Climate: Cherries are not the only fruit

Man using "cherry-picker" Image copyright Reuters
Image caption Accusations of selective data use abound... but who's right?

Just about the most predictable event of the week was the tempest of opinion created by the analysis of global temperature changes published in the Proceedings of the National Academy of Sciences (PNAS) on Monday.

As we (and a number of other mainstream news outlets) reported, Robert Kaufmann and colleagues analysed the impact of growing coal use, particularly in China, and the cooling effect of the sulphate aerosol particles emitted into the atmosphere.

They concluded that with a bit of help from changes in solar output and natural climatic cycles such as the El Nino Southern Oscillation (ENSO), the growth in the volume of aerosols being pumped up power station chimneys was probably enough to block the warming effect of rising greenhouse gas emissions over the period 1998-2008.

For some commentators, such as the UK Daily Mail's Christopher Brooker, this was further proof that the "climate scaremongers" had got it wrong.

"Global warming? A new ice age? YOU'RE paying for the hysteria of our politicians," the headline - er - whispered.

On the other side of the opinionosphere, Climate Progress's chosen headline was "Study: Hottest Decade on Record Would Have Been Even Hotter But for Chinese Coal Plant Sulfur Pollution"... which is consistent with what Kaufmann and his colleagues are saying, although they said it in more restrained tones.

Although it doesn't slam the study, in fact calling it "clever", Climate Progress also asks whether doing the research was wise: "What's not clever about this study is that it repeats the myth that there was a 'hiatus' [in global warming] in the first place".

Image copyright AP
Image caption Rising coal use put more warming CO2 into the atmosphere - but also more cooling aerosol particles

According to some e-mails I've had, the same point is being raised between climate scientists.

Whether the conclusions of the study are right or wrong, the argument goes, you're stepping into factually shaky ground - and the belief-systems of your "opponents" - if you start from the argument that temperatures haven't risen since 1998, the strongest El Nino year on record.

Concern was exacerbated by the wording of the PNAS press release, whose first sentence read: "The atmospheric concentration of carbon dioxide increased steadily between 1998 and 2008 even as the Earth's temperature declined..." - a line that reporters unfamiliar to the issue might not have seen fit to question.

The next few paragraphs will be so familiar to anyone who follows this stuff that I almost apologise for including them... but the point about it is that if you want to deduce the underlying trend, you have to remove the annual bumps caused by things such as ENSO.

A common method of doing this is to use a "moving average" (aka "running mean"), where - for example - each year's data point is the average of the 10 years around that year.

And when you do that, you see clearly that the underlying trend of temperature rise continues.

This has been the standard approach of mainstream scientists - and their standard response when challenged that 1998 remains, depending on your dataset, the warmest single year on record.

Cherry in the pie

One thing that everyone in the climate blogosphere seems to agree on is that the best fruit in the world is the cherry, judging by the number that are picked.

And the Kaufmann paper has brought a few more down from the tree.

The Global Warming Policy Foundation (GPWF), the UK-based pressure group, said researchers "tweak an out-of-date computer model and cherry-pick the outcome to get their desired result".

To which the opponents' rejoinder is, and long had been: "well, choosing 1998 as the baseline is cherry-picking, to start with".

To illustrate the point, I've been through a quick exercise using the approach that groups such as GPWF favour - and that Kaufmann's research group adopted - of using annual temperatures rather than any kind of smoothed average, and looking for the temperature change over a decade.

I took the record of global temperatures maintained by Nasa's Goddard Institute for Space Studies (GISS) which is one of the three main global datasets, and calculated the rate of change over each of the most recent 10 decades - ie, 1991-2001, 1992-2002, and so on up to 2000-2010.

I've summarised the results in a table on this page. What it basically shows is two things:

  • the numbers vary quite a bit from year to year; and
  • all but one give a temperature rise - the only one that shows a small drop being 1998-2008.

Seeing as it's logically impossible that the world warmed between 1997 and 2007, cooled between 1998 and 2008, and warmed again from 1999 to 2009, one conclusion you might reach is that using annual temperatures is not a sensible thing to do as it gives you a set of answers that does not make sense.

... which is why most scientists use the running mean approach.

Image copyright NASA
Image caption Annual data (black line) are smoothed by a five-year running mean (red) - a 10-year mean would smooth more. Graph from Nasa GISS

But if you do go with the annual method - does it show that global warming has stopped?

Only if you use the one dataset that starts in 1998, and ignore the other nine that give the contradictory answer; which sounds like quite a good definition of "cherry-picking" to me, and certainly nothing to indicate a coming ice age.

And in case anyone feels like arguing that I've cherry-picked a favourable dataset, I went through the same exercise with the UK Hadley Centre/University of East Anglia record, which shows the same thing - lots of annual variability, and only one of the 10 periods that indicates a global cooling.

Whether Robert Kaufmann's analysis of coal's impact is eventually adopted or refuted through the formal avenues of science is hard to predict at the moment.

Prolific commentators such as Judith Curry suggest there are flaws; and Climate Central has a good discussion of counter-arguments to the coal theory.

When I spoke to Dr Kaufmann, he flagged up a few areas himself where data was lacking, such as the rate at which China and other developing countries were fitting their power stations with equipment that removes the cooling sulphate particles while allowing the warming carbon dioxide through.

However this plays out, I suspect the majority of researchers will go back to the use of long-term, smoothed temperature curves, and stay away from annual datasets - especially if they begin in 1998.

Cherries may be delicious when they're fresh; but the shelf life of this one has surely been exceeded.