Air pollution may
be driving expansion of tropics
17 May 2012 The University of New
Black carbon aerosols and ozone, both man-made pollutants emitted predominantly
in the Northern Hemisphere’s low- to mid-latitudes, are most likely pushing
the boundary of the tropics further poleward in that hemisphere, new research by
a team of scientists shows.
While depletion of the ozone
layer high up in the stratosphere has already been shown to be the primary
driver of the expansion of the tropics in the Southern Hemisphere, the
researchers are the first to report that black carbon and ozone pollution nearer
the Earth’s surface are probably doing likewise in the Northern Hemisphere.
The study results appear in the
May 17 issue of the journal Nature. Climatologist Robert J. Allen, at the
University of California, Riverside, led the team. It included Professor Steven
Sherwood, co-director of the UNSW Climate Change Research Centre.
The observed tropical belt
expansion by 0.7 degrees latitude per decade has been greater than climate
models predicted, but this study considered newer estimates of regional trends
in air pollution. When this data was included, the models more closely agreed
with the observations.
The researchers note that an
unabated tropical belt expansion would have an impact on large-scale atmospheric
circulation, especially in the subtropics and mid-latitudes.
“Both black carbon and
tropospheric ozone warm the tropics by absorbing solar radiation,” Allen says.
“Because they are short-lived pollutants, with lifetimes of one to 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.
“If the tropics are moving
poleward, then the subtropics will become even drier. 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.”
Climate scientists have observed
that the tropics have widened by 0.7 degrees latitude per decade in recent
times, with warming from greenhouse gases also contributing to the expansion in
“This has typically been blamed
on global warming, although in the Southern Hemisphere some studies have
suggested the ozone hole may be playing a lead role,” says Sherwood.
“We have confirmed the finding
that ozone loss has been the main driver in the Southern Hemisphere, but also
that air pollution trends - mainly increases in China and to some extent India -
appear to have been the dominant counterpart driver in the Northern Hemisphere.
Both are beating out global warming driven by greenhouse gas increases.
“The ability of air pollutants
to cause such an expansion derives from the geographic pattern of the changes,
with increased emissions from Asia and decreased emissions from Europe, rather
than any trend in the worldwide amount of pollution.”