The early instrumental warm-bias: a solution for long central European temperature series 1760–2007

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, we developed three correction models Climatic Change (2010) 101:41-67 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.

Climate Warming and the Recent Treeline Shift in the European Alps: The Role of Geomorphological Factors in High-Altitude Sites

“…Geomorphological factors should always be taken into account in studies on climate change impacts on treeline dynamics. The temperature trend based on daily temperature data since 1975 is consistent with the generally constant trend of increasing mean temperatures recorded in the historical homogenized time series of Aosta (Brunetti et al 2006) and in the temperature anomaly series HI-STALP of the grid point 45°N 7°E (Böhm et al 2010) (Fig. 6).…”

“…Pollen Fig. 6 The mean annual temperature and annual precipitation sum presented as 11-year running means for Aosta (black) and for the grid point 45°N 7°E (grey); the original series are courtesy of the Società Meteorologica Italiana (Mercalli et al 2003) and of the HISTALP project (Böhm et al 2010; http://www.zamg.ac.at/histalp/), respectively. The regression lines for the mean temperatures are calculated on data referring to the common period sequences from bogs and lake sediments indicate past altitudes similar to present potential climatic treelines (in the Vanoise massif; David 1995) or about 100 m above it (in the Thabor-Galibier massif; Wegmüller 1977).…”

The reliability of global and hemispheric surface temperature records

“…Climatologists have recently begun to collect modern parallel measurements to attempt to resolve these instrument exposure issues. Two examples of this type of work are studies in the Greater Alpine Region (GAR) by Böhm et al (2010) and in Spain by Brunet et al (2011). The former used parallel measurements at one site in Austria, which enabled the differences between the old and modern exposure methods to be estimated and related to the directional exposure of all earlier sites in the GAR.…”

Extreme grape harvest data of Austria, Switzerland and France from A.D. 1523 to 2007 compared to corresponding instrumental/reconstructed temperature data and various documentary sources

“…Instrumental temperature data used for the stations ViennaHohe Warte (starting in 1775), Basel-Binningen (starting in 1760), Geneva-Cointrin (starting in 1760) and StrasbourgEntzheim (starting in 1801) are from the HISTALP data collection (Auer et al 2007) in the bias-corrected version (Böhm et al 2010). The monthly Central European temperature (CEuT, 1500(CEuT, -2007 reconstruction, developed by Dobrovolný et al (2010) from documentary index series from Germany, Switzerland and the Czech Republic (1500-1854) and 11 instrumental temperature records , was used for the pre-instrumental period.…”

“…Vienna-Hohe Warte from 1841 onwards) do not go back beyond the 18th century. Besides, homogenizing early instrumental (temperature) measurements means facing extraordinary complexities (Manley 1974;Böhm et al 2010), such as the effect of urban growth, changes in observation hours or in the immediate vicinity of the instruments, undetected instrumental errors, varying instrumentation and the results of imperfect or over-protected exposures.…”