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Global ‘cooling’: how will the IPCC explain 15-year temperature hiatus?

Climate sceptics have a favourite war cry: temperatures have not risen for 15 years. Do scientists have an answer?

Pic: Flickr / Hans Põldoja

By Sophie Yeo

If the UN’s much anticipated climate science report is to win around a small but increasingly vocal group of climate deniers, it will have to explain one particular ‘puzzle’.

Why, when humans continue to pump vast amounts of CO2 into the atmosphere, does the rate of global warming appear to have slowed?

Deniers frequently call attention to the fact that, since 1998, climate change has levelled off considerably after a steep upward trend throughout the eighties.

“The IPCC must address difficult issues like this, absolutely,” said former IPCC chairman, Bob Watson, in an interview with RTCC last month.

But the slowdown, or ‘hiatus’, as it is sometimes called, is not as puzzling to scientists as climate sceptics like to proclaim. Here is RTCC’s roundup of how the AR5 is expected to explain the last fifteen years of climate change.

Long term trends

The AR5 is expected to confirm that each of the last three decades has been warmer than all preceding decades since 1850, with the first decade of the 21st century topping all the charts.

This contrasts sharply to the idea that the fifteen years between 1998 and 2012 has been a period of global “cooling”, as some sceptics like to claim.

This demonstrates the necessity of looking at longer term trends, rather than cherry picking data over a short period of time. This is particularly in climate science, where trends are observed over centuries, rather than year on year.

In fact, according to documents leaked to the Associated Press, Germany even called for the reference to the slowdown to be deleted, saying that it was misleading to even acknowledge a hiatus that only occurred only fifteen years, when climate change is normally measured over decades and centuries.

Most scientists will argue that taking 1998 as the starting point automatically begets a false conclusion, as this year was particularly hot, thanks to strong El Nino conditions transferring heat from the oceans to the atmosphere.

“Taking 1998 as the starting year is a joke,” says Pieter Tans, a climate scientist who worked on the IPCC report. “Why not 1997 or 1999? Anyone doing this gets an ‘F’ grade in introductory statistics.

“It is too early for us to be able to say that the human-caused warming has stopped.  I fully expect the long-term warming to continue because we know that our activities are causing the greenhouse gases to increase, and we can calculate based on very well understood physics, how the GHGs retain heat in the atmosphere.”

He adds: “There is no ‘Greenhouse Warming Hypothesis’. The warming expectation follows directly from established physics and chemistry.”

Natural variability

The natural variability of the climate can account for considerable fluctuation in global temperatures year on year, in spite of an overall upward trajectory.

“Over relatively short, non-climate timescales (less than 20-30 years), these patterns of natural variability can lead to all kinds of changes in global and regional near-surface air temperature: flat, increasing, or even decreasing trends,” Katharine Hayhoe, a climate scientist at Texas Tech University, tells RTCC.

“This short-term variability reflects natural patterns of heat and energy exchange between the different components of the Earth’s system. Only over climate timescales (typically, 30 years or more), do the long-term trends emerge that reflect the influence of changes in atmospheric levels of carbon dioxide.”

Oceans are thought to be one natural feature of the planet’s system which has a particularly strong influence on the climate.

A leaked draft of the IPCC report says that it is “is virtually certain that ocean warming dominates the global energy change inventory.”

It adds that the heating of the atmosphere is relatively small, though still present, when the whole system is taken into consideration.

“Warming of the ocean accounts for more than 90% of the extra energy stored by Earth between 1971 and 2010; melting ice (including Arctic sea ice, ice sheets, and glaciers) and warming of the continents and atmosphere account for the remainder,” it says.


The cooling effect of aerosols has led to suggestions that they are responsible for tempering some of the effect of warming over the years,

But on this occasion it doesn’t look like aerosols can be counted on to explain away the 15-year slowdown in the AR5.

Improved modelling since the last Assessment Report in 2007 means that scientists now think that aerosols have offset less of the global warming than previously thought.

Leaked drafts say: “The uncertainty in the aerosol contribution dominates the overall net uncertainty in RF [radiative forcing].”

But, it adds: “There is high confidence that aerosols have offset part of the forcing caused by the well-mixed greenhouse gases.”

Climate sensitivity

No one is denying that humans are continuing to pump vast quantities of greenhouse gases into the atmosphere, which has led many to question why the temperature hasn’t also risen accordingly.

The answer the climate sceptics have arrived at is that the climate must simply not be as sensitive to carbon dioxide as the models suggest.

“The deniers are saying we’ve obviously got the theory wrong, because greenhouse gases are still increasing – which they are, at a rapid rate – so why isn’t the temperature responding?” Bob Watson told RTCC.

Climate sensitivity is how much warming can be expected if the amount of greenhouse gases in the atmosphere is doubled.

The currently available drafts of the IPCC report have added fuel to the fire by dropping the lower range of climate sensitivity from 2C to 1.5C.

But, as climate scientist Scott A. Mandia writes in his blog, “All these values are bad news so we should not take joy in the unlikely lower end. In fact, it is accepted that a warming of even 2C would put people and ecosystems at extreme risk.”

He adds that he received an email from climate expert Dr. John Harte contextualising the problem: “Suppose you had been told for many years that drinking a small amount of a pesticide would result in a probability of consequent death that was somewhere in the interval 15% – 75%.

“If tomorrow you were told the actual range is 10% – 75% would you be any more likely to take a swig?”

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  • Otter

    At the height of the Midieval Warm Period, there was very definitely one decade which topped all others. And it was quite likely- according to hundreds of peer-reviewed papers- even warmer than this decade.
    And then, things got colder again.
    What goes around…..

  • Billy___Bob

    ““There is no ‘Greenhouse Warming Hypothesis’.”

    Nonsense. AGW is a two part theory.

    Part one is based on physics and suggests a small amount of warming will occur if everything else is equal.

    Part two suggests there are are massive positive feedbacks that will results in huge amounts of warming.

    Part two is falsified by the pause.

  • papertiger0

    The currently available drafts of the IPCC report have added fuel to the fire by dropping the lower range of climate sensitivity from 2C to 1.5C.

    It’s a rare thing for an entire field of scientific endeavor to regress after 20 + years of lavish government funding. Yeah. If I were Scott Mandia I’d be embarassed too. He has every reason to be ashamed.

    Only plus side for him – he has a lot of company.

    Oh and you know what?

    When you pull your head out of computer models and look at actual evidence, Life expectancy has doubled as CO2 has risen.

    Forget this nonsense about it being poison.

  • handjive

    If I5 years is a too short time scale, what does that say about the time scale between the end of the ice age of the end of the 70′s and Hansen announcing AGW in 1988?

  • Flash

    Hmm. So lets say there are two competing hypotheses in climate change:
    - Natural cycles
    - CO2 induced
    For a number of years the natural cycle and CO2 concentration seem to have been going in the same direction. It is difficult to tell which is dominant.
    Now the natural cycle and CO2 are going in different directions, we might be able to better see which is dominant.
    That should allow us to really determine CO2 sensitivities from observed data.
    Pointing out that 2000-2010 is a warmer decade than previously measured tells us nothing. It is consistent both with being at the top of a natural cycle and if we are on a run-away CO2 driven temperature increase.
    Climate seems to be driven by both natural cycles (sea, sun, ice-cover …) and likely also some man-induced influences (CO2, aerosols). It is also driven by non-cyclical events (volcanic eruptions, massive coronal subursts, meteor strikes).
    How much each contributes is not yet known.

  • http://www.facebook.com/mbraganca Meagan Hessick Braganca

    Part of the problem has been cherry-picking data, as you mention in your article. In 2011, Dr. Richard Muller completed a study that incorporated a complete-scale of historical temperature & CO2 concentration data. Muller had been a climate change skeptic until this study was completed. He is now firmly aware that the globe is warming and that humans are the cause.

  • brucehall

    Examine the trend from the 1920s to present rather than beginning in the 1880s which were an abnormally cool period. Also consider that the trend at long-term rural weather stations has been flat to down while the trend at urban station has been up. Then think about those daily weather forecasts that go something like “tonight will be in the upper 50s in the heat sink [city] and lower in outlying areas. Most of the increase in average temperature has been at the minimums, not maximums. Now who is drinking the Kool Aid?

  • Gary Ward

    These statistics do not do much good for the credibility of the climate scientists saying that climate change (particularly heating) is real. Their credibility is further harmed by the rumours that there are attempts to squelch this information.

    From my view, if the model is not predicting the phenomena, then the model is bad. I’m still not getting that warm, fuzzy feeling of belief. I don’t need another article to try to convince me, I need a good, well-argued, statistical and phenomenological analysis that shows either side is correct. Till I see it , I remain skeptical.

  • David

    Your global warming religion is just that, factless drivel to disguise socialism. Your precious UN has already been caught fudging the numbers and the globe has cooled while the ice on the pole is hitting record levels. Aren’t facts a bitch?

    • Razzle

      You are missing THE fact, we are not just talking about the past 15 years, climate varies naturally and overall the temperature is increasing, we are contributing towards it. no question about it. What? You thought we could just add whatever we like to the air we breathe, the atmosphere that regulates our whole planet, the thing we need to survive and it wouldnt make a jolt of difference. Seriously get over yourself and the rest of the human race.I bet you think we can just take whatever we like from the ecosystems around us and that it wont change anything, thats funny because I thought it was simple mathematics. You add or subtract something and the outcome is different from what you started with. Also im not into religion and im not really into socialism either. I just care about my descendants and myself being able to live in a world that is appreciated for being what it is. amazing. Why dont you face the real facts; the ones that have more than 15 years worth of evidence behind them?


        Hey Razzle, nobody disputes that pollution is bad for the planet, however, to stand there and claim that, but without humanity, the Earth would not be warming, or just slower? Also, that if we somehow shutdown then the Earth will not continue to warm and then cool again as it always does throughout history?

        Click here to take a tutorial on the “Milankovich Cycles theory”

        • darcy_2k

          Agree with you on the Milankovich cycles but you kinda put words in Razzle’s mouth! :-[)

  • Canuck57

    What I find strange is how volcanic activity is missing from any discussion. Volcanic activity has drastically affected global climate all through history, causing major temperature fluctuations, famines, wars, and disease. And yet, we’re currently in a period of increased volcanic activity suddenly this has no affect on climate. Go figure.

    • Stuart Ratcliff

      It does. It has a cooling effect because most of what volcanoes release are aerosols.

      • Bay0Wulf

        Hunh!?! Huge amounts of ash, smoke and other particulate matter be ejected high into the atmosphere and carried about in the jet stream … gasses as nasty as any that come out of anything that man has created …

        You describe all these things as “aerosols”?

  • Daryl McGonigle Logan

    I’m not going to get into the politics of global warming but I can say for sure that I’m seeing it happen right at home in Fort Lauderdale Florida and in Miami. There’s a section of the beach where the ocean took back the strip and they had to reconfigure the road that use to run through that section. Miami gets completely flooded with just an inch of rain. It’s getting bad down here.

    • Ken Trout

      Miami has been sinking for thousands of years, in fact the whole Gulf coastline has been falling into the gulf for centuries, this is not climate, its erosion.

  • Graham Ford

    Here are five key questions, and the answers I understand from the AR5:
    1. Has the world warmed? Yes, around 0.5 deg C since pre-industrial time.
    2. Is it going to go on warming? The models said yes, but the models predicted continual rise over the past 15 years. This has not happened. If they were wrong about this, why are they not wrong about their answer to Question 2?
    3. How warm is it going to get? Assuming scenario RCP 4.5, around 2.5 C, according to the mid range of the models. But 4.5 does not appear to require any special action, and fossil fuel trading imbalances are likely to limit consumption anyway through financial limits, so do we need a policy at government level, at least for now? And anyway, the models now look much to pessimistic.
    4. Who is going to be affected? Populations in low lying areas. Farmers who don’t adapt.
    5.What do we do about it? Who’s we? As far as I can see, local solutions will work OK. Perhaps we consumers need to spend less on fossil fuels and more on insulation and efficiency and renewables. But these are local decisions, not supranational ones.

  • cperg

    What have these scientist done to take the cycles of the Sun’s heat as it cycles into account for the changes in climate and average temperatures. Also the fact that from ice cores taken in Antarctica illustrate that CO2 levels follow the rise in temperature rather than lead. Of course in any scientific method the lead is most always considered as the cause in any “cause and effect” relationship. If this is true then the rise in temperature is the cause of the rise in CO2 levels over the history shown in the ice cores that go back not centuries but thousands if not millions of years.

  • WSLuper

    There’s no doubt that the atmosphere is warming; the question is why. Is it due to natural climate variation or greenhouse gasses?

    Climate warmists will tell us that it’s due to the burning of fossil fuels and the resultant emission of carbon dioxide into the atmosphere. So, it’s important to consider a few facts about carbon dioxide:

    1. It’s a trace gas, accounting for merely 4 parts in 10,000 of the atmosphere.
    2. It’s a minor greenhouse gas, comprising only 4% of greenhouse gasses, compared to water vapor’s 95%.
    3. It’s a weak greenhouse gas; only about 1/5 as efficient as water vapor at retaining heat.

    Shouldn’t this make us question the premise that carbon dioxide is responsible for the warming?

    Most climate change deniers will remind us that the earth has been warmer than today during at least two periods in history, the Roman Warming Period, lasting from 400 BC to 100 AD and the Medieval Warming Period lasting from 950 AD to 1250 AD. In fact Greenland gained its namesake from the fact that it was green during the latter period, while it’s almost all ice today.

    In light of the above it would seem that climate variability and not mankind is responsible for the warming.

    • ski63

      Not to mention receding glacier in Alaska is giving up it’s trapped forest for over a 1000 years. Rock get hot, rock gets cold..

    • 41progress41

      wow…ignorance is strong with this one. I see how you threw some numbers in there to make it seem like you had knowledge…even though they are made up. You’re pretty.

  • 41progress41

    um…except that part where 2000-2010 was the warmest decade since they started recording temperatures. http://www.wmo.int/pages/mediacentre/press_releases/pr_976_en.html

    and extreme warm temperatures out paces extreme cold temperatures 2:1 in the same time period. https://www2.ucar.edu/atmosnews/news/1036/record-high-temperatures-far-outpace-record-lows-across-us

    the good news is…the scientific, academic, energy, and business sectors have moved past “the world is flat” climate denying community. They are now considered annoying gnats by the adult world.

  • PCalith

    The amount of disinformation in these comments is astonishing. Global Warming’s happening folks. Its our fault. No matter what your Heartland Institute funded studies may show.

    • darcy_2k

      Exactly. Our climate is changing. We understand enough to know we must act. The rest is just semantics

      • Gannojo

        Climate changes. Always has always will. What are you going to do about it? Stop a hurricane? If you think you can change the weather perhaps your over shooting a bit… But you probably believe CO2 (man made) is the key? Even thought in ALL ice core samplings show the rise of CO2 followed the warming period. It could not be the cause of warming. Maybe warming is the cause of higher CO2 following the warming? This makes sense when you consider the oceans hold and emit the most CO2. So when they are cold they hold CO2, when they get warm they release into the atmosphere CO2. That is what the ice core samples tell us isn’t it?

  • Bay0Wulf

    To reference the concept that climate has to be looked at over decades and even centuries for which there is an astounding lack of data, and then try to reference this non-existent data as a support for the theory of Global Warming / Climate Change is trying to have it both ways.

    While it is undeniable that mankind is having an impact on the ecology of the earth, the problem is that there is little evidence of exactly what kind of impact. Most of the noise currently in the Warming Globe debate is being driven by those who seek to gain a financial advantage by indicating that there is a crisis at hand when there is no provable crisis at hand to repair.

    Many of the studies being made to support these theories are being made by those whose self interest is to make work for themselves. Look at the EPA in the US, if there were no immediate crisis of Environment, there would be no need for it to be so large and to have gathered so much power to itself. It becomes its own self fulfilling prophecy. There are many commercial businesses that are making rather large fortunes on the theory of Global Warming and it is in their best interest to keep the thought process going. Then there are the “scientists” and “researchers” whose job it is to “study” the impact of … it would little serve them to find that there is no crisis at hand because they would then find their careers having come to a dead end (find themselves out of a job).

    While saying that “deniers” have an agenda on one hand it is hard to believe or agree that the proponents do not have exactly the same agenda but in the opposite direction.

    • darcy_2k

      There is actually quite a lot of information about the the impacts we’re having. Here’s an excellent paper on Biodiversity loss and how that will affect ecosystems ability to support human life – http://www.ecoservices.asu.edu/pdf/Cardinale%20et%20al,%20Nature%20(2012).pdf

      Also, the Boreal Forest which has already warmed by 3-4C and in the process, areas of it have become a global Carbon source, putting out more CO2 than they take in largely due to permafrost thaw….. ihttp://earthobservatory.nasa.gov/Features/BOREASCarbon/

      Yet the ‘debate’ continues. Regardless of the timescales of change, if we don’t do something radical with our way of life, the planet will alter whilst we stand still. Earth will change with or without us. The question is, will we adapt or die….. We can keep on arguing over causes, but as you said yourself, there is no doubt we 7 billion people affect the planet. Believe you and me, most people who research this stuff despair that it is happening. The main problem we face is that society organises to prevent change.

  • Devika Rani

    Global warming and anthropogenic sources could create climate changes rapidly in populated urban zones. Countries experiencing pollution related smogs during thermal inversions are directly affected. Longer periods of change could lead to 1-2 degrees increase globally but locally it could mean 4-5 degrees if emission factors are not checked. CO2 increase in the atmosphere could be beneficial and adverse depending on weather, season and a drastic temperature change created by land and water masses. No one wants to freeze over either locally or globally. Global policies for control over air and water quality change should be supported scientifically and accepted by the educated.

  • Doug Nusbaum

    Assume that you have a beautiful plant. Poison Ivy. It keeps giving you sores.
    Are you going to argue with it? No, because trying to argue with a plant makes you dumber than a plant. But the deniers of global warming are dumber than plants.

    Plants know that GW exists, and are acting accordingly. One of these ways is to move up (at anywhere from 1 to 10 feet per year over the past 100 years) Another way is to move towards the poles. This is a global phenomenon, and is well known to any person with a triple digit IQ who studies plants, botany, and ecologies. People living in mountanious areas for extendet periods of time have observed this movement of species of plants.

    So best to ignore the deniers, cause you have the same chance of changing their mind as you of using logic to change the behavior of poison ivy. Are you really going to tell the plant that if it keeps giving you sores than you are going to stop watering it? Really???

    • Jimmy Dean

      First off, you’re behind the times with respect to the latest warming jargon. It’s no longer “Global Warming” but “Climate Change” that the zealots employ in their attempt to convince us that the sky is falling. A modicum of critical thinking might illuminate that light bulb over one’s head as to why the change in nomenclature. it’s a sharp right turn due to the unexplained interruption in the scientists’ graphed path of warming that prompted a change in terminology. Easier to smokescreen the issue by saying (as non-scientist nitwits like Al Gore have been saying for years) that unforeseen decreases in temperature are just apart of the climate change dynamic—predictable even though not predicted. Very funny.

      Secondly, I believe that the term “Climate Change” is accurate, but not as significant as the true believers suggest it to be. The earth has been undergoing warming and cooling trends for millennia—FAR longer than any humans have been in existence to influence said climate change. The main objection I have with the alleged science behind the doom and gloom predictions involves the complexity of our planet’s climate system and man’s erroneous assertion that a flawed climate change computer model is adequate to predict future conditions.

      Google it and explain it away if you can.


      Knowing what I know about how these computer model programs are set up, I predicted a decade ago that they would be discredited and it has happened. The East Anglia scandal wasn’t one for the ages, but it did prove one thing; that scientists have allowed an agenda to trump hard science in this debate, and as the years go by and the disaster scenarios become less and less credible, more and more people are taking it less and less seriously. In 1997, our Senate voted 95-0 to reject the Kyoto Protocols. It was unanimous—Republicans AND Democrats voted it down, and that was during the administration of a stalwart defender of our “fragile” environment (a Democrat, of course!). Do you think that tally would be any different if the same protocols were brought to a vote today? Think again:


      • darcy_2k

        The US senate isn’t always correct you know. I’m sure you know that. And the US is arguably the greatest beneficiary or even perpetuator of the status quo / business as usual. Conflict of interest?

    • Jimmy Dean

      Plants know that GW exists, but what about the researchers on the continent of Antarctica? Someone get the news to those folks down there, okay, cuz at the moment things aren’t looking too warm or even slushy at the South Pole:



    I highly suggest that everyone commenting here learn
    about the Milankovich Cycle Theory then we can discuss.

  • Scott Sinnock

    “even 2C would put people and ecosystems at “extreme” risk” according to Mandia. What’s the problem. People move to warmer climates, higher T, higher CO2 = more food for people, less death due to winter freezing (like now in Chicago where it is now 20 below), longer growing seasons, less heating oil used, etc, etc. The climate debate is currently a “risk analysis” but should be a risk-benefit analysis. There are a lot of benefits to a warmer climate. Would you prefer an ice age?.

  • Scott Sinnock

    I love a theory where every fact supports it. Temperature increases, simple, due to global warming. Temperature decreases, simple, due to global warming. If not by data, how is the theory to be refuted, ala Karl Popper?

  • Scott Sinnock

    I wonder what the “agenda” is of the climate change zealots. I guess I can never know another’s reasons, but the idea of original sin ranks high on my list. A more Machiavellian reason might be to justify a carbon tax to pay for all the pensions, heart operations, and cancer treatments we have promised ourselves, i.e. all us old folk. Such a tax will just add to our theft from the younger generations, because they still outnumber us, but that is changing, so the debts pile up. Solution? — Bad Carbon — another sin tax.

  • SA_NYC

    That’s funny, I just read today that the globe just experienced its warmest average month ever–for the second month in a row. Hottest years ever on record are 2010 and 1998 (2010 above 1998), with 2014YTD on pace for third.

    So, how’s that cooling trend looking?