Anthropogenic heat
Anthropogenic heat, usually written , is heat released to the atmosphere directly by human activity: building heating and cooling, vehicles, industry, and ultimately almost all energy consumed in a city, since nearly every use ends as heat. It enters the urban atmosphere as an additional source term in the energy balance.
What makes it distinctive among urban climate effects is that it does not work through the urban surface. Albedo, heat storage, and imperviousness all modify how incoming solar radiation is processed — no sun, no effect. Anthropogenic heat is an independent input. It continues at night, in winter, and under cloud, and it is the only urban warming mechanism that grows when the weather gets colder.
Magnitudes, which vary enormously
The number that matters is the flux density, and its range across a single city is larger than most single-value summaries suggest:
- Dense city centres in cold climates during winter reach the order of tens of watts per square metre, and central business districts in the densest cities can exceed 100 W m⁻².
- City-wide annual averages are typically an order of magnitude smaller, commonly in the range of a few to ~20 W m⁻².
- Suburban and residential areas are smaller again.
For context, net radiation over a mid-latitude city averages on the order of 100 W m⁻² across a year. So city-wide is usually a modest fraction of the radiative term, while in the densest cores at the coldest times it becomes comparable to it. Averaging over a city therefore hides exactly the situations where the term dominates — which is a good reason to be suspicious of city-mean figures in general.
Why it is hard to pin down
There is no instrument for . It has to be built up from inventories — energy consumption statistics, traffic counts, building stock, population density — and then distributed in space and time. Every step is an estimate:
- Energy statistics are reported by administrative unit and billing period, not by grid cell and hour.
- Traffic emissions need diurnal profiles that vary by day of week and season.
- Building heat release depends on the building stock’s thermal properties, which are poorly known at scale.
- Metabolic heat from people is small but not always negligible in dense areas.
The resulting uncertainty is large, and it is systematic rather than random — an inventory method tends to be biased in a consistent direction rather than noisy. I treat estimates as useful for spatial pattern and diurnal shape, and as considerably weaker evidence about absolute magnitude.
Consequences
It strengthens the urban heat island, most clearly at night and in winter when the radiative contribution is weakest and the anthropogenic contribution is at its seasonal peak. This is part of why heat island intensity in cold-climate cities often peaks in winter, which surprises people expecting a summer phenomenon.
For modelling, it has to be prescribed, since a model cannot derive it from physics — it depends on what people did. Sensitivity to that prescription is worth testing explicitly rather than assuming the default is adequate.
There is also a feedback worth naming: air conditioning moves heat from indoors to outdoors, raising outdoor temperature, raising cooling demand. The magnitude is modest at present but it is the one term expected to grow with both warming and rising cooling adoption.
See also: urban climate, urban heat island, urban meteorology, and urban surface for the mechanisms this one bypasses.