El Niño is stronger this century, but the shift can’t be linked to climate change just yet
- 04 Jan 2013, 09:00
- Roz Pidcock
New research shows that a natural climate phenomenon called the
El Niño Southern Oscillation (ENSO) was unusually strong in the
20th century compared to the past 7,000 years. But while the
research suggests the ENSO cycle could be sensitive to
human-induced climate change, the researchers warn more data is
needed to prove a link.
ENSO is a natural climate pattern that occurs in the equatorial Pacific Ocean and
affects the climate worldwide. Every
five years or so, a change in the winds causes a shift to
warmer than normal ocean temperatures - known as El Niño - or
cooler than normal - known as La Niña, moving briefly back to
normal in between. Each time a switch occurs, the changes to ocean
and atmospheric circulation affect temperature and rainfall
patterns across the globe.
Sea surface temperature during El Niño (left) and La Niña
(right). Red and blue show warmer and cooler temperatures than the
long term average, respectively. [Image courtesy of Steve Albers,
NOAA Source:
ClimateWatch Magazine]
ENSO is responsible for a large part of the natural variation we
see in the global climate from one year to the next. To make
accurate climate predictions, scientists need to know how ENSO
varies in the short term, and if it is affected by
rising global temperatures caused by
human activity in the long term. As professor Kim Cobb, lead
author of the
new study published in the journal Science, told Carbon
Brief:
"Preparing for large swings in
temperature and rainfall is critical to adaptation strategies,
especially if such variability will increase in the future."
At the moment, climate models don't agree about how sensitive
ENSO is to climate change. And in the past there hasn't been enough
data available to test which model predictions are right.
Capturing corals
In the new study, a team of US researchers led by the Georgia
Institute of Technology examined a set of fossilised corals
collected from the central tropical Pacific. They used radioactive
dating techniques to determine their age, finding them to be
between 1.3 and 6.7 thousand years old.
The scientists examined the chemical makeup of each coral
sample. Cooler sea surface temperatures during La Niña cause a
particular form of oxygen to build up in the coral skeletons.
Warmer conditions during El Niño have the opposite effect. So
looking at changes in the amount of this form of oxygen can tell
scientists a lot about the timing and strength of ENSO events
during the coral's lifetime.
Scientists use a large drill to remove parts of the coral to
analyse for information about changes in rainfall and sea surface
temperature. [Image courtesy of Roland Klein, Norwegian Cruise
Lines].
By spanning the past 7,000 years - part of a period known as the
Holocene - the new study triples the amount of data available for
scientists to analyse. This means that any patterns detected in the
data are likely to be more reliable than in previous studies.
Natural highs and lows
The scientists found the strength of ENSO events - both El Niño
and La Niña - seems to have fluctuated widely over the corals'
7,000 year history. In some years, ENSO was fairly muted while
others saw large changes in global climate for several years at a
time.
However, the 20th century stood out as unusual. In the later
part of the 20th century, ENSO events were, on average, 42 per cent
stronger than the 7,000-year average.
In theory, human-caused warming could increase the strength of
ENSO events through a number of complex and interlinked processes.
As Cobb told us:
"It is important to remember that
climate change is much more than "global warming" … Because ENSO
involves feedbacks between the wind strength, ocean temperature,
and circulation, a change in any related climate parameter would
arguably have some effect on ENSO strength".
Signal or noise?
Even though ENSO was unusually strong in the late 20th century,
it was not unprecedented. Stronger ENSO events occurred in the 17th
century - at a time when anthropogenic greenhouse gas forcing was
negligible.
But, as Cobb told Carbon Brief, this doesn't rule out
human-induced climate change as a factor affecting ENSO. The recent
spell of stronger than usual events has persisted significantly
longer than the one 400 years ago.
"Misconceptions about climate science
are rife with those who confuse weather with climate ... the
presence of large El Niño events before 1850 AD does not mean that
climate change has no effect."
But the jury is still out. As Cobb continues in the paper:
"[Our results] may reflect a sensitivity
to anthropogenic greenhouse forcing, but definitive proof of such
an effect requires much longer data sets than are currently
available."
In other words, there could be a signal from greenhouse
gas forcing in the last century, but because ENSO strength has
varied so much in the past, such a signal is still not clear above
the natural noise.
Big challenge
Accurately predicting the timing and strength of ENSO events is
a major challenge for climate modellers. Although it may be too
soon to say definitively whether there is a link between climate
change and ENSO, the research points towards an intensification of
the cycle, bringing more extreme weather to many regions of the
globe. The extra data spanning many thousands of years that this
study uncovers will go a long way to matching model projections
with past observations, helping scientists identify the most
accurate models for making predictions of future climate
change.