By Steve McIntyre, Climate Audit

In a recent CNN interview discussed at RC here, Joe D’Aleo said: “Those global data sets are contaminated by the fact that two-thirds of the globe’s stations dropped out in 1990. Most of them rural and they performed no urban adjustment. And, Lou, you know, and the people in your studio know that if they live in the suburbs of New York City, it’s a lot colder in rural areas than in the city. Now we have more urban effect in those numbers reflecting - that show up in that enhanced or exaggerated warming in the global data set. “

Gavin Schmidt excoriated this claim as follows: “D’Aleo is mis-directing through his teeth here. He also knows that urban heat island effects are corrected for in the surface records, and he also knows that this doesn’t effect ocean temperatures, and that the station dropping out doesn’t affect the trends at all (you can do the same analysis with only stations that remained and it makes no difference). Pure disinformation.

Later in the comments (#167), an RC reader inquired about UHI adjustments, noting the lack of discusison of this point as follows: #167 In all of the above posts there is no mention of the urban heat island effect, nor of the effect of rural station drop out nor of the effect the GISS data manipulation has on surface temperature. Why is that?

To which Gavin replied: [Response: Because each of these ‘issues’ are non-issues, simply brought up to make people like you think there is something wrong. The UHI effect is real enough, but it is corrected for - and in any case cannot effect ocean temperatures, retreating glaciers or phenological changes (all of which confirm significant warming). The station drop out ‘effect’ is just fake, and if you don’t like GISS, then use another analysis - it doesn’t matter. - Gavin]

Neither CRU nor NOAA have archived any source code for their calculations, so it is impossible to know for sure exactly what they do. However, I am unaware of any published documents by either of these agencies that indicate that they “correct” their temperature index for UHI effect (as Gavin claims here) and so I’m puzzled as to how Gavin expects D’Aleo to be able to “know” that they carry out such corrections.

And as to GISS adjustments, as we’ve discussed here in the past (and I’ll review briefly), outside the US, they have the odd situation where “negative UHI adjustments” are as common as “positive UHI adjustments”, raising serious questions about whether the method accomplishes anything at all, as opposed to simply being a Marvelous Toy. And CRU and NOAA don’t even bother.

CRU Urban Adjustments?
The most recent exposition of CRU methodology is Brohan et al 2006, which stated in respect to UHI that they included an allowance of 0.1 deg C/century in the uncertainty, but does not describe any “correction” to the reported average temperature.  To make an urbanisation assessment for all the stations used in the HadCRUT dataset would require suitable meta-data for each station for the whole period since 1850. No such complete meta-data are available, so in this analysis the same value for urbanisation uncertainty is used as in the previous analysis[Folland et al., GRL 2001].

For greater certainty that CRU makes no “correction” for UHI in the actual temperature (only an allowance in the “uncertainty"), Folland et al (GRL 2001) stated: “We add independent uncertainties due to urbanisation, changing land-based observing practices and SST bias corrections. The uncertainties given by RSOA due to data gaps and random errors were augmented using published estimates of global uncertainties associated with urbanization effects (e.g. Jones et al., 1990). We assume that the global average LAT uncertainty increased from zero in 1900 to 0.1C in 1990 (Jones et al, 1990), a value we extrapolate to 0.12C in 2000.”

NOAA UHI Adjustments?
The homepage for the NOAA temperature index is here. It cites Smith and Reynolds (2005) as authority. Smith and Reynolds, in turn, state that they use the identical procedure as CRU, i.e. they make an allowance in uncertainty, but do not correct the temperature index itself.

Contrary to Gavin’s assertion ("Urban heat island effects are corrected for in the surface records"), there is no evidence that CRU or NOAA correct their records for urban heat island effects. They make a very slight allowance in their “uncertainty” for UHI relying ultimately on an estimate made in Jones et al 1990, a study which made untrue (and impossible) claims about quality control steps.

The only network where a plausible adjustment is made is the GISS US network (representing less than 2% of the world’s surface, as NASA GISS reminds us.) Unlike CRU and NOAA, GISS makes a decent effort to adjust for UHI in the U.S. (outside the USA, its efforts are risible.) A few days ago, I showed the notable difference between the GISS (UHI-adjusted) version in the US and the NOAA unadjusted version, where the difference is much more than 0.1 deg C/century asserted by CRU/NOAA.

While GISS US results are plausible, outside the US, the GISS adjustment is a pig’s breakfast and no sane person can claim that they live up to the warranty. What makes this frustrating is that the US temperature history (GISS version) had 1934 as a record year - a result that was at variance with the other indices and other parts of the world. Is this because this is the only network/country combination with an effective UHI adjustment or because of a unique “regional” climate history in the US?

Whether or not urban heat islands have a material impact on the surface records is a different question. The difference between GISS US results and NOAA US results is strong evidence that there is a noticeable impact - one which needs to be addressed by CRU and NOAA and by GISS outside the US. In my opinion, Gavin’s own statement that “urban heat island effects are corrected for in the surface records” is, to borrow a phrase from realclimate, “disinformation”. Read more from Steve with links to his many posts on this topic here. See our response to realclimate here.