Réf. Böhm & al. 2010 - A

Référence bibliographique complète

BÖHM, R., JONES, P.D., HIEBL, J., FRANK, D., BRUNETTI, M., MAUGERI, M. 2010. The early instrumental warm-bias: a solution for long central European temperature series 1760–2007. Climatic Change, Vol. 101, 41–67. [Etude en ligne]

Abstract: Instrumental temperature recording in the Greater Alpine Region (GAR) began in the year 1760. Prior to the 1850–1870 period, after which screens of different types protected the instruments, thermometers were insufficiently sheltered from direct sunlight so were normally placed on north-facing walls or windows. It is likely that temperatures recorded in the summer half of the year were biased warm and those in the winter half biased cold, with the summer effect dominating. Because the changeover to screens often occurred at similar times, often coincident with the formation of National Meteorological Services (NMSs) in the GAR, it has been difficult to determine the scale of the problem, as all neighbour sites were likely to be similarly affected. This paper uses simultaneous measurements taken for eight recent years at the old and modern site at Kremsmünster, Austria to assess the issue. The temperature differences between the two locations (screened and unscreened) have caused a change in the diurnal cycle, which depends on the time of year. Starting from this specific empirical evidence from the only still existing and active early instrumental measuring site in the region, [the authors] developed three correction models for orientations NW through N to NE. Using the orientation angle of the buildings derived from metadata in the station histories of the other early instrumental sites in the region (sites across the GAR in the range from NE to NW) different adjustments to the diurnal cycle are developed for each location. The effect on the 32 sites across the GAR varies due to different formulae being used by NMSs to calculate monthly means from the two or more observations made at each site each day. These formulae also vary with time, so considerable amounts of additional metadata have had to be collected to apply the adjustments across the whole network. Overall, the results indicate that summer (April to September) average temperatures are cooled by about 0.4°C before 1850, with winters (October to March) staying much the same. The effects on monthly temperature averages are largest in June (a cooling from 0.21° to 0.93°C, depending on location) to a slight warming (up to 0.3°C) at some sites in February. In addition to revising the temperature evolution during the past centuries, the results have important implications for the calibration of proxy climatic data in the region (such as tree ring indices and documentary data such as grape harvest dates). A difference series across the 32 sites in the GAR indicates that summers since 1760 have warmed by about 1°C less than winters.

Mots-clés

 

 

Organismes / Contact

• Central Institute for Meteorology and Geodynamics (ZAMG), Hohe Warte 38, 1190 Vienna, Austria (reinhard.boehm@zamg.ac.at)
• Climatic Research Unit (CRU), School of Environmental Sciences, University of East Anglia, Norwich, UK
• Swiss Federal Research Institute WSL, Birmensdorf, Switzerland
• Istituto di Scienze dell’Atmosfera e del Clima (ISAC)–CNR, Bologna, Italy
• Dipartimento di Fisica, Università degli Studi, Milan, Italy

 

(1) - Paramètre(s) atmosphérique(s) modifié(s)

(2) - Elément(s) du milieu impacté(s)

(3) - Type(s) d'aléa impacté(s)

(3) - Sous-type(s) d'aléa

 

 

 

 

 

Pays / Zone

Massif / Secteur

Site(s) d'étude

Exposition

Altitude

Période(s) d'observation

Greater Alpine Region (GAR)

 

 

 

 

 1760–2007

 

(1) - Modifications des paramètres atmosphériques

Reconstitutions

 

Observations

The new HISTALP LSS-2008 versions of long-term temperature variability in the GAR is presented. The regional averages of single years (half-years) temperatures from 1760 (1760/61) to 2007 (2007/08) along with the annual range (summer minus winter half-years) of temperature show a general 2◦C annual temperature increase from the late nineteenth to the early twenty-first century in two steps which followed a weaker decrease of 1◦C from 1790 to 1890. The stepwise warming of the twentieth century was more accentuated in summer than in winter. Winters do not show the accentuated summer cooling phase from the 1950s to the 1970s which was nearly 1◦C in magnitude. Interannual variability in the region is higher in the cold season, but not stable in all subregions. In the Mediterranean part, winter and summer half-years have equal variance.

The annual range of temperature (represented here by the summer- minus winter-half-year differences) was reduced in the early instrumental period by the asymmetric “early instrumental period” (EIP. i.e. prior to 1850) corrections. Nevertheless there still is a significant change from a stronger annual cycle before 1900 to a weaker one in the twentieth century. The smallest summer-winter differences occurred in the 1910s and in the 1960s and 1970s. These two (oceanic) phases typically went along with the last two glacier advances in the Alps, which were triggered by cool summers with higher albedo due to more frequent snow cover on the glacier surface. During the last 25 years, winters and summers have simultaneously warmed at comparable rates: non-typical features for the regional climate evolution of the past 250 years. This has caused the extraordinary warming of the annual means by 1.2◦C/25 years which is unprecedented in the instrumental period.

The EIP corrections have re-ranked also the individual (seasonal) extremes. 1816 is now the coolest summer half year of the central European instrumental period (which it was not in the LSS-2007 version). 2003 is even more extraordinary compared to a series of warm summers in the early period. After EIP-correction 1811 (the second warmest) was surpassed by 1947—the latter now being second warmest. Winter half-years saw no changes in the ranks of extreme winters. 1829/30 is still the coldest of the past 250 years. Only the update to 2007/08 performed in this study brought a new record at the other end of the scale. The winter half-year of 2006/07 was +3.6◦C above average (1851–2000) and 6.8◦C warmer than the coldest of 1829/30 (−3.2◦C). The range of the annual means is smaller. It spans only 3.8◦C between the coldest year on record (1829, −1.7◦C below 1851–2000 average) and the warmest (1994, +2.1◦C above).

Modélisations

 

Hypothèses

 

 

Informations complémentaires (données utilisées, méthode, scénarios, etc.)

Instrumental temperature recording in the Greater Alpine Region (GAR) began in the year 1760. Prior to the 1850–1870 period, after which screens of different types protected the instruments, thermometers were insufficiently sheltered from direct sunlight so were normally placed on north-facing walls or windows. It is likely that temperatures recorded in the summer half of the year were biased warm and those in the winter half biased cold, with the summer effect dominating. Because the changeover to screens often occurred at similar times, often coincident with the formation of National Meteorological Services (NMSs) in the GAR, it has been difficult to determine the scale of the problem, as all neighbour sites were likely to be similarly affected. This paper uses simultaneous measurements taken for eight recent years at the old and modern site at Kremsmünster, Austria to assess the issue. The temperature differences between the two locations (screened and unscreened) have caused a change in the diurnal cycle, which depends on the time of year. Starting from this specific empirical evidence from the only still existing and active early instrumental measuring site in the region, we developed three correction models for orientations NW through N to NE. Using the orientation angle of the buildings derived from metadata in the station histories of the other early instrumental sites in the region (sites across the GAR in the range from NE to NW) different adjustments to the diurnal cycle are developed for each location. The effect on the 32 sites across the GAR varies due to different formulae being used by NMSs to calculate monthly means from the two or more observations made at each site each day. These formulae also vary with time, so considerable amounts of additional metadata have had to be collected to apply the adjustments across the whole network.

 

(2) - Effets du changement climatique sur le milieu naturel

Reconstitutions

 

Observations

 

Modélisations

 

Hypothèses

 

 

Sensibilité du milieu à des paramètres climatiques

Informations complémentaires (données utilisées, méthode, scénarios, etc.)

 

 

 

(3) - Effets du changement climatique sur l'aléa

Reconstitutions

 

Observations

 

Modélisations

 

Hypothèses

 

 

Paramètre de l'aléa

Sensibilité des paramètres de l'aléa à des paramètres climatiques

Informations complémentaires (données utilisées, méthode, scénarios, etc.)

 

 

 

 

(4) - Remarques générales

 

 

(5) - Syntèses et préconisations

 

Références citées :