But none of the climate simulations carried out for the IPCC produced
this particular hiatus at this particular time. That has led sceptics —
and some scientists — to the controversial conclusion that the models
might be overestimating the effect of greenhouse gases, and that future
warming might not be as strong as is feared. Others say that this
conclusion goes against the long-term temperature trends, as well as
palaeoclimate data that are used to extend the temperature record far
into the past. And many researchers caution against evaluating models on
the basis of a relatively short-term blip in the climate. “If you are
interested in global climate change, your main focus ought to be on
timescales of 50 to 100 years,” says Susan Solomon, a climate scientist
at the Massachusetts Institute of Technology in Cambridge.
But
even those scientists who remain confident in the underlying models
acknowledge that there is increasing pressure to work out just what is
happening today. “A few years ago you saw the hiatus, but it could be
dismissed because it was well within the noise,” says Gabriel Vecchi, a
climate scientist at the US National Oceanic and Atmospheric
Administration’s Geophysical Fluid Dynamics Laboratory in Princeton, New
Jersey. “Now it’s something to explain.”
Researchers have followed various leads in recent years, focusing mainly on a trio of factors: the Sun
1, atmospheric aerosol particles
2 and the oceans
3.
The output of energy from the Sun tends to wax and wane on an 11-year
cycle, but the Sun entered a prolonged lull around the turn of the
millennium. The natural 11-year cycle is currently approaching its peak,
but thus far it has been the weakest solar maximum in a century. This
could help to explain both the hiatus and the discrepancy in the model
simulations, which include a higher solar output than Earth has
experienced since 2000.
An unexpected increase
in the number of stratospheric aerosol particles could be another
factor keeping Earth cooler than predicted. These particles reflect
sunlight back into space, and scientists suspect that small volcanoes —
and perhaps even industrialization in China — could have pumped extra
aerosols into the stratosphere during the past 16 years, depressing
global temperatures.
Some have argued that
these two factors could be primary drivers of the hiatus, but studies
published in the past few years suggest that their effects are likely to
be relatively small
4, 5.
Trenberth, for example, analysed their impacts on the basis of
satellite measurements of energy entering and exiting the planet, and
estimated that aerosols and solar activity account for just 20% of the
hiatus. That leaves the bulk of the hiatus to the oceans, which serve as
giant sponges for heat. And here, the spotlight falls on the equatorial
Pacific.
Blowing hot and cold
Just before the hiatus took hold, that region had turned unusually warm
during the El Niño of 1997–98, which fuelled extreme weather across the
planet, from floods in Chile and California to droughts and wildfires in
Mexico and Indonesia. But it ended just as quickly as it had begun, and
by late 1998 cold waters — a mark of El Niño’s sister effect, La Niña —
had returned to the eastern equatorial Pacific with a vengeance. More
importantly, the entire eastern Pacific flipped into a cool state that
has continued more or less to this day.
Just before the hiatus took hold, that region had turned unusually
warm during the El Niño of 1997–98, which fuelled extreme weather across
the planet, from floods in Chile and California to droughts and
wildfires in Mexico and Indonesia. But it ended just as quickly as it
had begun, and by late 1998 cold waters — a mark of El Niño’s sister
effect, La Niña — had returned to the eastern equatorial Pacific with a
vengeance. More importantly, the entire eastern Pacific flipped into a
cool state that has continued more or less to this day.
This
variation in ocean temperature, known as the Pacific Decadal
Oscillation (PDO), may be a crucial piece of the hiatus puzzle. The
cycle reverses every 15–30 years, and in its positive phase, the
oscillation favours El Niño, which tends to warm the atmosphere (see
‘The fickle ocean’).
After a couple of decades of releasing heat from the eastern and
central Pacific, the region cools and enters the negative phase of the
PDO. This state tends towards La Niña, which brings cool waters up from
the depths along the Equator and tends to cool the planet. Researchers
identified the PDO pattern in 1997, but have only recently begun to
understand how it fits in with broader ocean-circulation patterns and
how it may help to explain the hiatus.
One
important finding came in 2011, when a team of researchers at NCAR led
by Gerald Meehl reported that inserting a PDO pattern into global
climate models causes decade-scale breaks in global warming
3.
Ocean-temperature data from the recent hiatus reveal why: in a
subsequent study, the NCAR researchers showed that more heat moved into
the deep ocean after 1998, which helped to prevent the atmosphere from
warming
6.
In a third paper, the group used computer models to document the flip
side of the process: when the PDO switches to its positive phase, it
heats up the surface ocean and atmosphere, helping to drive decades of
rapid warming
7.
A
key breakthrough came last year from Shang-Ping Xie and Yu Kosaka at
the Scripps Institution of Oceanography in La Jolla, California. The duo
took a different tack, by programming a model with actual sea surface
temperatures from recent decades in the eastern equatorial Pacific, and
then seeing what happened to the rest of the globe
8.
Their model not only recreated the hiatus in global temperatures, but
also reproduced some of the seasonal and regional climate trends that
have marked the hiatus, including warming in many areas and cooler
northern winters.
“It was actually a
revelation for me when I saw that paper,” says John Fyfe, a climate
modeller at the Canadian Centre for Climate Modelling and Analysis in
Victoria. But it did not, he adds, explain everything. “What it skirted
was the question of what is driving the tropical cooling.”
![[image]](http://barrons.wsj.net/public/resources/images/ON-BD502_GT1012_NS_20140129152756.jpg)
