Urban heat islands are a contentious phenomenon in the study of global climate change, but a new University of California-led study suggests that city heat can reach far enough into the upper atmosphere to alter high-altitude winds, impacting climate thousands of kilometres away from where energy is consumed.
The study, published in Nature Climate Change, sought to resolve a long-standing problem in climate modelling: the apparent disparity between observed warming over the last fifty years on the one hand, and the predictions made by computer climate models.
As noted by AFP, northern hemisphere winters have, somewhat awkwardly, been warmer than predicted by existing computer models.
The problem, according to the new study, is that those models focus on predicting the impact of anthropogenic greenhouse gases and aerosols, and to date have not included the vast amount of heat released by cities.
The authors explain that their calculations suggest that of the 16 Terawatts of global energy consumption in 2006, 6.7 TW was consumed in the 86 metropolitan areas they examined in the study.
By incorporating that heat release, the researchers found “the inclusion of energy use at 86 model grid points where it exceeds 0:4Wm -2 can lead to remote surface temperature changes by as much as 1K [El Reg - Kelvin] in mid- and high latitudes in winter and autumn over North America and Eurasia”.
An announcement at Florida State University quotes Cai as saying: “The world’s most populated metropolitan areas, which also have the highest rates of energy consumption, are along the east and west coasts of the North American and Eurasian continents, underneath the most prominent atmospheric circulation troughs and ridges.
“The concentrated and intensive release of waste energy in these areas causes a noticeable interruption to normal atmospheric circulation systems, leading to remote surface temperature changes far away from the regions where the waste heat is generated.”
The paper describes the impact on atmospheric jets as “a robust change in atmospheric circulation in response to the anthropogenic heating due to energy consumption. There is an equatorward shift of the winter mid-latitude jet, with increasing westerly wind at 20°N and decreasing westerly wind at 40°N.”
Interestingly, the study says the notion that urban heat islands could affect high-altitude atmospheric currents was first considered fifty years ago in two late-1960s studies. It was excluded from later models because early studies suggested the “heat island forcing” was indistinguishable from natural fluctuations.
Those studies, the authors note, only lasted a few months. The new results suggest that longer-scale modelling of the impact of urban heat release produces a quantifiable, if localised, impact. The global impact, however, remains trivial at 0.01°C.
The work, part-funded by the National Oceanic and Atmospheric Administration, was conducted by Florida State University’s Ming Cai, the University of California’s Guang Zhang, and Aixue Hu of the National Center for Atmospheric Research. ®