CLIMATE CHANGE FROM METEOROLOGICAL OBSERVATIONS ANDUNDERGROUND TEMPERATURES IN NORTHERN ITALY

V. PASQUALE, M. VERDOYA, P. CHIOZZI

Department of Earth Sciences, University of Genova, Italy*

S u m m a r y : The change in the air temperature recorded at the Genoa University observatoryover 155 years followed the change in the wind direction. A warming occurred both from 1838 to1869 and from 1890 to 1950. The latter warm period was followed by a cooling in the 1950s, whenthe lowest mean annual temperature occurred. Generally, a warm period corresponds to a growthin the southern components of the ground wind direction; the increase in the southerly circulation inthe Mediterranean yields increased stability in dry weather. Temperature-depth data measured in ageothermal borehole reflect this climate change. A comparison between the measurements and asynthetic temperature profile, based on the air temperature history recorded at the observatory,indicates that the average temperature prior to the meteorological time series was about 0,6 Khigher than the average of the decade 1973-1982.

Keywords : Meteorological data time series, climatic signal, synthetic temperature-depthprofile, climate variation

1. INTRODUCTION

Air temperatures recorded at meteorological observatories provide the most studied evidence atleast of the most recent climate changes, but they suffer from several deficiencies. In particular therelatively short time span of the record, the unequally spaced coverage, and the urban heat islandsmay bias the data, and hinder them from being properly evaluated as to the global warming startingwith the climatic minimum at the end of the nineteenth century being due to industrialisation. Otherclimatic indicators such as the wind direction and velocity, precipitation, solar radiation andmoisture are usually recorded at the meteorological stations, and may contribute to discriminatingbetween natural and anthropogenic causes. Provided that no other thermal variation due to internalor external phenomena occurs, further information on the recent climate change can be inferredfrom the temperatures in the uppermost hundred metres of the underground.

In this paper we attempt to combine meteorological data and borehole temperatures to constrainthe climate change which has occurred in the last two centuries in northern Italy. The time series ofthe air temperature recorded at the Genoa University observatory from 1833 to 1988 has beenanalysed. Additional meteorological data such as the wind direction and the precipitation rate havebeen considered to understand the characteristics of the climate variation and the relatedmechanisms better. Temperature data from a geothermal borehole have been used to extend back theclimate history and in particular to determine whether the air temperature of the first decades of thenineteenth century is significantly different from that of the last decades.

* Address: Dipart. di Scienze della Terra, Settore di Geofisica, Universita di Genova, VialeBenedetto XV, 5, I-16132 Genova, Italy (Fax: +39(0)-10-352169; e-mail:verdoya@dister.unige.it)

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2. EVIDENCE FROM METEOROLOGY

M e a s u r e d s u r f a c e a i r t e m p e r a t u r e

An evident temperature increase in the Northern Hemisphere during the last hundredyears has already been recognised in many studies based on the analysis of temperaturetime series, recorded at several meteorological stations (see, e.g., Jones et al, 1986;Kucerova, 1994). In Europe additional information has been reported by Pasquale et al,(1992) about the temporal pattern of warming and cooling trends from 1850 to 1980through the analysis of the air temperature data derived from eleven stations within a beltstretching from 6° and 12°E and running from northern Germany to northern Italy. Thedata, collected from numerous published sources for Italy (Rapetti and Vittorini, 1988;Flocchini, 1989) and from the meteorological observatories of Germany (Bayreuth,Hamburg, Hannover, Karlsruhe, Kiel) and Switzerland (Basel, Geneva, Great St.Bernard), show that during the period analysed large changes have occurred. Thetemperature rose by about 0.4 K from 1850 to 1865 and underwent a substantial drop until1885; then, a main rising trend of about 0.8 K started again and continued until 1945. Aweak cooling of 0.2 K was observed for about 25 years, followed by a warming, whichseems to persist. The recent rise has increased the temperature to levels as high as in 1865and 1945.

These variation characteristics are somewhat different in spatial nature and maydepend on different mechanisms. The genesis of the main warming phase before 1945could be explained by the rate of heat pumping (calculated from temperatures and fromlow-level winds estimated from pressures) of the lower atmosphere towards the Arcticwhich was greater than that which occurred during the subsequent period of cooling (vanLoon and Williams, 1976; Jones and Kelly, 1983). Albeit everywhere detectable, thiswarming was predominantly concentrated at high latitudes. The persistence of large-scaleatmospheric circulation patterns could be due either to the basic inertia of the atmosphere,or to additional physical controls, such as extraterrestrial impulses, terrestrial effectsmodified by feedback processes, and inherent characteristics of the atmosphere whichcause the circulation to switch abruptly from one regime to another.

Changes in the general circulation of the atmosphere are strictly connected to theclimatic variations as confirmed by a comparison between the temperature and wind datarecorded at the Genoa University observatory (44°24.9'N, 8°54.7'E). Figure 1 shows thetime series of the average annual air temperature in the period 1833-1988. These data,whose reliability and accuracy have been presented and discussed in other papers(Flocchini, 1988,1989), represent one of the longest and most complete time seriesavailable in Italy. The mean annual values have been derived from the daily maximum,minimum and mean values of the air temperature. The time series shows sequences ofyears characterised by increased temperatures alternating with sequences characterised bylower temperatures. The variations of the annual averages are chronologically consistentwith those observed at many sites in the Northern Hemisphere, but the values aredifferent. The small range between the maximum and minimum annual averages (3.2 K)is due to the thermal regulating effect of the sea.

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Fig. 1. Mean annual temperature at the Genoa University observatory.

From 1838 to 1869 the time series were characterised by a general temperatureincrease (0.7 K at the most). This sequence was followed by a cooling which lasted until1890 when the mean annual temperature was only about 14.5°C. In the decade 1880-1890summer and autumn were characterised by low temperatures and weak fluctuations,different from the previous decades. Then a long warmer period started during which thehighest mean annual temperature of the time series was attained (17.3°C in 1943). Asudden drop took place from 1954 to 1958. The lowest temperature (14.1°C) was recordedin 1956 which was the coldest year of the whole time series. During this year there wereseveral months in which the temperatures were below the seasonal average, and themonthly mean of February (1.5°C) was the lowest value in the time series.

These temperature fluctuations closely follow the variations in wind direction,deduced from Lambert's formula disregarding the speed. Long-term fluctuations can bestudied by analysing the 10-year means of these two parameters (Fig. 2). Generally, awarm period corresponds to a growth of the southern components of the ground winddirection. An increase in this circulation in the Mediterranean leads to increased stabilityof dry weather. This can also be observed in recent decades, during which climaticmodifications due to intense human activity took place (Budyko et al, 1988).

In the period 1888-1900, although the wind direction showed prevailingly northerncomponents, the temperature increased; this contrasting behaviour may be accounted forby the exceptional events which cause abrupt changes in the global circulation of theatmosphere. The quinquennium preceding this period was affected by a severe coolingascribed to the Krakatoa eruption in 1883, whose dust ejected in the atmosphereattenuated the incoming solar radiation (Landsberg, 1974). Although there is scarceevidence of this cooling due to the small number of stations in the world networkoperating at that time, the eruption must have modified the normal alternation of climatechange adequately (Sneyers, 1982).

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Fig. 2. Comparison between the temperature line 1) and wind direction (line 2) time series basedon the annual means observed at Genoa. Z is the standardised value for the 10-year periods,

After 1888, on the one hand, a temperature rise occurred and, on the other, northerncomponents persisted in their trend till 1900 and then increased. The temporal shiftobserved between the two meteorological parameters is probably only an expression ofthe atmosphere-ocean interaction and can be explained by the remarkable lag effect due tooceanic thermal inertia which is greater than that of the continental areas. The inertia ofoceanic masses, dynamic accumulators of heat and essentially atmosphere's memory,induced long-term delayed interactions with the atmosphere (Landsberg, 1974).

P r e c i p i t a t i o n r a t e

Observations on the daily precipitation amount as well as the count of dry days withprecipitation lower than 0.3 mm recorded at Genoa since 1833 enable us to determine therelated monthly and annual average, and the precipitation rate, defined as the ratiobetween the amount of precipitation and the number of rainy days. The availableobservations are used here to define the weather trend which can also be inferred from theaverage duration of drought periods.

During the time span studied, the annual precipitation amount was rather stable, i.e. itfluctuated about the mean value of 1292 ±308 mm (mean deviation 17.6%), withoutshowing any significant trend. The frequency distribution is close to normal and its fitting,verified by the chi-squared test, is good at the 5% significance level. With 95%confidence the mean precipitation in the population falls between 1244 and 1440 mm peryear. Annual precipitation greater than 1800 mm has a probability of occurrence of about5%, and that lower than 900 mm of 10%. The highest precipitation amount (2752 mm)was recorded in 1872; this value is more than twice the average value of the time series.That year was characterised in October-December by rainfalls which produced about1850 mm and caused floods. This kind of event has a very long return period and regionalcharacter as it affected the whole of north-western Italy and the surrounding countries.

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The lowest value (543 mm) occurred in 1921, which was preceded by a year with aprecipitation amount exceeding the mean of the time series by about 500 mm.

The annual precipitation rate shows short-period fluctuations, with a maximum of22.3 mm/day in 1970 and a minimum of 8.0 mm/day in 1884, and a clear increase withtime (Fig. 3a). The time series averages 12.6 mm/day with standard deviation of2.6 mm/day. A general increase of 0.23 day/year can be inferred for dry days. The numberof dry days is on the average 261.9 ± 17.9 and ranges from values of 307 to 205 whichcorrespond to the years of absolute minimum and maximum precipitation amounts,respectively (Fig. 3b). Even though the number of rainy days tends to rise in the yearswith the largest amount of precipitation, a close relation between the annual precipitationamount and the number of rainy days cannot be established.

The increase of periods with abnormally dry weather confirms the fact that theprecipitation rate is characterised by a global rise. If we take account of the periods ofconsecutive dry days, we can say that they have progressively grown in average length,except during the winter, with a remarkable difference between the period 1841-1930 andthe subsequent period of 60 years (Table 1). This is confirmed by the global number ofrainy periods and by their progressive decrease, which indicate that the average length ofsteady weather has increased, particularly from the last to this century. An analogousbehaviour is observed on the coast, east of Genoa. The comparison between these resultsand data from inland stations having typical continental climate shows that dry periods arelonger, on the average, near the Ligurian Sea coast (Bossolasco, 1950; Pasquale et al,1994).

The increase of drought periods and the stability of dry weather, observed in thiscentury, account for the rise of the air mean temperature which occurred in Europe duringthe same period and which is continuing. This phenomenon is nothing else but the effectof the increase in southerly circulation which is clearly illustrated by the change in themean annual wind at the ground.

Fig. 3. Rate of precipitation (a) and number of dry days (b) at Genoa together with the relatedinterpolation curves showing a quadratic and linear trend, respectively.

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Table 1. Average duration of the period with consecutive dry days at the Genoa Universityobservatory.

Time span

1841-18701871-19001901-19301931-19601961-1990

Winter

11.411.811.511.711.9

Spring

9.88.99.2

11.410.5

Summer

11.112.111.914.013.8

Autumn

8.910.210.210.911.8

Year

10.3010.7510.7012.0012.00

3. EVIDENCE FROM TEMPERATURE-DEPTH DATA

B o r e h o l e t e m p e r a t u r e

To extend climate information back beyond the available time series, undergroundtemperatures were used. As temperatures measured in the uppermost hundred metres ofthe solid earth reflect the recent climate change, meteorological observations can beintegrated with temperature-depth data from boreholes. Provided that a meteorologicalstation is reasonably close to a given borehole, a time series of surface air temperature canbe used to produce synthetic temperature-depth profiles to be compared with the measuredborehole temperatures, making it possible to infer the surface temperature history prior tothe time of meteorological observations. In this section we analyse the temperature-depthdata from one borehole in relation to the air temperature data recorded at Genoa.

The borehole was drilled in 1981 at Santuario (44°20.6'N, 8°26.1'E, 100 m a.s.l), afew tens of kilometres from Genoa, for heat-flux studies within the framework of anational project for geothermal potential evaluation. One year later, good qualitytemperature logs were carried out by Delia Vedova et al. (1982) and the thermalconductivity was measured. The geological background consists of a tectonic windowbelonging to the easternmost sector of the western Alps where crystalline rocks of the pre-Westfalian basement (amphibolites, gneisses and metagranodiorites) outcrop, overlaidwith their Permo-Carboniferous metasedimentary cover (phyllites, carbonaceousmicaschists, quartz-micaschists and schistose metabasites). Highly fractured phyllites withquartz nodules and veins, 0.5 m thick at the most, were encountered down to 182 m (holebottom). No significant water circulation was found along the borehole.

Table 2 reports the results of the temperature measurements. The measurements werecarried out at 5-m intervals by means of a precision bridge and Pt-resistance elements.The accuracy of the probe was estimated to be better than ± 0.005 K and that of the databetter than ± 0.01 K. The 5-m interval temperature gradient shows a decrease with depthwhile the average thermal gradient is equal to 26 mK m-1 in the range of 25-180 m.

The thermal conductivity was measured on five core samples, 1.5 m long and spaced30-35 m apart, starting from a depth of 50 m, by means of three different standardmethods which gave coherent results. Due to diffuse cracking, only 7 specimens weresuitable for laboratory analysis, which gave a weighted mean thermal conductivity of3.1 Wm-1K-1. This value, combined with the average thermal gradient yields a surface

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Table 2. Underground temperatures observed at the Santuario hole in June 1982.

Depth [m]

51015202530354045505560657075808590

Temperature [°C]

12.0912.7513.2913.6413.8213.9814.1414.2914.4714.6514.8215.0015.1515.3015.4015.5515.6715.79

Depth [m]

95100105110115120125130135140145150155160165170175180

Temperature [°C]

15.9716.1016.2316.3816.5416.6916.7916.8816.9617.0917.2617.3817.4917.5717.6717.7717.8417.90

heat flux of 81 mWm- 2. The topographic effect does not seem significant as the boreholeis located in a subdued relief. We suspect that the temperature-depth distribution is mostlyinfluenced by the climate history.

S y n t h e t i c t e m p e r a t u r e - d e p t h p r o f i l e

To combine the borehole and meteorological data, we followed a procedure similar tothat described by Lachenbruch et al. (1988) and by Chapman et al. (1992). Thetemperature perturbation in the underground was computed by means of the mathematicalformulation of Birch (1948) and Carslaw and Jaeger (1959) considering individual stepchanges in the air temperature which occurred prior to the borehole temperaturemeasurement as shown in Fig. 4. Such temperature changes result from the 10-yearaverages of the Genoa time series expressed as a departure in K from the mean of theperiod (1973-1982) before the borehole temperature measurement. The least squaresfitting of the decadal averages shows a very negligible increasing trend. A thermaldiffusivity value of 1.0 x 10~6m2s~1 has been assumed. The effect of the air temperaturehistory in the underground was then added to the background thermal gradient of theborehole, derived from the lowermost (140-170 m) quasi-linear section of thetemperature-depth profile which, at least in a first approximation, is considered to beunaffected by the recent changes in the surface air temperature. The least squares fittingyielded a background temperature gradient of 22.7 mKm-1.

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Fig. 4. Calculated (bold curve) and observed (dots) underground temperatures at the Santuariohole. Thin straight line: background thermal gradient extrapolated between 140-170 m. In the leftcorner, the 10-year average air temperature variation recorded at Genoa (continuous step line) andthe inferred pre-observational average temperature variation (dashed line). In the right corner,position of the Santuario hole (1) and the Genoa University observatory (2).

The calculated temperature-depth profile together with the experimental temperaturelog and the extrapolated thermal gradient are plotted in Fig. 4. The best agreementbetween the calculated and observed temperatures is obtained for the pre-observationalmean air temperature 0.6 K higher than the last decade average. The level of agreement islower below the depth of 80 m, but the average difference between the calculated andobserved temperatures is 0.07 K, which compares well with a reasonable level ofgeological noise in the data. Some discrepancy could also be due to the fact that the airtemperature variation at the meteorological station is not an exact replica of the groundtemperature variation at the borehole. The results show ultimately that the cooling of0.8 K, which occurred after a relatively warmer period at the beginning of the last century,can account well for the temperature-depth distribution in the borehole.

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This analysis has been carried out under the assumption that the lowermost section ofthe borehole reflects the equilibrium thermal gradient. If the temperature gradient and theextrapolated surface temperature are incorrect, our inferred ground temperatures are alsoincorrect. However, the depth range considered actually reflects the thermal diffusiontimes of the climatic changes which occurred 153-226 years ago for a level of temperaturedisturbance of 0.1 K.

4. CONCLUSIONS

The variation in the analysed air temperature time series substantially agrees with thetemperature change reported in the literature for the whole Northern Hemisphere (Jones etal, 1986) and in Europe (Hansen and Lebedeff, 1987; Pasquale et al,, 1992). Thenineteenth century data show a cooling between the late 1870s and the late 1880s. Around1900-1920 a rapid warming took place, culminating in the early 1940s. A new cooling isevident in the late 1940s, again followed by a temperature increase. However, there aresome peculiar features which differentiate our time series from the Northern Hemispheretrend. The temperature increase between 1920 to 1950 was only 0,2 K, i.e. thetemperature did not differ so much from the warm period between 1840 and 1870. Theincrease in the, last century, therefore, seems to be slightly lower than that estimated fromglobal warming studies. A comparison between the annual minimum temperatures atGenoa with the closest rural station in the last 30-year period shows the existence of anurban heat-island effect (Flocchini, 1988) which has often been reported for otherEuropean stations (see, e.g., Safanda and Kubik, 1992). On the other hand, thetemperature trend compared to the wind direction indicates that the changes in the airtemperature history are primarily related to the general circulation of the atmosphere.

The precipitation rate shows a clear increase versus time, but no particular trend hasbeen observed in the annual rainfall. The increase in the number of dry days, on theaverage 0.23 day/year, plays a decisive role in the occurrence of such processes. Theprogressive growth in stability of the dry weather depends on a corresponding increase inthe southerly circulation in the Mediterranean basin. The rather stable rate of the annualrainfall indicates that drought periods alternate with periods of more intense andconcentrated precipitation.

The air temperature variation at our meteorological observatory accounts well for theunderground temperature observed in the nearby geothermal borehole. The temperature-depth profile, calculated from the air temperature data as a forcing function, replicates in agood approximation the experimental data, especially in the first 80 m. The inferredclimatic signal prior to 1833 indicates a slightly warmer period than that of the decadebefore the temperature logging. The length of this warm period cannot be exactlyquantified, but it depends on the depth of our hole and represents a weighted meantemperature for a few decades before the meteorological records. Its existence matches atleast three independent pieces of evidence. First, a warmer period during 1820-1830 by upto 0.5 K, followed by a drop in the temperature of about 1 K has been estimated byHansen and Lebedeff (1987) for central Europe from meteorological station data. Second,a reconstruction of the air temperature histories from proxy data reported by Guiot (1994)shows that there are some temporal discrepancies in the cold periods between the westernMediterranean and central Europe. However, there is agreement between the different

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geographical regions all indicating that the decade preceding our time series wasdecidedly warmer. Third, a combined study on the tree-ring width in the Maritime FrenchAlps and the variation of the glacial fronts in the Mont Blanc area (Vivian, 1975; Serre,1978) supports the evidence of a warmer, positive and high-frequency climatic signalbetween the 1790s and the 1820s which occurred within the negative and low-frequencysignal of the so-called "Little Ice Age".

Manuscript received: 23 June 1997; Revisions accepted: 20 September 1997

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