Signs of a very different 21st-century climate are already showing up, and not just in the melting arctic. A new review in Nature Geoscience highlights reports that the 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. According to climate models, that’s as far as the tropical belt was supposed to creep by the end of this century. Five different methods used to measure the tropics all show more or less breakneck rates of expansion — one gives as much as 4.8 degrees in 25 years.
The problem with the observations being much more radical than the model predictions is that the scientists don’t know why this is happening. 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” (AP). If the trend continues, though, it will bring a lot of bad news. As with ice melt, the long-term worst-case scenario includes an irreversible climate tipping point from altered ocean circulation. Less remotely, the review warns:
The edges of the tropical belt are the outer boundaries of the subtropical dry zones and their poleward shift could lead to fundamental shifts in ecosystems and in human settlements. Shifts in precipitation patterns would have obvious implications for agriculture and water resources and could present serious hardships in marginal areas. 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. Widening of the tropics would also probably be associated with poleward movement of major extratropical climate zones due to changes in the position of jet streams, storm tracks, mean position of high and low pressure systems, and associated precipitation regimes. 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.
Image credit: Nature Geoscience doi:10.1038/ngeo.2007.38 (2007)