhance the productivity and resilience of ecosystems, support biodiversity, and improve climate change mitigation through carbon sequestration. A shift in land use, particularly by reducing meat consumption, can help alleviate pressure on natural resources, as livestock farming is a major driver of deforestation, habitat loss, and greenhouse gas emissions. Adopting plant-based diets or reducing meat intake can free up land for reforestation, afforestation, or other nature-based solutions. The Half-Earth Project, which proposes conserving half of the Earth’s land and sea to ensure the survival of the majority of species and maintain ecosystem health, further underscores the importance of protecting and restoring natural habitats. By integrating these approaches, we can create a synergistic effect that promotes biodiversity, mitigates climate change, and fosters a more sustainable coexistence between humans and nature.

Protected Areas (PAs) and Key Biodiversity Areas (KBAs) are vital instruments in the global effort to conserve Earth’s biodiversity and maintain healthy ecosystems. As of 2024, about 17.6% of the world’s land and inland waters, and 8.4% of the ocean, lie within documented protected and conserved areas — a significant safeguard for habitats and the species that reside in them, though still far short of what is needed. KBAs, identified as sites that contribute significantly to the global persistence of biodiversity, now number 16,602 worldwide, spanning more than 22 million km² of land and sea. By focusing on the conservation of these critical areas, we can make substantial progress towards the Kunming-Montréal Global Biodiversity Framework’s Target 3 — protecting at least 30% of the planet’s land, freshwater, and sea by 2030, an agenda widely known as 30×30. Strengthening the management and expansion of protected areas and KBAs is essential to safeguarding our planet’s rich biodiversity and ensuring long-term ecological sustainability.