How stable are twentieth-century calibration models? A high-resolution summer temperature reconstruction for the eastern Swiss Alps back to AD 1580 derived from proglacial varved sediments

Abstract

We found a significant positive correlation between local summer air temperature (May-September) and the annual sediment mass accumulation rate (MAR) in Lake Silvaplana (46°N, 9°E, 1800 m a.s.l.) during the twentieth century (r = 0.69, p < 0.001 for decadal smoothed series). Sediment trap data (2001-2005) confirm this relation with exceptionally high particle yields during the hottest summer of the last 140 years in 2003. On this base we developed a decadal-scale summer temperature reconstruction back to AD 1580. Surprisingly, the comparison of our reconstruction with two other independent regional summer temperature reconstructions (based on tree-rings and documentary data) revealed a significant negative correlation for the pre-1900 data (ie, late ‘Little Ice Age’). This demonstrates that the correlation between MAR and summer temperature is not stable in time and the actualistic principle does not apply in this case. We suggest that different climatic regimes (modern/‘Little Ice Age’) lead to changing state conditions in the catchment and thus to considerably different sediment transport mechanisms. Therefore, we calibrated our MAR data with gridded early instrumental temperature series from AD 1760-1880 (r = -0.48, p < 0.01 for decadal smoothed series) to properly reconstruct the late LIA climatic conditions. We found exceptionally low temperatures between AD 1580 and 1610 (0.75°C below twentieth-century mean) and during the late Maunder Minimum from AD 1680 to 1710 (0.5°C below twentieth-century mean). In general, summer temperatures did not experience major negative departures from the twentieth-century mean during the late ‘Little Ice Age’. This compares well with the two existing independent regional reconstructions suggesting that the LIA in the Alps was mainly a phenomenon of the cold season.

The IPCC on a heterogeneous Medieval Warm Period

“…2 Number of measurement series included in the six tree-ring proxy records shown in AR4. Tree-ring records are from SW Canada, W USA, N Sweden, NW Russia, N Russia, and Mongolia or lake sediments are typically affected by reduced dating control and associated lowered resolution back in time (Blass et al 2007;Fisher et al 1996;Tan et al 2006). These changes, however, can affect the estimation of long-term heterogeneity changes, as higher resolution climatic events are not accurately matched, and earlier periods appear less homogenous.…”

Quantifying human-induced eutrophication in Swiss mountain lakes since AD 1800 using diatoms

“…The TP concentration (depth-weighted average value for the entire water column) is 8 µg/l and indicates oligotrophic conditions, the lake-water pH is slightly alkaline (pH = 7.8) (both measured 11 September 2002;Bigler et al, 2006). The glacial meltwater supply is large during the summer months, as the lake catchment includes c. 5% (6 km 2 , status 1998) glaciated areas (Blass et al, 2007b).…”

Multiproxy study of anthropogenic and climatic changes in the last two millennia from a small mire in central Poland

“…Pronounced human impact occurs during the LIA, which is dated differently in various parts of the world. The LIA was recorded in many environmental archives in Europe (Mauquoy et al, 2002;Matthews, 2005;Weckström et al, 2006;Blass et al, 2007;van der Linden et al, 2008), and it is commonly dated to AD 1550-1850 (Bradley & Jones, 1992). In Poland, this important event is not well documented, but records of it may be more common than we suppose.…”