Spawning droughts, floods, and other weather disturbances
world-wide, the El Niño – Southern Oscillation (ENSO) impacts the daily life of
millions of people. During El Niño, Atlantic hurricane activity wanes and
rainfall in Hawaii decreases while Pacific winter storms shift southward,
elevating the risk of floods in California.
The ability to forecast how ENSO will respond to global
warming thus matters greatly to society. Providing accurate predictions,
though, is challenging because ENSO varies naturally over decades and
centuries. Instrumental records are too short to determine whether any changes
seen recently are simply natural or attributable to man-made greenhouse gases.
Reconstructions of ENSO behavior are usually missing adequate records for the
tropics where ENSO develops.
Help is now underway in the form of a tree-ring record
reflecting ENSO activity over the past seven centuries. Tree-rings have been
shown to be very good proxies for temperature and rainfall measurements. An
international team of scientists spearheaded by Jinbao Li and Shang-Ping Xie,
while working at the International Pacific Research Center, University of
Hawaii at Manoa, has compiled 2 222 tree-ring chronologies of the past seven
centuries from both the tropics and mid-latitudes in both hemispheres. Their
work is published in the June 30, 2013 online issue of Nature Climate Change.
What this means
The inclusion of tropical tree-ring records enabled the team
to generate an archive of ENSO activity of unprecedented accuracy, as attested
by the close correspondence with records from equatorial Pacific corals and
with an independent Northern Hemisphere temperature reconstruction that
captures well-known teleconnection climate patterns.
These proxy records all indicate that ENSO was unusually
active in the late 20th century compared to the past seven centuries, implying
that this climate phenomenon is responding to ongoing global warming.
"In the year after a large tropical volcanic eruption,
our record shows that the east-central tropical Pacific is unusually cool,
followed by unusual warming one year later. Like greenhouse gases, volcanic
aerosols perturb the Earth's radiation balance. This supports the idea that the
unusually high ENSO activity in the late 20th century is a footprint of global
warming" explains lead author Jinbao Li."Many climate models do not reflect the strong ENSO
response to global warming that we found," says co-author Shang-Ping Xie,
meteorology professor at the International Pacific Research Center, University
of Hawaii at Manoa and Roger Revelle Professor at Scripps Institution of
Oceanography, University of California at San Diego.
"This suggests that many models underestimate the
sensitivity to radiative perturbations in greenhouse gases. Our results now
provide a guide to improve the accuracy of climate models and their projections
of future ENSO activity. If this trend of increasing ENSO activity continues,
we expect to see more weather extremes such as floods and droughts."