long-term climate change and surface versus underground temperature measurements in paris

INTERNATIONAL JOURNAL OF CLIMATOLOGY

Int. J. Climatol. 25: 1619–1631 (2005)

Published online in Wiley InterScience (www.interscience.wiley.com). DOI: 10.1002/joc.1211

LONG-TERM CLIMATE CHANGE AND SURFACE VERSUSUNDERGROUND TEMPERATURE MEASUREMENTS IN PARIS

F. PERRIER,a,b,* J.-L. LE MOUEL,a J.-P. POIRIERa and M. G. SHNIRMANc

a Laboratoire de Geomagnetisme, Institut de Physique du Globe, Paris, Franceb Laboratoire Hydrogeochimie et Etudes de Sites, Commissariat a l’energie atomique, Bruyeres-le-Chatel, France

c International Institute of Earthquake Prediction Theory and Mathematical Geophysics, Moscow, Russia

Received 17 November 2004Revised 18 April 2005

Accepted 18 April 2005

ABSTRACT

Careful temperature measurements performed from 1783 to 1852 in underground galleries, 28 m below the ParisObservatory, are compared with current measurements performed in a limestone quarry, 20 m below ground surface,and with local and European surface temperature records. When averaged using a backward 11-year moving window,the surface temperature time series looks similar and exhibits the already well-known 1 °C temperature increase over thelast century. In addition, since about 1987, a steeper increase of about 0.07 °C per year is noticed on all surface records.Underground temperatures, unaffected by surface fluctuations and averaging procedures, show a 0.9 °C increase and thusconfirm the trend indicated by the surface records. The averaged time series of the temperature in Paris and of the Wolfnumber, an indicator of sunspot activity, were reasonably well correlated till 1987 but deviated significantly from eachother after that date. The long-term connection between surface temperature and solar cycles is further supported by atemporal analysis of the frequency content at 11 years and 5.5 years. Visual correlations between temperature and sunspotnumbers, unconvincing when using recent records, appear more striking with underground data from 1783 to 1852. Thisanalysis suggests that solar activity played an important role in temperature changes till the last century, but that differentprocesses, possibly related to human-induced changes in the climate system, have been taking place lately with increasingintensity, especially since 1987. Copyright 2005 Royal Meteorological Society.

KEY WORDS: caves; global warming; surface temperature; underground; sunspots

1. INTRODUCTION

Since the beginning of the nineteenth century, when Fourier (1827) pointed out that the atmosphere playedthe same role as the glass panes of a greenhouse, preventing the ‘obscure heat’ from escaping, it has beenknown that the so-called ‘greenhouse effect’ accounted for the Earth’s surface temperature being in average15 °C rather than −20 °C. Arrhenius (1896) calculated the temperature increase resulting from the increasingconcentration of carbon dioxide in the atmosphere. It is now generally accepted that the burning of fossil fuelscauses an increase in atmospheric CO2 and hence, together with other contributions such as methane, leadsto global warming (e.g. Petit et al., 1999; Mann et al., 1998). The question of when a significant temperatureincrease started being noticeable is, however, still a matter of debate, as is the question of whether the recentincrease might be, at least partly, due to changes in solar activity (e.g. Solanki and Krivova, 2003; Laut,2003; Friis-Christensen and Lassen, 1991).

The assessment of significant variations of the mean surface temperature is seen to be fraught withdifficulties, stemming from the unavoidable noise due mostly to atmospheric turbulence and artificial heatsources. In the present paper, we investigate whether underground temperature measurements in caves 20 to

* Correspondence to: F. Perrier, DASE/RCE/HES, B.P.12, F-91680 Bruyeres-le-Chatel, France; e-mail: [email protected]

Copyright 2005 Royal Meteorological Society

1620 F. PERRIER ET AL.

30 m deep are reasonably free from such inconvenience, and whether they could provide useful complementaryinformation for the assessment of a long-term climate change.

This question, raised in 1942 (Mironovitch, 1942), was already considered for the Paris area in 1970(Dettwiller, 1970a,b) using surface temperature records obtained in Montsouris and Saint-Maur parks, andunderground temperature measurements in a gallery below the Paris Observatory. The averaged temperaturein Montsouris and Saint-Maur were observed to be stable before 1850, then rising with a slope of about 1 °Cper century, in agreement with underground observations. As the Montsouris and Saint-Maur areas were notpopulated before 1850, most of this temperature increase was attributed to urbanization. Different views maybe proposed today with the inclusion of three decades of data. In particular, recent studies (e.g. Jones et al.,1999 and references therein), have compared urban and non-urban records and concluded that the temperaturetrend could not be attributed to local urban heating only, but that the observed temperature increase ratherreflected a global climatic trend. Therefore, the conclusions obtained in 1970 for the Paris area need to bere-evaluated. In addition, temperature records in the underground galleries of the Paris Observatory werepossibly affected by the presence of a heat source (petrol lamp) from 1912 to 1917. Recent experiments(Crouzeix et al., 2003) indeed indicated that a low power heat source can have lasting effects in undergroundsettings, possibly longer than ten times the heating duration.

In this paper, we make use of an updated temperature data set both at the surface and underground. First, weuse homogenized series of surface temperature measurements in Paris and all over Europe. We will show thatthese data, properly filtered, give surprisingly consistent results and smooth out most of the noise. Secondly,available underground temperature measurements in Paris will be carefully evaluated to be able to makea meaningful comparison with surface records. Data obtained between 1783 and 1852 in the undergroundgalleries of the Paris Observatory will be complemented by data obtained in a disused quarry in 2003 and2004.

We will compare surface and underground temperature measurements to the Wolf number, a good indicatorof solar activity, defined as s + 10 g where s and g are the spot and spot group numbers, respectively. Sunspotshave been consistently characterized in this way from Earth since 1749. Surface temperatures followed thetrend of the solar activity up to about 1987 and grew more rapidly after this date (Le Mouel et al., 2004b;Solanki and Krivova, 2003).

2. SURFACE TEMPERATURE MEASUREMENTS

2.1. Temperature series at Montsouris and Saint-Maur

Montsouris Observatory is located in the southern part of Paris intra muros (14th arrondissement), in a16 ha park, a short distance from the ‘Boulevard peripherique’ (Figure 1). When it was established in 1872,the Montsouris meteorological observatory was located at the edge of Paris (Dettwiller, 1970b), but it isnow well inside the highly urbanized city core (average density 20 × 103 person per sq. km in 1999). Theobservatory at Saint-Maur des Fosses was established in 1873 about 10 km east-south-east of Paris, then inthe countryside (Figure 1). It is now located in a 3 ha park, in a residential, suburban area of small detachedhouses (density 6.5 × 103 person per sq. km in 1999).

A long series of daily temperatures at Montsouris Observatory is available. It starts in 1873 and includes thedaily minimum and maximum values and the daily mean value. The method by which the values have beencollected, corrected and homogenized is described in Moisselin et al. (2002). The series of averaged dailyvalues are shown in Figure 2. The averages are computed using a backward running window of 11 years.This choice is designed to smooth out a possible influence of the solar cycle. A smooth continuous increaseis observed in the minimum temperature. The variation of the maximum temperature is different. It showsa similar increase over the first hundred years, then a decrease followed by an abrupt increase after 1987,with a slope of the order of 0.064 °C per year. Such recent large temperature increases have been reported byother workers (e.g. Jones et al., 1999; Solanki and Krivova, 2003; Le Mouel et al., 2004b). The different timeevolution of the minimum and the maximum temperatures in Paris, also pointed out previously (Moisselinet al., 2002), is a global phenomenon that has been observed at other non-urban locations and therefore

Copyright 2005 Royal Meteorological Society Int. J. Climatol. 25: 1619–1631 (2005)

CHANGES IN SURFACE AND UNDERGROUND TEMPERATURES 1621

Paris Vincennespark

Underground temperaturemeasurements

Surface temperaturemeasurements

Marneriver

Seineriver

5 km

Paris MontsourisSaint-Maur

Paris ObservatoryVincennes quarry

Figure 1. Map showing the locations of the sites in the Paris area mentioned in the paper

cannot be attributed to urbanization effects only (Jones et al., 1999). In France, the spatial organization ofthe minimum temperature trend suggests a change of cloud cover from the Atlantic Ocean (Moisselin et al.,2002). Contrary to expectations, the smoothed temperature record in Paris Montsouris primarily reflects theregional temperature evolution and does not appear to be contaminated greatly by the urban heat island. Thisconclusion was also reached after a detailed comparison of urban and rural stations in the United States(Peterson, 2003), a study which also suggested that the parks where the long-term observatories are locatedcould provide an efficient cooling mechanism against the urban heat island.

A series of minimum, maximum and average temperatures is also available at the observatory of Saint-Maurdes Fosses, starting in 1978. The Saint-Maur data, filtered as described above, are displayed in Figure 3,and compared with the Montsouris records. The two curves are strikingly similar, as noticed previously(Dettwiller, 1970a,b). The abrupt temperature increase around 1987, noticed in Figure 2, is also seen here.We note, however, a puzzling behaviour of the averaged temperature difference, which shows a minimumaround 1990 (Figure 3, black line).

16

14

Tem

pera

ture

max

imum

(°C

)

Tem

pera

ture

min

imum

(°C

)

Paris Montsouris

Minimum

Minimum

8

6

1900 1925 1950 1975

Figure 2. Daily maximum and minimum temperatures as a function of time. Data from the Paris Montsouris station, obtained frommeteo-France (Moisselin et al., 2002), are filtered using a backward moving window of 11 years. Thus, each data point corresponds to

the average over the 4015 previous days



Paris Montsouris

Saint-Maur

Saint-Maur−Montsouris

13.0

12.5

12.0

11.5

0.4

0.2

0.0

−0.2

−0.41975 1980 1985 1990 1995 2000

Tem

pera

ture

(°C

)

Tem

pera

ture

diff

eren

ce (

°C)

Figure 3. Mean daily temperature at Paris Montsouris and Saint-Maur. Data are filtered using a backward moving window of 11 years

2.2. Long temperature series in European cities

Long-term local surface temperature changes (Figure 3) are reasonably similar to changes averaged over amuch wider area. Thus, for instance, the time series of the averaged temperature at Montsouris is comparedwith the evolution of a reference mean temperature over Northern Europe (Jones et al., 1999) in Figure 4.

Uninterrupted series of daily temperature measurements since the eighteenth century (Figure 5) exist forMilan, Stockholm, Uppsala (65 km north of Stockholm) and the central part of Belgium. The correction ofsystematic errors and the homogenization to obtain these data are described in detail in the book by Camuffoand Jones (2002).

A slope of about 1 °C per century is observed in all cities after 1850, in agreement with borehole data(e.g. Huang et al., 2000; Beltrami and Bourlon, 2004) and tree ring reconstructions (Briffa et al., 2001). Sometransient coherent temperature variations are expected (e.g. Robock, 2000; Briffa et al., 1998) as consequencesof particularly strong volcanic eruptions such as Laki (1783) and Askja (1875) in Iceland, Tambora (1815)

Paris Montsouris

Northern Europe (Jones et al., 1999)

1900 1925 1950 1975

12.5

12.0

11.5

11.0

10.5

Tem

pera

ture

(°C

)

Figure 4. Mean daily temperature at Paris Montsouris and compiled temperature variation obtained by Jones et al. (1999). Both dataseries are filtered using a backward moving window of 11 years



Uppsala

Stockholm

Belgium

Paris

Milan

1750 1800 1850 1900 1950

6.5

6.0

5.5

5.0

4.5

10.5

10.0

9.5

9.0

14.0

13.5

13.0

12.5

12.0Tem

pera

ture

(°C

)

Tem

pera

ture

(°C

)T

empe

ratu

re (

°C)

Tem

pera

ture

(°C

)T

empe

ratu

re (

°C)

7.0

6.5

6.0

5.5

5.0

12.5

12.0

11.5

11.0

10.5

Figure 5. Mean daily temperature at Paris Montsouris and at selected European locations (Camuffo and Jones, 2002). Data series arefiltered using a backward moving window of 11 years

and Krakatoa (1883) in Indonesia, or Mount Pinatubo in the Philippines (1991). The forcing associated withsuch volcanic eruptions should be mainly interannual but a possible decadal-scale effect remains unclear(Robock, 2000). Despite some differences in the curves from place to place, the abrupt temperature increase,starting around 1987, is clearly seen on all curves (Figure 5), with slopes ranging from 0.045 to 0.086 °C peryear. Such rapid temperature increases have occurred locally several times in the past. However, they werenot simultaneously present at all sites.

3. PARIS OBSERVATORY UNDERGROUND DATA

At the end of the eighteenth century, a series of temperature measurements was performed in the undergroundgalleries of the Paris Observatory (Figure 1) by Jean-Dominique Cassini IV (1748–1845), director of theobservatory. The measurements are recorded in a manuscript kept in the observatory library (ms AD 5.33).Although Cassini recorded the temperature for only a few years (from August 1783 to March 1789, i.e. about2000 days), the extreme care with which he carried out his measurements and the accuracy achieved makethem worthy of mention here. The galleries below the observatory (indeed, below much of the left bank ofthe Seine) are those of underground quarries, dug in the Lutetian limestone, which served as building stonefor much of Paris. They lie 28 m deep below the surface and their temperature was known to be very stable.

Cassini, having decided to measure the evolution of the temperature over the years, recognized that thevariations were so small that he would have to use thermometers of exquisite precision and sensitivity. Healso demanded a finite response time, in order for the temperature to remain stable while being read by anexperimenter. There existed no thermometer with such extraordinary specifications, and Cassini writes thathe was fortunate enough to have the thermometer built by Lavoisier himself.

Indeed, Cassini could install two Reaumur mercury thermometers (for which the difference in temperaturebetween the melting ice and the boiling water was 80 degrees). The first one, the most precise and sensitive(Lavoisier, 1785), was graduated in such a way that one degree was 4 inches 3 lines long (10.8 cm), whichallowed a precision in reading of better than 0.005 degree (0. 01 degree Reaumur, or 0. 0125 °C, correspondedto about 1 mm). To remain of manageable length, the thermometer had to be graduated only for temperaturesclose to that of the gallery, which, of course, precluded direct calibration at melting ice and boiling water



temperature. Therefore, Cassini had another thermometer calibrated and used it as a standard for the moreaccurate one.

The two thermometers were installed underground at the beginning of July 1783, in a room of the formerquarry 101 ft long, 6 ft wide and 8 ft high (31 m, 1.8 m and 2.4 m, respectively). All openings were carefullysealed, except one which was closed by a door. The measurements began a month later and the results aredisplayed in Figure 6a. In order to introduce minimal perturbations, Cassini entered the room always alone,holding a weak candle and remaining only the short time needed to read both thermometers. Thus, Cassiniwas aware of the thermal effect associated with a person, which corresponds to a power of the order of100 W, a value large enough to produce lasting thermal effects in the delicate underground environment(Crouzeix et al., 2003). The temperature at the surface was also measured. Cassini notes that during anexternal temperature cycle of 30 °C (between summer and winter), the temperature of the room did not varyby more than 0.03 °C. The annual wave is indeed completely damped. A temperature increase of about 0.5 °Cwas observed (Figure 6a) over the 2000 days during which measurements were made.

Because of the French Revolution, Cassini had to interrupt his experiment, but measurements with theLavoisier thermometer were continued after 1795 and compiled by Francois Arago (1786–1853). Arago,director of the observatory at that time, suspected a drift of the absolute scale of the thermometer andordered another thermometer from Gay-Lussac. Data were recorded from both instruments from 1817 to 1852(Figure 6b). A systematic shift, whose origin cannot be ascertained, was observed between the temperaturescales of the two thermometers. The mean value, however, was compatible with a single earlier measurement(11.76 °C) performed at the same location by Messier in 1776 (Arago, 1854).

After Arago’s death, the Lavoisier thermometer was damaged and the temperature was no longer recordedcontinuously in the Cassini underground room. Additional temperature measurements are punctually availablefrom 1865 to 1954 but these data are affected by the presence of a petrol lamp and therefore have to bedisregarded for our study.

Nowadays, precise temperature records below the Paris Observatory are not meaningful any more becauseof the installation of underground heating pipes, probably between the First and the Second World War.

(a) Cassini IV data

(b) Arago data

1783 1784 1785 1786 1787 1788 1789

1790 1800 1810 1820 1830 1840

11.8

11.6

11.4

12.2

12.0

11.8 LavoisierGay Lussac

Tem

pera

ture

(°C

)T

empe

ratu

re (

°C)

Figure 6. Underground temperature records below the Paris Observatory (depth 28 m): (a) data from Cassini using the Lavoisierthermometer. (b) Data from Arago using the Lavoisier thermometer (diamonds) and using another thermometer built by Gay-Lussac

(triangles)



Indeed, we measured a temperature of about 14.9 °C in the beginning of 2004. To attempt a comparisonbetween the past temperature records at the Paris Observatory and present time surface temperature, we can,however, use another underground cavity, unaffected by the heating pipes present in Paris intra muros.

4. UNDERGROUND MEASUREMENTS AT VINCENNES

Since February 2003, we have been measuring the temperature at several locations in an underground Lutetianlimestone quarry, located below the 365 ha Vincennes Park, 3 km SE of Paris (Figure 1). The quarry, spreadover a surface of about 32 000 m2, was dug according to the room and pillar technique, in the eighteenth andearly nineteenth century. Its roof is 18 m thick. Natural ventilation operates mainly through the access pit, witha diameter of about 4.6 m (Perrier et al., 2004). Temperatures are recorded, in the atmosphere, by Seabird

autonomous sensors. These instruments are calibrated with a precision of 0.001 °C. The measurement pointsare spread over the entire quarry, the sensors being at distances from the floor varying from 10 cm to 1 m.

Time series are shown in Figure 7. Long-term variations are heterogeneous in the quarry: both the amplitudeof the annual variation and the long-term trend are different from point to point. The origin of this heterogeneityis not clear. While there is no correlation with the distance from the floor, or to the nearest wall, smallerannual variations are observed at the points most remote from the main access pit (point number 6). Thissuggests that natural ventilation (Perrier et al., 2004) may be dominating the amplitude of the annual variation.The slope of the long-term trend can be obtained by fitting these data to a function composed of an annualsine wave superimposed to a linear trend. A mean linear increase of about 0.02 °C per year is obtained.

5. COMPARISON OF THE UNDERGROUND AND SURFACE TEMPERATURES

Surface and underground temperature data in Paris are displayed in Figure 8. In order to check the consistencyof underground and surface temperatures, we have filtered the surface data using a diffusion filter, thus

6

2

3

45

1

Jan Feb Mar Apr May Jun Jul Aug Sep Oct Jan Feb Mar Apr May Jun Jul Aug Sep OctNov Dec N

20042003

12.70

12.60

12.50

12.40

12.30

Tem

pera

ture

(°C

)

Figure 7. Underground temperature records at the Vincennes quarry (depth 18 m). Measurement points (numbered 1 to 6 from north tosouth) are spread over the entire quarry



transforming the temperature series of daily means into the series that would have been observed at areference depth of 20 m, in a half-space of thermal diffusivity κ = 10−6 m2 s−1. The diffused time series T D

i

is calculated as follows. The surface temperature time series Ti is transformed into the frequency domain Tk

by Fast Fourier Transform. For one-dimensional diffusion in a half-space, the transfer function of a surfaceharmonic perturbation of frequency f at depth z (defined positive upwards) is

A(ω) = exp( z

λ(1 + i)

), (1)

where λ is the attenuation length at frequency f :

λ =√

κ

πf. (2)

The time series T Di is then obtained by multiplying the transformed series Tk by the harmonic transfer

function A(ω) given by Equation (1), transforming back into the time domain by inverse Fourier Transform.This filtering technique, which damps efficiently the annual amplitude and surface noise, has a relevant

physical basis. It could be used in general as we note that the obtained time series for Montsouris is similarto the series obtained using a backward running average of length 11 years (displayed in Figure 4). TheMontsouris time series starting in 1875 has been complemented by the average of the time series of Milanand Uppsala, also processed by the diffusion filter. This time series, which allows this extrapolation before1875, is matched with the Montsouris temperature between 1890 and 1900. Similar results are obtained if, forexample, the time series from Stockholm is used instead for this extrapolation, but the details of the temporalvariations before 1800 differ, as can be noticed in Figure 5. It is again seen in Figure 8 that the filtered surfacetemperatures show an increasing trend up to about 1987 and grow more rapidly after this date.

Underground temperatures in Figure 8 were corrected to take into account the geothermal gradient betweenthe actual depth of measurement and the surface. A value of 0.03 °C m−1, independent of time is assumedfor the geothermal gradient. The observed temperature in the underground sites has to be shifted downward,and this shift depends on the depth of the site. The shift amounts to 0.84 °C for the galleries below the ParisObservatory (28 m depth) and to 0.54 °C for the Vincennes quarry (depth 18 m). This correction is uncertain,as the near-surface temperature gradient can be affected by other causes, such as urbanization (e.g. Blocket al., 2004; Ferguson and Woodbury, 2004) or groundwater flow (Smith and Chapman, 1983). Such effects

12.0

11.5

11.0

10.5

Tem

pera

ture

(°C

)

1800 1850 1900 1950

Messier data

Arago−Lavoisier data

Arago−Gay Lussac data

Vincennes data

Paris Montsouris filtered

Average of Milan and Uppsala filtered

Cassini data

Figure 8. Underground and surface temperature records for the Paris area. Surface data have been filtered using a diffusion filter fora reference depth of 20 m, assuming a thermal diffusivity of 10−6 m2 s−1. Underground data have been corrected for the geothermal

gradient



should not affect our correction to first order. Indeed, dwellings are located at least 500 m away from thelocation of the quarry in the Vincennes Park. In addition, infiltration rate is small because of overlying claylayers.

In Figure 8, the mean temperature observed from 1776 to 1852 in the underground galleries of the ParisObservatory (about 11 °C) agrees well with the mean temperature obtained from the surface temperaturerecords in Montsouris, extrapolated in time from 1875. The time structures of the extrapolated temperatureand the measured temperature underground, however, are rather different. The present corrected undergroundtemperature at Vincennes (about 11.9 °C) is significantly larger than the corrected average temperature (11 °C)recorded from 1776 to 1852 at the Paris Observatory. The temperature increase between the two dates (0.9 °C)is in good agreement with the filtered surface temperature (Figure 8). We can thus conclude that, at first order,the temperature measured currently in the Vincennes quarry reflects the global surface temperature change.

6. TEMPERATURE EVOLUTION AND SUNSPOTS

The question of whether temperature (or climate) and solar activity might be related is a very timely one,all the more since it bears on the assessment of human activity on global warming. There is as yet no clearanswer (Laut, 2003).

James Jeans (1934) showed a good correlation between the number of sunspots, between 1896 and 1927,and the height of water in Lake Victoria Nyanza. He pointed out that ‘the height of water in the lake keepsin almost perfect step with the frequency of sunspots, and so exhibits a 11-year cycle, just as sunspots do.The water is of course higher after a wet year, providing proof that the weather is wettest when sunspotsare frequent, and vice versa’. Since precipitation bears some relation to temperatures, this observation maybe seen in the light of the work of Friis-Christensen and Lassen (1991), who showed a correlation betweenthe length of the solar cycle and the so-called temperature of the Northern Hemisphere. Indeed, Solanki andKrivova (2003) showed that the curve representing the time variation of the Sun total irradiance and the onerepresenting the global temperature (i.e. climate) present a rather strong similarity, since the beginning of thetwentieth century up to 1970. A similar conclusion can be obtained by using any geomagnetic parameter (LeMouel et al., 2004b).

Figure 9 shows the running averages over 11 years of the temperature at Montsouris and of the Wolfnumber, as a function of time since 1870. The two curves present similar trends, at least till 1987. After

Paris Montsouris

100

50

0

12.5

12.0

11.5

11.0

10.5

1875 1900 1925 1950 1975

Tem

pera

ture

(°C

)

Wol

f num

ber

Figure 9. Mean daily temperature at Paris Montsouris and Wolf number versus time. Data series are filtered using a backward movingwindow of 11 years



1987, the temperature rises sharply as already mentioned. Simultaneously, the averaged Wolf number startsdecreasing.

An efficient technique to reveal the correlation between two series consists in looking at the time variationof the frequency content of the series to be compared (Le Mouel et al., 2004a,b). The amplitude of the11-year solar cycle, as represented by the Wolf number, varies with time over the time span covered bythe temperature data; the amplitude of the harmonics (e.g. 5.5 years) is also expected to change. Figure 10displays the evolution of the amplitude of the 11-year line in both Wolf number and Montsouris temperatureseries. Figure 11 displays the time evolution, over the same time span, of the amplitude of the 5.5-year linein both series. The similarity is now striking.

If such a correlation has a general meaning, one may expect that visual inspection would confirm it, atleast for particular time sections. For example, in Figure 12, the evolution of temperature in the Cassini(Figure 12a) and Arago (Figure 12b) records shows an unmistakable parallel trend with the sunspot number.Note that, at a depth of 28 m, for a value of the heat diffusivity of 10−6 m2 s−1, the diffusive phase shift

11-year line

Paris Montsouris

Wolf number

1875 1900 1925 1950 1975

1.0

0.8

0.6

0.4

0.2

0.0

Arb

itrar

y un

its

Figure 10. Intensity of the 11-year line, in a centred moving window of 22 years, versus time, for the Paris Montsouris temperatureand the Wolf number

5.5-year line Wolf number

1875 1900 1925 1950 1975

1.0

0.8

0.6

0.4

0.2

0.0

Arb

itrar

y un

its

Paris Montsouris

Figure 11. Intensity of the 5.5-year line, in a centred moving window of 22 years, versus time, for the Paris Montsouris temperatureand the Wolf number



(a) Cassini IV data

(b) Arago−Lavoisier data

11.8

11.6

11.4

12.30

12.20

12.10

12.00

Tem

pera

ture

(°C

)T

empe

ratu

re (

°C)

1783 1784 1785 1786 1787 1788 1789

1815 1820 1825 1830 1835 1840 1845 1850

150

100

50

0

200

150

100

50

0

Sun

spot

num

ber

Sun

spot

num

ber

Figure 12. Underground temperature records (diamonds) below the Paris Observatory (depth 28 m) compared with sunspot number:(a) data from Cassini. (b) data from Arago using the Lavoisier thermometer

is about 4 years for a disturbance of period 11 years. Note also that the corresponding attenuation, given byEquation (1), would be of the order of 0.07 at period 11 years and of the order 1.5 × 10−4 at period one year.While long-term temperature trends induced by the sunspot cycles could be observed underground, higherfrequency fluctuations of the sunspot number are strongly damped at the observation depth.

7. CONCLUSIONS

In this paper, we have shown that surface temperature records can be filtered by running averages or, tocompare with underground temperature records, by using a diffusion filter. In Paris, underground temperaturerecords are in good agreement with filtered surface records.

Surface temperature series show that this evolution can be divided into three phases. The temperatureappears comparatively stable till about 1900, and then rises with a slope of about 0.01 °C per year till about1987. After 1987, a larger slope of 0.07 °C per year is present in all measuring sites in Northern Europe.Part of the monotonous temperature increase after 1900 could be attributed to an urban heat island effect.Note, however, that urbanization alone cannot provide a complete explanation. A similar trend is observed inUppsala and Stockholm, which have a population density of 43 and 4000 persons per sq. km, respectively,and thus where urbanization effects are expected to be much smaller than in the Paris area and differentbetween the two Swedish cities. In Milan, where urbanization effects, associated with a population density of7.5 × 103 person per sq. km, would be naively expected to be comparable to Paris, a smaller trend is observed.We therefore contend, in agreement with previous assertions (e.g. Jones et al., 1999; Peterson, 2003), that thetemperature trend since 1900 reflects a global modification of the temperature distributions rather than purelylocal urban island effects. After 1987, the faster temperature increase observed all over Europe, both at thesurface and underground, could not be attributed to urbanization, which had actually levelled off during thatperiod, in particular in Paris.

Temperatures observed currently in the underground quarry in Vincennes, near Paris, are greater thaneighteenth and nineteenth century extrapolated values below the Paris Observatory by 0.9 °C. This increase



must include some contribution from the urban heat island. However, the agreement with the surfacetemperatures, as discussed above, suggests that this contribution is not dominating. The lack of a significanturban heat island effect in our data may be due to the fact, as pointed out by Peterson (2003), that thequarry is located below the large Vincennes Park, which includes more than 150 000 trees. The bulk of theunderground temperature increase of 0.9 °C may thus be tentatively interpreted as a direct confirmation ofglobal warming.

Time variation of solar activity plays an important role in accounting for the long-term evolution oftemperature. During the first two phases described above, the averaged Wolf number follows roughly thecorresponding temperature time series. An analysis of the spectral content confirms this behaviour in aconvincing manner for the 11- and 5.5-year cycles. However, this correlation between filtered temperatureand Wolf number turns into an anti-correlation after about 1987. The fact that the temperature increases whilethe Wolf number decreases suggests that the recent steep increase of temperature might be of anthropogenicorigin, possibly an accelerated greenhouse effect. No prediction of the future temperature evolution, however,can be reasonably attempted.

ACKNOWLEDGEMENTS

The authors thank L. Bobis, librarian of the Paris Observatory, for communicating the Cassini manuscript,and the Inspection Generale des Carrieres and the City of Paris for access to the underground sites. Theythank M. Le Goff for the temperature records at Saint-Maur. They are also grateful to C. Crouzeix and P.Richon for their contributions in the experiments. The authors also acknowledge the help and advice fromD. Camuffo and V. Kossobokov. Anonymous reviewers are thanked for enlightening suggestions and forpointing out important references. This is IPGP contribution number 2043.

REFERENCES

Arago F. 1854. Oeuvres Completes, tome 8, Vol. 5, Barral JA (ed). Paris; Gide & Baudry 636–646.Arrhenius S. 1896. On the influence of carbonic acid in the air upon the temperature of the ground. Philosophical Magazine 41:

237–276.Beltrami H, Bourlon E. 2004. Ground warming patterns in the northern hemisphere during the last five centuries. Earth and Planetary

Science Letters 227: 169–177.Block A, Keuler K, Schaller E. 2004. Impacts of anthropogenic heat on regional climate patterns. Geophysical Research Letters 31:

L12211. DOI:10.1029/2004GL019852.Briffa KR, Jones PD, Schweingruber FH, Osborn TJ. 1998. Influence of volcanic eruptions on northern hemisphere summer temperature

over the past 600 years. Nature 393: 450–455.Briffa KR, Osborn TJ, Schweingruber FH, Harris IC, Jones PD, Shiyatov SG, Vaganov EA. 2001. Low-frequency temperature variations

from a northern tree ring density network. Journal of Geophysical Research 106(D3): 2929–2941.Camuffo D, Jones PD. 2002. Improved Understanding of Past Climatic Variability from Early Daily European Instrumental Sources.

Kluwer Academic Publishers: London; 392.Crouzeix C, Le Mouel J-L, Perrier F, Richon P, Morat P. 2003. Long term thermal evolution and effect of low power heating in an

underground quarry. Comptes Rendus Geoscience 335: 345–354.Dettwiller J. 1970a. Deep soil temperature trends and urban effects at Paris. Journal of Applied Meteorology 9: 178–180.Dettwiller J. 1970b. Evolution seculaire du climat de Paris: influence de l’urbanisation. Memoires de la Meteorologie Nationale, Paris

52: 83.Ferguson G, Woodbury AD. 2004. Subsurface heat flow in an urban environment. Journal of Geophysical Research 109: B02402.

DOI:10.1029/2003JB002715.Fourier J. 1827. Memoire sur les temperatures du globe terrestre et des espaces interplanetaires. Memoires de l’Academie Royale des

Sciences Institut de France 7: 569–604.Friis-Christensen E, Lassen K. 1991. Length of the solar cycle: an indicator of solar activity closely associated with climate. Science

254: 698–700.Huang S, Pollack HN, Shen P-Y. 2000. Temperature trends over the past five centuries reconstructed from borehole temperatures. Nature

403: 756–758.Jeans J. 1934. Through Space and Time. Macmillan: New York.Jones PD, New M, Parker DE, Martin S, Rigor IG. 1999. Surface air temperature and its changes over the past 150 years. Reviews of

Geophysics 37: 173–199.Laut P. 2003. Solar activity and terrestrial climate: an analysis of some purported correlations. Journal of Atmospheric and Solar-

Terrestrial Physics 65: 801–812.Lavoisier AL. 1785. Thermometre des caves de l’Observatoire: Precautions prises pour construire et graduer ce thermometre. Oeuvres

completes de Lavoisier, Ed JB Dumos, Grimaux and Fouque. Paris 1865, 421–426.Le Mouel J-L, Blanter E, Shnirman M. 2004a. The six-month line in geomagnetic long series. Annales Geophysicae 22: 985–992.



Le Mouel J-L, Kossobokov V, Courtillot V. 2004b. On long-term variations of simple geomagnetic indices and slow changes inmagnetospheric currents. The emergence of anthropogenic global warming after 1990? Earth and Planetary Science Letters 232:273–286.

Mann ME, Bradley RS, Hughes MK. 1998. Global-scale temperature patterns and climate forcing over the past six centuries. Nature392: 779–787.

Mironovitch V. 1942. Sur les variations de temperature dans les caves de l’Observatoire de Paris. La Meteorologie 37–39: 109–110.Moisselin J-M, Schneider M, Canellas C, Mestre O. 2002. Climate change over France during the 20th century, a study of long-term

homogenized data of temperature and rainfall. La Meteorologie 38: 45–56.Perrier F, Richon P, Crouzeix C, Morat P, Le Mouel J-L. 2004. Radon-222 signatures of natural ventilation regimes in an underground

quarry. Journal of Environmental Radioactivity 71: 17–32.Peterson TC. 2003. Assessment of urban versus rural in-situ surface temperatures in the contiguous United States: no difference found.

Journal of Climate 16(18): 2941–2959.Petit JR, Jouzel J, Raynaud D, Barkov NI, Barnola J-M, Basile I, Bender M, Chappellaz J, Davis M, Delaygue G, Delmotte M,

Kotlyakov VM, Legrand M, Lipenkov VY, Lorius C, Pepin L, Ritz C, Saltzman E, Stievenard M. 1999. Climate and atmospherichistory of the past 420,000 years from the Vostok ice core, Antarctica. Nature 399: 429–436.

Robock A. 2000. Volcanic eruptions and climate. Reviews of Geophysics 38: 191–219.Smith G, Chapman DS. 1983. On the thermal effects of groundwater flow 1. Regional scale systems. Journal of Geophysical Research

88: 593–608.Solanki SK, Krivova NA. 2003. Can solar variability explain global warming since 1970? Journal of Geophysical Research 108(A5):

1200. DOI:10.1029/2002JA009753.


long-term climate change and surface versus underground temperature measurements in paris

Documents