From YaleEnvironment360
A consensus is emerging among scientists that the rate of global warming has slowed over the last decade.
While they are still examining why, many researchers believe this phenomenon is linked to the heat being absorbed by the world’s oceans.
Whatever happened to global warming? Right now, that question is a good
way of starting a heated argument. Some say it is steaming ahead.
But
others say it has stalled, gone into reverse, or never happened at all —
and they don’t all run oil companies or vote Republican.
So what is going on?
First, talk of global cooling is palpable nonsense.
This claim relies on
the fact that no year has yet been hotter than 1998, an exceptional
year with a huge planet-warming El Nino in the Pacific Ocean.
Naysayers
pretend that 1998 was typical, when it was anything but, and that
temperatures have been declining since, which is statistical sleight of
hand.
Meanwhile consider this.
According to the National Oceanic and
Atmospheric Administration (NOAA), all 12 years of the new century rank
among the 14 warmest since worldwide record-keeping began in 1880. The
second-warmest year on record, after 1998, was 2010.
This is not
evidence of cooling.
But there is a growing consensus among temperature watchers that the
pace of warming in the atmosphere, which began in earnest in the 1970s and seemed to accelerate in the 1990s, has slackened, or stalled, or
paused, or whatever word you choose.
It may turn out to be a short blip;
but it is real.
“Although the first decade of the 21st century was the
warmest on record, warming has not been as rapid since 2000,” says Pete
Stott, head of climate monitoring and attribution at the UK’s Met
Office, one of the leading keepers of the global temperature.
He calls
it a “hiatus” in warming.
In a blog last week,
James Hansen, the retiring head of NASA’s Goddard Institute for Space
Studies (GISS), agreed that “the rate of global warming seems to be less
this decade than it has been during the prior quarter century”
Something is going on.
With heat-trapping greenhouse gases accumulating
in the atmosphere ever faster, we might expect accelerated warming.
So
it needs explaining.
There are a number of theories.
Hansen suggested that extra emissions of
particles in Asian smogs could be shading the Earth and camouflaging
the greenhouse effect.
In
a February post on RealClimate,
his Goddard Institute colleague Gavin Schmidt instead pointed to fewer
warming El Ninos and more cooling La Ninas.
He suggested that adjusting
for their influence produced an unbroken pattern of warming.
Schmidt’s analysis certainly hints at a role for the oceans in all this.
And most researchers on the case argue that, one way or another, the
most likely explanation for the heating hiatus is that a greater
proportion of the greenhouse warming has been diverted from the
atmosphere into heating the oceans.
A new study from Kevin Trenberth of
the National Center for Atmospheric Research in Boulder, Colorado,
published online in Geophysical Research Letters, found that ocean warming has been accelerating over the last 15 years.
Richard Allan of the University of Reading in England says simply:
“Warming over the last decade has been hidden below the ocean surface.”
If you take the oceans into account, he says, “global warming has
actually not slowed down.”
This should not come as a surprise, notes Chris Rapley of University
College London.
The oceans are the planet’s main heat sinks.
More than
90 percent of the extra heat trapped in the atmosphere by greenhouse
gases ends up there.
But, while climate models are good at calculating
atmospheric processes, they are poorer at plumbing the ocean-atmosphere interactions that determine how fast and how regularly this happens.
That makes those interactions a big source of uncertainty about
atmospheric global warming, especially over the short term.
If oceans
grab a bit more heat one year, they can shut down that year’s warming.
Equally, if they release a bit more they can accelerate atmospheric
warming.
This matters. “The way the ocean distributes the extra energy
trapped by rising greenhouse gases is critical... [to] global surface
temperatures,” says Allan.
For forecasters trying to figure out the next
decade or so, oceans could screw it all up.
Some bits of the puzzle have been known for a while.
For instance,
during El Nino years, warm water spreads out across the equatorial
Pacific and the ocean releases heat into the air, warming the air
measurably.
That is what happened in 1998.
But while El Ninos come and go within a year or so, there are other
cycles in heat distribution and circulation of the oceans that operate
over decades.
They include the Pacific Decadal Oscillation and the
Atlantic Multidecadal Oscillation (AMO), both of which have been
implicated in climate fluctuations in the 20th century.
So have these or
other ocean cycles been accelerating the uptake of heat by the oceans?
Virginie Guemas of the Catalan Institute of Climate Sciences in Barcelona, believes so.
In a paper
published in Nature Climate Change this week,
she claims to provide the first “robust” evidence linking ocean uptake
of heat directly to what she calls the recent “temperature plateau” in
the atmosphere.
By plugging detailed measurements of recent atmospheric and sea temperatures into
EC-Earth,
a European model of interactions between atmosphere, oceans, ice and
land surfaces, Guemas found that about 40 percent of the take-up was in
the tropical Pacific, and another 40 percent in the tropical and North
Atlantic.
She told me that it seems likely the changing thermohaline ocean
circulation, which starts in the North Atlantic, plus the cycles of El
Nino and perhaps the AMO,
may play a prominent role.
She thinks her model could have predicted the recent slowdown of atmospheric warming ahead of time.
That would be a breakthrough, but nobody has done it yet.
Meanwhile, the
climate modellers are skating on thin ice when they make predictions
that play out over the timescales of a decade or so on which ocean
cycles seem to operate.
These forecasters can claim that, all things
considered, they have done pretty well.
But the forecasts remain hostages to fortune.
If anything, the recent pause shows the model forecasts in a good light.
Myles Allen, a climate modeller at Oxford University in England,
reported in Nature Geoscience last month
on an audit of one of his own forecasts, which he made in 1999.
He had
predicted a warming of a quarter-degree Celsius between the decade that
ended in 1996 and the decade that ended in 2012.
He found that, in the
real world, temperatures got too warm too soon during the 1990s; but the
slackening pace since had brought the forecast right back on track.
That shows the forecast is performing well so far, but Allen admitted it
might not stay that way.
If temperatures flat-line out to 2016, his
model’s prediction for that year will look no better than a forecast
based on a series of random fluctuations.
Some in the mainstream climate community privately admit that they were
caught out by the slackening pace of warming in the past decade or so.
Back in the 1990s, some suggested — or at least went along with — the
idea that all the warming then was a result of greenhouse gases.
They
were slow to admit that other factors might also be at work, and later
failed to acknowledge the slowdown in warming. As Allen pointed out
earlier this year: “A lot of people were claiming in the run-up to the
Copenhagen 2009 [climate] conference that warming was accelerating. What
has happened since then has demonstrated that it is foolish to
extrapolate short-term climate trends.”
Not surprisingly they have been taken to task for this hubris.
Roger
Pielke Jr., an environmental studies professor at the University of
Colorado at Boulder, who enjoys baiting the mainstream, told me last
month: ”It is good to see climate scientists catching up with the
bloggers. They should ask why it took so long to acknowledge what has
been apparent to most observers for some time.”
But modellers are now responding more actively to the new real-world temperature data.
For instance, the Met Office’s Stott
reported last month
that global temperatures were following the “lower ranges” of most
model forecasts, and that higher projections were now “inconsistent”
with the temperature record.
And last December, the Met Office
downgraded its best guess for temperatures in the five years to 2017 from 0.54 degrees C higher than the average for the late-20th century average to 0.43 degrees higher. It
said the new forecast was the first output of its latest climate model,
HadGEM3, which incorporates new assessments of natural cycles.
But the problem is that these cycles are not well integrated into most
climate models.
Natural cycles could switch back to warming us again at
any time, admits Stott.
But he has no clear idea when.
The stakes for the climate forecasting community are high.
It may be
unfair, but the brutal truth is that if the climatologists get their
forecasts for the coming decade badly wrong, then a great many in the
public will simply not believe what they have to say about 2050 or 2100 –
even though those forecasts may well be more reliable.
Forecasters badly need a way to forecast the ocean fluctuations, and it
could just be that Guemas’s new study will help them do that.
She claims
that her findings open the way to the future delivery of “operational
decadal climate predictions.”
For now she is cautious about making firm
predictions, but told me she believes that “the heat that has been
absorbed recently by the ocean might very well be released back to the
atmosphere soon. This would be the scenario of highest probability. It
would mean an increased rate of [atmospheric] warming in the next
decade.”
It would indeed.
If natural cycles start pushing towards strong warming,
they will add to the continued inexorable upward push from rising
concentrations of heat-trapping greenhouse gases.
In that case, we would
see climate change returning to the rapid pace of the 1990s.
Whatever happened to global warming?
The odds may be that by 2020 it
will have come roaring back.
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