News

According to a new study in the Journal of Geophysical Research: Atmospheres, Earth’s tropical region is expanding towards the poles and that expansion is being driven by human-induced changes to the oceans—as opposed to direct changes to the atmosphere.

The warm, wet belt that wraps around the planet—where the vast majority of the world’s coral reefs are found—has a lush interior with hot, arid edges, and these parched outer bands have been gradually expanding poleward over the past 15 years. Scientists have noticed that these dry areas have extended more in the Southern Hemisphere compared to the Northern Hemisphere, and the new research explains why: Ocean warming is driving the change and the shift is larger in the Southern Hemisphere because it has more ocean surface area.

The expansion of the tropics was first revealed in a scientific paper back in 2006, but climate models using different anthropogenic drivers—greenhouse gas emissions, ozone depletion—have been underestimating the speed of the shift and have not been able to account for why expansion is happening in some regions but not others. The new work instead analyzed water temperatures in the major ocean gyres, and it was discovered that tropical expansion was occurring in places where the gyres were also moving poleward.

Tropical expansion is bad news for regions such as California and Australia, where the phenomenon could shift storm paths, drive more severe wildfires, and cause harsher droughts—impacts many would argue are already being felt.