Environment Counts | New data suggest that a cooling trend in Earth’s climate over the past 2000 years may be stronger than previously thought in northern Scandinavia
Author: Geoff Zeiss – Published At: 2012-09-22 06:07 – (1056 Reads)
Tree-ring width (TRW) records are the most widely used temperature proxy for the last 2,000 years of Earth’s history. However, the ability of TRW records to reliably track recent warm episodes has been questioned. It has been argued that maximum latewood density (MXD) is a better proxy for summer temperatures than simple ring width, which tends to reflect average annual temperatures. This study reports a 2,000-year summer temperature reconstruction MXD series from northern Scandinavia. The final reconstruction was calibrated against instrumental regional June-August (JJA) temperatures and spans the 138â€‰BCâ€“2006 AD period. The results indicate that in northern Scandinavia over the past 2000 years the cooling trend was stronger, âˆ’0.31â€‰Â°C per millennium, than previously reported and suggests that that in northern Scandinavia Medieval and Roman times may have been warmer than previously thought. The authors also suggest that surface air-temperature reconstructions based on tree-ring width (TRW) data may underestimate pre-instrumental summer temperatures. Nature Climate Change (2012)
As reliable modern records of climate only began in the 1880s, climate proxies provide a means to reconstruct climatic conditions that prevailed during much of the Earth’s history before record-keeping began. Examples of climate proxies include ice cores, tree rings, sub-fossil pollen, coral and lake and ocean sediments.
For the late-Holocene tree-ring width (TRW) records are the most widely used climate proxy. However, the ability of TRW records to reliably track recent warm episodes has been questioned. It has been argued that other properties of the annual rings, especially maximum latewood density (MXD), may be better proxies for summer temperatures than simple ring width.
This study reports a 2,000-year summer temperature reconstruction based on 587 high-precision MXD series from northern Scandinavia. The record was developed using living and subfossil pine trees from 14 lakes and 3 lakeshore sites >65Â°â€‰N, making it not only longer but also much better replicated than any existing MXD time series. The final reconstruction (N-scan) was calibrated against instrumental regional June-August (JJA) temperatures and spans the 138â€‰BCâ€“2006 AD period.
Variability of MXD (grey curves) and instrumental temperature measurements (red and blue curves) over the earliest (AD 1876â€“1890) and latest (AD 1992â€“2006) 15-year periods common to these data.
a. Summer temperature construction extending back to 138 BC highlighting extreme cool and warm summers (blue curve), cool and warm periods (black curve) and a long-term cooling trend (dashed red curve). Estimation of uncertainty of the reconstruction (grey area) integrates the validation standard error (Â±2 Ã— root mean square error) and bootstrap confidence estimates.
b. Regression of the MXD chronology (blue curve) against JJA temperatures (red curve) over the 1876â€“2006 common period.
The results indicate that over the past 2000 years the cooling trend in northern Scandinavia was stronger (âˆ’0.31â€‰Â°C per 1,000â€‰years, Â±0.03â€‰Â°C) than previously reported, suggesting that in northern Scandinavia Medieval and Roman times may have been warmer than previously thought.
Comparison with TRW data show that that this trend is missing in published tree-ring proxy records which suggests that large-scale near-surface air-temperature reconstructions relying on tree-ring width (TRW)data may underestimate pre-instrumental summer temperatures.
To put this in context Cook reviewed temperature trends globally in long tree-ring and coral proxy temperature histories and found that the histories mostly reflect regional variations in summer warmth from the tree rings and annual warmth from the corals.
In the Northern Hemisphere, the North American tree-ring temperature histories and those from the north Polar Urals, covering the past 1000 or more years, indicate that the twentieth century has been anomalously warm relative to the past.
In contrast, the tree-ring history from northern Fennoscandia indicates that summer temperatures during the Medieval Warm Period were probably warmer on average than those than during this century.
In the Southern Hemisphere, the tree-ring temperature histories from South America show no indication of recent warming, which is in accordance with local instrumental records.
In contrast, the tree-ring records from Tasmania and New Zealand indicate that the twentieth century has been unusually warm particularly since 1960.
The coral temperature histories from the Galapagos Islands and the Great Barrier Reef are in broad agreement with the tree-ring temperature histories in those sectors, with the former showing recent cooling and the latter showing recent warming that may be unprecedented.
Overall, the regional temperature histories broadly support the larger-scale evidence for anomalous twentieth century warming based on instrumental records. However, this warming cannot be confirmed as an unprecedented event in all regions.