Detecting urbanization effects on surface and subsurface thermal ...

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ava i l a b l e a t w w w. s c i e n c e d i r e c t . c o m
w w w. e l s ev i e r. c o m / l o c a t e / s c i t o t e n v
<strong>Detecting</strong> <strong>urbanization</strong> <strong>effects</strong> on surface and subsurface
thermal environment — A case study of Osaka
Shaopeng Huang a, ⁎, Makoto Taniguchi b , Makoto Yamano c , Chung-ho Wang d
a Department of Geological Sciences, University of Michigan, Ann Arbor, Michigan 48109-1005, USA
b Research Institute for Humanity and Nature, 457-4 Motoyama, Kamigamo, Kita-ku, Kyoto, 603-8047, Japan
c Earthquake Research Institute, University of Tokyo, 1-1-1 Yayoi Bunkyo-ku, Tokyo, 113-0032, Japan
d Institute of Earth Sciences, Academia Sinica, P.O.B. 1-55, Taipei Nanking, 11529, Taiwan, ROC
A R T I C L E I N F O
Available online 2 June 2008
Keywords:
Surface air temperature
Diurnal temperature range
Borehole temperature
Urban heat island <strong>effects</strong>
Subsurface thermal environment
A B S T R A C T
Tremendous efforts have been devoted to improve our understanding of the anthropogenic
<strong>effects</strong> on the atmospheric temperature change. In comparison, little has been done in the study
of the human impacts on the subsurface thermal environment. The objective of this study is to
analyze surface air temperature records and borehole subsurface temperature records for a better
understanding of the urban heat island <strong>effects</strong> across the ground surface. The annual surface air
temperature time series from six meteorological stations and six deep borehole temperature
profiles of high qualities show that Osaka has been undergoing excess warming since late 19th
century. The mean warming rate in Osaka surface air temperature is about 2.0 °C/100a over the
period from 1883 to 2006, at least half of which can be attributed to the urban heat island <strong>effects</strong>.
However, this surface air temperature warming is not as strong as the ground warming recorded
in the subsurface temperature profiles. The surface temperature anomaly from the Osaka
meteorological record can only account for part of the temperature anomaly recorded in the
borehole temperature profiles. Surface air temperature is conventionally measured around 1.5 m
above the ground; whereas borehole temperatures are measured from rocks in the subsurface.
Heat conduction in the subsurface is much less efficient than the heat convection of the air above
the ground surface. Therefore, the anthropogenic thermal impacts on the subsurface can be more
persistent and profound than the impacts on the atmosphere. This study suggests that the
surface air temperature records alone might underestimate the full extent of urban heat island
<strong>effects</strong> on the subsurface environment.
© 2008 Elsevier B.V. All rights reserved.
1. Introduction
20th century global warming is well documented in the
instrumental record (Brohan et al., 2006) and a wide range of
proxies including borehole temperatures (e.g., Huang, 2004). It
has been recognized that anthropogenic forcing is at least
partially responsible for the recent warming (IPCC, 2007). The
human influences on climate change are particularly significant
in urban areas. Urbanization alters the thermal properties of the
land, changes the energy budget at the ground surface, changes
the surrounding atmospheric circulation characteristics, generates
a great amount of anthropogenic waste heat, and leads to
a series of changes in the urban environmental system.
The impacts of <strong>urbanization</strong> on the thermal environment are
generally termed as urban heat island <strong>effects</strong>. Those <strong>effects</strong>
mostly originate near the ground surface and first result in
surface temperature anomalies. The anomalous urban ground
surface temperature anomalies will unavoidably propagate
both upward into the atmosphere (Jones et al., 1990; Kalnay
and Cai, 2003; Kim and Baik, 2004; Zhou et al., 2004; Brazel et al.,
⁎ Corresponding author.
E-mail address: shaopeng@umich.edu (S. Huang).
0048-9697/$ – see front matter © 2008 Elsevier B.V. All rights reserved.
doi:10.1016/j.scitotenv.2008.04.019

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2007; Liu et al., 2007; Stathopoulou and Cartalis, 2007) and
downward into the subsurface (Bodri and Cermak, 1999;
Changnon, 1999; Baker and Baker, 2002; Ferguson and Woodbury,
2004; Taniguchi et al., 2005).
With the increasing trend in <strong>urbanization</strong> worldwide,
tremendous efforts have been devoted to improve our understanding
of the urban heat island <strong>effects</strong>. However, most of the
research efforts so far have been focused on the atmospheric
aspects of the urban heat island <strong>effects</strong> (Jones et al., 1990; Kalnay
and Cai, 2003; Peterson, 2003; Trenberth, 2004; Zhou et al., 2004;
Brohan et al., 2006; Parker, 2006; Jenerette et al., 2007). The urban
heat island <strong>effects</strong> on the subsurface temperature and their
environmental consequences are poorly understood.
The objective of this study is to analyze meteorological
records and surface temperature data for the detection of the
<strong>urbanization</strong> <strong>effects</strong> on the thermal environment across the
ground surface in Osaka metropolitan area in Japan.
Osaka is the second largest metropolis in the country and
has a population growing from 8.67 million in 1985 to
8.82 million in 2006 (http://www.citypopulation.de/). We
analyze the trends in the annual mean temperature, the
mean maximum and minimum temperatures, and the mean
diurnal temperature range recorded in 6 meteorological
stations around Osaka. We use a high quality subset of
borehole temperature data reported in two previous studies
(Taniguchi and Uemura, 2005; Taniguchi et al., 2005) to detect
the subsurface warming in Osaka. We calculate the transient
impacts of the urban annual time series on subsurface
temperature distribution, and compare the calculated subsurface
temperatures to the borehole measurements.
Although borehole temperatures can be used for a reconstruction
of surface temperature history (e.g., Huang et al., 2000),
climate reconstruction is the focus of the paper by Yamano et al.
(this issue) in this special issue. Readers interested in climate
reconstruction based on borehole temperatures from Osaka and
several other Asian cities are referred to Yamano et al. (this
issue) for additional information and results.
This study is part of an on-going effort of the multidisciplinary
research project “Human impacts on urban
subsurface environment” which has Tokyo, Osaka, Bangkok,
Seoul, Taipei, Manila, and Jakarta as its target research areas.
2. Meteorological records
In addition to the surface air temperature time series from the
Osaka meteorological station, we select meteorological
records from two urban stations (Kyoto and Kobe) and one
suburban station (Nara) around Osaka, and two rural stations
(Tsurugisan and Ibukiyama) in more remote areas for this
study (Fig. 1). All the selected meteorological stations are the
member stations of the Global Historical Climatological Network
(GHCN) (Peterson and Vose, 1997).
The Osaka meteorological observatory is located in the
downtown area of the Osaka city. The Kyoto, Kobe, and Nara
stations are the GHCN stations closest to Osaka, located
respectively in the northern Kyoto city, the shore area of the
Kobe city, and the hilly area of the Nara city. The two rural
stations selected for comparison are the rural stations closest
to Osaka among the 167 Japanese meteorological stations
included in the GHCN. Unfortunately, there are no other rural
stations in the GHCN that are located within the immediate
vicinity of the Osaka metropolitan area.
For this study, we analyze the trends in annual mean
temperature, annual mean maximum temperature, annual
mean minimum temperature, and the diurnal temperature
range for the selected stations. The monthly mean temperature,
mean maximum temperature, and mean minimum temperature
time series of the selected GHCN stations are available at
the Japan Meteorological Agency (JMA) website http://www.
data.jma.go.jp/obd/stats/data/en/smp/index.html. These
meteorological records have been aggregated to generate
corresponding annual time series. We further derived the
annual mean diurnal temperature range time series from the
aggregated annual maximum and minimum records.
All of the four urban/suburban stations are still in active
service and have annual data up to 2006 by the time of this
study. The Osaka time series started in 1883, and the Kyoto
record started two years earlier in 1881. However, both of the
two rural stations have been out of service since 2001. The
temporal lengths covered by the selected meteorological
records range from 52 years (Nara) to 125 years (Kyoto). All of
the selected meteorological records show overall warming
trends in the annual mean temperature, maximum temperature,
and minimum temperature time series over their
observation periods (Fig. 2).
The Osaka station shows a warming trend of 1.99 °C/100a
over the 124 year period from 1883 to 2006, more than triple
the 20th century global warming rate 0.6 °C/100a (IPCC, 2001).
The anomalous urban warming is consistently recorded in the
records from the nearby urban/suburban stations, of which
the warming rates are 2.24 °C/100a for Kyoto, 1.45 °C/100a for
Kobe, and 1.96 °C/100a for Nara, respectively. In comparison,
the warming rates recorded in the two rural stations are more
diverse. Over its 55-year life span, the Tsurugisan station
showed a warming rate of 0.47 °C/100a which is slightly lower
than the global average; whereas the 82-year Ibukiyama
record showed a 1.60 °C/100a warming rate that is much
greater than the global average.
To reduce the ambiguity of comparing records of different
temporal lengths, we have also analyzed the records over the
1954–2000 period covered by all of the 6 selected stations. The
trends derived from the records over the entire observation
periods and over the 1954–2000 common-period are summarized
in Table 1 for comparison. A more vigorous yet consistent
urban warming trend and a more moderate yet diverse rural
warming are reconfirmed by the common-period data
analysis.
The change in the diurnal temperature range appears to be
another important indicator of urban heat island <strong>effects</strong> in
this region. In both entire-period and common-period analyses,
Osaka and the other two urban stations show a
decreasing trend in the diurnal temperature range, apparently
due to nighttime warming being more significant than
daytime warming in urban areas (Kalnay and Cai, 2003; Zhou
et al., 2004). In contrast, the trends in the diurnal temperature
range for the two rural stations are mild over their entire
observation periods, and positive over the 1954–2000 period
(Table 1). In these two rural sites, the nighttime (minimum
temperature) warming trends are comparable to or less

3144 S C I E N C E O F T H E T O T A L E N V I R O N M E N T 4 0 7 ( 2 0 0 9 ) 3 1 4 2 – 3 1 5 2
Fig. 1 – Satellite photos showing the location of the six selected meteorological stations. The locations of the individual stations
are detailed in the six smaller photos. Each small photo covers an area of about 5 km×7 km. The photos are generated with
online mapping tool Google Earth.

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Fig. 2 – The annual mean (green dot), maximum (purple diamond), minimum (blue cross), and annual temperature range (red
triangle) time series and the linear regression trends for the selected meteorological stations. The equations for the trend lines
are shown with the same color scheme. The meteorological data were retrieved from the Japan Meteorological Agency website
at http://www.data.jma.go.jp/obd/stats/data/en/smp/index.html. (For interpretation of the references to color in this figure
legend, the reader is referred to the web version of this article.)
significant than the daytime (maximum temperature) warming
trends.
The record from the Nara station embraces a strong
warming trend in the annual mean temperature that is
similar to the urban stations and a positive trend in the
diurnal temperature range that is characteristic of the two
rural stations. The Nara station is classified in the GHCN
inventory as an urban station based on the population of Nara

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Table 1 – Characteristics of the selected meteorological records.
Station
name
Setting Latitude Longitude Period
covered
Trends (°C/100a) over entire
observation period
Trends (°C/100a) over the
1954–2000 period
North East Mean Max Min Range Mean Max Min Range
Osaka Urban 34°40.9′ 135°31.1′ 1883–2006 1.99 1.35 2.71 −1.36 2.90 1.58 4.17 −2.59
Kyoto Urban 35°0.9′ 135°43.9′ 1881–2006 2.24 0.94 3.55 −2.61 2.43 0.68 3.25 −2.57
Kobe Urban 34°41.8′ 135°12.7′ 1897–2006 1.45 0.66 1.95 −1.29 1.81 1.49 1.57 −0.08
Nara Suburban 34°41.6′ 135°49.6′ 1954–2006 1.96 1.93 1.43 0.50 1.81 1.69 1.13 0.55
Tsurugisan Rural 33°51.2′ 134°5.8′ 1946–2000 0.47 0.71 0.11 0.41 0.18 0.89 −0.33 1.22
Ibukiyama Rural 35°25′ 136°24.4′ 1919–2000 1.60 1.72 1.88 −0.17 1.62 1.95 0.89 1.06
city it is associated with. The GHCN uses a tripartite
classification of stations by population, with less than 10,000
as Rural, 10,000 to 50,000 as Small Town and over 50,000 as Urban.
The city of Nara has a population of over 300,000. However, in
contrast to Osaka, Kyoto, and Kobe, the Nara meteorological
station is not within an urban center. We reclassify Nara station
as suburban for its location in the hilly area near the margin of the
city (see Fig. 1). The positive trend in the diurnal temperature
range is consistent with this reclassification.
3. Borehole data
The six boreholes from which temperature measurements are
analyzed in this study are located within the densely
populated Osaka urban area (Fig. 3). Osaka city occupies an
area of 220 km 2 of the 1600 km 2 Osaka alluvial plain which is
open to the Osaka Bay to the west, and surrounded by
Hokuetsu and Rokko Mountains to the north, Ikoma Mountain
to the east, and Izumi Mountain to the south. The plain is
covered by Pleistocene Osaka group and the later sediments to
a thickness over 600 m in the central part of the plain. The
underlying bedrocks of the Osaka plain are Mesozoic volcanic
rocks and granites. All the boreholes were drilled at least
20 years ago in the Quaternary sedimentary rocks.
The borehole temperature profiles (Fig. 4) used in this
analysis are selected from the borehole temperatures measured
during the field campaign from August to October 2003.
Subsurface temperatures from these boreholes were measured
at 1 m intervals with a logger of 0.01 K accuracy.
Additional information regarding the geological settings of the
borehole sites and the technical aspects of the temperature
measurements can be found in Taniguchi and Uemura (2005).
In principle, the distribution of temperature in the subsurface
is controlled in part by the heat flowing from deep interior
of Earth, and in part by the temperature history at the ground
surface (Cermak et al., 1992; Huang et al., 2000; Pollack and
Huang, 2000; Harris and Chapman, 2001; Smerdon et al., 2006). If
the surface temperature never changes, the distribution of the
subsurface temperature would be a linear function of depth
with the geothermal gradient as its slope (Fig. 5). However, if the
surface temperature changes with time, the distribution of the
subsurface temperature would depart from linearity. A progressive
cooling at the ground surface would result in a
temperature profile bending towards lower temperature as
illustrated in Fig. 5 in blue. Conversely, a warming at the ground
surface would force a temperature profile to bow towards higher
temperature at shallow depths (red curve in Fig. 5).
In the field, however, subsurface temperature is also
subject to other perturbations related to site-specific environmental
settings including land cover, land use, and groundwater
movement (Pollack and Huang, 2000; Goto et al., 2005;
Taniguchi and Uemura, 2005; Bartlett et al., 2006). 6 out of the
34 borehole temperature profiles obtained by Taniguchi
project team over the 2003 field campaign (Taniguchi and
Uemura, 2005) in Osaka meet the basic data requirements for
the objective of detecting urban heat island <strong>effects</strong>.
Borehole drilling is very expensive and is generally unaffordable
to researchers from an academic institution. Temperature
logging for scientific research has to rely mostly on the boreholes
drilled for industrial or civil purposes. As such, many borehole
temperature measurements from undesired settings may be not
useful to the specific research purpose due to high level of noise.
The boreholes available for logging in Osaka at the time are
groundwater monitoring wells. Many borehole temperature
data are excluded from the analysis of this study due to severe
perturbations from groundwater flows (Taniguchi and Uemura,
2005; Taniguchi et al., 2005). Temperature changes at the ground
surface can only impose smooth transient perturbations to the
subsurface temperature field. However, groundwater flows can
cause both smooth and abrupt changes to a temperature profile.
In our borehole data selection, an abrupt change is taken as an
identifier of ground water perturbation.
Another factor leading to the rejection of some borehole
temperature profiles is the borehole depth. Temperature measurements
from shallow boreholes would not allow separation of
the steady steady-state geothermal gradient from the climate/
<strong>urbanization</strong> related transient components. In this study, we only
use temperature profiles that extend to the depth of 180 m.
Although the temperature profiles selected for this study
comprise a high quality subset of the Osaka borehole data archive,
they are not free of non-climate perturbations, as evidenced in the
irregular variation in the profiles. Nevertheless, the level of the
noise is much lower than the transient signals in the selected
profiles. All six profiles bend towards higher temperature at
shallower depths, a clear signature of ground warming in Osaka.
4. Urban heat island <strong>effects</strong> in surface air
temperature
A wide range of the <strong>urbanization</strong> <strong>effects</strong> in surface air
temperature have been reported by different groups, from
negligible (Peterson, 2003; Parker, 2004; Peterson and Owen,
2005; Parker, 2006) to very significant (Kato, 1996; Kalnay and
Cai, 2003; Li et al., 2004; Zhou et al., 2004; Kalnay et al., 2006).

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Fig. 3 – Satellite photos showing the location of the six selected boreholes. The location of the Osaka (o), Kobe (b), and Nara (n)
meteorological stations are shown on the top map for reference. The locations of the individual borehole sites are detailed in the
five smaller photos. Each small photo covers an area of about 5 km×7 km. The photos are generated with online mapping tool
Google Earth.

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Fig. 4 – Temperature - depth profiles selected for this study.
Included in the legend labels are the borehole ID numbers
followed by borehole names and logging dates.
The Osaka station documented a strong warming trend of
about 2.0 °C/100a over the period from 1883 to 2006, and 2.9 °C/
100a over the 1954–2000 period, much greater than the 20th
century global warming rate 0.6 °C/100a (IPCC, 2001). The
anomalous urban warming is consistently recorded in the records
from the nearby urban/suburban Kyoto, Kobe, and Nara stations.
This excess warming rate in Osaka and its surrounding urban
areas is also substantially greater than the regional warming rate
estimated by the Japan Meteorological Agency (JMA).
The JMA carefully selected 17 meteorological stations
across the country that are considered not to have been
much influenced by <strong>urbanization</strong> and have continuous records
for the regional climate trend. The mean surface temperature
in Japan is estimated to have been rising at a rate of about
1.06 °C per 100 years since 1898 (JMA, 2006). This Japanese
regional mean is close to the average of the warming trends of
the two rural stations Tsurugisan and Ibukiyama, despite that
the warming rates of these two stations are more diverse.
The JMA (JMA, 2006) cautions that its regional estimate
might be not entirely free of <strong>urbanization</strong> perturbation. Based
on the records from the urban stations around Osaka and the
JMA regional estimate, a conservative estimate of the urban
heat island <strong>effects</strong> in Osaka would be in the range of 1–2 °C/
100a. This estimate agrees in general with the early analysis of
Kato (1996). Based on principal component score analysis of
monthly mean temperature data for the period from 1920 to
1992 from 51 meteorological stations in Japan, Kato suggests
that the maximum urban <strong>effects</strong> with a population of over
100,000 in 1993 were 1.0–2.5 °C/100a in Japan (Kato, 1996).
It is worthy of pointing out that climate change is not
homogenous, temporally and spatially. Based on land-only
meteorological records, for example, Huang (2006a,b) show that
on a global scale, the 20th century warming comprised two
warming phases and a slightly cooling transition phase, with the
first warming phase over the earlier four decades and the second
warming phase started in around the beginning of the 1970s.
However, on a continental scale, a two-phase warming was more
typical of North America, Europe, and Africa than Asia, Australia,
and South America (see Fig. 1 of Huang, 2006b). The mean surface
air temperature appeared to be flat over the first half of the 20th
century in Asia. Among the three long long-term urban records
analyzed in this study, Osaka and Kobe show no significant trend
over the first half of the 20th century, which is consistent with the
Asian continental record. However, due to the shortage of longterm
rural records for comparison, it is unclear at this point
whether the missing of an early 20th century warming phase is
characteristic of rural areas as well.
In addition to the warming rate substantially greater than the
global and regional means, the urban heat island <strong>effects</strong> in Osaka
appear to be reflected in a decreasing trend in the diurnal
temperature range, as opposed to an increasing trend in the rural
areas. The overall decreasing rate of 1.36 °C/100a in the diurnal
range for the entire observation period and 2.59 °C/100a for the
later half of the 20th century also exceed the global mean of
0.84 °C/100a (Easterling et al., 1997) and the rates in many areas in
Asia-Pacific region (Zhou et al., 2004; Griffiths et al., 2005).
The diurnal temperature range is an important climate
parameter widely used in the study of <strong>urbanization</strong> <strong>effects</strong>
(Easterling et al., 1997; Kalnay and Cai, 2003; Braganza et al.,
2004; Zhou et al., 2004; Sun et al., 2006). Land use and land
cover associated with roads and buildings tend to enhance
daytime storage and nighttime release of thermal energy.
Therefore, the diurnal temperature range is commonly
expected to decrease with <strong>urbanization</strong>.
It should be pointed out, however, that the spatial and temporal
variations in the diurnal temperature range in the study
areas are not always coherent. For examples, superimposed on
the long-term decreasing trends are decadal increasing trends in
the Osaka and Kyoto series. Moreover, given a significantly decreasing
trend in the urban areas, it seems logical to expect the
diurnal temperature range to be greater in rural areas than in
urban areas. But this is not the case for the meteorological records
we analyzed in this study. Fig. 2 shows that the diurnal temperature
ranges at the two rural sites are consistently smaller than
the diurnal temperature ranges at the four urban/suburban sites.
Over the 1954–2000 common-period, the average diurnal temperature
ranges for the two rural stations are 5.58 (Ibukiyama)
and 6.44 (Tsurugisan) °C, as opposed to the 8.1 (Osaka), 9.34
(Kyoto), 7.54 (Kobe), and 9.96 (Nara) °C for the urban/suburban
stations. The relationship between the diurnal temperature range
and <strong>urbanization</strong> process in Osaka remains to be further
investigated.
Fig. 5 – Scheme illustrating the distribution of subsurface
temperature controlled in part by geothermal gradient (G)
and ground surface temperature condition. (For interpretation
of the references to color in this figure legend, the reader
is referred to the web version of this article.)

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5. Air warming and ground warming
Both meteorological record and borehole temperature data
show an on-going warming trend in Osaka which is expectable.
However, when we compare the synthetic temperature
profiles projected from the Osaka annual surface air temperature
time series to the actual borehole temperature profiles,
we also see discrepancies.
For a comparison of the meteorological projection and borehole
observation, we first used the Shen and Beck functional
space inversion method (Shen and Beck, 1991, 1992) to process the
borehole data for the best estimates of the steady-state geothermal
gradient and surface temperature for each of the boreholes
(Table 2).
The functional Space inversion method of Shen and Beck
(1991, 1992) allows for incorporation of a priori information for
the ground surface temperature history, subsurface temperature
distribution, thermal physical properties, and the steady
steady-state basal heat flow (Shen et al., 1995). In addition to
the measured borehole temperatures, the a priori information
we incorporated in the estimation of the steady steady-state
components include a regional thermal gradient of 0.034 K/m
(Taniguchi and Uemura, 2005) and a terrestrial heat flow of
85 mW/m 2 .
We then assumed that the ground surface temperature can be
approximated by the surface air temperature (Harris and Chapman,
1997; Smerdon et al., 2004; Huang, 2006a) and calculated the
transient subsurface temperature anomalies that the Osaka
surface air temperature series can be responsible for.
Temperature variation over time t at the ground surface
imposes a downward-propagating transient perturbation to the
steady-state background temperature field. In a semi-infinite
solid medium, the subsurface transient temperature anomaly
ΔT z at a given depth z due to a temperature anomaly history on
the surface T 0 (t) is given by (Carslaw and Jaeger, 1959)
Z
z
DT z = p
2 ffiffiffiffiffi T
pa 0 ðtÞt − 3 = 2 e − z2 = ð4at
Þ dt;
where α is thermal diffusivity which is assumed to be
0.8×10 − 6 m 2 /s, and the surface temperature anomaly T 0 (t) is
referred to the pre-1900 mean of the Osaka meteorological
record. The calculated subsurface temperature anomalies
are superimposed on to the estimated steady-state components
to synthesize the calculated temperature profiles.
We note that the calculation of the transient perturbations
from a surface temperature time series is dependant on the
choice of the initial temperature or the pre-observational
mean (Harris and Chapman, 1997). In this analysis the pre-
1900 mean is taken as the initial temperature for two major
reasons: it is the best estimate we can get from the
meteorological data available; and it is consistent with our
focus on the borehole data down to the depth of 180 m.
We reiterate that the objective of this study is to detect
possible <strong>effects</strong> of <strong>urbanization</strong> on thermal environment
through comparative analyses of meteorological and borehole
temperature records. Given that the Osaka meteorological time
series started in 1883 and that the transient perturbations of a
ground surface temperature history over the past one and a
quarter centuries or so are expected to be confined within the
depth of 150 m, we zoom on the borehole temperatures down to
a depth of 180 m. A slightly greater cutoff depth of the borehole
data is to allow for the determination of the quasi steady state.
Fig. 6 shows that the subsurface temperature anomaly
calculated from the meteorological record is substantially
smaller than the ground warming documented in the borehole
temperature profiles in Osaka. Given that the meteorological
observatory and the boreholes are all located within the urban
area and that the daily mean temperature rarely falls below
frozen point in Osaka, the discrepancy cannot be attributed to
snow cover effect (Chapman et al., 2004; Bartlett et al., 2005;
Smerdon et al., 2006). Instead, the apparent discrepancy
between the meteorological and borehole records could be
characteristic of an urban thermal environment.
As noted earlier, the anthropogenic urban warmth mainly
originates on the ground surface and propagates upward to the
atmosphere and downward to the earth. The so called surface air
temperature from a meteorological observatory is conventionally
recorded at a height of around 1.5 m above the ground surface. A
meteorological observatory is usually located in an open space
where air can flow easily. The air flow around the observatory can
prevent the measuring instruments from feeling the full extent of
warming on the ground surface. This could explain partially that
although the assembles of the surface air temperatures at
regional and global scales include numerous urban records,
they are not substantially contained by urban heat island <strong>effects</strong>,
according to the world wide and US data analyses (Peterson, 2003;
Brohan et al., 2006; Parker, 2006).
On the contrary, borehole temperatures are measurements
of temperatures of rocks in the subsurface. It has long been
recognized that the intensity of urban heat island <strong>effects</strong>
decreases with increasing wind speed [e.g., see Arnfield, 2003
for a review]. The influence of the large scale wind decreases
to zero at the ground surface. The heat transfer below the
ground surface is dominated by conduction which is orders
less efficient than air convection above the surface. Therefore,
Table 2 – Estimated steady-sate components of the selected borehole temperature profiles.
Borehole
ID
Borehole
name
Latitude Longitude Geothermal
North
East
gradient
(°C/100m)
Surface
temperature
(°C)
1 Niwakubo2-3 34° 43′ 48″ 135° 34′ 00″ 2.72 15.12
2 Minato2-B 34° 39′ 27″ 135° 26′ 56″ 3.57 15.99
3 Sakai2 34° 34′ 39″ 135° 27′ 27″ 3.98 16.20
4 Sakai3 34° 34′ 39″ 135° 27′ 27″ 4.49 15.28
5 Izumiotu 34° 30′ 16″ 135° 24′ 37″ 2.69 14.46
6 Kishiwada3 34° 28′ 30″ 135° 24′ 37″ 2.89 16.07

3150 S C I E N C E O F T H E T O T A L E N V I R O N M E N T 4 0 7 ( 2 0 0 9 ) 3 1 4 2 – 3 1 5 2
Fig. 6 – Comparison of the hypothetic temperature profiles
projected from the Osaka meteorological time series (dashed
purple curves) and the observed temperature profiles (solid
red curves). Shown in green lines are the quasi-steady sate
temperatures. The profiles are offset to avoid overlaps. (For
interpretation of the references to color in this figure legend,
the reader is referred to the web version of this article.)
the urban heat island <strong>effects</strong> could be more persistent and
profound in the subsurface than in the atmosphere.
6. Conclusions
The analysis of the meteorological records from the six GHCN
stations around Osaka shows that there is a strong warming in
the annual mean temperature and a decreasing trend in the
diurnal temperature range in the urban areas, as opposed to a
more moderate warming and an increasing trend in the
diurnal range in rural areas. This general characterization is
more obvious in the trends derived from the period from 1954–
2000 commonly covered by all six stations. The mean warming
rate in Osaka surface air temperature is about 2.0 °C/100a over
the period from 1883 to 2006 and 2.6 °C/100a from 1954 to 2000.
Based on the comparison of the urban and regional trends, it is
estimated that the urban heat island <strong>effects</strong> are responsible for
at least half of the observed surface air warming.
The overall diurnal temperature range in Osaka–Kyoto–
Kobe urban areas has decreased considerably over the past
century. However, the spatial and temporal variations in the
diurnal temperature range in the study areas are not always
coherent. There are increasing trends of decadal scales superimposed
on the long long-term decreasing trends in the Osaka
and Kyoto diurnal temperature range records. Additionally,
the fact that the diurnal temperature range is greater in urban
areas than in rural areas seems to contradict the decreasing
trend characteristic of urban stations. The relationship
between the diurnal temperature range and the <strong>urbanization</strong>
process in Osaka remains to be further investigated.
Six deep borehole temperature profiles obtained within the
urban area of Osaka, a high quality subset of the borehole data
used in the two previous studies (Taniguchi and Uemura, 2005;
Taniguchi et al., 2005), are selected for the inspection of the
urban heat island <strong>effects</strong> on the subsurface thermal environment.
All these borehole profiles comprise positive transient
temperature component at shallower depths that can be
attributed to an on-going land warming in Osaka.
Both meteorological records and borehole data show
strong positive temperature anomalies in Osaka. However,
the urban warming in Osaka recorded in the subsurface
temperature profiles is significantly greater than what is
recorded in the near surface air temperature time series. The
air flow around a meteorological observatory might have
prevented the measuring instrument from recording the full
extent of anthropogenic warmth in the Osaka urban areas.
Surface air temperature is conventionally measured at a
height of around 1.5 m above the ground while borehole
temperatures are measured from rocks in the subsurface. The
dominating heat transport mechanism in the subsurface is heat
conduction, which is much less efficient than the heat convection
of the air above the ground surface. Therefore, the anthropogenic
thermal impacts on the subsurface are more persistent and
profound than the impacts on the atmosphere. This study
suggests that the surface air temperature records alone might
underestimate the extent of urban heat island <strong>effects</strong>.
Temperature is a fundamental parameter controlling
various physical, chemical, and geological processes. The
subsurface urban heat island can adversely impact a city's
ecosystem, air quality, public health, energy demand, and
metropolitan infrastructure costs. Recent studies of Knorr
et al. (2005) and Knorr et al. (2007) on the possible effect of
warming of soil on the microorganisms illustrates the
importance of a better understanding of subsurface thermal
environment. With the world wide <strong>urbanization</strong> growing at an
unprecedented pace, there is an urgent need to improve our
understanding of the subsurface urban heat island and its
environmental, social, and economical consequences.
Acknowledgements
The project “Human impacts on urban subsurface environment”
is sponsored by the Research Institute for Humanity
and Nature in Kyoto. Huang is supported in part by the NOAA
Climate Change Data and Detection Program (Award
NA07OAR4310059) and the Knowledge Innovation Program
of the Chinese Academy of Sciences (Award KZCX2-YW-BR-
03). This study is benefited from the constructive comment
from Henry Pollack. The authors thank Drs. Vladimir Cermak
and Robert Harris for their constructive comments that helped
to improve the quality of this paper.
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