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All IPCC definitions taken from Climate Change 2007: The Physical Science Basis. Working Group I Contribution to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change, Annex I, Glossary, pp. 941-954. Cambridge University Press.

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The Pacific Decadal Oscillation (PDO) is not causing global warming

What the science says...

Select a level... Basic Intermediate Advanced

The PDO shows no trend, and therefore the PDO is not responsible for the trend of global warming.

Climate Myth...

It's Pacific Decadal Oscillation

"The Pacific Decadal Oscillation (PDO) is a temperature pattern in the Pacific Ocean that spends roughly 20-30 years in the cool phase or the warm phase. In 1905, PDO switched to a warm phase. In 1946, PDO switched to a cool phase. In 1977, PDO switched to a warm phase. In 1998, PDO showed a few cool years. Note that the cool phases seem to coincide with the periods of cooling (1946-1977) and the warm phases seem to coincide with periods of warming (1905-1946, 1977-1998)." (The Reference Frame)

At a glance

Oscillate. To move repeatedly from side to side or up and down between two points, or to vary between two states or amounts. To vary above and below a mean value. To move or travel back and forth between two points. To swing backward and forward like a pendulum.

These and similar definitions are to be found if you look up the meaning of 'oscillate' online. Yet global warming is wobbling its way up a one-way course. We've just witnessed the hottest year since temperature records began (2023). Every few years that record goes again. Conclusion: global warming is not an oscillation.

The Pacific Decadal Oscillation or PDO is one of a number of phenomena that affect the world's major oceanic basins. It is a good example of heat being moved around within the ocean and atmosphere. Like all climatic oscillations it has warm, neutral and cool modes and these may endure for years or decades. Oscillations like this do not correspond to a timetable, but are irregular in nature.

The PDO is directly driven by conditions in the northern Pacific but has considerable reach in its effects. Prevailing winds and atmospheric pressure-patterns over that ocean dictate the mode. When winds are predominantly from the southwest, warmer conditions occur along the western USA seaboard. That is due to the onshore transport of warm, subtropical waters. Conversely, when winds are mainly from the north, upwelling of cool and nutrient-rich waters occurs in the open ocean, with cooler conditions prevailing.

Notable long, warm modes of the PDO include 1925-1946 and 1977-1998. 1947-1976 was a lengthy cool phase. More recently, the flip-flopping has been of a much shorter duration with cold and warm phases lasting just a few years. The reason for this switch is incompletely understood.

Like the El Nino Southern Oscillation or ENSO, which flips around over annual timescales, the PDO affects weather patterns, particularly in Asia and North America. It also has considerable impacts on fisheries and if there was one good reason to understand the PDO, it's right there. However, despite the loose coincidence with global temperatures in the early and mid-20th Century, that apparent relationship is no more. For example, a negative PDO mode commenced at the end of 2019 and was still ongoing in mid-2023, the latter having been the warmest year globally since records began.

Like all oscillations, there is no net gain or loss of heat involved in the PDO. It is merely a pattern involving how the heat in the system is being moved around within it. Global warming is different because it involves impeding the loss of heat, originally reaching the planet as sunshine, back out to space. That makes it a climate forcing agent. Big difference.

Oscillate. It's all in the name.

Please use this form to provide feedback about this new "At a glance" section. Read a more technical version below or dig deeper via the tabs above!

Further details

The Pacific Decadal Oscillation (PDO) is a climate phenomenon that occurs primarily in the North Pacific Ocean. It has wide ranging effects on weather patterns, especially over North America and Asia. Like other ocean-basin oscillations, it has a warm mode, expressed as positive values in the PDO index, and a cool mode, with negative values. These modes last anywhere from a few years to multiple decades and feature changes in sea surface temperatures.

While the causes of the PDO are still poorly understood, the primary effects seem to be changes in northeast Pacific marine ecosystems and therefore fisheries. Also they affect the position of the jet stream's path, that may in turn impact agriculture.

During the PDO positive mode, winters in the southern and eastern US states tend to have above average temperatures and higher rainfall. In the western and north-western states, the opposite is the case. Asian winters tend to be cooler and dryer, although above normal temperatures and higher rainfall tend to occur over India.

In the negative PDO mode, warmer and drier winters occur through much of the contiguous USA, with cool conditions confined to the north-west, although parts of the central USA may see notably wet conditions. Over in Asia, India and China see relatively cool and wet winters, whilst Japan has both the warmth and the rainfall. Clearly, a key impact of the PDO is on agriculture, hence its extensive study and the substantial scientific literature surrounding it.

It is important to note, however, that the PDO modes are not set in stone. Frequently, especially in recent years, short sets of 1-5 warm years have occurred during a cool phase and vice-versa. In addition, the warm and cool modes are less descriptive than they would appear. The cool mode, for instance, is in fact associated with high sea surface temperatures in the Northern Pacific (Fig. 1). Another important point is that the hottest year in the global temperature record, 2023, has occurred within a negative PDO mode.

Example of the PDO warm mode.Example of the PDO cool mode.

Figure 1: Examples of the PDO warm mode (above) and cool mode (below). During the positive PDO mode, sea surface temperature anomalies over the North Pacific Ocean form a vast cool area north of Hawaii. At the same time, warmer than normal waters are present near the North American coast. During negative PDO conditions, warm waters are found north of Hawaii and cooler than normal waters are encountered near the North American coast. Images courtesy of World Climate Service.

Because the PDO is an oscillation, it does not present a clear trend. If you compare the Global Temperature Anomaly alongside the PDO Index (fig. 2), you will see that although the PDO index appears to influence short-term temperature changes, global temperatures have had a distinct upward trend, especially since the late 1970s.

 Global temperature anomaly 1850-2023.

PDO time series.

Figure 2: Top-panel: global temperature anomaly 1850-2023. Graphic: Realclimate. Bottom-panel: Pacific Decadal Oscillation index, 1870-2023. Smoothed data (thicker black line) included. Graphic: NOAA.

Natural oscillations like the PDO simply move heat around from oceans to air and vice-versa. They don't have the ability to either add or remove heat to or from the overall system. Therefore, they're not capable of causing a long-term warming trend like that of the last 50+ years. Instead they are another example of a process causing short- to medium-term temperature variations. Basically they're good examples of internal climate variability. If the PDO was responsible for warming the surface, the oceans would be cooling, which is not the case.

The long term warming trend on Earth is due to increasing greenhouse gas levels. These constitute an external radiative forcing, creating an energy imbalance. In contrast, the PDO is an internal process and does not increase or decrease the total energy in the climate system. Essentially, like other such oscillations, it cancels itself out. The fact that its name defines it as an oscillation should communicate that fact.

Last updated on 31 March 2024 by John Mason. View Archives

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Argument Feedback

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Many thanks to John Cross who co-authored this post. Thanks also to Josh Willis for his advice on this topic.


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Comments 126 to 137 out of 137:

  1. Sorry, please replace "Tom" with "Sphaerica" in #124 above. Sorry Sphaerica!
  2. 123, Eric the Red, Gee, how very nefarious of me. Okay, lets start at 1995, which was during an El Nino, and includes the world temperature record 1998 El Nino at the start of the period. We'll end in 2009 (during a La Nina). or, if you prefer, we can end in 2010, again in the midst of a moderate La Nina. Any other cherries you'd like unpicked?

    [DB] Perhaps this is relevant?

  3. Sphaerica, You still have a time frame when the ENSO index is essentially zero. The goal posts have not moved. As I stated previously, the period starting in 2007 may suffice, if it persists. Something similar to 1950 - 1977.
  4. Eric the Red @114, again to just make sure we are on the same page, unless the climate system is very close to a tipping point (within 0.2 degrees c), ENSO fluctuations cannot result in a "step change" in global mean temperatures. Another way to look at it is this, if you have a period of predominantly La Ninas and follow with a period of predominantly El Ninos, in a period of no underlying trend, in the immediate period of the transition that will introduce a spurious trend to global mean temperatures. But outside of any interval that ranges across the point of transition, that change will introduce no trend to the series. Do you agree? I believe that this is a straightforward conclusion from the fact that there are no long term consequences from an ENSO fluctuation. I am seeking this clarification because you qualify your response by mentioning La Ninas, whereas the lack of long term effect of an El Nino is independent of any follow on La Ninas.
  5. Tom, In essence we agree. There are no long term consequences from an ENSO fluctuation, but the effects are evident in the short term. The fluctuations do not produce a "step change," but rather a relatively smooth transition.
  6. Eric the Red So after all this, it seems your position is that ENSO causes short terms fluctuations in surface temperatures and we need to consider this in attributing climate changes to the various forcings? That has been known for a very long time and news to nobody. It is the "skeptics" you need to be telling that to, as it is them that use the fluctuations of ENSO to cherry pick arguments such as "did global warming stop in 1998". The effects of ENSO are fairly straightforward to remove via regression, giving results like this It seems to me you have just been backing away from your original point about PDO as people have refuted your argument and ended up arguing for a completely bland position that we all argee with anyway, and which has virtually nothing to do with PDO!
  7. Eric the Red @130, you will note that I said, and that you have now agreed that:
    " if you have a period of predominantly La Ninas and follow with a period of predominantly El Ninos, in a period of no underlying trend, in the immediate period of the transition that will introduce a spurious trend to global mean temperatures. But outside of any interval that ranges across the point of transition, that change will introduce no trend to the series."
    (Emphasis added) The interval from 1977 (chosen because it was a high value for the period) to 2001 (chosen because Trenberth claims 25 years is the minimal period for a significant effect) follows after the end of the period of frequent La Ninas that occurred during the 60's and early 70's. It includes no transition from frequent La Ninas to frequent El Ninos, and included no transition from negative to positive PDO phase. Therefore on the principle quoted above, neither ENSO nor PDO have introduced a trend to that period. Never-the-less the trend over that interval (HadCRUT3) is 0.156 degrees C per decade. Further, the period 1951-1975 does not include a transition from frequent El Ninos to frequent La Ninas, and has a Negative PDO phase throughout. Therefore on the quoted principle, these ocean fluctuations have introduced no trends to that period, and hence the negative trend over that interval (see graph in 131) is not explained by ENSO states or the PDO. Finally, even if we are undergoing a transition between frequent El Ninos to frequent La Ninas, and/or from positive to negative phase PDO, on the quoted principle that would at most introduce a spurious reduction of the trend in the immediate decade of the transition. Over the following decades, and until the next transition (which would introduce a temporary spurious increase to the trend), they cannot be expected to effect the trend at all. Consequently we would predict a resumption of the preceding trend, and hence have no reason to expect ENSO or the PDO to result in a reduce trend for a sustained period into the future. I think these three points follow straight forwardly from the quoted principle, which as I have said follows straight forwardly from the discussion in 109, and to which you have agreed. Given that, why would we not be predicting an increase in temperature by at least 1.56 degrees by the end of this century, and far probably more because of the increasing GHG forcing?
  8. Tom, You seem to be arguing for a step-like shift, whereas I was claiming a more gradual change. Instead of a jump around 1977, the change would slowly increase to a maximum around the trasition year, and slowly subside. If you plot the 5-year moving average for ENSO, the value rises steeply from 1975-1983, crossing zero in 1979. Hence, I would expect the greatest change over that interval. Currently, the moving average is falling, and crossed zero in 2009. We have yet to see if a repeat of the 1940s will occur. Once again, long term, the effects will cancel. As I stated previously, the 130-year near-linear trend is 0.6C / century, with early 2011 CRU data falling slightly below the trend line. I see no reason to deviate from this trend. Remember, an exponential rise in CO2 leads to a linear increase in temperature. Atmospheric CO2 concentrations have not kept pace with the expponential rise recently, so I see no reason for an increase in the temperature trend. Using a shorter time frame to determine a long-term trend may not be the best. Therefore, I think you estimate of 1.56 C by the century is high.
  9. Recommended supplemental reading:

    Going out for ice cream: a first date with the Pacific Decadal Oscillation by Tom Di Liberto, NOAA, Aug 25, 2016

  10. "the 130-year near-linear trend is 0.6C / century"

    The trend is certainly not near-linear since 1887.

    Warming since 1880

    As for why global warming is predicted to accelerate, see here.


    [RH] Fixed image width. Please limit images to 500px.

  11. Recommended supplemental reading...

    Following three record years for global surface mean temperature in 2014-2016, the observed recent slowdown in average global temperature has ended.

    A Pacific flip triggers the end of the recent slowdown, News Release, Met Office, Sep 18, 2017 

  12. Please note: the basic version of this rebuttal has been updated on March 31, 2024 and now includes an "at a glance“ section at the top. To learn more about these updates and how you can help with evaluating their effectiveness, please check out the accompanying blog post @

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