The boundary of the Earth's tropics is governed by a fundamental physical law rather than just weather patterns and plant life.
April 23, 2026
Original Paper
First-principles theory for Earth's tropical-midlatitude climate boundary
EarthArXiv · 10.31223/X5JZ01
The Takeaway
Geography books usually define the tropics based on arbitrary lines of latitude or average rainfall. This new theory uses a dimensionless parameter based on the first law of thermodynamics to pin down the exact edge of the tropical zone. It proves that the expansion of the tropics is a predictable response to changes in global energy levels. This shift from description to physics allows for much more accurate predictions of how climate change will move these boundaries. Knowing the true physical limit of the tropics is essential for protecting the world's most vulnerable ecosystems.
From the abstract
In Earth's climate, the boundary between the tropics and midlatitudes is a key determinant of temperature and precipitation characteristics, influencing human societies through daily weather, atmospheric chemistry, carbon cycling, and vegetation distribution. The physical origin of these climate zones has been investigated through idealized simulations, observations, and state-of-the-art climate models. However, a first-principles theory to explain the boundary position remains largely unexplore