That P-word

This post will deal with the word which, until recently, no self-respecting climate scientist would dare utter. The word on using which there was no consensus among those holding stakes in the global warming debate. This word is PAUSE.

Has there been a PAUSE in the global warming in the last 15-17 years? If so, why has the global temperature stopped responding to the relentless increase in the CO2 levels in the atmosphere? Does this materially change the long-term projections of a continued, dangerous warming linked to the human emissions? The blogsphere has been abuzz with the issue for a while now and the mainstream media reluctantly followed. The PAUSE, once a dirty word manipulatively used by the sceptics, has now become an accepted fact. The global warming has PAUSED but – according to the climate scientists – it does not matter! True, the warming trend may have PAUSED temporarily but it will come back with a vengeance:

http://www.dailymail.co.uk/sciencetech/article-2374777/Global-warming-paused-natural-causes-continue-rise-warn-scientists.html

This has raised two interesting issues. The first question is whether the PAUSE was expected by the climate science. If it was then this is business as usual and we should all move on with carbon taxes, subsidising wind farms etc, cheered along by the obliging media. But there are some signs that the journalists are beginning to question the PR statements from the likes of Met Office:

http://www.bbc.co.uk/news/science-environment-23409404

Gradually the words ‘pause’ and ‘hiatus’ which first featured in the blogs have crossed to the media and then to the scientists professionally engaged in researching the global climate. (…) On top of that, the scientists say, pauses in warming were always to be expected. This is new – at least to me. It is common sense that climate change would not happen in a neat, linear away but instead in fits and starts. But I’ve never heard leading researchers mention the possibility before. (…) I asked why this had not come up in earlier presentations. No one really had an answer, except to say that this “message” about pauses had not been communicated widely. But what about another possibility – that the calculations are wrong? What if the climate models – which are the very basis for all discussions of what to do about global warming – exaggerate the sensitivity of the climate to rising carbon dioxide?

These acerbic comments, coming from the Science Editor of BBC David Shukman, are quite remarkable. No longer are the words of the climate gurus taken as Gospel and the journalists are beginning to cautiously do what they were always supposed to be doing – investigate by critical questioning. Even a few years ago anyone asking inconvenient questions like why had no one told us that a PAUSE was on the cards before it started would have been branded a climate change denier. Looks like the tide may be turning.

But the issue which puzzled me even more when the PAUSE first began was more fundamental in nature. The official stance, exemplified by the IPCC reports:

http://www.ipcc.ch/publications_and_data/ar4/syr/en/mains2-4.html

is that:

Most of the observed increase in global average temperatures since the mid-20th century is very likely due to the observed increase in anthropogenic GHG concentrations.

If we accept this we should have an upward sloping wiggle in the global temperature graph. The wiggling would represent natural variability – sometimes warming, sometimes cooling – trumped by the consistent rise resulting from the greenhouse gas forcing. A plateau of any significant length in the graph would tell me that other factors are temporarily overriding the GHG concentrations influence. But, if other influences can negate the GHG signal, how can we be sure that the warming since mid-20th century was not due to the very same influences magnifying it? If you think about it, a weaker signal cannot completely cancel out a stronger, long term trend. If it does it is not weaker! This bugged me in early 2000s but then I stopped thinking about it. Until the news about the release of a new climate science paper this morning:

http://www.bbc.co.uk/news/science-environment-23854904

Scientists say the slowdown in global warming since 1998 can be explained by a natural cooling in part of the Pacific ocean.

The bombshell is buried in the body of the report where it was picked up by Judith Curry:

http://judithcurry.com/2013/08/28/pause-tied-to-equatorial-pacific-surface-cooling/#more-12751

My mind has been blown by a new paper just published in Nature.

(…)

 I’m not sure how good my eyeball estimates are, and you can pick other start/end dates.  But no matter what, I am coming up with natural internal variability associated [sic] accounting for significantly MORE than half of the observed warming. Like I said, my mind is blown. I have long argued that the pause was associated with the climate shift in the Pacific Ocean circulation, characterized by the change to the cool phase of the PDO.  I have further argued that if this is the case, then the warming since 1976 was heavily juiced by the warm phase of the PDO. I didn’t know how to quantify this, but I thought that it might account for at least half of the observed warming, and hence my questioning of the IPCC’s highly confident attribution of ‘most’ to AGW.

What Dr Curry is saying is that the graphs in the report appear to show that the same Pacific Decadal Oscillation which negated the recent GHG warming was also responsible for over half of the pre-1998 observed global warming. If this is correct, the IPCC report attributing “Most of the observed increase in global average temperatures since the mid-20th century” to GHG is spectacularly wrong! The climate scientists have painted themselves into a corner – they cannot explain the PAUSE without PDO but a PDO strong enough to negate GHG warming is also responsible for most of the warming pre-1998, which they thought was attributable to GHG.

Watch this space!

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