When Luke Howard discovered heat islands in 1818, it was not his first scientific breakthrough. In 1802, the thirty-year-old chemist (pharmacist) and amateur scientist presented “The Modification of Clouds” capping his childhood fascination with clouds by creating the classification system that we still use today. He won worldwide fame, with no less a figure than Johann Wolfgang von Goethe writing poetry, “In Honour of Howard”1.
Years later, his interests transitioned to climate. He spent several years collecting temperature records in and around London. Howard calculated that London was 1.570 F warmer than outlying areas2. His publication of “The Climate of London Deduced from Meteorological Observations” was the first identification and analysis of heat islands. Heat islands today are typically 50F to 90F warmer, but a doctoral dissertation cites an example of 260F.3
Howard’s primary contribution was determining the conditions that increased temperatures. We called them “Urban Heat Islands” because, at the time, those conditions only existed in urban environments, which made the calibration against “outlying areas” valid. That is where we took our eyes off the ball.
A decade later, Fourier recognized the “greenhouse effect” but left it to later researchers to determine how it operated.4 That discovery, although completely valid in itself, shifted the focus of the study of climate from the land, to the atmosphere.
Instead of recognizing that these two-phenomenon act together, the physics of heat islands; the stove generating heat, with the atmosphere; the pot lid, trapping heat, American science and the IPCC chose the pot lid as the critical variable.
And this in spite of a largely European group, the World Meteorological Organization, also funded by the IPCC, that favored a two-legged approach: compiling the science in a book published by MIT and the Royal Swedish Academy of Science in 1971, and in the papers from the 1979 organizations conference.5 Unfortunately, the ecological data that the WMO used was far too complex to be modeled on a computer, while the greenhouse gas data was easy to model. The WMO folded in 2015.
So, what are the causes of heat islands? Modern studies largely confirm Howard’s conclusions.6
The first cause is “reduced evaporation”. Water evaporates by absorbing heat from ambient air, thus cooling the air. So, the primary cause of temperature rise is reduced cooling. As the largest source of evaporation is transpiration through the leaves of vegetation, the first cause of heat islands is actually de-vegetation; the loss of plants and trees.
The second cause of heat islands is “increased heat storage”, conflating the conversion of energy into heat, with the storage of heat, but explaining both increased daily temperatures and warmer evenings and sometimes nights. Bare ground stores heat, while construction and paving material have even higher heat-storage capacities. Take a bare-foot walk on any summer day and you will understand what this means.
The third heat island cause is the heat trapping mechanism responsible for generating the highest temperatures. As building surfaces get hot, they radiate heat into their surroundings, not unlike how greenhouse gases radiate heat into the atmosphere. Where surrounded by other buildings, heat is re-radiated from one building surface to another; trapping it.
The fourth cause of heat islands is also the result of building proximity. Tight building configurations block air flow that would disperse heat. The reduced air-flow reduces the cooling effects that would have been achieved in less constricted conditions.
The fifth cause of heat islands is “anthropogenic heat” or thermal pollution. All energy eventually degrades into heat, and our society is built on massive energy consumption. From heat lost from buildings in the winter, to all the electricity used to make life more comfortable, to the heat generated for commercial and industrial processes, to cars powered by converting heat into physical power. Our energy consumption is huge, and not getting smaller. AI is expected to increase it significantly.
The sixth cause of heat islands is the interactions between all these causes, but if we reduce the negative impacts, we reduce the interactive warming.
As noted, heat island impacts are calculated by measuring the temperatures created in city centers, against the temperature of “outlying areas”. This assumes that “outlying areas” are not impacted by the physics that increases temperatures in cities.
While the heat-trapping mechanism that concentrates temperatures, and the restricted air flow that reduces cooling, are limited to cities, the primary causes of heat islands: de-vegetation, increased heat storage, and thermal pollution, have become global phenomenon.
Unless someone has repealed the laws of physics, these factors are raising temperatures worldwide. The first implication is that “outlying areas” are warmer than they would otherwise be, and do not represent a valid “zero” base line to measure against. This means we are underestimating the temperature increase due to heat islands.
But more significant, the physics that create heat islands are raising temperatures globally. We have created a Heat Planet.
We will not resolve our climate woes without addressing the actual cause of warming. We must turn down the stove!
The easiest and cheapest means of reducing warming is increasing vegetation in rural areas; eliminating bare soil, especially the millions of acres produced by industrial agriculture, addressing erosion and aridification, and restoring forests, which will also increase fire-resistance, reducing the need for the far-more complex and expensive changes required in suburban and urban areas.
A two-lane road covers almost three acres, more than two football fields, for every mile. We must reverse this love-affair with paving and take on a serious de-paving program, replacing only absolutely-necessary foot and wheel traffic surfaces with half-soil-filled grids, planted for growth below traffic level.
Merely increasing albedo in cities will be counter-productive. Reducing temperatures will require green-roofs, green-walls and green-streets to levels no one has ever contemplated. Building renovations could increase energy efficiency while reducing heat-island impacts; but will require technical expertise and individual focus applying building science principles to prevent mold. Corner cutting could be disastrous. The work generally gets more complicated and expensive as it moves higher.
References
- Hamblyn, R, (2001) the Invention of Clouds: How an Amateur Meteorologist Forged the Language of the Skies’. New York. Picador.
- Howard, L, (1818) the Climate of London: Deduced from Meteorological Observations Volume 1. London, W. Phillips, George Yard. (2012, New York, Cambridge University Press.)
- Kim, Jun-Pill & Guldmann, Jean-Michel (2014) Land-use planning and the urban heat island, Environment and Planning B: Planning and Design, v 41:1077 – 1099.
- Henson, R, (2019) The Thinking Person’s Guide to Climate Change, 2nd Boston, Ma. American Meteorological Society.
- Lewis, R. https:/www.resilience.org/stories/2023-07-17/Millan-millan-and-the-mystery-of-the-missing-mediteranean-storms/
- Garland, L (2011) Heat Islands: Understanding and Mitigating Heat in Urban Areas. Washington D.C.: Earthscan Publishing.
