Global warming since 1995 “statistically significant”
Professor Phil Jones
The claim that global warming has stopped - one of the most
overused and deeply flawed climate skeptic arguments - can finally
be laid to rest today, following the publication of new data
analysis by one of the country's leading climatologists.
Phil Jones, from the Climate Research Unit at the University of
East Anglia, agreed in a BBC
interview last year that the warming trend between 1995 and 2009
was statistically non-significant - but "only just".
This caused quite a stir with the Mail on Sunday
proclaiming that Jones had made a "climate change retreat".
Skeptics appropriated his statement as part of an on-going campaign
to undermine climate science.
However, the BBC
reports today that Jones has added the temperature data for
2010 to the record since 1995, and has found that this gives a
warming trend that is statistically significant.
Global temperature has warmed by around 0.19 ˚C between 1995 and
2010. This surely must be the end of the 'no more warming'
claim.
Jones has also updated the trend since 1979, finding that
between 1979 and 2010 there has been an overall warming of 0.5 ˚C.
That's between 0.14 and 0.16 ˚C per decade, as shown in the graph
below:
The 'no more warming' claim has previously been widely used by
climate skeptics, and parroted by
journalists.
For example, Dr Benny
Peiser, director of the Global Warming
Policy Foundation (GWPF), wrote in a
letter to the Telegraph:
"The data on 2010 confirms that since
1998 there has been no overall warming… Climate theory is at a loss
to explain this."
And Lord Nigel Lawson, GWPF
founder, wrote in the
Times:
"Certainly, it is curious that, whereas
their models predicted an acceleration in global warming this
century as the growth in emissions accelerated, so far this century
there has been no further warming at all."
This skeptic tactic lacks any scientific credibility, as we have
discussed in detail in a previous post.
Using such short time periods to draw conclusions about global
warming is flawed since scientists can only rely on global
temperature trends that are statistically significant, and trends
from small amounts of data tend not to be statistically
significant.
As Phil Jones told
the BBC environment correspondent Richard Black:
"It just shows the difficulty of
achieving significance with a short time series, and that's why
longer series - 20 or 30 years - would be a much better way of
estimating trends and getting significance on a consistent
basis."
In statistics 'significant' does not mean an important result,
rather it means a result that is unlikely to have come about by
chance. In this case, the 1995 - 2010 temperature trend is
statistically significant to the 95 percent level, meaning that
there are one in twenty odds that the trend came about by
chance.
Furthermore, a short-term hiatus in warming does not detract
from a longer warming trend. In fact such pauses are only to be
expected, as US
scientists demonstrated in 2009:
"…Climate over the 21st century can and
likely will produce periods of a decade or two where the globally
averaged surface air temperature shows no trend or even slight
coolingin the presence of longer-term warming."
In spite of these fundamental problems with their argument,
skeptics have kept repeating it. However, even the most blinkered
among them will now have to concede that this is an argument that
cannot be substantiated.