Did Someone Say ‘PDO’?

by Heidi Cullen, Philip Duffy and Claudia Tebaldi

Earlier this month, The New York Times ran a front-page story looking into why climatologists and TV meteorologists are at odds over global warming.  (In terms of full disclosure, one of the authors of this blog – Dr. Heidi Cullen – was quoted in the Times story).  The article pointed out that while climate scientists almost universally agree that human activities, such as the burning of fossil fuels, are warming up the planet, a significant percentage of TV meteorologists do not. In fact, a recent study from George Mason University and the University of Texas at Austin showed that out of 571 TV meteorologists surveyed, only about half believed that global warming was happening and fewer than a third accepted the proposition that climate change was “caused mostly by human activities.” The survey also suggested that TV meteorologists view climate change as mostly a natural phenomenon.

Joe Bastardi, a senior meteorologist at Accuweather, stands squarely in the natural causes camp and he offered up his own explanation recently on Comedy Central's The Colbert Report. On the comedy show, Bastardi said the global warming trend is just temporary and caused by a mix of volcanic activity, solar cycles, warmer ocean temperatures and specifically a natural climate pattern known as the “PDO” or  Pacific Decadal Oscillation.

We welcome that Bastardi is investigating and communicating information about climate change. In fact, one point mentioned in the George Mason survey that got very little traction in the mainstream media was that “many of the weathercasters said that having access to resources such as climate scientists to interview and high-quality graphics and animations to use on-air would increase their ability to educate the public about climate change”.  

Bastardi has provided a great opportunity to educate the public about climate change. And as climate scientists, we’d like to take a moment to talk about natural climate variability specifically. The solar cycle and volcano arguments Bastardi gravitates toward are fascinating. But when it comes to climate change – these natural sources of climate variability are incapable of doing the heavy lifting. In fact, they’ve been raised, tested, and solidly laid to rest by the climate science community. Variations in solar output are too weak, and in any case repeat every 11 years, and so cannot explain a steady warming trend over 40+ years 1. As for the volcano argument, eruptions are also too puny. Globally, volcanoes, like Iceland’s Eyjafjallajokull volcano as well as those under the sea release a total of about 200 million tonnes of CO2 annually. That may sound like a lot, but it’s trivial when compared to human activity. According to the U.S. Department of Energy's Carbon Dioxide Information Analysis Center (CDIAC), global fossil fuel CO2 emissions for 2003 tipped the scales at 26.8 billion tonnes—over 100 times more. Let’s just say human activity can bench press a whole lot more warming than the sun and volcanoes combined.

Before we move on to the role of the Pacific, we want to first thank Bastardi for daring to mention the phrase P-D-O on television. While geeks like us find the Pacific Decadal Oscillation fascinating, acronyms have a tendency to make the public’s eyes glaze over – so kudos to him for putting it out there.  

The PDO is just one of many natural oscillations in the climate system. It is characterized by a positive or “warm”  phase, and a negative or “cool” phase, which refer to the pattern of anomalies in sea surface temperatures and air pressure between the north central Pacific Ocean and the northeastern Pacific. The El Niño/La Nina cycle, for example, is another natural oscillation. Its period, about three to seven years, is shorter than the PDO’s, but in fact, the PDO is often thought of a slower version of El Niño, as some of the manifestations are similar.

For example, in the warm phase of the PDO temperatures in the Northwest of North America tend to be warmer than average, while the southeastern US tends to be cooler than average.

Bastardi believes the warming trend (shown below) is only temporary because the phase in which the PDO has predominantly been at the same time, with its warmer than average tropical Pacific Ocean temperatures, is temporarily juicing the system. He forecasts the global temperature trend will dip back down once the PDO shifts back.

Bastardi believes the warming trend (shown below) is only temporary because the PDO, with its warmer than average northern Pacific Ocean temperatures, is temporarily juicing the system. He forecasts the global temperature trend will dip back down once the PDO shifts back.  


Here’s the problem. First and foremost, while the PDO is important in driving regional climate variations, it has no clear effect on global temperatures. And although the PDO was in its warm phase during the majority of the time from the mid 1970s through the present, it also shifted sharply in multiple instances (see the chart above), which is inconsistent with the steady global warming trend during the same period. For example, the decade from 2000 to 2009 was the warmest on record globally, but the PDO was not positive throughout that period.2 3.

It has been said that the truth is stubborn. This idea gives climate scientists a small sense of relief in that eventually, the stubborn truth will be recognized – that the recent global warming trend is real and caused mostly by human activities.


1Duffy, P.B., Santer, B.D., & Wigley, T.M.L., Solar Variability does not explain late-20th-century warming. Physics Today (January 2009), 48-49 (2009)

2Chen, J., Del Genio, A.D., Carlson, B.E., & Bosilovich, M., The Spatiotemporal Structure of Twentieth-Century Climate Variations in Observations and Reanalyses. Part II: Pacific Pan-Decadal Variability. Journal of Climate 21, 2634-2650 (2008).

3Meehl, G.A., Hu, A., & Santer, B.D., The Mid-1970s Climate Shift in the Pacific and the Relative Roles of Forced versus Inherent Decadal Variability. Journal of Climate 22, 780-792 (2009).]]