Biomes are shaped by a layered set of physical and biological forces, starting with how much solar energy a region receives and extending through atmospheric circulation, mountain barriers, ocean currents, soil chemistry, fire regimes, and even the evolutionary history of the plants that live there. No single factor works alone. The interplay between these drivers explains why a patch of land at one latitude and elevation supports tropical rainforest while another at the same latitude but on the opposite side of a mountain range is bone-dry desert. Understanding the full set of contributors also reveals why biome boundaries are not fixed lines on a map but shifting, contested zones that respond to both geological timescales and rapid human alteration.
Solar Energy and Latitude
The sun is the starting point for almost everything that determines a biome. The amount of solar radiation a place receives depends primarily on its latitude: regions near the equator get intense, near-vertical sunlight year-round, while polar regions receive the same energy spread over a much larger surface area and lose much of it during months of darkness. This gradient in incoming energy drives the basic temperature differences between the tropics and the poles, which in turn determine how much moisture the atmosphere can hold and how fast plants can grow.
In Angola, for example, the interplay of latitude, altitude, and seasonal shifts in sunlight produces an entire spectrum of biomes within a single country, from lowland rainforest in the north to arid savanna and desert in the south. The seasonal swings in temperature and rainfall that create these patterns are themselves powered by how the Earth’s tilt changes the angle of incoming sunlight over the course of a year.1Springer Link. Solar Energy, Temperature and Rainfall This solar-driven gradient is so fundamental that climate classification systems almost always begin with latitude bands.
Atmospheric Circulation and the Hadley Cell
Raw sunlight alone does not decide where it rains and where it does not. The atmosphere redistributes that solar energy through massive circulation loops, and the most influential of these is the Hadley cell. In this system, intensely heated air near the equator rises, sheds its moisture as heavy tropical rainfall, then flows poleward at high altitude, cools, and sinks back to the surface around 30 degrees latitude. Where the air descends, it compresses and dries out, which is why so many of the world’s great deserts, from the Sahara to the Arabian Peninsula to the Sonoran, sit in a rough band around that latitude. The Hadley cell is considered the primary cause of both tropical rainforest and subtropical desert formation.2Journal of Climate. Influence of Baroclinic Eddies on the Hadley Cell Edge
Beyond the Hadley cell, mid-latitude weather is driven by a different regime of swirling storm systems (sometimes called baroclinic eddies) that mix warm tropical air with cold polar air. These storms bring the variable, season-driven precipitation that supports temperate forests and grasslands. Closer to the poles, the air is cold and dry enough that even modest precipitation sustains tundra rather than forest. The boundaries between these circulation regimes roughly correspond to the boundaries between biome types, though the correspondence is far from neat because other factors intervene.
Seasonal Shifts and the Monsoon Connection
Atmospheric circulation is not static. One of the most dramatic seasonal changes involves the Intertropical Convergence Zone (ITCZ), a belt of low pressure near the equator where trade winds from the northern and southern hemispheres collide. As the ITCZ migrates north and south with the seasons, it delivers intense wet seasons to regions that would otherwise be semi-arid. In South Asia, the ITCZ undergoes a pronounced northward shift during the summer monsoon, driven by large-scale atmospheric dynamics that pull moisture-laden air deep into the continent.3Atmospheric Research. Climatological characteristics of ITCZ over the South Asian monsoon domain This seasonal pulse of rain sustains monsoon forests and tropical grasslands across a huge swath of southern and southeastern Asia. Without the ITCZ’s migration, much of India and mainland Southeast Asia would be far drier, and the biomes there would look very different.
A similar seasonal dynamic shapes the savannas of sub-Saharan Africa and northern Australia. In these regions, the wet season arrives when the ITCZ passes overhead, and the dry season sets in when it retreats. The length and intensity of these wet and dry seasons determine whether a region supports dense woodland, open grassland, or something in between.
Mountains and the Rain Shadow Effect
Topography can override latitude. When moisture-laden air encounters a mountain range, it is forced upward, cools, and drops its rain on the windward slopes. By the time the air descends on the opposite side, it has lost most of its moisture, creating a dry “rain shadow.” This effect is powerful enough to place lush forest on one side of a mountain and near-desert on the other, sometimes only tens of kilometers apart.
The uplift of the northern Tibetan Plateau, for instance, caused large reductions in annual precipitation across inland Asia to its north, mainly by enhancing the rain shadow and altering regional wind patterns.4Quaternary Science Reviews. Impacts of uplift of northern Tibetan Plateau and formation of Asian inland deserts on regional climate and environment Similarly, the Sierra Nevada in western North America creates a pronounced rain shadow that has helped maintain arid conditions in the Great Basin for millions of years.5Tectonics. Stable isotopic evidence for a Pre‐Middle Miocene rain shadow in the western Basin and Range Modeling work has shown that once a mountain range grows tall enough, downstream precipitation can essentially vanish, with cloud mass on the lee side dropping by as much as 90%.6Journal of Geophysical Research: Earth Surface. Rain shadow development during the growth of mountain ranges
Altitude also introduces its own biome layering independent of rain shadows. In Taiwan, vegetation mapping reveals clear altitudinal zonation: subalpine communities give way to montane cloud forests, then to subtropical mountain forests in the north and tropical montane forests in the south.7Applied Vegetation Science. Climate‐based approach for modeling the distribution of montane forest vegetation in Taiwan Climbing a tall tropical mountain can feel like traveling from the equator to the subarctic in the span of a few thousand meters, because temperature drops with elevation at a fairly predictable rate. Coriolis forces can even affect precipitation patterns on different flanks of elongated mountain ranges, contributing to asymmetric vegetation on opposite sides.8Quarterly Journal of the Royal Meteorological Society. Coriolis effects on orographic and mesoscale flows
Ocean Currents and Distance From the Coast
The ocean acts as both a thermostat and a moisture delivery system. Warm currents (like the Gulf Stream in the Atlantic) transfer tropical heat poleward, keeping coastal climates milder than they would otherwise be. Cold currents running along continental west coasts, such as the Humboldt Current off South America or the Benguela Current off southwestern Africa, cool the air above them and suppress rainfall, contributing to coastal deserts. Along the hyperarid Atacama Desert coast, where annual rainfall is negligible, the Pacific Ocean still delivers moisture in the form of marine advective fog. This fog supports highly specialized ecosystems dominated by the bromeliad Tillandsia landbeckii, which survives by harvesting water directly from foggy air.9SpringerLink (Plant Systematics and Evolution). Spatial distribution and interannual variability of coastal fog and low clouds cover in the hyperarid Atacama Desert These fog-dependent ecosystems fluctuate with global climate patterns like El Niño, a reminder that even niche biomes are wired into planetary-scale processes.
Distance from the ocean matters independently of currents. Continental interiors tend to have hotter summers, colder winters, and less total precipitation than coastal areas at the same latitude, because land heats and cools faster than water and because moisture decreases the farther air travels over land. A recent global analysis found the highest continentality values over arid inland regions, with an absolute maximum of about 50 days of air-mass travel time from the ocean over the Taklamakan Desert in central Asia. Seasonal variation is also striking, with summer values often roughly double those in winter, except in areas where monsoon systems temporarily pull marine air deep inland.10Geophysical Research Letters. A New Climatology of Continentality Based on Lagrangian Air Mass Travel Times From the Ocean This gradient from maritime to continental climate helps explain why the interiors of large continents tend to support grasslands and deserts rather than forests.
Soil as a Biome Filter
Climate sets the broad envelope of possibilities for a region, but the soil underfoot often decides which biome actually takes hold within that envelope. Two places with nearly identical temperature and rainfall can support completely different vegetation if one has deep, nutrient-rich clay soils and the other sits on nutrient-poor sand. Research on Brazilian biomes illustrates this neatly: a single biome type can span a wide range of climatic conditions but occupy only a narrow range of soil types, or the reverse. Combining soil properties with climate data produces substantially better predictions of biome distribution than climate alone.11PubMed. Combining climatic and soil properties better predicts covers of Brazilian biomes
In the Brazilian Cerrado, for example, the climate could theoretically support forest, but the aluminum-rich, acidite soils favor fire-tolerant grasses and shrubs instead. In parts of the Amazon basin, patches of white-sand soil support stunted, nutrient-poor “campina” vegetation even though surrounding areas with richer soil support towering rainforest. Soil drainage matters too: waterlogged soils in otherwise forested climates produce wetlands and peatlands rather than upland forest. Geology and soil are sometimes called the “forgotten” biome determinants because they operate quietly beneath the more visually obvious forces of temperature and rainfall.
Fire, Grazing, and Disturbance Regimes
Some biomes exist precisely because they are periodically disturbed. Savannas and grasslands, which together cover a vast share of the Earth’s land surface, depend on fire and herbivory to prevent tree cover from closing in. Without regular burning or grazing, many grasslands would gradually become woodland, and many savannas would thicken into forest. A long-term study in Minnesota oak savannas found that frequent burning, at least three fires per decade, prevented the development of a sapling layer and kept canopy cover from filling in over a 32-year period.12Ecological Applications. Prescribed Fire in Oak Savanna: Fire Frequency Effects on Stand Structure and Dynamics
Grazing adds another dimension. When fire and grazing work together, they appear to control woody encroachment far more effectively than fire alone. A study of savanna management found that burn-only treatments actually increased woody plant establishment fourfold compared to treatments that combined burning with grazing. The movement of large herbivores decreased the frequency of woody species that spread vegetatively, and grazed plots accumulated more fuel that was subsequently burned, keeping shrubs shorter.13Ecosphere. Restoring the fire–grazing interaction promotes tree–grass coexistence by controlling woody encroachment This fire-grazing interaction is ancient. For millions of years, grasslands and savannas co-evolved with both lightning-ignited fire and herds of large herbivores. Removing either one can tip the system toward a different vegetation state.
Evolutionary History and Biome Assembly
Biomes are not simply imposed on passive landscapes by climate. The organisms that define each biome carry evolutionary baggage that shapes where they can and cannot persist. Plants in the succulent biome, the dry tropical regions dominated by fleshy-stemmed species like cacti and euphorbias, provide a striking case. Despite being scattered across multiple continents, succulent biome plants show strong evolutionary convergence in drought-related traits. Research has found that the succulent biome forms a coherent evolutionary arena for drought-adapted lineages that have conserved their biome affinity across continents over deep time, in contrast to savannas, which were assembled by repeated local recruitment of lineages from adjacent biomes.14Global Ecology and Biogeography. Biomes as evolutionary arenas: Convergence and conservatism in the trans‐continental succulent biome
This distinction matters because it means not all biomes respond to environmental change in the same way. A biome whose characteristic lineages readily shift between vegetation types may reassemble quickly under new climate conditions. A biome whose lineages are deeply conservative, having stayed in the same ecological niche for tens of millions of years, could be more vulnerable to rapid shifts because its plants lack the evolutionary flexibility to colonize novel conditions.
Permafrost and the Frozen Boundary
In high-latitude regions, the presence or absence of permanently frozen ground (permafrost) acts as a hard constraint on vegetation. Trees struggle to root in soil where only the top meter or so thaws each summer. As permafrost thaws, though, the rules change. Experimental work has shown that deep-rooting plant species can rapidly exploit newly thawed soil, accessing organic matter that was previously locked away from biological processes. Root growth tracks the seasonally receding thaw front, meaning plants effectively chase the permafrost downward as it melts.15PubMed. Dwelling in the deep – strongly increased root growth and rooting depth enhance plant interactions with thawing permafrost soil This interaction has direct implications for the boundary between tundra and boreal forest: as permafrost degrades under warming temperatures, trees gain access to deeper soil, potentially allowing forest to advance into what was previously treeless terrain.
The Human Footprint
For most of Earth’s history, biomes were shaped entirely by the physical and biological processes described above. That is no longer the case. Human land use has become one of the dominant forces shaping global vegetation patterns. By one comprehensive estimate, nearly 39% of the Earth’s ice-free land surface has been converted to agriculture and settlements. An additional 37% of land that has not been directly converted now sits embedded within agricultural and settled landscapes, altering its ecological character even without being plowed or paved.16Global Ecology and Biogeography. Anthropogenic transformation of the biomes, 1700 to 2000 These “anthromes,” or anthropogenic biomes, now cover most of the terrestrial biosphere and have done so increasingly over thousands of years.17Land. Anthropogenic Biomes: 10,000 BCE to 2015 CE
Human alteration does not just replace biomes wholesale. Fragmentation, selective logging, altered fire regimes, introduction of non-native grazers, and nutrient pollution all reshape the composition and function of remaining natural vegetation. A temperate grassland that is no longer burned or grazed may look “natural” on a satellite image but function very differently from its historical state. Tropical forests that are selectively logged often shift toward a more open, vine-dominated structure. In many parts of the world, the biome you see today reflects centuries of human management as much as it reflects climate and soil.
Climate Change and Shifting Biome Boundaries
As global temperatures rise and precipitation patterns shift, biome boundaries are already moving. Climate change has triggered poleward expansions in the distributions of various groups of organisms, including tree species that are beginning to colonize higher latitudes as conditions warm.18PubMed. Potential migration pathways of broadleaved trees across the receding boreal biome under future climate change Modeling studies project substantial shifts in vegetation over the coming decades. In tropical regions, increasing aridity may convert roughly 170,000 square kilometers of tropical forest to savanna by 2080. Around the Arctic Circle, tundra is projected to give way to boreal forest across some 240,000 square kilometers over the same period.19PubMed Central. Biomes of the world under climate change scenarios: increasing aridity and higher temperatures lead to significant shifts in natural vegetation Under high-emissions scenarios, the tundra biome could shrink by up to 47%, while various dryland biomes expand overall.20Scientific Reports. Climate vulnerability of Earth’s terrestrial biomes
These shifts are not guaranteed to be smooth. Vegetation often lags behind climate because trees grow slowly, soils take centuries to develop, and disturbance patterns need time to adjust. A region whose climate has already become suitable for forest may remain grassland for decades if no seed sources are nearby or if fire continues to suppress tree establishment. This lag creates “climate debt,” where current vegetation no longer matches the climate it is living in but has not yet been replaced.
The Ghost of Ice Ages Past
Today’s biome map also carries the fingerprints of past climates, especially the glacial periods of the Pleistocene. During the Last Glacial Maximum roughly 20,000 years ago, ice sheets covered much of the northern continents, sea levels were far lower, and many biomes were pushed into smaller, fragmented refugia. The genetic signatures of those refugia persist in modern species. A study of boreal land snails found a striking correspondence between the number of distinct genetic lineages within modern species and the number of discrete subpopulations predicted by habitat models for the Last Glacial Maximum. Similar patterns show up in insects, birds, mammals, amphibians, and plants that experienced elevated rates of diversification during Pleistocene glacial cycles.21PubMed Central. The ghost of ice ages past: Impact of Last Glacial Maximum landscapes on modern biodiversity
This means that the species composition within any modern biome is not simply a product of today’s climate. It reflects which populations survived in refugia, which corridors allowed recolonization as glaciers retreated, and which barriers kept populations isolated long enough to diverge. Regions that served as refugia often have higher genetic diversity today and may respond differently to future climate change than areas that were recolonized more recently. The past is, in a real sense, still alive in the distribution of life around the planet.