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:
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.