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Carbon Brief Staff

Carbon Brief Staff

10.06.2011 | 4:30pm
ScienceGlobal warming since 1995 “statistically significant”
SCIENCE | June 10. 2011. 16:30
Global warming since 1995 “statistically significant”

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 Ë?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 Ë?C. That’s between 0.14 and 0.16 Ë?C per decade, as shown in the graph below:

T rise since 1979

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 warmingâ?¦ 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:

“â?¦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.


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