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Autumn 2006 is Very Likely the Warmest of Over More Than Half a Millennium
23 Dec, 2006 05:03 pm
Detailed insight into high-resolution temporal and spatial patterns of climate change during previous centuries is essential for assessing the degree to which late 20th/early 21st century changes may be unusual in the light of pre-industrial natural climate variability. Regional and temporal high-resolution reconstructions show important climatic features, such as regionally very hot or cool summers or autumns and very mild or cold winters or springs that may be masked in a hemispheric or global reconstruction [1,2]. Thus, regional studies and reconstructions of climate change are critically important when climate impacts are evaluated.
Past temperature variations for spring and autumn have not been investigated so far neither at hemispheric nor continental scales due to limited information in climate proxies for these seasons. In a recent approach, Xoplaki et al.  combined long monthly instrumental temperature time series from Europe and temperature index series derived from documentary evidence (written sources, historical information, phenological evidence, etc., ) to reconstruct statistically the temperature distribution over European land areas for each month (back to 1659) and each season (1500-1658). , Such analyses are always an interdisciplinary approach including climatologists, historians, physicists, statisticians, modellers and scientists from adjacent fields that optimally incorporate their knowledge for a common understanding of past climate change and climate change impacts . The statistical model using long instrumental data and indexed temperature information derived from documentary evidence is calibrated to [conditions in] the 20th century instrumental data and then applied to reconstruct European climate back in time. Apart from the reconstructions, the results also provide uncertainties inherent in those reconstructions taking into account unexplained variance in the instrumental calibration period, partly uncertainties in the documentary data as well as the decrease of data back in time.
We have updated the data of Xoplaki et al. (2005) using monthly values of September, October and November 2005 and 2006 using data from NASA Goddard Institute for Space Studies (http://data.giss.nasa.gov/gistemp/maps/). The updated data indicate that the autumn of 2006 (September-November average) over European land areas was very likely the warmest for at least the last half millennium. The seasonal anomaly averaged all over Europe was around 2.5°C higher when compared to the autumns of the period 1971-2000. Autumn 2006 was the warmest autumn on record in Belgium, the Netherlands, the United Kingdom, parts of Ireland, Denmark, southern Norway and southern Sweden, Switzerland, Germany, as well as most parts of Austria.. In those areas, the temperature anomaly exceeded 3 to 4 times the 1971-2000 standard deviation. Only the southern central and eastern part of Europe including southern Italy, Albania, Bulgaria and Greece did not show much difference to the long-term mean. It was drier from southern France over Italy to the Balkans, Greece and western Turkey and wetter in parts of western and northern Europe. A glance at the large scale autumn 2006 temperature distribution indicates that most areas over the Northern Hemisphere were warmer than usual, except for the US and parts of northern Russia.
Rather unusual is the fact, that all three single autumn months were much warmer than the long-term mean and contributed to the overall autumn warmth. The previous warmest autumns covering the last 507 years were in 1938, 1772 and 2000. The autumn of 2006 however, exceeds those values clearly, [which is also true] when the uncertainties of the earlier reconstructions are taken into consideration. In the context of the last half millennium, the warmest autumn decade occurred in 1997-2006 (0.7°C warmer than the 1971-2000 mean), while the mildest 30-year period for autumn was in 1977-2006 (0.3°C warmer than in 1971-2000). There is also a significant warming trend of European scale autumn temperatures over the last 30 years. The strongest autumn warming over that period was experienced over Scandinavia and the British Isles. It is interesting to note that the previous autumn warming from World War 1 to the late 1930s did also show strongest warming over northern Europe. The physical understanding and the role of internal and external forcings at decadal to multidecadal scale during different periods to understand continental scale climate change is a challenge for future research.
Extreme climate anomalies for a single season, such as the hot European summer 2003 or the warm autumn of 2006, cannot be easily pinned to a single cause. However, the autumn warming trend over the past few decades, including the currently warmest conditions in the context of the past, are both in agreement with the general warming trend over the Northern Hemisphere, also during other seasons over similar periods. Recent studies describe evidence that indicates an anthropogenic influence on surface temperature during the last decades at continental and subcontinental scales.
An increasing trend of warm autumns or an increase of extreme warm European autumns (that is also very likely a scenario for the future) can have impacts on the phenology. For instance, some flowering trees, such as horse chestnuts, may spring into blossom before winter arrives. That can cause problems later in the year. Also butterflies, migratory birds and other animals may face troubles if they miss the signal to reduce their activity for the winter or do leave too late in warmer areas. A disruption in the common pattern between food availability and temperature could also cause starvation.
In summary, the new 507 years European scale autumn surface temperature time series provides evidence of current European climate change. Comparing recent temperature changes with those of the past and taking into account uncertainties in the reconstruction shows that the autumn of 2006 is very likely the warmest over more than half a millennium. Furthermore, the late 20th and early 21st century warmth very likely exceeds that of any time during at least the past five centuries with similar results having been found for the other seasons. The high-resolution reconstruction also sheds light on the spatial structure of regional temperature anomalies and extremes at monthly to seasonal resolutions back in time.
The European seasonal temperature data can be downloaded from the NOAA Paleo Climatology program:
[Author’s Note: Important work on those documentary weather and climate information to obtain temperature and precipitation for European areas have been done for instance by Emmanuel Le Roy Ladurie (Paris), Christian Pfister (University of Bern); Rudolf Brázdil (University of Brno); Ruediger Glaser (University of Freiburg); Mariano Barriendos (University of Barcelona); Ricardo Garcia Herrera (University of Madrid); Fernando Rodrigo (Unversity of Almeria); Maria Joao Alcoforado (University of Lisbon); Dario Camuffo (University of Padova); Rajmund Przybylak (Nicolaus Copernicus University, Torun); Lajos Racz (Szeged); Judit Bartholy (Eötvös Loránd University, Budapest) Astrid Ogilvie (INSTAAR); Aryan van Engelen (KNMI) and many others. Those data are the most reliable sources of independent temperature reconstructions prior to the late seventeenth/early eighteenth centuries to reconstruct monthly and seasonal spring and autumn temperature fields over Europe back to AD 1500 [see for instance  and references therein. Typical indicators for warm autumns from the past (before instrumental information) in central Europe are for instance an extended growing season, re-blossoming of plants, no reports about snow in low altitudes from September to November (e.g. Pfister, 1999, Wetternachhersage, Haupt Bern)]
 Xoplaki, E., Luterbacher, J., Paeth, H., Dietrich, D., Steiner N., Grosjean, M., and Wanner, H., 2005: European spring and autumn temperature variability and change of extremes over the last half millennium. Geophys. Res. Lett. 32, L15713.
 Luterbacher, J., Dietrich, D., Xoplaki, E., Grosjean, M., and H. Wanner, 2004: European seasonal and annual temperature variability, trends and extremes since 1500. Science. 303, 1499-1503.
 Brázdil, R., Pfister, C., Wanner, H., von Storch, H., and Luterbacher, J., 2005: Historical climatology in Europe – The State of the Art, Climatic Change, 70, 363 - 430.