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
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.
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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.
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.
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.
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.
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