Scafetta on UHI

By Rud Istvan – Re-Blogged From WUWT

Detection of UHI bias in China climate network using Tmin and Tmax surface temperature divergence

Nicola Scafetta a, Shenghui Ouyang b

Abstract Near-surface temperature records show that China warmed by about 0.8 °C from 1950 to 2010. However, there exists an ongoing debate about whether this warming might have been partially due to urbanization bias. In fact, homogenization approaches may be inefficient in densely populated provinces that have experienced a significant urban development since the 1940s. This paper aims to complement previous research on the topic by showing that an alternative approach based on the analysis of the divergence between the minimum (Tmin) and maximum (Tmax) near-surface temperature records since the 1940s could be useful to clarify the issue because urban heat island (UHI) effects stress the warming of nocturnal temperatures more than the diurnal ones.

Then, the significance of the divergence observed in the data could be evaluated against the expectations produced by the CMIP5 general circulation model simulations. From 1945–1954 to 2005–2014, on average and over China, these models predict that Tmin had to warm 0.19 ± 0.06 °C more than Tmax. However, during the same period, the climatic records show that Tmin warmed 0.83 ± 0.15 °C more than Tmax.

A similar analysis demonstrates that the effect is more pronounced during the colder months from November–April than during the warmer ones from May to October. A comparison versus China urbanization records demonstrates that the regions characterized by a large Tmin-Tmax divergence are also the most densely populated ones, such as north-east China, that have experienced a diffused and fast urbanization since the 1940s. The results are significant and may indicate the presence of a substantial uncorrected urbanization bias in the Chinese climate records. Under the hypothesis that Tmax is a better metric for studying climatic changes than Tmean or Tmin, we conclude that about 50% of the recorded warming of China since the 1940s could be due to uncorrected urbanization bias. In addition, we also find that the Tmax record from May to October over China shows the 1940s and the 2000s equally warm, in contrast to the 1 °C warming predicted by the CMIP5 models.


The time period suffices. Per published CMIP5 ‘experimental protocol’ documentation[1], the mandatory 30 year best hindcast is back from 2005 YE plus a forecast.

The diurnal heat difference between Tmin and Tmax has been discussed here many times. No WUWT explanation should be necessary. UHI is conclusively shown by the night-time and seasonal differences.

Chinese Tmin warmed mostly per UHI, not GW. So the Chinese can build as many USC/HELE scrubbed coal plants as they want with no significant climate impact per their ridiculous Obama agreement.

The only viable criticism of this new paper is the well-known problem that global climate models do NOT reliably regionally downscale (here, to China). For those references, see footnotes to essays ‘No Bodies’ and ‘Last Cup of Coffee’ in my ebook Blowing Smoke, or the review summary by Pielke and Wilby, Regional Climate Downscaling, What’s the Point? in EOS 93:52-53 (2012).

[update 9:45 pm Pacific Time.  There’s a much more comprehensive discussion of this over at JoNova]


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