Key Word: Oscillation

The skeptic argument “it’s Pacific Decadal Oscillation (PDO)” suggests that maybe the true cause of recent warming is the PDO.

The PDO is a climate phenomenon that occurs primarily in the North Pacific Ocean.  The “oscillation” happens between warm phases (positive values) and cool phases (negative values) that last anywhere from 10 to 40 years.  The phases are associated with changes in sea surface temperatures (SST).  While the causes of the PDO are still unknown, the primary effects seem to be changes in northeast Pacific marine ecosystems and a changing jet stream path.

Important to note, however, is that the phases are not set in stone; there are frequently short sets of 1-5 warm years during a cool phase and vice-versa.  Secondly, the “warm” and “cool” phases are less descriptive than they would appear.  The cool period, for instance, is actually associated with extremely high sea surface temperatures in the Northern Pacific (see image below).

Figure 1: PDO warm phase (left) and cool phase (right). Image courtesy of JISAO. 

One way to test this skeptic theory is to plot the Global Temperature Anomaly alongside the PDO Index (shown below).  What we find is that although the PDO index appears to influence short-term temperature changes, global temperatures have a distinct upward trend, while the PDO Index does not. 

Figure 2: Pacific Decadal Oscillation index (blue, University of Washington) versus Global Temperature Anomaly (Red - GISS Temp). Smoothed data (thicker blue and red lines) and trend lines (thick straight line) are added.

Natural oscillations like PDO simply move heat around from oceans to air and vice-versa.  They don't have the ability to either create or retain heat, therefore they're not capable of causing a long-term warming trend, just short-term temperature variations.  Basically they're an example of internal variability, not an external radiative forcing.  If PDO were responsible for warming the surface, the oceans would be cooling, which is not the case.

These results are expected.  The long term warming trend is a result of an energy imbalance caused primarily by an increase of greenhouse gases in the atmosphere.  In contrast, the PDO is an internal process and does not increase or decrease the total energy in the climate system.

Good one for taking this on Nicholas - but it seems to me that some explanation is required of the physical effects of the PDO. You only mention the counter-intuitive surface temp issue, but it might be the case that people don't really understand what the PDO does, so the argument becomes a little muddy.

I'm also curious about the last statement. Is it not the case that the PDO uses available energy? That energy cannot be decoupled from the overall climate energy available, surely, so while it may not be said to increase or decrease the total energy, does it reflect the nature, availability or level of that energy - or, more's the point, do the physical effects? Is there a linkage between PDO and storms? (You may need to ignore the second para - I'm starting to get the feeling I'm asking you to research my questions - sorry if that's the case).

Robert - This comment was brought up in the comments section of the Intermediate version of this argument.  As Tom points out there, this would seem to address the skeptic argument "the PDO is the cause of recent warming" but not "the PDO shows a long term warming trend."  I really dont have the technical understanding / expertise to address this issue - if you have any links/help that would be great.  I think the answer may be that as long as it is defined as an oscillation then it inherently has no trend; there would need to be some evidence to suggest that it is in fact not an oscillating phenomenon to suggest otherwise.  

Graham - I added some very basic PDO information into paragraph two and copied the figure from the intermediate argument to highlight the point that the 'warm' and 'cool' phases are more in name than in reality.  

As for your second paragraph (similar to my response to Robert) I'm not really sure anyone knows the answer to these questions (though I would be very interested to find them.)  There is no reason to suggest (or at least ive never heard it) that the PDO in any way changes the incoming/outgoing energy to the climate system - contrasting that with the clear change in energy from the increase in greenhouse gases is what I was going for in the conclusion.

http://www.jisao.washington.edu/pdo/

http://sealevel.jpl.nasa.gov/science/pdo.html 

Dana - great points - I was going for that in the conclusion paragraph but I think explaining it improved the overall argument.  I liked your points so much that I used no original wording (hope thats ok?)

Doug - Though I'm not sure it is essential to the argument I would be open to adding something along those lines but not exactly sure where to put it / how to phrase it.

The PDO index is:

"[The] standardized values for the PDO index, derived as the leading PC of monthly SST anomalies in the North Pacific Ocean, poleward of 20N. The monthly mean global average SST anomalies are removed to separate this pattern of variability from any "global warming" signal that may be present in the data."

 So I agree with Robert, the lack of trend is not a good point. From the definition above, the PDO index is just a simple way to describe the time evolution of a geographic patter of sea surface temperature anomaly. The idea of warm and cold phases is defintely Alaska-centric. Although scientists use this terminology (they know what they're talking about) to avoid confusion we should use positive and negative phase. Finally, the PDO is probably not even an independent pattern of variability but may be strongly related to ENSO.

I suggest to drop the story of the trend and focus on its nature, i.e. a pattern of variability which, as dana said,  "simply move heat around".

P.S. dana your wording turns out to be very popular here :)

One rebuttal I've made against this argument is that the PDO and AMO are temperature indices -- that is, they are derived exclusively from SST data. Thus they reflect temperature change but do not themselves cause temperature change. So saying PDO causes global warming is like saying the heat wave was caused by the thermometer going up.

This is also useful in explaining the (moderately high) correlation between PDO and global temp, and AMO and global temp: it's pretty much required.

An intelligent skeptic might suggest that PDO/AMO is also a measure of an underlying oceanic circulation pattern (which is true), but in that case, I point out the increasing heat content of the oceans, which matches that of the surface. BTW, a good graphic on oceanic heat content can be found here:

http://www.ncdc.noaa.gov/bams-state-of-the-climate/2009-time-series/ohc 

The fundamental point is well-made that the PDO neither adds or subtracts energy, rather it transfers it from sea to air or air to sea.  Therefore since average global temperature is calculated as the average of land and sea temperatures, the PDO can have no effect on that calculation – or can it? 

Heat absorbed by the ocean surface causes convection currents to flow and some of those currents transport heat from the surface (top 700m) to the deep ocean.  If heat transferred to the deep ocean heats cooler deeper water, that means it is no longer available for transfer from the ocean surface to the atmosphere.  If that occurs, does the PDO produce a net reduction in atmospheric temperature?  If so, can it be argued that an oscillation such as the PDO merely transfers heat from air to sea with no effect on average global temperature as defined? 

Sorry if this simply shows my ignorance of the subject but it is an argument that may be made by other equally uninformed readers.

agnostic, I reckon that issue is a commenter's ( or moderator's) opportunity to refer people to the next level up.