By the Mail's reasoning, temperatures have "barely risen" - by 9 degrees

  • 25 Apr 2012, 12:32
  • Christian Hunt

An article that implies global warming is "just hot air" and that global temperatures have "barely risen" over the last thirty years should probably try and get the basics of how global temperature rise is measured right.

So it's unfortunate that today's Daily Mail confidently argues that "world temperatures have barely risen in the last two decades" in an article which hangs on a fairly basic error in interpreting global temperature data.

The piece begins:

"World temperatures have barely risen in the last two decades, figures reveal."

"Temperatures across the globe rose by around a third of a degree last year from the average of 14 degrees Celsius recorded between 1961 and 1990."

"In some years, temperatures rose by just 0.29 degrees C while in others they rose by .53 degrees."

Given that temperatures have risen in total by around 0.8 degrees celsius since the early twentieth century, the Mail's suggestion that temperatures rose by half a degree in a single year might already be ringing alarm bells.

So where do these new figures come from? The Mail says they are in a parliamentary answer given by Minister of State for Energy and Climate Change Greg Barker MP to a written question by Conservative backbencher Anne Main MP.

Actually, the figures are not particularly new, nor do they show year-on-year temperature rise as the Mail claims. This becomes clear when you check the parliamentary answer, where Barker references Hadley Centre figures for global surface temperatures over the past ten years released just over a month ago.

He writes:

"Latest estimates made by the Met Office Hadley Centre of the annual difference in global average surface temperatures, relative to the 1961 to 1990 average of 14.0°C, are shown in the following table for each year since 1997..."

He then provides the Hadley Centre summary:

Screen Shot 2012-04-25 At 12.26.37

For those who want to dig deeper, this is a limited summary of the recent HADCRUT4 data. The full dataset is here.

The Mail mixes up temperature rise with temperature anomaly

It's quite important to note that this table shows 'temperature anomalies' - the difference in temperature each year from the long term (1961-1990) average temperature.

As NOAA puts it, a temperature anomaly:

"means a departure from a reference value or long-term average. A positive anomaly indicates that the observed temperature was warmer than the reference value, while a negative anomaly indicates that the observed temperature was cooler than the reference value."

So to take the figure for 2000 shown above, an anomaly of 0.29 degrees celsius means that the year 2000 was 0.29 degrees warmer than the average global temperature calculated over 1961-1990.

It doesn't mean that temperatures in 2000 were 0.29 degrees warmer than in 1999 - which is what the Mail believes the data shows:

"In some years, temperatures rose by just 0.29 degrees C while in others they rose by .53 degrees."

This is completely wrong. The Mail has not spotted that the chart shows temperature anomalies, stating instead that it shows temperature rise from year to year. Perhaps an easy mistake to make if you're casting a quick eye over the data, but a pretty serious clanger if you're writing an article implying that global warming is "just hot air".

Flowing from this, the Mail incorrectly assumes that temperatures go up every year. In fact, they fluctuate from year to year due to natural variability in the climate. This explains why the anomaly for 2011 is lower than for 2010 - global average temperature dropped over the one year time period.

Of course, measuring big things like global temperatures over very short time periods like a year doesn't tell you very much (see here for some more detail). So scientists measure temperature rise over longer time periods - over decades and multiple decades.

Apocalypse now?

To appreciate the scale of just how wrong the Mail has got this one, let's just assume that temperatures were rising by anything as much as 0.29 - 0.53 degrees per year. If we followed that reasoning, it would mean a temperature rise over the past three decades of between about 9 and 15 degrees celsius - truly catastrophic global warming!

In fact, as Barker correctly notes, Hadley figures suggest 0.17 degrees of warming per decade since 1979. On that basis, that amounts to, on average, about 0.51 degrees over the 30 years.

The questioner, Anne Main MP, was one of the 101 Conservative MPs that wrote to the Prime Minister demanding that the £400 million-a-year subsidies paid to the "inefficient" onshore wind turbine industry should be "dramatically cut".

Since March, she's also submitted various written questions to the Department for Energy and Climate Change (DECC) about the "credibility" of the IPCC, and how much various green policies will cost consumers. 

She's also asked DECC about the " evidential basis" for a section about global temperatures on their website.

Given her concern with the accuracy of information about global temperatures, perhaps she is slightly disappointed by the Mail article.

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