Urban Heat Island Effect Caused 50% of Warming in China, New Study Finds

  • Date: 22/07/19
  • Nicola Scafetta and Shenghui Ouyang

New study finds that past temperature records have failed to accurately detect urbanisation biases, which may account for about 50% of the recorded warming in China since the 1940s.

Urban Heat Island China
Fig. 6. Map of the divergence (ΔTMin − ΔTMax) between the warmings registered by the minimum and maximum temperature records (CRU TS4) between 1945 and 1954 and 2005–2014. The cyan dots indicate the 200 most populated cities in China according to the Free World City Database.

Highlights

  • Mean surface temperature in China has warmed by 0.8 °C since 1950.
  • Atmospheric boundary layer physics predicts higher UHI warming during night.
  • The divergence between Tmin and Tmax is used to detect urbanization biases.
  • The China regions with the strongest Tmin-Tmax divergence are also the most populated.
  • Thus, the temperature warming in China is partially due to UHI biases.

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.

Reference

Nicola Scafetta and Shenghui Ouyang (2019) Detection of UHI bias in China climate network using Tmin and Tmax surface temperature divergence. Global and Planetary Change, Volume 181. https://doi.org/10.1016/j.gloplacha.2019.102989


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