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Climate Oscillations and the Global Warming Faux Pause

Filed under: — mike @ 26 February 2015

No, climate change is not experiencing a hiatus. No, there is not currently a “pause” in global warming.

Despite widespread such claims in contrarian circles, human-caused warming of the globe proceeds unabated. Indeed, the most recent year (2014) was likely the warmest year on record.

It is true that Earth’s surface warmed a bit less than models predicted it to over the past decade-and-a-half or so. This doesn’t mean that the models are flawed. Instead, it points to a discrepancy that likely arose from a combination of three main factors (see the discussion my piece last year in Scientific American). These factors include the likely underestimation of the actual warming that has occurred, due to gaps in the observational data. Secondly, scientists have failed to include in model simulations some natural factors (low-level but persistent volcanic eruptions and a small dip in solar output) that had a slight cooling influence on Earth’s climate. Finally, there is the possibility that internal, natural oscillations in temperature may have masked some surface warming in recent decades, much as an outbreak of Arctic air can mask the seasonal warming of spring during a late season cold snap. One could call it a global warming “speed bump”. In fact, I have.

Some have argued that these oscillations contributed substantially to the warming of the globe in recent decades. In an article my colleagues Byron Steinman, Sonya Miller and I have in the latest issue of Science magazine, we show that internal climate variability instead partially offset global warming.

We focused on the Northern Hemisphere and the role played by two climate oscillations known as the Atlantic Multidecadal Oscillation or “AMO” (a term I coined back in 2000, as recounted in my book The Hockey Stick and the Climate Wars) and the so-called Pacific Decadal Oscillation or “PDO” (we a use a slightly different term–Pacific Multidecadal Oscillation or “PMO” to refer to the longer-term features of this apparent oscillation). The oscillation in Northern Hemisphere average temperatures (which we term the Northern Hemisphere Multidecadal Oscillation or “NMO”) is found to result from a combination of the AMO and PMO.

In numerous previous studies, these oscillations have been linked to everything from global warming, to drought in the Sahel region of Africa, to increased Atlantic hurricane activity. In our article, we show that the methods used in most if not all of these previous studies have been flawed. They fail to give the correct answer when applied to a situation (a climate model simulation) where the true answer is known.

We propose and test an alternative method for identifying these oscillations, which makes use of the climate simulations used in the most recent IPCC report (the so-called “CMIP5” simulations). These simulations are used to estimate the component of temperature changes due to increasing greenhouse gas concentrations and other human impacts plus the effects of volcanic eruptions and observed changes in solar output. When all those influences are removed, the only thing remaining should be internal oscillations. We show that our method gives the correct answer when tested with climate model simulations.

2015-02-12-Sci15FigHuffPost.png
Estimated history of the “AMO” (blue), the “PMO (green) and the “NMO” (black). Uncertainties are indicated by shading. Note how the AMO (blue) has reached a shallow peak recently, while the PMO is plummeting quite dramatically. The latter accounts for the precipitous recent drop in the NMO.

Applying our method to the actual climate observations (see figure above) we find that the NMO is currently trending downward. In other words, the internal oscillatory component is currently offsetting some of the Northern Hemisphere warming that we would otherwise be experiencing. This finding expands upon our previous work coming to a similar conclusion, but in the current study we better pinpoint the source of the downturn. The much-vaunted AMO appears to have made relatively little contribution to large-scale temperature changes over the past couple decades. Its amplitude has been small, and it is currently relatively flat, approaching the crest of a very shallow upward peak. That contrasts with the PMO, which is trending sharply downward. It is that decline in the PMO (which is tied to the predominance of cold La Niña-like conditions in the tropical Pacific over the past decade) that appears responsible for the declining NMO, i.e. the slowdown in warming or “faux pause” as some have termed it.

Our conclusion that natural cooling in the Pacific is a principal contributor to the recent slowdown in large-scale warming is consistent with some other recent studies, including a study I commented on previously showing that stronger-than-normal winds in the tropical Pacific during the past decade have lead to increased upwelling of cold deep water in the eastern equatorial Pacific. Other work by Kevin Trenberth and John Fasullo of the National Center for Atmospheric Research (NCAR) shows that the there has been increased sub-surface heat burial in the Pacific ocean over this time frame, while yet another study by James Risbey and colleagues demonstrates that model simulations that most closely follow the observed sequence of El Niño and La Niña events over the past decade tend to reproduce the warming slowdown.

It is possible that the downturn in the PMO itself reflects a “dynamical response” of the climate to global warming. Indeed, I have suggested this possibility before. But the state-of-the-art climate model simulations analyzed in our current study suggest that this phenomenon is a manifestation of purely random, internal oscillations in the climate system.

This finding has potential ramifications for the climate changes we will see in the decades ahead. As we note in the last line of our article,

Given the pattern of past historical variation, this trend will likely reverse with internal variability, instead adding to anthropogenic warming in the coming decades.

That is perhaps the most worrying implication of our study, for it implies that the “false pause” may simply have been a cause for false complacency, when it comes to averting dangerous climate change.

158 Responses to “Climate Oscillations and the Global Warming Faux Pause”

  1. 151
  2. 152
    JH says:

    I agree that there is NO slowdown in forced climate change in response to CO2 forcing.

    ” Note the term “global warming” in the context of global warming hiatus and in Intergovernmental Panel on Climate Change (IPCC) reports is not equivalent to forced climate change (FCC). Global warming is measured by the anomaly of globally averaged SAT relative to a reference climate mean state, say, the average over 1961–1990. Defined in this way, the transient SAT anomaly (or global warming) involves not only the long-term trend related to FCC but also internal variability at various time scales. Therefore, the global warming hiatus may be caused by either the slowdown in FCC, or a negative SAT trend due to internal variability that cancels out the forced trend on decadal and interdecadal time scales, or the combined effect of FCC and internal variability. ” ( Lu,Hu,and Zeng http://onlinelibrary.wiley.com/doi/10.1002/2014GL059908/full )

    ” The global warming hiatus does not necessarily mean a hiatus in anthropogenic greenhouse gas forcing and forced climate change (FCC). Here we show that both Atlantic Multidecadal Variability (AMV) and Pacific Decadal Oscillation (PDO) may cause fluctuations of the global mean surface air temperature (SAT) and hence alternately positive and negative trends on (inter)decadal time scales. However, the forced SAT change under a mean state due to greenhouse gas forcing does not depend on different phases of AMV and PDO, implying the effect of the FCC and internally generated climate variability can be linearly added up to the observed transient global mean SAT. ” ( http://onlinelibrary.wiley.com/doi/10.1002/2014GL059908/abstract)

  3. 153
    jgnfld says:

    @140

    1. Re. “This is plenty long enough to constitute climate and climate cycles, and if today’s climate is put into the context of the last couple of millennia and the Holocene there is nothing unusual going on. If you are going to cite Mann, Briffa, Jones, et al, or the IPCC as a rebuttal don’t bother.”

    This is simply another way of saying: “I don’t think anything unusual is happening. Don’t bother to cite the scientific literature that shows I am wrong.” If you are not willing to accept the scientific literature and posit some “establishment” conspiracy, go ahead. But then you really should be posting on conspiracy sites, not on a science site.

    If you don’t accept Mann’s research what about the large number of other reconstructions by multiple independent groups using multiple independent proxies and analyzed by multiple methods all showing general agreement? (See IPCC report for list complete to several years ago.)

    2. To try to state cherrypicking points made by more skilled others in terms perhaps even you can understand with 2 undergrad stats courses: If you flip a coin and the first 7 come up heads are you surprised? (Answer is yes to better than .05) If you flip 100 times are you surprised to see 1 or more runs of 7 heads? (Answer is most definitely no. Actually 52% likely.)

    Further, given you start with a series of 7 heads, what is the probability of seeing 8 heads in a row? Is it 2^8? No. It is 50%. Such is how cherrypicking bias works.

  4. 154
    jgnfld says:

    1/2^8 in final paragraph, obviously.

  5. 155
    wili says:

    Another reason why we may be seeing an increase in the rate of global warming in the coming years and decades:

    http://thinkprogress.org/climate/2015/03/12/3632373/carbon-sinks-climate-action/

    “uptake rate, kS, declined over 1959–2012 by a factor of about 1/3”

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    Peter Reason says:

    What is the general view of the argument put forward by Richard Seager & others that the North Atlantic Currents do not bring sufficient warmth to NW Europe to create the temperate climates as is so often claimed? I ask not as a climate scientist but (hopefully) informed activist/writer

  8. 158
    Hank Roberts says:

    > Richard Seager

    Found this:
    http://www.ldeo.columbia.edu/res/div/ocp/gs/
    Lamont-Doherty Earth Observatory of Columbia University

    at the latitudes of Europe, the atmospheric heat transport exceeds that of the ocean by several fold. In winter it may even by an order of magnitude greater. … how regional climates are determined around the Atlantic turned out to be the Rocky Mountains. Because of the need to conserve angular momentum, as air flows from the west across the mountains it is forced to first turn south and then to turn north further downstream. As such the mountains force cold air south into eastern North America and warm air north into western Europe. This waviness in the flow is responsible for the other half of the temperature difference across the North Atlantic Ocean.
    … The Gulf Stream and future climate change
    A slowdown of the Gulf Stream and ocean circulation … would probably be overwhelmed by the direct radiatively-driven warming by rising greenhouse gases.