Man-made pollution is helping to push the tropics northwards, research suggests.
The effect could impact weather and climate, making sub-tropical regions drier and creating wetter and stormier conditions further north.
Scientists already knew that the tropics have been widening by around 0.7 degrees of latitude per decade.
Ozone depletion in the stratosphere is thought to be the main driver of this expansion in the southern hemisphere.
But the new findings indicate that tropic widening in the northern hemisphere is mainly due to black carbon and ozone lower in the atmosphere.
While stratospheric ozone provides vital protection against harmful solar radiation, the same gas in the lower troposphere is a man-made pollutant and harmful to health.
Professor Robert Allen, from the University of California at Riverside, who led the climate modelling study, said: "Both black carbon and tropospheric ozone warm the tropics by absorbing solar radiation.
"Because they are short-lived pollutants, with lifetimes of one-two weeks, their concentrations remain highest near the sources: the northern hemisphere low-to-mid-latitudes. It's the heating of the mid-latitudes that pushes the boundaries of the tropics poleward."
Tropical expansion northwards could affect large-scale atmospheric circulation, especially in the subtropics and mid-latitudes, said the researchers, whose findings are reported in the journal Nature.
"If the tropics are moving poleward, then the sub-tropics will become even drier," said Dr Allen. "If a poleward displacement of the mid-latitude storm tracks also occurs, this will shift mid-latitude precipitation poleward, impacting regional agriculture, economy, and society."