News Section
Stories from Climate Central's Science Journalists and Content Partners

Study Ties Global Warming ‘Hiatus’ to Pacific Cooldown

Scientists probing the mystery of the so-called "global warming hiatus" may have made a breakthrough. According to a new study published Wednesday in the journal Nature, a persistent area of unusually cool sea surface temperatures in the tropical Pacific Ocean could explain why, despite ever-increasing amounts of manmade greenhouse gases in the atmosphere, global average surface temperatures have increased at a slower rate during the past 15 years.

By running sophisticated computer models that looked at observed energy coming into the climate system, plus observed sea surface temperatures in the central and eastern Pacific Ocean, researchers from the Scripps Institution of Oceanography found that the decadal-scale cooling of a portion of the tropical Pacific accounts for most of the current plateau in global average surface temperatures. 

Global-mean temperature (ºC) and CO2 (ppm) for 1971-2012.Temperature is represented in terms of deviation from 1980-1999 average. Both are based on annual mean values.The temperature for the hiatus period is highlighted.
Click image to enlarge. Credit: Scripps Institution of Oceanography.

That is remarkable considering that the region of ocean under scrutiny comprises just 8.2 percent of the globe’s surface, the study said. That such a small area of the planet could have such a large influence on the climate has much to do with how the Pacific Ocean helps influence global weather patterns and the transfer of heat into the air and deep oceans. For example, El Niño events, which take place in the same area of the Pacific, help boost global temperatures.

While scientists lack a full understanding of all the processes involved, they know enough to list this part of the world as a chief suspect in contributing to the temporary warming slowdown.  

According to a forthcoming report from the U.N. Intergovernmental Panel on Climate Change, the rate of warming during the past 15 years was about 0.09°F per decade, which is a smaller increase than the temperature trend since 1951. During the 100-year period from 1901-2012, the globe warmed by about 1.6°F, a trend scientists attribute largely to manmade emissions of greenhouse gases.

MORE: This Is What Global Warming Looks Like

Study co-author Shang-Ping Xie told Climate Central that by running computer models with observed ocean temperatures from the 1940s onward, with a particular emphasis on the recent 15-year period, “We got very good agreement with the observed record, including the current hiatus that started in the late 1990s.” In fact, not only did the models reproduce the overall warming plateau, but they also showed continued warming during the summer months, and a lack of warming during the winter, which has also been shown by observations.

“This was a remarkable result telling us that we are on the right track,” Xie said in an interview.

While global surface temperatures have not warmed significantly since 1998, other studies have shown that Earth’s climate system continues to warm, with emerging evidence indicating that the deep oceans may be taking up much of the extra heat. That extra heat is expected to be released back into the atmosphere in the coming decades. Even with a slowed rate of warming, the first decade of the 21st century was still the warmest decade since instrumental records began in 1850.

Xie said that for now, Pacific Ocean temperatures are dampening the increase in global temperatures, but that will change soon, perhaps even in the next several years. “Now it is swinging down, eventually it is going to swing up, and when it swings up we are going to see much, much stronger warming” on par with the accelerated warming seen during the period from the 1970s to 1990s, “if not bigger,” Xie said. “When it swings up we’re going to be in big trouble.”

The new study is part of a surge in research examining the recent warming hiatus.

Gavin Schmidt, a scientist at NASA’s Goddard Institute for Space Studies in New York who was not involved in the Nature study, said the results are, “A part of the discussion, but not the last word.”

Other climate scientists contacted by Climate Central who were not involved in the new study said the new work is consistent with other emerging evidence showing that natural climate variability involving the equatorial Pacific Ocean may be a key factor in modifying the rate of surface warming worldwide.

Simulated temperature trend patterns (right) created by climate modelers at Scripps Institution of Oceanography, showed strong agreement to observed summer temperatures (left) for 2002-2012. (JJA stands for June, July, and August.)
Click image to enlarge.  Credit: Image courtesy of Nature Magazine.

“I think this is a very well designed study that strongly supports the notion that internal variability of the climate system can influence the rate of global warming on timescales ranging from years to decades,” said John Michael Wallace of the University of Washington, in an email. “It argues that not only could the current hiatus in the warming be due to natural causes: so also could the rapidity of the warming from the 1970s until the late 1990s.”

In other words, natural climate variability affects the rate of global warming like a dial controlling the sound on a speaker — at some points the volume is turned up, and the planet warms faster, while at other times it is dialed back down, with a slower rate of warming. Scientists have also warned that at some point, the speaker may go all the way to 11, with warming at rates never before seen in human history, depending how high greenhouse gases climb.

Viewing Study in Context of Emerging Research

The study is consistent with the findings of a 2011 study published in Nature Climate Change and a 2013 paper in the Journal of Climate, which together showed that a natural climate cycle that affects the climate on decadal timescales, known as the Interdecadal Pacific Oscillation, can lead to periods of accelerated global warming as well as “hiatus decades,” depending on what phase the cycle is in. Since the late 1990s, the cycle has been in a phase that promotes cooler temperatures at the surface, but a more rapid buildup of heat in the deep ocean.

Kevin Trenberth, a climate scientist at the National Center for Atmospheric Research in Boulder, Colo., who was not involved with the new study, told Climate Central in an email conversation that the paper provides valuable, but relatively limited insight.

“This paper identifies a pattern which we agree is an important part of what is going on. The main areas where it has not warmed is the central and eastern tropical Pacific,” Trenberth said. “What this paper does not do is analyze the role of changes in 'forcing': effects external to the system such as from small volcanoes, changes in the sun, etc. Nor does it deal with where the energy is going, such as our work on the deep ocean. Nor does it deal with why.”

Global ocean heat content since the 1950s, showing a more rapid increase in deep ocean heat content in recent years.
Click image to enlarge. Credit: NOAA.

Susan Solomon, a climate scientist at MIT, echoed Trenberth’s skepticism about the broader implications of the study’s findings, saying the study “is a chicken vs. egg problem as it stands . . .” because it feeds the models with observed sea surface temperatures, shedding no light on why the temperatures have evolved as they have.

“Did the sea surface temperatures cool on their own or were they forced to do so by, for example, changes in volcanic or pollution aerosols, or something else?” Solomon said.

Another explanation that has been put forward for the global warming plateau, one that climate skeptics have seized upon, is that the climate system is simply less sensitive to increasing amounts of greenhouse gases, such as carbon dioxide, than previously thought. Human activities have been pushing carbon levels ever higher, surpassing 400 parts per million this year, the highest level on record in modern human history.

“It's possible that the answer is 'door No. 3,' which would mean less future warming,” Solomon said. “But I think that 'door,' (a lower climate sensitivity to greenhouse gases) is the least consistent with observations, not just of the past decade, but the previous 40 years.”

Benjamin Santer, a climate scientist at Lawrence Livermore National Laboratory, expressed a similar view. "My take on the "warming pause" is that a 15-year period with relatively muted warming does NOT provide a scientifically compelling reason for fundamentally re-evaluating our estimates of the sensitivity of the climate system to human-caused increases in greenhouse gases," he said in an email.

A leaked draft of the forthcoming IPCC report said that the slowdown is “due in roughly equal measure” to natural climate variability, such as La Niña events in the Pacific Ocean, and reduced incoming energy from the sun by volcanic eruptions that help scatter incoming solar energy back into space. It also contains a slightly lower figure for climate sensitivity to a doubling of carbon dioxide levels in the atmosphere compared to the panel's last comprehensive report, which was published in 2007. The new IPCC report is due to be finalized by September 27 at a meeting in Stockholm.

Related Content
Warming Plateau Likely Due to Deep Ocean Warming
This is What Global Warming Looks Like
NOAA: 2012 One of Globe’s 10 Hottest on Record
Why Globe Hasn’t Warmed Much for the Past Decade
The Last Time CO2 Was This High, Humans Didn’t Exist


By Erik
on August 28th, 2013

So what I’m reading here is AGW-funded climate scientists, in a desperate attempt to explain why their original models have been unable to explain MEASURED phenomena, have switched to “better models” in order to perpetuate the AGW hypothesis and thus retain credibility.  Ah, science at its very worst.

Reply to this comment

By Porch Monkey (Anytown,USA,90210)
on August 28th, 2013

  So lets see, CO2 rises and temps fall flat. There goes the Global Warming Argument.
Screw the Windmills and Solar Panels, it’s time for Coal baby.

Reply to this comment

By Chas
on August 28th, 2013

This is going the way of the global “cooling” scare of the 70’s. Google Dr. Hubert Lamb, Director of Climate research at University of East Anglia in the 1970’s.

Reply to this comment

By Caroll (Rogers, MN)
on August 28th, 2013

I started to telecommute instead of driving into work, leave the AC set at 78 or off when it’s not in the 90’s.  I’ve switched all my lights to either CFL or LCD’s and all these changes have done for me was save me money.  I guess all the global warming deniers can laugh at me for driving an economy car or biking when possible.  It keeps me healthy and I suppose that must make me a wimp.

Reply to this comment

By Dr.Ferguson (77964)
on August 28th, 2013

  Assuredly, the vents of the deep or a hole in the atmosphere must have siphoned away excess carbon , such an assertion would be impossible, but the facts are astoundingly contrary!

Reply to this comment

By Big Jim Slade
on August 28th, 2013

There is one other explanation of course.

It involves the word…


Reply to this comment

By Asok Asus (Denver, CO 80021)
on August 28th, 2013

That’s odd. I don’t remember any of their models predicting any “hiatus”, at least BEFORE the “hiatus” has occurred. Oh wait, that’s right, because none of them did

But that’s easy to fix, just go back and “fix” the model AFTER THE FACT to fit the observed data. Presto Chango, Abracadabra, and hey, global warming is STILL really happening! Really! It is! Our models say so! Sort of.

Reply to this comment

By irving krinlap (north monmouth, me 04265)
on August 28th, 2013

I have wintered in fl for many years. this year I got stuck in fl for the summer. it’s hot here in the summer. it’s supposed to be - the sun says so.

Reply to this comment

By randmor (Shenzhen/Guangdong/China)
on August 28th, 2013

If it’s the tropical Pacific that’s cooling the Earth, then something must be feeding it. Most likely the upwelling of the deep cold arctic and antarctic currents which are in turn being driven by the increased melt of the polar ice sheets. Has the research team looked into this? When the polar ice runs out, so will the tropical Pacific cooling effect.

Reply to this comment

By Paul Davis (Tn)
on August 28th, 2013

This cool area is very probably due to increased melting in Greenland causing bottom level currents to come to the top of the ocean sooner, and bottom currents in the oceans are all due to the thermal conveyor that begins in Greenland.

I’d wager a penny or two that salinity in those areas is reduced.

Reply to this comment

By Herrie Odots (Cape Town 7975)
on August 29th, 2013

Sigh. When are they going to run out of excuses and admit that the causal link is not CO2—heating, but heating—CO2? It’s embarrassing to see scientists getting ever more inventive in looking for post facto explanations rather than keeping up their bold sweeping predictions. They must really get over their millenarian guilt and stop flagellating us. This is 2013, not the year 1000!

Reply to this comment

By dan_in_illinois
on August 29th, 2013

“Scientists probing the mystery of the so-called “global warming hiatus” may have made a breakthrough. According to a new study published Wednesday in the journal Nature, a persistent area of unusually cool sea surface temperatures in the tropical Pacific Ocean could explain why, despite ever-increasing amounts of manmade greenhouse gases in the atmosphere, global average surface temperatures have increased at a slower rate during the past 15 years.”

Either that or this whole theory about AGW is just a crock.

Reply to this comment

By Jm
on August 29th, 2013

Could this in any possible way be related in part to jet stream changes due to Arctic amplification?  NOAA recorded record warming of the Atlantic in 2012.

Reply to this comment

By Salvatore Del Prete (Bay City ,Michigan 48708)
on August 29th, 2013

Salvatore Del Prete says:

August 29, 2013 at 12:40 pm

They are saying a natural event such as ENSO has been responsible for almost all of the temperature variations, which proves (sorry AGW theory) AGW theory is invalid.

According to AGW theory not only are natural causes not to be the prime movers of the climate but actually the man made co2 /watervapor positive feedbacks would create a condition that would favor more El Ninos going forward adding to the warmth.

So this study shows natural forces not man made co2 drives the climate and further it proves AGW theory wrong once again, which said one of the results from man made global warming would be a siginificant increase in El Ninos, due to the positive co2/water vapor feedback which they contended was tied into their missing lower troposheric hot spot near the equator, which is also missing in action.

Reply to this comment

By Eric Peterson (Front Royal, VA 22630)
on August 29th, 2013

“with emerging evidence indicating that the deep oceans may be taking up much of the extra heat. “

I would not call Levitus (shown in the chart above) evidence since it relies on a model that assumes that there is extra heat based on assumptions of CO2 causing positive feedback in water vapor.  It is somewhat circular logic to say that the “missing” heat is in the ocean when there may not be any “missing” heat at all.

All it takes is for Levitus to be wrong about clouds or some other poorly measured quantity.  More low clouds (for example) would mean less heat from the sun and less “missing” heat deep in the ocean.

Levitus integrates some ocean temperature measurements into the model.  But those measurements are sparse and very inadequate for the deep ocean.

Reply to this comment

By Magoo
on August 29th, 2013

If a cooling Pacific ocean is the cause of the ‘hiatus’ in warming, does that mean that the warming was due to a warming Pacific? They can’t have it both ways

Reply to this comment

By Paul (Melbourne)
on August 29th, 2013

If La Nina and El Nino are the drivers of the changes, what drives them? Oh that’s right, climate.
So it’s the climate’s fault the climate paused.

Reply to this comment

By Andrew
on August 30th, 2013

Magoo and many others who have commented along similar lines,

A cooling in part of the Pacific may be one of the causes of the hiatus, but no, that does not mean that the warming prior to the hiatus was due to a warming Pacific.

The important point to realize is that this mechanism in the Pacific modulates the RATE of global warming, but is not the factor causing global warming in the first place. At times the Pacific acts to speed up the rate of warming, at times it holds it back, but the warming trend viewed over longer periods is still there, and still due largely to manmade emissions of greenhouse gases.

In other words, the variability at shorter timescales is dwarfed by the long-term trend of increasing temps.

In more technical terms:  For the multi-decadal trend, the Pacific Decadal Oscillation only accounts for up to 0.2 deg C of the trend since 1950-2010, according to the study’s co-author Shang-Ping Xie.

Here’s a comment from Susan Solomon, via email, that might help explain this in simpler terms than I have above.

“There is an animation in this posting that seems more or less right to me (See Figure 2).  Please look at it long enough to see the change from one image to the second one.

If this is correct, it’s an easy way to explain this for the general public and for scientists.  The recent La Ninas are among the warmest La Ninas ever.  The recent El Ninos also are.  So there are strong trends in
both.  If you have a series of years with lots of El Ninos in the front and lots of La Ninas in the back, then it appears fairly flat but the global warming is clearly seen in each type of dynamical situation individually.”

Reply to this comment

By Eric Peterson (Front Royal, VA 22630)
on August 30th, 2013

“A cooling in part of the Pacific may be one of the causes of the hiatus, but no, that does not mean that the warming prior to the hiatus was due to a warming Pacific.”

I would recommend reading Bob TIsdale.  In a nutshell, strong El Ninos following neutral or mild La Nina can (and did) warm the entire planet.  In contrast, La Nina, whether weak or strong does not affect global temperature as much.  In evidence we have the profound effects of the 1997/98 El Nino on North America and much of the Atlantic, a dramatic increase in temperature and moisture, unequaled since then.  No such impact is evident for La Nina.

Reply to this comment

By Kit Stolz (Ojai CA 93060)
on August 31st, 2013

Thanks Andrew for digging deeper (and your extra-clear second sentence as well). As a Californian, I note that in the study the authors talk about a “La Nina-like” phenomenon, which suggests that the nation might be stuck with a La Nina-like weather consequences for some time…no?

Reply to this comment

By Pablano (RICHFIELD/MN/55423)
on August 31st, 2013

Amazing, they stumbled upon the Pacific Decadal Oscillation hiding in plain sight.

Reply to this comment

By Earthling
on September 1st, 2013

Dr. Phil Jones – CRU emails – 5th July, 2005

“The scientific community would come down on me in no uncertain terms if I said the world had cooled from 1998. OK it has but it is only 7 years of data and it isn’t statistically significant….”

Dr. Phil Jones – CRU emails – 7th May, 2009

‘Bottom line: the ‘no upward trend’ has to continue for a total of 15 years before we get worried.’

Jone’s 15 year period has passed.

Reply to this comment

Name (required):
Email (required):
Enter the word "climate" in the box below:

[+] View our comment guidelines.

Please note: Comment moderation is enabled. Your comment will not appear until reviewed by Climate Central staff. Thank you for your patience.