More than 2000 years ago Aristotle described the world as being divided into three zones – the Frigid Zone, the Temperate Zone and the Torrid Zone – deeming that humans could only live and work productively in the temperate zone. Following Aristotle, for much of western history the Tropics has been viewed as a place of pestilence and, from time to time, as a place of paradise, but rarely has it been considered on its own terms. While Aristotle’s negative view of the Tropics was misguided, modern approaches to understanding the world – as east/ west, north/ south and developed/ developing – have papered over Aristotle’s critically important insight about the lateral nature of the world. While there is great diversity across the nations and peoples of the Tropics, climate and geography mean they share many challenges and opportunities.
The Tropics is commonly defined as the region of the Earth surrounding the Equator within the latitudes of the Tropics of Cancer and Capricorn at +/– 23.5 degrees. With its origins in astronomy, these latitudes are the limit of where the Sun reaches a point directly overhead at least once during the solar year, and are used to define the Tropics in this paper.
Although topography and other factors contribute to climatic variation, tropical regions are typically warm and experience little seasonal change in day-to-day temperature. An important feature of the Tropics is the prevalence of rain in the moist inner regions near the equator, and that the seasonality of rainfall increases with distance from the equator.
In the Köppen-Geiger climate classification the Tropics is dominated by ‘equatorial’ and ‘arid’ climates, with the balance of the world being primarily ‘warm temperate’, ‘snow’ and ‘polar’ climates. Equatorial climates have a mean temperature for all months above 18°C (64°F), and arid zones are defined with reference to both temperature and rainfall, but are characterised by a lack of water which inhibits plant and animal life.
Nations identified as being in the Tropics for the purposes of the Report are listed in the project overview.