New study: Pacific Ocean holds the key to surface warming 'hiatus'
- 28 Aug 2013, 18:15
- Roz Pidcock
The earth's atmosphere has warmed more slowly over the last
decade or so than in previous years - and a big question in climate
science right now is why.
A new paper links the so-called hiatus to natural changes in the
climate, saying it's all down to what's going on in the tropical
Pacific Ocean. But we should expect faster warming to make a
comeback, the authors warn.
greenhouse gases are driving up
global temperature. But data on land and from the surface of
the ocean in the last decade and a half show surface temperatures
have risen somewhat
slower than expected.
According to a
new paper in Nature, understanding natural changes in the
Pacific Ocean is key to finding out what's causing the "hiatus" and
how long it's likely to stick around.
Pause for thought
When scientists talk about
what's causing the slowdown in surface temperature rise, a
couple of explanations usually come up. As the authors explain in
the new paper:
"Two schools of thought exist regarding
the cause of this hiatus in global warming: one suggests a slowdown
in radiative forcing ... and the other considers the hiatus to be
part of natural variability."
When volcanoes erupt they spit reflective particles into the
atmosphere. One suggestion is that an increase in these particles
together with a dip in the amount of solar energy reaching earth
could be contributing to less-than-expected warming. That's what's
meant by a change in radiative forcing.
But the new paper sits squarely in the second camp, saying the
"hiatus" is part of natural climate variability. It's all to do
with sea surface temperatures in the tropical Pacific Ocean cycling
between warm and cool phases, the paper says.
A good match
The paper looks at the period 1970 to 2012, which includes the
current "hiatus" and a period of faster
global warming between the 1970s and the late 1990s.
When the scientists included measurements of Pacific sea surface
temperature over the 42-year period, they found their model
reproduced recent global temperatures much better than when they
only included changes in radiative forcing.
This shows the tropical Pacific plays an important role in
climate, despite the fact it only accounts for 8.2 per cent of the
earth's surface, the authors say.
This graph shows the good match between model temperatures
in the last few decades (in red) and measured temperatures (in
black). Just accounting for radiative changes doesn't reproduce the
recent surface warming slowdown (in purple).
The scientists found the recent period of slower surface warming
coincided with lower than usual sea surface temperatures in the
tropical Pacific, caused by a tendency towards the cool phase of a
natural cycle, known as La Niña. The paper explains:
"Our results show that the current
hiatus is part of natural climate variability, tied specifically to
La-Niña-like decadal cooling … For the recent decade, the decrease
in tropical Pacific sea surface temperature has lowered the global
temperature by about 0.15 degrees Celsius compared to the
In contrast, a tendency towards the opposite warm phase of the
Pacific cycle, known as El Niño, caused global temperatures to rise
much faster between the 1970s and the late 1990s.
This may all sound familiar - it's not the first time scientists
have proposed a link between the "hiatus" and Pacific surface
temperatures. Lead author on a
recent paper in the Journal of Climate, Professor Gerard Meehl,
"This paper basically confirms, with a
novel methodology, what we originally documented in our Nature
Climate Change paper in 2011 and followed up with in our Journal of
But Meehl adds, the new paper only tells half the story. For
example, it doesn't address why the tropical Pacific ocean shifts
from cool to warm phases, or where the missing heat is ending up.
He tells us:
"We went beyond [the new paper] to show
that when the tropical Pacific was cool for a decade ... more heat
is mixed into the deeper ocean, something the new paper doesn't
Ocean temperature data - not part of the new study
indicates that at the
same time surface temperature rise has slowed over the past decade
or so, the rate at which the deep oceans take up heat has
accelerated. The graph below is from the
first of three special Met Office reports last month.
Change in globally averaged annual ocean heat content (blue)
for the upper 700 m of the
ocean and global average near-surface temperature (black)
relative to 1970-2011.
Co-author on the earlier paper, Professor Kevin Trenberth,
echoes this point, telling us:
"The [new] paper has captured some
essential parts of the cause of the apparent hiatus in global mean
surface temperatures … However, it does not deal with why the sea
temperatures have changed as observed".
The earlier paper by Meehl, Trenberth and colleagues suggests a
reason why the deep ocean shifts between taking up more or less
heat - a natural cycle in the ocean known as the Interdecadal
Pacific Oscillation. We wrote more about the paper,
According to the new paper, the link with natural variability
suggests the slower surface warming we're seeing is temporary and
"global warming will return when the tropical Pacific swings back
to a warm state."
Though similar "hiatus" events may occur in the future,
temperatures will only be affected in the short term, the paper
adds. As the Met Office said in another
special report last month:
"[T]he heat in the ocean is merely being
rearranged; it is effectively 'hidden' from the surface only to
re-emerge at some later date."
When looking at how global temperatures have changed, it's easy
to focus on the atmosphere because that's where most measurements
are made. But this paper adds to growing evidence that we need to
look to the deep ocean to really take earth's temperature.
Kosaka, Y. & Xie, S. Recent global-warming hiatus tied to
equatorial Pacific surface cooling. Nature,