Nothing New: On natural variability and global mean surface temperature

Denier blogs are going through a hiatus so I wandered a bit and got to reading a conversation, of sorts, at Judith Curry's blog (archived here). A couple of pro-science types decided to join in to help inform lurkers, while deniers were busy flogging their dead horses.

The subject was the slowdown on global mean surface temperature over the past few years, when various influences came together in a particular way. (Eg the PDO in a cool phase, volcanic aerosols in the stratosphere etc).

At one point someone wrote how he didn't think scientists anticipated natural variability in climate. He was questioning Tamsin Edwards who he reported as writing: "That pause in warming of the atmosphere surprised the media and public, even though scientists always expected this kind of thing could happen in the short term".

Nick Stokes pointed out that the recent slowdown was "simply a consequence of known variability", but the person who called himself the "Editor of the Fabius Maximus website" was having none of it. (If you're familiar with the climate blogosphere, imagine a cross between obsessive blogger Steve McIntyre and word-game player "nevaudit" Willard). Playing word games, and quoting Nick Stokes, he wrote:

“Many different things could happen, and there is no reason why anyone would speculate about this particular form in advance.”

So you believe Edwards is incorrect and that a pause was not expected. 

Nick set him straight again, pointing out that expecting internal variability and predicting the specific timing and nature of it, aren't the same thing, but are not contradictory either. (It turned out that Fabius Maximus was more interested in playing games than having a genuine conversation, so the to-ing and fro-ing went on for a bit but quickly devolved in the manner that scientist/denier conversations tend to do.)

That little exchange did get me thinking, in part because deniers are so black and white when it comes to climate. They are not known for understanding subtlety (or understanding climate science). So, I thought that for the record, I'd put together some relevant extracts from the IPCC reports over the years, to show that from the outset scientists were telling policy makers and the general public that the surface temperature won't always increase in a straight line. That there is natural variability for one thing. Bold italics are mine. Other bold is from the text.

From the Policymakers Summary from the First Assessment Report (FAR - 1990):

The projected temperature rise out to the year 2100, with high, low and best-estimate climate responses, is shown in Figure 8. Because of other factors which influence climate, we would not expect the rise to be a steady one....

...Because of long-period coupling between different components of the climate system, for example between ocean and atmosphere, the Earth’s climate would still vary without being perturbed by any external influence. This natural variability could act to add to, or subtract from, any human-made warming, on a century time-scale this would be less than changes expected from greenhouse gas increases. 

From the Summary for Policymakers in the Second Assessment Report (SAR - 1995):

In all cases the average rate of warming would probably be greater than any seen in the last 10,000 years, but the actual annual to decadal changes would include considerable natural variability. Regional temperature changes could differ substantially from the global mean value.

And from the SAR Technical Summary:

The model results exhibit "natural" variability on a wide range of time- and space-scales which is broadly comparable to that observed. This "natural" variability arises from the internal processes at work in the climate system and not from changes in external forcing. 

From the Summary for Policymakers in the Third Assessment Report (TAR - 2001):

The response to anthropogenic changes in climate forcing occurs against a backdrop of natural internal and externally forced climate variability. Internal climate variability, i.e., climate variability not forced by external agents, occurs on all time-scales from weeks to centuries and even millennia. Slow climate components, such as the ocean, have particularly important roles on decadal and century time-scales because they integrate weather variability. Thus, the climate is capable of producing long time-scale variations of considerable magnitude without external influences. Externally forced climate variations (signals) may be due to changes in natural forcing factors, such as solar radiation or volcanic aerosols, or to changes in anthropogenic forcing factors, such as increasing concentrations of greenhouse gases or aerosols. The presence of this natural climate variability means that the detection and attribution of anthropogenic climate change is a statistical “signal to noise” problem.  

The AR4 report (2007) is written in a different style to the previous reports. The summary sections are written bullet-point style, missing nuance and subtleties. Those sections seems to be written for a scientifically illiterate audience. By contrast the report proper is mostly highly technical and assumes a depth of scientific knowledge that few laypersons would have. There isn't much in between the two extremes. The main report misses the explanations a reasonably informed member of the general public might look for, although a scientist wouldn't need them to be spelt out. The figures and charts of model projections in AR4 demonstrate that they provide for internal variability. However I couldn't find any passage as clear as the ones in FAR, SAR and TAR. One that comes close, for anyone who gets that far into the guts of the report is in Chapter 10, which is about climate models:

Interannual variability is evident in each single model series, but little remains in the ensemble mean because most of this is unforced and is a result of internal variability, as was presented in detail in Section 9.2.2 of TAR. Clearly, there is a range of model results for each year, but over time this range due to internal variability becomes smaller as a fraction of the mean warming. 

The most recent report, AR5 (2013), does discuss internal variability and even has a section on the so-called "hiatus". The second point in the Executive Summary of Chapter One is:

The processes affecting climate can exhibit considerable natural variability. Even in the absence of external forcing, periodic and chaotic variations on a vast range of spatial and temporal scales are observed.

I'd say that AR4 is the only IPCC report that didn't have the section for Dummies, spelling out that the climate is subject to natural variability as well as natural forcings as well as human-caused forcings. All the others did spell it out quite clearly. It was assumed in AR4, for example, in the charts of model projections, but it wasn't spelt out in big letters.

I've said this before and I'll say it again, because it's a good idea :D As its reports get more complex, the IPCC needs scientific writers involved - and it has said it will.

It needs people who specialise in writing science for consumption by the general public. Not the researchers themselves, but specialist writers. That's essential in my view, so that more people can more easily understand the scientific advances. There's got to be a balance between i dotting and t crossing, and communicating meaning to the reader. AR4 wasn't quite there. It was more concerned with i's and t's. AR5 WG1 is a bit better although some sections are really pretty horrible from a communication perspective. It's awful to think of all the hours put in by scientists volunteering their time to the report preparation, only to have their effort wasted on most of the readers, because they can't understand what's been written.