The project presented here sought to determine whether changes in anthropogenic thermal emission can have a measurable effect on temperature at the national level, taking Japan and Great Britain as type examples. Using energy consumption as a proxy for thermal emission, strong correlations (mean r 2 = 0.90 and 0.89, respectively) are found between national equivalent heat output (HO) and temperature above background levels Δ t averaged over 5‐ to 8‐yr periods between 1965 and 2013, as opposed to weaker correlations for CMIP5 model temperatures above background levels Δ mt (mean r 2 = 0.52 and 0.10). It is clear that the fluctuations in Δ t are better explained by energy consumption than by present climate models, and that energy consumption can contribute to climate change at the national level on these timescales.

1 Introduction It has long been known that within large cities, thermal emission from heated buildings, industry, and transport can contribute to a microclimate up to 12°C warmer than background levels in the surrounding area, a phenomenon known as the urban heat island (UHI) effect [Howard, 1833; Arakawa, 1937; Oke, 1973; Knight et al., 2010]. However, some of this heat difference is attributed to contrasts in evaporative cooling and albedo [Taha, 1997], absorbed and re‐emitted solar radiation [Rizwan et al., 2008], and convection [Zhao et al., 2014]. Here, we consider thermal emission alone, but our study is not restricted to cities, but extends the concept to encompass heat generated by entire nations, thus including heat from smaller urban areas, rural districts, and transport networks. Weather systems do not respect political boundaries, so heat generated in one country could affect nations downwind. Japan and Britain are particularly suited to such a study, both being high population‐density island nations largely isolated from the heat output (HO) of neighboring countries by the surrounding ocean.

2 Data and Methods All data used in this study are derived from existing published values. Figure 1 shows this primary data, as plots of annual mean temperature, background global temperature, local data extracted from global CMIP5 model temperatures, and annual energy consumption of Japan and the United Kingdom between 1965 and 2013. Figure 1 Open in figure viewer PowerPoint Source data used in this study. (a) Black circles are annual mean temperature t for Japan (top) and Central England (bottom) between 1965 and 2013, together with global temperature H4 (gray crosses) from the HadCRUT4 data set. (b) Mean multi‐model temperatures mt extracted from 42 global CMIP5 models for the areas (top) 131°.25–138°.75E long. and 33°.75–36°.25N lat. (Japan), and (bottom) 4°W–1°E long. and 51°–53°.5 N lat. (Central England), also with HadCRUT4 global temperatures. (c) Annual energy consumption in millions of tons of oil equivalent (toe). Temperatures for Great Britain are from the Central England Temperature series, which combines data from four stations carefully selected to be representative [Manley, 1974; Parker et al., 1992]. Those for Japan are the means of the 19 longest unbroken data sets from weather stations on Japan's main island (Honshu), and published by the Japan Meteorological Agency [2015]. Specifically, these are Fukuoka, Gifu, Hamada, Hikone, Ishigakujima, Kochi, Kumagaya, Maebashi, Matsuyama, Naze, Sakai, Shimonoseki, Tadotsu, Tokushima, Tokyo, Tsu, Tsuruga, Wakayama, and Yokohama. Global temperatures are from the HadCRUT 4 data set [Morice et al., 2012]. Model temperatures are the means of 42 CMIP5 (Coupled Model Intercomparison Project) global models [Taylor et al., 2012; Collins et al., 2013], from which model temperatures for Japan's main island were extracted (131°.25–138°.75E long. and 33°.75–36°.25N lat.), and those for central England (4°W–1°E long. and 51°–53°.5N lat.). The CMIP5 suite of coupled atmospheric models is the most sophisticated so far developed [Taylor et al., 2012], incorporating not only the effects of changes in greenhouse gas concentrations but also dust from volcanic eruptions [Robock, 2000] and solar variations [Lean and Rind, 2008; Foster and Rahmstorf, 2011] on climate. All the three effects can be taken into account at global levels [Jones et al., 2013], which improves correspondence with observed values, despite discrepancies between different models [Räisänen and Ylhäisi, 2013] and uncertainties associated with methodology [Jones et al., 2013]. The anthropogenic heat flux was derived from annual primary energy consumption for both countries, given in the Statistical Review of World Energy [2014], the longest consistent data set for both countries, that lists values back to 1965. Virtually, all energy consumed is dissipated as heat on timescales of less than a few days [Flanner, 2009]. To isolate national changes of temperature from world‐wide changes, global temperature is subtracted from the annual Japan and United Kingdom temperature to give residual temperature Δt, i.e., Δt = t – H4, where t is observed mean monthly temperature and H4 is mean monthly global temperature from the HadCrut4 data set. The changes in Δt can then be compared with CMIP5 model residual temperatures Δmt. In this context, Δmt = mt – H4, where mt is the CMIP5 multi‐model mean. Finally, Δt can also be compared directly with HO, derived from energy consumption of the two nations considered, so that the effect of anthropogenic heat on local temperature changes can be quantified. Both nations have large year to year variations in annual temperature, so in Figure 2, 7 yr means are plotted to minimize the effects of weather, solar cycles, and El Niño events and thus isolate climate change. Seven years is less than a quarter of the 30 yr period for measurement of climatological normals [Trewin, 2007], but close to the maximum needed to retain sufficient data points to characterize the major temperature changes in Figure 1. Seven years also avoids any possible synchronization with the 11 yr solar cycle, which might affect means of 5 or 6 yr [Friis‐Christenesen and Lassen, 1991]. Figure 2 Open in figure viewer PowerPoint (a) Seven years means of observed Japan (top) and United Kingdom (bottom) residual temperature (Δt) 1965–2013. (b) Similar presentation, but showing mean residuals Δmt of 42 CMIP5 global model temperature simulations for a 7½° long. × 2½° lat. quadrangle over Japan (top), and a 5° long. × 2½° lat. quadrangle over Central England (bottom), instead of Δt. (c) Variations in Japanese and United Kingdom energy consumption (expressed as equivalent heat output (HO) in ExaJoules) over the same period. See text for further details.

3 Results Figure 2a shows the 7 yr mean residual temperature Δt between 1965 and 2013. Figure 2b shows the multi‐model mean Δmt from 42 CMIP5 models for Japan and Central England. Plotting Δmt against observed Δt yields varying fits of r2 = 0.71 and r2 = 0.05 for Japan and Central England, respectively (Figure 3a). Figure 2c shows 7 yr means of Japan's and Britain's equivalent HO from 1965 to 2013, derived from national primary energy consumption in tonnes of oil equivalent (toe), using the relation 1 toe = 41.87 GJ [International Energy Agency, 2015]. In contrast to Δmt, the changes in HO match observed Δt changes over this period (Figure 2a), with high correlations in both nations of r2 = 0.90 and 0.97 (Figure 3b). Figure 3 Open in figure viewer PowerPoint t) 1965–2013 for Japan (top) and Britain (bottom) plotted against mean Δmt of 42 CMIP5 global model temperature simulations as in Figure HO) in ExaJoules instead of Δmt. Correlations of observed Δt with Δmt are r2 = 0.71 and r2 = 0.05 in Japan and United Kingdom, respectively, and r2 = 0.90 and r2 = 0.97 for HO. Values of p (see text) are 0.001 for Japan and 0.00007 for the United Kingdom. (a) Seven years means of observed residual temperature (Δ) 1965–2013 for Japan (top) and Britain (bottom) plotted against mean Δof 42 CMIP5 global model temperature simulations as in Figure 2 . (b) Similar plots using national equivalent heat output () in ExaJoules instead of Δ. Correlations of observed Δwith Δare= 0.71 and= 0.05 in Japan and United Kingdom, respectively, and= 0.90 and= 0.97 for. Values of(see text) are 0.001 for Japan and 0.00007 for the United Kingdom. The choice of 7 yr as the averaging interval A is not critical. A was set at all values between 1 and 16 yr, and the r2 values for Δmt and HO models plotted against A for the two countries in Figure 4. When A is small there are lots of data points, so even modest values of r2 can be highly significant (as they are for Japan in relation to HO), but r2 itself is low for both countries: the CMIP5 and HO models (for the United Kingdom at least) are as good as each other, and neither accounts for the fluctuations in observed Δt. When A is large, we have high values of r2 for HO but very few data points, and for A > 16 only two data points so r2 must equal 1. Values of p (the probability of such a large value of r2 occurring by chance, on the null hypothesis that HO and residual temperature are unrelated) for the United Kingdom are low when A = 5–8 (p < 0.004), but greater when A lies outside this range; for Japan, p < 0.004 for A = 1–9. Figure 4 Open in figure viewer PowerPoint r2 values for heat output (HO) and CMIP5 multi‐model Δmt against observed Δt for different averaging intervals A for the United Kingdom (a) and Japan (b). All intervals A above 3 yr show higher r2 values for HO, those with the lowest probability p of occurring by chance being between 5 and 8 yr for the United Kingdom, and up to 10 yr for Japan. Mean r2 values for 5–8 yr averaging intervals for the United Kingdom and Japan are 0.89 and 0.90, respectively, for HO and 0.10 and 0.52 for CMIP5 Δmt. Values of p in this range of A are all <0.004 for both nations. The heat equivalent of Japanese energy consumption averages 17 EJ yr−1 1965–2013, which reduces to 1.5 J m−2 s−1. Equivalent values for the United Kingdom are 8 EJ yr−1, which because of its smaller land mass reduces to 1.2 J m−2 s−1. Much less than 1% of this energy consumption is liable to be lost as radiation without warming the atmosphere–land–ocean system [Flanner, 2009]. Japan and the United Kingdom have a mean vertical velocity component in the low troposphere at 700 hPa between about 0.01 and 0.05 Pa s−1 in a downward direction [Kallberg et al., 2005], aiding heat generated to remain at lower levels. An interesting further test is to look at seasonal changes in temperature and energy consumption. Unfortunately, annual seasonal energy consumption values are not available for either country, but what evidence shows is that in Japan, where summer temperatures are more than 8°C higher than the United Kingdom, electricity demand is 10–25% higher in summer than in winter [Kempton and Kubo, 2000; Akil and Miyauchi, 2013] because of air conditioning. In the United Kingdom, where air conditioning is almost absent, the opposite is true, winter electricity demand being about 30% higher than in summer [U.K. Dept. of Energy and Climate Change, 2014]. When summer (May–September) and winter (November–March) residual temperatures Δt against annual energy consumption for both countries were plotted, as expected Japan shows stronger correlations in summer than in winter, and the United Kingdom stronger correlations in winter than in summer for most averaging intervals (Figure 5). However, r2 values, especially those for the United Kingdom, are lower than for annual Δt (Figure 4), as both winter and summer values are perforce included in the energy consumption data. Figure 5 Open in figure viewer PowerPoint r2 values for annual heat output (HO) plotted against observed summer and winter Δt for different averaging intervals A for the United Kingdom (a) and Japan (b). Japan shows stronger correlations in summer than in winter, because of the cooling load of air conditioning. See text for details.

4 Discussion and conclusions Both countries are rather extreme cases, Japan having a mean annual energy consumption per unit area during 1965–2013 of 1114 toe km−2, the 8th highest in the world during 1965–2013, and the United Kingdom 870 toe km−2, the 13th highest [Statistical Review of World Energy, 2014]. Of the two nations, Japan has a warmer climate and consequently lower heating requirements, and 60–65% cloud cover [Norris and Wild, 2009]. However, Japan's more consistently increasing energy consumption parallels world CO 2 levels, meaning that the correlations with Δmt are consistently higher than the United Kingdom, so the distinction between the two models is not so pronounced. Britain is better suited to this study, being cold enough to require indoor heating for about 6 months per year, and with 75% cloud cover [Kontoes & Stakenborg, 1990], meaning that less surface‐generated heat is lost by radiation. Most importantly, Britain is a country where annual energy consumption has fallen significantly as well as risen during the time period considered, so that the greater effect of HO than other causes on United Kingdom temperature can be more clearly distinguished (Figure 4a). First, our hypothesis was not suggested by the data, but by its qualitative reasonableness. Second, our results are reproducible, in that our statistical study of the United Kingdom data was completed, and the results noted, before testing our conclusions by consideration of the Japan data. Third, we carried out no other statistical study of these or any other data sets. Fourth, the effect appears large, in that variations of HO correlate (Figure 2, bottom row) with temperature changes of a few tenths of a degree. The reliability and importance of our conclusions does not rest on the probabilities returned by our statistical tests, significant though these are by conventional standards: It may appear that, reasonable though it is, our hypothesis is harder to justify quantitatively, in the sense that HO (of order 1 J m−2 s−1) is much smaller than insolation, by two orders of magnitude. On the other hand, what is at issue is the relative importance of fluctuations in these quantities. The fact that the statistically significant results require averaging over several years is because of the small area of the Earth's surface being sampled in both locations. At this scale, temperatures vary widely from one year to the next compared with world values (Figure 1a). Our results are strong evidence that changes in energy consumption contribute to temperature change over sub‐decadal timescales in the two nations considered. Britain has experienced a drop in temperature of about 0.5°C since the early years of the millennium (Figure 2, lower left) at a time when world temperatures have remained virtually stable, whereas Japan experienced a rise in Δt of 1.0°C between the early 1980s and 2000 (Figure 2, upper left), double the world rise in temperature over the same period. Both these changes reflect changes in energy consumption in each country. These conclusions might be perceived to be in contrast to recent studies of the UHI effect that relate to large cities, where warming of only ∼0.1°C per decade or less is detected, compared with nearby rural districts [Parker, 2010; McCarthy et al., 2011]. However, such studies are designed to detect urban/rural contrasts, not the effects of overall increases or decreases in heat emission in entire nations. UHIs are most pronounced in calm weather [Oke, 1973; Wilby, 2003], and are best measured at such times [Knight et al., 2010]. Under average conditions, generated heat will drift downwind and may affect rural weather stations [Parker, 2010]. In addition, the problem of nearby road and urban development at long‐lived rural control stations, which may have affected recorded temperatures, is discussed by Hansen et al. [2001]. Certainly in Japan, Fujibe [2009] detected temperature anomalies from towns of population less than 1000. Because anthropogenic heat is generated close to where temperatures are measured in both countries, we have not used a climate model to investigate the transport of such released heat further afield. Early attempts to do this globally found temperature variations of a similar order to the model's natural fluctuations [Washington, 1972], and Flanner [2009] found no significant effect for the present day. Oleson [2012] used CMIP5 simulations to model future changes in urban minus rural temperatures in response to changing climate over the 21st century, rather than the effects of changing energy consumption. More recently however, Zhang et al. [2013] despite including only 42% of world energy consumption in their model, found significant winter and autumn temperature changes up to 1°C in mid‐ and high‐latitudes, far from heat sources, that correspond well to areas of previously unexplained differences between observed and modeled temperatures. Chen et al. [2014], entering anthropogenic heat flux into a refined model that included long wave radiation, found higher and more widespread increases over standard models: 1–2°C in mid‐ to high‐latitude areas of Eurasia, North America, and parts of the southern hemisphere, and concluded that anthropogenic heating is an important factor in global warming that should not be ignored. Our study is the first of its kind that provides direct observational evidence of this. If projections of energy consumption prove to be true, then future contributions of anthropogenic heat to climate change in Japan and the United Kingdom will have fallen by 2040. Japan is predicted to have an 18% fall [U.S. Energy Information Administration, 2016], corresponding to a temperature drop of about 0.3°C, and the United Kingdom a 3% fall [U.K. Dept. of Energy & Climate Change, 2015], producing a negligible drop in temperature.

Acknowledgments We thank Jouni Räisänen for the CMIP5 simulation data for the United Kingdom and Japan, and C.G.G. Aitken and B.J. Worton for advice on statistical matters, and an anonymous referee for a constructive and informative report. The authors declare no competing financial interests. Correspondence and requests for materials should be addressed to J.B.M. (J.B.Murray@open.ac.uk). The data supporting our conclusions can be obtained from the listed references below, as described in Section 2.