boundaries of the tropics, defined by temperature, rainfall, wind, and ozone patterns, have shifted poleward by at least 2 degrees latitude in the last 25 years.
It could be warming of the ocean surface, ozone depletion, El Nino changes, or climate change in the stratosphere (NOAA), among other ideas. In fact, the review's lead author "said this expansion may only be temporary, but there's no way of knowing yet"
Of particular concern are the semi-arid regions poleward of the subtropical dry belts, including the Mediterranean, the southwestern United States and northern Mexico, southern Australia, southern Africa, and parts of South America. A poleward expansion of the tropics is likely to bring even drier conditions to these heavily populated regions, but may bring increased moisture to other areas.
An increase in the width of the tropics could bring an increase in the area affected by tropical storms, or could change climatological tropical cyclone development regions and tracks.