Tropics

The tropics constitute the broad geographical zone of Earth lying between the Tropic of Cancer in the Northern Hemisphere and the Tropic of Capricorn in the Southern Hemisphere. This region surrounds the Equator and is distinguished by the possibility of the Sun appearing directly overhead at least once each year. The position of the tropical boundaries reflects the Earth’s axial tilt, and thus these latitudes change slightly over time. Known also as the torrid zone, the tropics receive the greatest annual amount of solar energy and exhibit some of the highest temperatures on the planet. Despite this heat, the region contains an extraordinary range of climates and ecosystems, from rainforests to deserts and alpine environments.

Astronomical Basis and Definition

The defining characteristic of the tropics is tied to the Sun’s angle relative to Earth. Because the planet is tilted on its axis, only within this equatorial belt can the Sun ever stand at the zenith. The width of the tropics is approximately twice the value of the Earth’s axial tilt. The two limiting parallels of latitude are:

• the Tropic of Cancer in the Northern Hemisphere
• the Tropic of Capricorn in the Southern Hemisphere

These boundaries correspond to the maximum latitudes where the Sun may appear directly overhead. They are not immobile; the Earth’s axial tilt undergoes slow variation, causing the tropical and polar circles to migrate over time.
The term tropic originates from an Ancient Greek word meaning turn or change of direction, acknowledging the apparent turning points of the Sun in its annual cycle.

Solar Energy and Seasonal Patterns

Because the Sun remains high in the sky throughout the year, the tropics receive intense insolation. Annual temperature contrasts are therefore minimal, and classical temperate seasons of winter and summer are muted or absent. Instead, seasonal divisions rely largely on precipitation patterns, most commonly the distinction between wet and dry seasons.
Many tropical areas experience heavy rainfall during a defined wet season that may last for several months. This pattern is shaped by the annual migration of the Intertropical Convergence Zone (ITCZ) or monsoon trough. Under the Köppen climate classification, a region qualifies for a tropical climate if at least one month registers an average precipitation of 60 mm or more. In some areas, mid-season breaks in rainfall occur when the ITCZ shifts poleward during the warmest period.
The wet season brings expanded vegetation growth, improved air and water quality, and often increased crop yields. However, the intense rainfall may lead to flooding, erosion, and greater incidence of waterborne diseases such as malaria. The preceding dry season typically produces food shortages as new crops have yet to mature.

Diversity of Local Climates

Despite being unified by solar geometry, the tropics contain an immense range of climates. These include:

• tropical rainforests, characterised by year-round rainfall and high biodiversity
• seasonal tropical forests, which experience alternating wet and dry periods
• monsoonal climates, where seasonal winds govern precipitation cycles
• savannahs, dominated by grasses and scattered trees
• deserts, such as the Sahara or Atacama, found in the tropics due to atmospheric subsidence
• alpine climates, occurring on high mountains such as Kilimanjaro or the Andes

Climatic variation is influenced by altitude, continentality, sea-surface temperatures, and atmospheric circulation patterns.
Large regions of the geographical tropics do not qualify as tropical climates under Köppen’s system. Vast deserts in Africa, Australia, and South America fall within the tropical latitudes but are classified as arid or semi-arid climates. At high altitudes within tropical latitudes, alpine tundra and year-round snowfields may be present.

Human Populations and Land Area

The tropics cover around 39.8 per cent of Earth’s total surface area and contain approximately 36 per cent of its landmass. As of the early twenty-first century, about 40 per cent of the world’s population lived within the tropics, with projections suggesting this figure could reach half of the global population by mid-century. This demographic prominence reflects both historical settlement patterns and the presence of major developing regions in Asia, Africa, and Latin America.

Climate Change and Environmental Stress

Climate change is altering conditions within the tropics. Although long-term warming has been slightly less than the global average, with an increase of around 0.7–0.8°C over the past century, variability driven by the El Niño–Southern Oscillation complicates the detection of trends. Strong El Niño events have produced markedly warm years, most notably 1998.
Climate models project further warming of 1–2°C by 2050, rising to 1.4–4°C by 2100, depending on emissions pathways. The changing climate may produce more frequent heatwaves, stronger storms, and shifts in rainfall patterns. Some subtropical regions adjacent to the tropics are already experiencing these changes, raising concerns that certain areas may become less habitable.

Ecosystems and Biodiversity

Tropical ecosystems rank among the most biologically diverse on Earth. They include rainforests, dry broadleaf forests, savannahs, spiny forests, deserts, and grasslands. Biodiversity hotspots with high species endemism include:

• El Yunque National Forest in Puerto Rico
• Central American and Nicaraguan rainforests
• the Amazon Rainforest, stretching across several South American nations
• Madagascar’s dry deciduous forests and eastern rainforests
• the Waterberg Biosphere in South Africa

Despite their biological richness, tropical soils are often nutrient-poor. Rapid nutrient cycling maintains forest productivity, but this also renders the environment vulnerable to slash-and-burn agriculture and other forms of land clearance.
Biogeographically, the tropics divide into two broad realms:

• the Paleotropical Kingdom, covering Africa, Asia, and Australia
• the Neotropical realm, including the Caribbean, Central America, and South America

Together, these regions are sometimes called the Pantropics.

Flora of the Tropics

Tropical regions produce a wide array of distinctive flora, including fruit-bearing plants such as citrus varieties (oranges, lemons, and mandarins) and other economically important fruits such as mango, avocado, and sapote. Many of these plants have spread widely due to cultivation but remain strongly associated with tropical origins.

Tropicality

The notion of tropicality refers to the cultural and imaginative representations of the tropics created by those outside the region. This concept encompasses idealised depictions of the tropics as a paradisiacal Garden of Eden, characterised by lush vegetation and exotic wildlife, as well as contrasting portrayals of the tropics as wild, dangerous, or unruly. Such imagery featured prominently in Western literature of the nineteenth and early twentieth centuries.
Scholarly interest in tropicality grew particularly after the publication of Pierre Gourou’s work Les pays tropicaux (The Tropical World) in the late 1940s. Over time, interpretations of the tropics have become more nuanced, integrating ecological, cultural, and historical perspectives. Earlier theories that linked climatic conditions directly to social or civilisational development have been largely superseded by more complex understandings of environmental and human interactions.