PNNL

Abstract

Observations collected over recent decades indicate that, as the Earth's surface has warmed, its tropical belt has widened. This poleward expansion of dry belts on either side of the Equator has the potential to impact a large fraction of the world's population, and its timing begs the question of whether it is caused by human activity. While theory and numerical modeling suggest that increasing greenhouse gas concentration should widen the tropics, the observed widening is near the upper range of model simulations. The mixture of forced changes and natural swings in climate, along with the challenge of observing the width of the tropical belt itself, complicate the attribution of tropical widening. In this article, we review the possible causes of tropical widening, including greenhouse gas concentration increases from a growing body of attribution research. We find that, while the tropics have been widening, this widening is likely smaller than previously thought, and the widening due to climate change will only become clear in coming decades.