Heat

Anyone who has lived in or around a city will know that the temperature in the city itself often feels quite a bit warmer than surrounding communities. In reality, it doesn’t just feel warmer, it is warmer! This is because of a phenomenon known as the Urban Heat Island effect (UHI). It arises because materials that comprise buildings, roads, and other urban infrastructure absorb and re-emit more of the incoming solar radiation than natural surfaces, like soils and greenery, do [1]. This means that urban areas can range from 0.6° to 12°C hotter than rural areas just outside the city [2]. It is important to note that the urban heat island effect is not a consequence of climate change. While the mechanisms are inherently related, heat islands will almost certainly still exist in a zero-carbon world.

    There is a wide variety of literature on the negative effects of heat islands. For example, warm weather periods result in a higher energy consumption (for cooling), and higher temperatures may lead to more illness or at least discomfort [1]. Additionally, heat islands promote the formation of ground-level ozone (smog), which has negative effects on air quality and human health. There are numerous other consequences of heat islands, and there are lots of strategies underway to curb the effect, such as more vegetation, cooler pavements, and smarter development [1]. However, as much as these mitigation strategies may help reduce the UHI effect, there is almost no getting around the fundamental fact that cities require the use of heat-absorbing materials. The use of these materials will also only grow as cities do. So, it is worth finding ways to take advantage of it instead. 

One such benefit is that heat islands provide new agricultural opportunities. In temperate-to-cold regions, cities may provide a climate suitable for growing crops that otherwise could not be grown in surrounding areas. One study from Toronto found that urban soil temperatures were almost 8°C warmer than a site just outside the metro region, and artificial mulches could increase this an additional 3° [3]. These higher temperatures accelerate germination and seed growth, while reducing frost risks, resulting in a 50% increase of ‘growing degree days’ since the mid-1800s. Notably, the comparison site outside the urban area saw almost no increase. Additionally, crop yields can be increased even further by using the glass in buildings to concentrate sunlight on the growing area [3], providing an otherwise environmentally-unfriendly material a way to ‘give back’ its energy use and get closer to net-zero.

There are numerous benefits to urban agriculture. Socioeconomically, it improves the quantity and quality of food in cities, providing an important tool to overcome ‘food deserts’ in low-income neighborhoods [4]. Moreover, it brings sustainable, unskilled jobs to cities. One major barrier for a renewable energy transition is that almost all developments require investments in education; there is little future for unskilled workers. Urban agriculture provides one outlet that doesn’t require relocation. This may also contribute to a slight change in ideology - or at least perception - of a carbon-free world. Such a world is not just sci-fi or a vision of elitist academics who have the ability to imagine it, but it can be real, tangible, and one in which everybody can play a crucial role.

Urban agriculture is just as important from an energy perspective. Heat island-induced agriculture should cut down massively on transportation costs, as these crops would have had to be grown in a place with a different climate far away. Transportation is a large contributor to CO2 and other GHG emissions, so reductions will get closer to a carbon-free economy. Another counter-intuitive benefit is the fact that heat islands make certain crops just viable climatologically. This means that, in a cold period, the UHI effect might not provide enough heat. Therefore, excess heat produced at power plants or heavy industrial plants could be re-routed to the crops instead of being put into the atmosphere. Such ideas are already in place for greenhouses in the Netherlands [5]. Finally, any crops grown in urban areas are less crops that need to be produced on traditional farmland. While food production has historically escaped the Malthusian Trap scenarios, world population growth will see an increased food demand. This is especially problematic when certain renewable energies - notably solar and biofuels - require extensive arable land use. If the warm-weather, ethanol-producing sugar cane, for instance, could be produced in urban areas in colder climates, it would go far in alleviating land use debates. 

There is no doubt that urban heat islands have significant negative impacts. But in an increasingly urbanized world, it would do us well to look at its advantages, ones which could help in the quest for 100% renewable energy.

Citations

[1] “Heat Island Effect,” US EPA, 2020. https://www.epa.gov/heatislands. Accessed 30 Nov, 2020.
[2] Memon, RA., Leung, DYC., and Liu, C. “A Review on the Generation, Determination, and Mitigation of the Urban Heat Island,” Journal of Environmental Studies, Vol. 20, 2008. 
[3] Waffle, A., Torry, R., Gillespie, T., and Brown, R. “Urban heat islands as agricultural opportunities: An innovative approach,” Landscape and Urban Planning, Vol. 161, 2017.
[4] Ackerman, K., Conard, M., Culligan, P., Plunz, R., Sutto, MP., and Whittinghill, L. “Sustainable Food Systems for Future Cities: The Potential of Urban Agriculture,” The Economic and Social Review, Vol. 45, No. 2, 2014.
[5] boxxen. “What are we waiting for?”, YouTube, 2006.