Climate Reason - The Little Ice Age Thermometers
A study of Climatic Variability from 1660-2009

The Urban Heat island effect (UHI)

1) The official view.

This was the IPCC’s interpretation of the urban heat island effect (UHI) in 2001:

"Clearly, the urban heat island effect is a real climate change in urban areas, but is not representative of larger areas. Extensive tests have shown that the urban heat island effects are no more than about 0.05°C up to 1990 (from 1900) in the global temperature records used in this chapter to depict climate change. Thus we have assumed an uncertainty of zero in global land-surface air temperature in 1900 due to urbanisation, linearly increasing to 0.06°C (two standard deviations 0.12°C) in 2000."

In the IPCC statement (SPM 2007, page.5): "Urban heat island effects are real but local, and have a negligible influence (less than 0.006°C per decade over land and zero over the oceans)."

The Urban Heat effect-The IPCC view and the urban reality of warming.

2) What is the Urban Heat Island effect?

To understand the principles of UHI we first need to understand the manner in which temperature data is collected.

Gathering Raw Data

It might be thought that obtaining accurate instrumental temperature records which represent a country's weather, that reflects its changing climate, should be relatively simple. When such records are collated with those from every other country that has been obtained in the same manner, these should become a reasonable representation of the world's evolving climate as viewed through its temperature data.

The principle to achieve this end result is that someone places a sufficient number of reliable thermometers in a representative number of places in order to record the numerous micro climates that make up a country's climate profile. They ensure they are housed inside a suitable screen so they don't catch the full glare of the sun, operatives are trained to read them accurately at specified times over a period of many decades (most are automated these days) and consistency is achieved by ensuring the equipment used remains the same (more or less sensitive equipment can play havoc with data) Lastly it needs to be ensured there are no adverse external factors. These can include the site becoming less rural, inappropriate siting, or the thermometer being moved to a location that no longer represents the original microclimate -airports are increasingly favoured and are often some distance away.

Fluctuating station numbers in the country, or worldwide, also radically affect the desired end result- a 'global' temperature supposedly accurate to fractions of a degree.

In the real world very little of this apparently logical manner of collecting information happens at all. Original data is highly processed and 'adjusted' by complex calculations to try to take into account a variety of factors that will theoretically take the data back to an 'unaffected' reading, so like can be compared for like when comparing modern to older readings. The graphic below illustrates the astonishing rise and fall of station numbers as they migrate round their respective country.

Growth and Decline of Weather Stations Globally
Fig 1. Graphic by Verity Jones and Lucy Skywalker from material supplied by EM Smith

As can be seen, station numbers rose from zero to a peak around 1980/90 and have fallen back substantially since. Stations were historically far more numerous in the Northern Hemisphere than the South, making the concept of accurate global temperatures back to 1850/1880 (Hadley/Giss) rather hypothetical.

The Impact of UHI

One of the biggest factors to consider when trying to compile an accurate temperature is the UHI effect - something that appears to be substantially downplayed by the IPCC, but particularly relevant as a significant proportion of the worlds records increasingly come from urbanised areas. The link to population growth (here) illustrates how the environs of a station may have changed over the years.

The precise amount of artificial warming through proximity of buildings, vehicles, tarmac etc will vary according to whether the station remains in a large open space or becomes crowded in by development. There is clearly a limit to the UHI effect - as urbanisation grows artificial warming is likely to be spread more widely but will not necessarily be more concentrated. However, a difference of several degrees between urban and adjacent rural areas is common and the effect is particularly marked on still clear nights when the difference can be substantially greater. This renders many modern temperature records rather suspect as the correction factor (if applied) often appears to be inadequate. The end result is that the recorded temperature may read rather higher than it should do when trying to compare like for like with older less affected records.

The warming caused locally by the UHI effect is unambiguous. Some urban areas will welcome the additional warming, but to others it will be a real issue. However it appears to be one that does not attract as much attention as the much less obvious impact of co2.

Further Reading

This page will eventually contain many general studies of the UHI effect. Where they are available studies of the UHI effect in specific cities are linked through the appropriate city graph accessed from the home page graphic.

A Description of UHI by Professor Ole Humlum

Professor Humlum has been the Professor of Physical Geography at the Department of Geosciences in University of Oslo, as well as the Adjunct Professor of Physical Geography at the University Centre in Svalbard (UNIS), since 2003