GREENHOUSE BULLETIN NO 125

The surface temperature record, which plots annual temperature anomalies with respect to a given reference period, has an in-built upwards bias because of population increase and expansion of human activity. Although this bias is difficult to identify and quantify, evidence is presented that it is nonetheless a major cause of the reported surface temperature rise over the past century, both in urban and in rural areas.

The pronounced different phases in the surface temperature record over the past century, and its highly regional character are incompatible with theoretical explanations based on steady global change, such as the proposed effects of the increase of greenhouse gases in the atmosphere.

Comparison of land surface regional hot spots identified in the surface record from 1976 to 1998 with the MSU satellite temperature anomalies for the 1979 to 1999 period shows that these regional effects are not detectable from the lower troposphere, although the MSU satellites do detect cooling in the South Indian and South Atlantic oceans and other climatic anomalies due to volcanic action, ocean circulation, and solar activity, evident in the surface record. The regional surface rises must therefore be due to individual local anomalies. The fact that these hot spots often occur in cold climates, and are mainly rises in night-time and winter temperatures, suggests that they are mainly due to local heating. Differences that have recently emerged between sea-surface and land-surface temperatures confirm this conclusion.

The MSU units evidently record the genuine changes in global temperature, but do not detect the supposed greenhouse effect, or its local manifestations.

  1. The Surface Temperature Record

The surface temperature record (Figure 1.) as compiled by Jones et al (1999) is familiar as the most convincing evidence that the surface of the earth is becoming warmer. Yet it does not represent a single continuous temperature record, or an average of continuous records. It represents a compilation of very many individual land and sea-surface temperature records, for different places and periods; each influenced by methods and times of measurement and by elevation and location. Converting these records into an overall average global record is a difficult task.

Figure 1. Global Surface Temperature record (land and sea), according to Jones (1997)

The procedure that has evolved, first proposed by Hansen and Lebedeff (1987), and subsequently refined by Parker et al. (1994) consists of dividing the surface of the earth into grids (currently 5°x5° latitude by longitude) and calculating a weighted average of the monthly averaged temperature records of the qualified measurement stations or sea-surface measurements within the grid. This average is then subtracted from the mean value of a similar average for the same grid over a reference period (currently 1961 to 1990). The figure obtained is the temperature anomaly for that grid for that month and year. The weighted annual average anomalies are then plotted as in Figure 1.

Figure 1 incorporates sea-surface temperatures (Parker et al 1994, 1995). The sea-surface data include many incomplete observations where assumptions have to be made about the method and time of measurement and size of ship before they can be aligned with the land surface observations. The procedure used is regarded as unreliable by US workers. Hansen and Lebedeff (1987) pinned their faith on surface measurements alone; "thus we avoid the ambiguity inherent in combining sea surface temperatures with surface air station data as well as the difficulties encountered with any marine (surface air or sea surface) temperatures due to temporal changes in the nature of ships" To this day, the US global temperature record (Hansen et al 1999) is for surface stations only. Recently Hansen et al (1996) have developed a Land-Ocean temperature index that combines land measurements with primarily satellite-derived sea surface measurements (Reynolds and Smith, 1994 and Smith et al 1996) but only from 1990. This procedure is an admission by the US workers that the satellite temperature record is more reliable than the surface record. But why only for sea-surface temperatures? Logically, they should admit that the satellite measurements are more reliable for the land surface as well.

Quayle et al (1999) have recently provided an improved combined record of land-based and sea-surface temperatures which incorporate surface and satellite measurements from 1880. The various US records do not differ significantly from Figure 1.

The comparison between temperature averages from land-based stations from different periods inevitably introduces an upwards bias. The population increases as the years go by, and human activity and economic prosperity increase accordingly. Each grid box has an increased population, coverage of buildings, and consumption of energy. This must inevitably have an influence on the temperature readings. The influence is a subtle one, not easy to identify or measure. Long-term measurement stations will have slow changes, such as growth of vegetation and protection from wind (Hessell 1980), sealing of roads, increased vehicle use, better heating in buildings, besides occasional upgrading with larger, more comfortable buildings. Stations that are closed would be replaced with more modern, better equipped facilities. Although most of these changes cause an increase in measured temperature, some changes, such as the removal to an airport site, would cause a fall in temperature. But this would be temporary, as the airport traffic increases, as found by Duchon (1986). Sea surface temperatures would also be influenced by size and comfort of ships taking the readings.

Measurement stations set up to measure carbon dioxide in the atmosphere have made a deliberate attempt to measure background levels by being situated in remote areas. Often wind direction is restricted to the open ocean or to mountainous areas so that the record is not contaminated with the effects of industry or agriculture. The prototype station at Mauna Loa Hawaii is often taken as typical of global measurements.

Weather stations are almost never set up to measure background surface temperature. They serve the local community by providing the temperature within that community. The temperature that they record is thus affected by the community, and it will increase as a result of increases in community activity. There are no requirements on the distance that the equipment should be from buildings or roads. There is no control on surrounding vegetation. Even the height of the equipment from the ground may very between 1.25 and 2 meters. The nature of the surface upon which it is placed is not standardised. Some earlier stations were on the roof of buildings or attached to their side and they were not always shielded from the sun or rain.

The regional distribution of temperature change from 1901 to 1996 is shown in Figure 2 (from Karl 1998) which shows the increase in °C for each of the qualifying 5°x5° grids. It is even less explicable as being caused by a steady overall climate change, as might be produced by radiative forcing. The increase in temperatures from 1901 to 1996 of 0.59°C took place predominantly in a few regions. The largest increase was in Russia/Siberia with a rise of 1.23°C for 3.67% of the earth’s surface. This should be compared with a rise of only 0.5°C for Western Europe (1.84% of the earth) and 0.41°C for the contiguous United States (1.67% of the area). Several small regions (N Atlantic, SE USA, Bolivia, SW China/Tibet) fell in temperature. Important regions (Pacific Ocean, Africa, Brazil, Antarctic, S Indian Ocean) are mainly without records. The records suffer from various levels of unreliability. Many display gaps during the two world wars, and movements of sites do not seem to be accounted for. The extreme example is Svalbard which has a recorded increase, shown in Figure 2., of 4.06°C from 1901 to 1996. The recorded annual figure for the year 1917 is 7.63°C below the 1961-1990 average. All this depends on a single record, that from Isfjord Radio (78.1N, 13.6E), which evidently completely changed its temperature level in the year 1920 and showed little overall trend until it was shut down in 1976 (GISS 2000).

The global temperature record from 1870 (Figure 1) falls into four sections. The first, (1870-1910) was variable with no definite trend. The second from 1910 to 1945 showed a temperature increase of about 0.5°C. The third, from 1946 to 1975, showed a fall of about 0.15°C, and the fourth, from 1976 to the present, has shown a rise of about 0.5°C.

This behaviour is not easily reconciled with any explanation which predicts a steady increase in temperature, such as might be related to the increase of atmospheric concentration of greenhouse gases.

The four sections of Figure 1, can, however, readily be interpreted as being caused by increases in human activity. Before 1910 urban development and heating standards were modest. From 1910 to 1945 was a period when measurement stations were mainly urban and population and heating standards were increased. Two world wars interrupted many temperature records, but they probably provided incentives for more frequent replacement with better facilities. The result would be a measured temperature rise.

1945 to 1976 saw the expansion of the aviation industry, with removal of many weather stations to airports, and an expansion of the system to rural ares. The slight fall in the temperature record follows from this.

After 1976 their was a rapid expansion of human population, motor traffic and economic wealth. Airports were surrounded by new buildings, paved with concrete and asphalt, with increasing numbers of large aircraft. Many weather stations were closed, predominantly in rural areas. As a consequence the surface temperature record rose.

  2. Urbanization

There is a voluminous literature which shows that urban development causes a rise in local surface temperature (Houghton et al 1990, 1996).

Hansen and Lebedeff (1987) found that removal from their dataset of stations associated with population centres of more than 100,000 people in 1970 lowered the global temperature rise by about 0.1°C. Balling and Idso (1989) criticised the technique used by Hansen and Lebedeff (1987) and showed that by taking into account changes in population for the eastern United States the temperature increase over 64 years attributable to "urbanization": was 0.39°C by their method instead of 0.02°C.

Karl et al (1988) in a comprehensive comparison of "urban" and "rural" sites in the United States found that there could be a significant warming effect (0.1°C from 1901-1984) from a population as low as 10,000. They found a non-linear relationship between urban warming and population,

where T is the average annual temperature difference (°C) (1901-1984) between a station located in an urban area with a known population (POP) and a rural station with an average of 750 residents. This amounts to an urban warming of 0.32°C for a town of 100,000 and 0.91°C for 1 million.

Karl and Jones (1989) found an urban bias of between + 0.1°C and +0.4°C for United States stations between 1901 and 1984, more than the overall temperature trend (+0.16°C) over the period. In a subsequent comment (1990) they corrected an error regarding Hansen and Lebedeff’s calculations which now indicate an urbanization effect of 0.12°C over 84 years.

Hessell (1980) identified several reasons why measurement sites display an upwards bias. These included extra surrounding buildings, increased shelter, and overheating of Stevenson screens in sunlight. These effects may be present in places with populations as small as a few thousands. He designated a few "B:" stations in New Zealand which showed no temperature change between 1950 and 1980.

Salinger (1982) designated further New Zealand "B" stations which did show an increase since 1950. It is unclear whether current New Zealand averages are based exclusively on "B" stations, or whether they preserve their status.

Goodridge (1992) in a study of California records found that large counties (644 to 8,710 thousand people) showed a warming effect of 1.44°C per century; mid-size counties (109-635 thousand people) 0.61°C per century and small counties (4-99 thousand people) 0.28°C per century.

Goodridge (1996) later commented that "Long-term temperature trends are clearly a function of urban population density. There are few temperature-measuring stations located in places with no heated buildings, pavement, or night lights in their view shed".

Christy and Goodridge (1997) showed that differences between MSU satellite temperature records in California and local surface records was due to urbanization effects.

Hughes and Balling (1996) found a difference of about 0.15°C temperature change per decade between large and small towns in South Africa.

Chow Shu Djen (1992) found a difference in the surface temperature increase between an urban station in Shanghai and a neighbouring rural station of 0.44°C between 1980 and 1984. The difference between the urban station and a suburban station was even higher, at 0.46°C

Balling et al (1998) analysed long-term European temperature records from 1751-1995 and found, by comparing the land-based European set and an ocean set that the land-based stations warmed twice as fast as the oceanic observations between 1890 and 1950.

Santamouris et al (1999) found differences as high as 15°C between temperature measurements in one month from urban and rural sites in the Athens area

Despite this evidence for urban warming two recent studies have concluded that it is negligible.

Jones et al (1990) compared the mean surface temperature change between urban and rural sites in the former Soviet Union , Eastern China, and Eastern Australia, and compared it also with data for the contiguous United States.

For Western Former Soviet Union two methods were used for assessing the urban sites, those from their own study (Jones et al 1966) and those from the time series of Vinnikov et al (1990). For the period 1901 to 1987 they found no difference between the rural trend and urban trend (Jones) and the urban trend (Vinnikov) was actually lower than the rural trend by 0.07°C. For the period 1930-87 the rural stations were 0.12°C below urban (Jones) but only 0.01°C below urban (Vinnikov). The rural sites were those with a population less than 10,000.

For Eastern Australia, for 1930-1987, urban (Jones) was 0.04°C above rural, but urban (Vinnikov) was 0.01°C below the rural figure. "Rural" meant a maximum population of 33,369 and a mean of 5,775.

The Eastern China figures had two different urban averaging systems. The first averaged the stations themselves, and when compared with the same method for averaging rural stations, it showed an increase of 0.16°C from 1954 to 1983. However, the second urban average, based on grid points, was 0.04°C below the rural figure, and the Vinnikov urban figure was 0.10°C below the rural figure. "Rural" meant less than 100,000 people from the 1984 census..

They compared their results with those that had been obtained by Karl and Jones (1989) and Jones et al (1989) who found an urbanization effect of 0.15°C in the continental United States between 1901 and 1984

The conclusion from this study was that urbanization effects were, at most. an order of magnitude lower than the measured effects. As a result, no correction for these effects has been made in Figure 1.

Wang , Zeng and Karl (1990) found an average difference of 0.23°C between urban and rural stations in China from 1954 to 1983, in disagreement with Jones et al (1989)

The more recent study by Peterson et al (1998) has confirmed the relative unimportance of urbanization effects. They used the 7,280 stations in the recently developed Global Historical Climatology Network (GHCN, Paterson and Vose 1998) for their global surface record and removed from it 2,290 stations which were both below 10,000 in population and fell below a defined level of night illumination. They found that the temperature records obtained from gridded 5°x5° boxes .for the two sets were almost identical.

  3. Local Heating

All the above studies suffer from a common fallacy which is embodied in the term "urbanization" It is automatically assumed that the only local influence on surface temperature measurement is urban development, as measured by population. There have been few investigations into possible distortions on local temperature measurement in "rural" measurement sites.

The definition of "rural" has fluctuated. Although there is evidence (Hessell 1980) that even communities with 2,000 people can exert an "urban" influence on local temperature, Jones et al (1990) accepted a figure in Eastern China of 100,000 as "rural". This might be a small town in China, perhaps, but it could hardly be regarded as "rural" and in New Zealand it would be regarded as a sizeable city.

There are many reasons why so-called "rural" measurement sites might be subject to increased local heating influences. Very few places would not have installed better heating in the neighbouring buildings over the past century. Roads would be sealed. Motor traffic would increase. Shrubs, trees and other vegetation around the site are likely to increase in size.

The effects would vary with the location. A site in a small Pacific island or in a tropical desert location would be expected to change less than one subjected to cold winters, where better heating in the buildings, the presence of sealed roads, increased vegetation and motor traffic would be welcomed. Jones et al (1990) make much of the possible effects of transferring measurement stations to airports, which would therefore become "rural" This may have been true in the early days of aviation, but a visit to any major airport today would find palpable evidence that they are heat islands, with many surrounding buildings and a large fuel consumption from aircraft and other vehicles.

  4. Evidence For a General Local Heating Effect

What is the evidence that many measurement stations, both "urban" and "rural" are affected by local heating effects, whose increase over the years has contaminated the surface temperature record.

4.1. Latitude

Figure 2. Shows the temperature change from 1901 to 1996 for each of the 5°x5° grids on the earth’s surface (from Karl 1998). Apart from the cool region of the North Atlantic there is a distinct increase with increasing latitude., both in the Northern and Southern Hemispheres, and for both urban and rural regions. North America and Western Europe shows the effect most convincingly. Details of the increase with latitude are given by Hansen and Lebedeff (1897)

Russia/Siberia, which showed the greatest temperature increase calls for special treatment, but it might be noted here that there are severe winter temperatures over most of the region which would provide an incentive for improved heating, and many of these grids would be in areas classified as "rural" by Peterson et al. If they were omitted from their study they would detect "urbanization" in the rest of the world.

There are many "rural" sites which do not display significant change over the past century. Seventy examples are given by Daly (2000). Many are on a coast, where the sea may dominate, even when there is local urban development. .

4.2 Hot Spots

Figure 3 shows the temperature trend in °C per decade for 5°x5° grids on the earth’s surface between 1976 and 1998. It shows, much more clearly than in Figure 2, that the temperature rise over that period was not uniform, but was dominated by a limited number of hot spots. The most prominent was Central and Northern Siberia, where the temperature increase was between 2 and 3°C. The next most prominent was Northern Europe which showed a rise of about 2°C, followed by smaller centres in SW USA, SE Africa and SE Australia. This behaviour is unlikely to be caused by an overall global warming. It is best explained as a series of local effects, mainly of localised heating. Its dominant position in the cold Northern climates highlights this probability

4.3 Comparison with MSU

Figure 4 shows the global average temperature record as measured by MSU (Microwave Sounder Units) carried by NASA satellites since 1978. There is no evidence of a significant temperature trend. The National Research Council (2000) recently considered in detail the possible reasons why the MSU record differs from the surface temperature record. They concluded that the difference between the two is "probably at least partially real:". However, they did not consider seriously the most plausible explanation for the difference, which is that the surface record is affected by local heating

Figures 5 and 6 show the individual MSU temperature records for the "hot spots" on Figure 3. The MSU record shows no sign of an overall temperature change in N/Central Siberia, Northern Europe, SE Africa, or SE Australia. The large temperature rises in these regions is thus not detectable in the lower troposphere. There is a similar inability of the MSU to detect the minor hot spots in the SE USA and in SW Africa.

On the other hand, the MSU satellite units do detect the fall in surface temperature shown in Figure 3 for the South Indian Ocean and the South Atlantic Ocean.

The land-based "hot spots", responsible for most of the temperature increase between 1976 and 1998, are therefore not detectable in the lower troposphere by the MSU units. On the other hand, the MSU units do detect the ocean based cool spots in the Southern Indian Ocean and the South Atlantic. The land-based hot spots must therefore be a purely local phenomena whose cause is related to behaviour of the local community. Their most likely explanation is improved local heating.

It might be noted that the MSU units do detect warmer regions in the lower troposphere over central and eastern Siberia and over the North Atlantic ocean (National Research Council 2000, Figure 7.1.) however, the maximum increases detected are 0.5°C per decade compared with the 1.5°C rise per decade shown by the surface measurements in Figure 3.

Jones et al (1997), who presented maps of the differences between surface and MSU measurements from 1979 to 1996 identified the large difference (at that time 0.5-1.0°C) for Siberia. They also found smaller differences (less than 0.5°C) for several tropical countries (Africa, Indonesia, W Africa and Brazil) which are probably due to unreliable local measurements.

It is evident that the MSU units detect genuine global climate variations which are also evident on the surface record, such as volcanic activity (notably that of Mount Pinatubo in 1992), ocean circulation (such as the El Niño event of 1998), and changes in solar activity (recently documented by Soon et al (2000)). The MSU units do not, however, identify the mainly local hotspots which are the main reason for the rise in the surface temperature record. These, therfore, cannot be regarded as a genuine climate change, but merely a local result of human activity, primarily improved comfort conditions.

4.4 The Temperature Range

A feature of the Surface temperature record (Figure 1) is that much of the observed temperature increase has been in the minimum temperature, and in the winter months. Easterling et al (1997) have examined this phenomenon and found that it is at least in part due to a narrowing of the diurnal temperature range (DTR). The most obvious explanation for this trend is that some weather stations have improved local heating in the winter, and on cold nights. Easterling et al (1977) show that the effect is just as evident in "non-urban" stations (below 50,000 population) as in the whole population.

4.5 Sea Surface Temperature

The land surface air temperature is currently 0.14°C above the Sea Surface Temperature (SST) and 0.21°C above the Night-time Marine Air Temperature (NMAT), as compiled by the UK Met. Office (Figure 7). The gap between the land and sea- surface readings has only opened up since 1980 as surface readings have increased, and it shows that there are heating effects on land which are not observed over the oceans.

4.6 Choice of period

As pointed out with Figure 1 the past global temperature record passed through four distinct phases, each with a different temperature profile. In order to study the possible influence of local heating the Urban/rural records to be compared should be confined to only one of these phases. Since 1910-1945 shows a rise, and 1946-1976 a fall in temperature, a study which contains both of these phases is likely to confuse two different aspects of behaviour. The study by Jones et al (1989) was for 1901 to 1987 and 1930 to 1987 for Russia, 1930 to 1987-8 for Australia, and 1954-1983 for China, a comparison being made with 1901 to 1984 for the USA. The authors themselves comment that the results might have been influenced by the move of measurement sites to airports halfway through the period of their investigation, thus confusing the results.

  5. Russian Temperatures

The temperature rises over the former Imperial Russia/Soviet Union form one of the most puzzling features of the entire temperature record. Figure 2 shows that the major sources of temperature increase over the period 1901 to 1996 was the whole of the records for Russia/Siberia. Figure 3, shows that the major source of temperature rise from 1976 to 1998 was Northern and Central Siberia.

The former Russian Empire/Soviet Union occupies 1.7% of the earth’s surface, 8.6% of the land. Its people have suffered enormous disruption and destruction during the past century. The very cold winter climate places great difficulties on temperature measurement. Monthly temperature records for the Russian stations show an extreme temperature range of around 60°C Early measurements are likely to have been in primitive or deprived conditions. Stations would have been operated by political prisoners

The Russia/Siberia figures stand out from all the others by the very larger annual variability. For example the Siberian station at Verhojansk (67.5N, 133,4E) showed an increase in annual mean temperature of 4.46°C from 1987 to 1988 and a fall of 1.94°C between 1991 and 1992. Many remote stations are near airports, so the weather station is likely to be there, and could be affected by measures used to keep the airport running during the winter. Although some Russian stations have excellent records over a very long time, the service has deteriorated in recent years, together with the rest of the Russian economy. In 1988 there were 244 temperature stations, but in 1989 135 were closed; mainly the smaller ones, leaving only 109 stations. Most of the 91 5°x5° grids in Russia/Siberia in Figure 2 will be represented by single stations. Recent monthly records from Russian stations show many gaps and doubtful figures.

Since the Russian figures have played a large part in establishing an overall global surface warming, it is important that their authenticity should be established.

  6. Conclusions

The records of annual global surface temperature anomalies and their regional distribution are not explicable by a theory of steady almost uniform global temperature increase, such as the supposed effects of increases in greenhouse gases. The surface temperature behaviour is much more readily explained by local effects, particularly heating, which can take place in both urban and rural sites, and is most likely in cold locations.

The MSU satellite temperature records of the lower troposphere detect important climate effects also evident in the surface record, such as those of volcanos, ocean circulation (El Niño, and ocean cooling) and the sun. They do not detect, however the regional hotspots which are largely responsible for the rise in surface temperature. The differences between the surface temperature record since 1978 and that recorded by the MSU satellites in the lower troposphere must therefore be largely due to local heating which is highly regional, and is particularly evident in cold climates.

The indications are that the MSU satellites give a true indication of global and regional temperature change, whereas the surface record is contaminated by local effects. of which improved heating is the main contributor.

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Vincent R. Gray , M.A.,Ph.D., F.N.Z.I.C.     
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January 24th 2000