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Carbon Brief Staff

Carbon Brief Staff

21.10.2011 | 9:00am
ScienceBEST reconfirm: warming is happening
SCIENCE | October 21. 2011. 9:00
BEST reconfirm: warming is happening

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.

Screen Shot 2011-10-20 At 22.58.47

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.


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