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10 June 2011 Last updated at 13:59
Global warming since 1995 'now significant'
Richard Black By Richard Black Environment correspondent, BBC News
Climate warming since 1995 is now statistically significant, according
to Phil Jones, the UK scientist targeted in the "ClimateGate" affair.
Last year, he told BBC News that post-1995 warming was not significant -
a statement still seen on blogs critical of the idea of man-made climate
change.
But another year of data has pushed the trend past the threshold usually
used to assess whether trends are "real".
Dr Jones says this shows the importance of using longer records for
analysis.
By widespread convention, scientists use a minimum threshold of 95% to
assess whether a trend is likely to be down to an underlying cause,
rather than emerging by chance.
If a trend meets the 95% threshold, it basically means that the odds of
it being down to chance are less than one in 20.
Last year's analysis, which went to 2009, did not reach this threshold;
but adding data for 2010 takes it over the line.
"The trend over the period 1995-2009 was significant at the 90% level,
but wasn't significant at the standard 95% level that people use,"
Professor Jones told BBC News.
"Basically what's changed is one more year [of data]. That period
1995-2009 was just 15 years - and because of the uncertainty in
estimating trends over short periods, an extra year has made that trend
significant at the 95% level which is the traditional threshold that
statisticians have used for many years.
"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."
Professor Jones' previous comment, from a BBC interview in Febuary 2010,
is routinely quoted - erroneously - as demonstration that the Earth's
surface temperature is not rising.
Globally consistent
The dataset that Professor Jones helps to compile - HadCRUT3 - is a
joint project between the Climatic Research Unit (CRU) at the University
of East Anglia (UEA), where he is based, and the UK Met Office.
It is one of the main global temperature records used by bodies such as
the Intergovernmental Panel on Climate Change (IPCC).
HadCRUT shows a warming 1995-2010 of 0.19C - consistent with the other
major records, which all use slightly different ways of analysing the
data in order to compensate for issues such as the dearth of measuring
stations in polar regions.
Shortly before the UN climate summit in Copenhagen, Phil Jones found
himself at the centre of the affair that came to be known as
"ClimateGate", which saw the release of more than 1,000 emails taken
from a CRU server.
Critics alleged the emails showed CRU scientists and others attempting
to subvert the usual processes of science, and of manipulating data in
order to paint an unfounded picture of globally rising temperatures.
Subsequent enquiries found the scientists and their institutions did
fall short of best practice in areas such as routine use of professional
statisticians and response to Freedom of Information requests, but found
no case to answer on the charges of manipulation.
Since then, nothing has emerged through mainstream science to challenge
the IPCC's basic picture of a world warming through greenhouse gas
emissions.
And a new initiative to construct a global temperature record, based at
Stanford University in California whose funders include "climate
sceptical" organisations, has reached early conclusions that match
established records closely.
http://www.bbc.co.uk/news/science-environment-13719510
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Mario Huet
Libertarian Alliance Forum
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