Question to the Department for Business, Energy and Industrial Strategy:

To ask Her Majesty's Government, further to the Written Answer by Lord Callanan on 6 October (HL8378), what reasons they have for assuming that the set of residuals is stationary.

The assumption of stationarity of the residuals is well established in the peer reviewed literature. Recent examples of this can be found in the following papers:

Kew, Sarah F., Sjoukje Y. Philip, Geert Jan van Oldenborgh, Gerard van der Schrier, Friederike EL Otto, and Robert Vautard. "The exceptional summer heat wave in southern Europe 2017." Bulletin of the American Meteorological Society 100, no. 1 (2019): S49-S53.

Yiou, Pascal, Julien Cattiaux, Davide Faranda, Nikolay Kadygrov, Aglae Jézéquel, Philippe Naveau, Aurelien Ribes et al. "Analyses of the Northern European summer heatwave of 2018." Bulletin of the American Meteorological Society 101, no. 1 (2020): S35-S40.

Leach, Nicholas J., Sihan Li, Sarah Sparrow, Geert Jan van Oldenborgh, Fraser C. Lott, Antje Weisheimer, and Myles R. Allen. "Anthropogenic influence on the 2018 summer warm spell in Europe: the impact of different spatio-temporal scales." Bulletin of the American Meteorological Society 101, no. 1 (2020): S41-S46.

Further to the written answer provided on 6 October (HL8377), the plot placed in the Library of the House shows that the GEV distribution is a good fit to the data, which supports the assumption that the distribution of residuals may be adequately modelled as stationary.