A new research has suggested that the three-year drought in California, US, may be a consequence of the expanding tropics, which are a gradual result of human emissions of greenhouse gases that are warming the planet.
Climate scientists have documented a slow progression of low-latitude weather systems towards the poles, and this has been matched by rising temperatures in many temperate regions.
According to a report in New Scientist, deciding whether this broadening of the tropical belt is linked to the greenhouse effect has been difficult, however.
Part of the reason is that there are many ways of defining the tropics, explained Thomas Reichler of the University of Utah.
Geographically, the tropical belt is contained between the Tropics of Cancer and Capricorn. It is also the region on either side of the equator where temperatures tend to be hot and humid all year.
But, the simplest and most easily tracked characteristic of the tropics lies high above, at the boundary between the troposphere, where weather systems form, and the stratosphere above it.
Over the tropics, the tropopause, as this boundary is known, tends to lie several kilometers higher up in the atmosphere.
The change in altitude is relatively easy to measure.
"It is much more difficult to detect significant changes in the lower levels of the atmosphere and surface rainfall pattern," said Jian Lu of the US National Center for Atmospheric Research in Colorado.
Using the tropopause, Lu and Reichler tracked the position of the tropical belt since the 1960s and found it has slowly been getting wider.
"There is a lot of natural variation from year to year, but we see a slow, gradual change," said Reichler.
On average, the tropical boundaries are moving 0.7 degrees towards the poles each decade. This amounts to roughly 70 kilometers per decade, or 350 kilometers in 50 years.
The team then plugged their data into a leading climate model. If the model included human emissions, it matched the real data. Without the emissions, it didn't.
"Our main conclusion is that greenhouse gases and [the depletion of stratospheric] ozone are the culprits for the widening," said Lu. "These two work in the same direction, both pushing the boundary of the tropics polewards," he added.
According to Reichler, the expansion of the subtropics is more feared, with climate models quoted by the Intergovernmental Panel on Climate Change (IPCC) predicting that the Mediterranean region and the south-west of the US are heading towards devastating droughts.
Reichler said that this latest study suggests this is a result of the poleward march of the tropics.