BEST reconfirm: warming is happening
- 21 Oct 2011, 09:00
- Robin Webster
The controversial Berkeley
Earth Surface Temperature Project, originally inspired by
skeptic claims about 'dodgy data', promised to produce a
definitive statement on the Earth's surface temperature record. Now
the BEST team has released
their analysis on their website, and they agree with previous
groups of researchers in identifying a warming trend. The project
has found that the earth is warming by about 1 °C since
the middle of the last century - and that the the Urban Heat
Island's effect on global temperatures is "nearly negligible" or
"spurious".
It is an important caveat that this work has not yet been
published in a peer reviewed journal - unusually, the BEST team are
releasing their paper for a form of 'wiki review' before submitting
it. But it shouldn't come as a great shock that the results confirm
what scientists had already told us about how the Earth's surface
temperature is changing.
The 'urban heat island effect' (UHI) - first documented in London
in the early 1900s - describes how temperatures in a city are often
higher than in rural surroundings. Allegations that scientists are
therefore over-estimating global warming has achieved iconic status
amongst climate skeptics, and has even been used to level
accusations of "
scientific fraud" against researchers.
In the USA, the author of prominent skeptic website Watts Up With That (total
hits since launch:
more than 37 million) Anthony Watts, who sells weather
measurement equipment himself, has long argued that the use of data
from poorly
cited weather stations had skewed the global temperature record
- and has accused climate scientists of ignoring results that are
inconvenient to their data.
The Berkeley Earth Surface
Temperature (BEST) project took these concerns seriously. Set
up with the the explicit intention of addressing criticisms of
the existing temperature records and constructing a new record
of global average temperatures, the project was launched and
chaired by Professor Richard
Muller.
The project raised some red flags - the only climate scientist
involved in the project was Professor Judith Curry (author of the
skeptic-friendly blog Climate
Etc); Muller was known to
question the seriousness of global warming; and worst of all
for some the project was
part-funded by the Koch Brothers. Anthony Watts was involved in
the project and
expressed strong support for it.
So what did the project do differently to the scientific bodies
who are already monitoring the planet's temperature, and what have
they found?
To study the effects of UHI BEST used a statistical methodology
specifically designed to address the differences between urban and
rural stations. They split the 39,028 weather monitoring stations
(from 10 publicly available sources) into "very rural" and "not
very rural" using satellite data that can classify land use as
urban or not. They then worked out linear temperature trends for
each station and compared the results for all of the data with just
the "very rural" dataset. 16,132 of the sites were classified as
"very rural".
The graph below, taken from BEST's paper on the "Influence of
Urban Heating on the Global Temperature Land Average" illustrates
what this looks like in practice. The graph on the left shows the
temperature trend from all of the temperature stations; that on the
right from only those in rural areas.
A cursory glance would seem to indicate that the two
distributions are virtually the same. And indeed the paper just
released by Berkeley backs this up. It says that their research
"supports the key conclusion of prior
groups that urban warming does not unduly bias estimates of recent
global temperature change."
and that
"The trend analysis also supports the
view that the spurious contribution of urban heating to the global
average, if present, is not a strong effect; this agrees with the
conclusions in the literature we cited previously"
Slightly weirdly, this is not actually the whole story. The BEST
team reconstructed global land temperature since 1850 for both the
whole set of weather stations, and solely the very-rural stations,
and compared the two. Over the last half-century (when most of the
temperature change from human-induced climate change has been
observed), the BEST project did find an impact from locating
weather stations in urban areas - but in the opposite direction to
that claimed by the skeptics. BEST say:
"We observe the
opposite of an urban heating effect over the
period 1950 to 2010, with a slope of -0.19 ± 0.19 °C/100yr". [our
emphasis]
In other words, their "rural" weather stations recorded slightly
higher temperatures than the urban ones, rather than vice versa.
The difference is statistically significant but only very
slight.
The BEST researchers say that they "hesitate" to offer a definite
explanation for this before more investigation is undertaken. They
suggest that it may reflect "some recent form of 'urban cooling
and/or 'rural warming'" or relate to "a subtle difference in the
spatial coverage or rural and non-rural sites" - although the
latter explanation looks less likely.
In any case, the authors of the BEST study are keen to emphasise
that the key point is that the impact so small ("nearly
negligible") that it is "almost insignificant" on the scale of
observed warming". They add that this is "not surprising",
because
"the fraction of the Earth's land area
denoted as urban by the MOD500 analysis is only 0.5%. Even if all
these urban areas had a heat island effect as large as that of
Tokyo, roughly 3C per century, the contribution to the world
average once properly weighted for land area would be only 0.5% of
that, or 0.015C per century."
So the results are totally unsurprising and in agreement with
the previously published and extensively peer reviewed literature
on the subject.
The key result from the BEST study is encapsulated in the
following comments in their paper:
"In this table we see evidence of
"global warming. Using all the records there is a median warming
trend of 0.98 ± 0.04 °C/100yr."
This confirms a preliminary result (when 2% of the data had been
analysed) which was
leaked by the project , and then
outlined by Muller in front of a Congressional Committee in
April of this year. Muller told the Committee that
"We see a global warming trend that is
very similar to that previously reported by the other groups."
Given the makeup and background of the project, this result may
be important politically, but it doesn't represent any significant
change in the scientific factbase on climate. Rather, if and when
it is published in a peer reviewed journal, it will have
reconfirmed - in a highly public way - what has already been found
(see for example
here,
here,
here and here).
Anthony Watts had
originally indicated a close working relationship with the BEST
team and stated that
"I'm prepared to accept whatever result they produce, even if
it proves my premise wrong. I'm taking this bold step because the
method has promise."
However, when Professor Muller gave evidence before the House of
Representatives Committee, Watts defended his own work in a letter
to the Committee,
arguing "It is evident that such siting problems do in fact
cause changes in absolute temperatures, and may also contribute to
new record temperatures." The sensationalist skeptic blog Climate
Depot also subsequently
accused Muller of being a "front man for geoengineering org".
In reaction to this latest release of papers, Watts has written a
long blog post criticising the methodology of the paper and the
fact that it has been released prior to peer review.
When the project first started
Muller expressed the hope to the Guardian that
"...if the only thing we do is allow a
consensus to be reached as to what is going on with global warming,
a true consensus, not one based on politics, then it will be an
enormously valuable achievement."
Now that it seems that the BEST project has come to the same
conclusions as the climate scientists it purported to critique,
will this now lay the matter to rest? It remains to be seen.