High-resolution palaeoclimatic records for the last millennium: interpretation, integration and comparison with General Circulation Model control-run temperatures

Abstract

Palaeoclimatology provides our only means of assessing climatic variations before the beginning of instrumental records. The various proxy variables used, however, have a number of limitations which must be adequately addressed and understood. Besides their obvious spatial and seasonal limitations, different proxies are also potentially limited in their ability to represent climatic variations over a range of different timescales. Simple correlations with instrumental data over the period since ad 1881 give some guide to which are the better proxies, indicating that coral- and ice-core-based reconstructions are poorer than tree-ring and historical ones. However, the quality of many proxy time series can deteriorate during earlier times. Suggestions are made for assessing proxy quality over longer periods than the last century by intercomparing neighbouring proxies and, by comparisons with less temporally resolved proxies such as borehole temperatures. We have averaged 17 temperature reconstructions (representing various seasons of the year), all extending back at least to the mid-seventeenth century, to form two annually resolved hemispheric series (NH10 and SH7). Over the 1901–91 period, NH10 has 36% variance in common with average NH summer (June to August) temperatures and 70% on decadal timescales. SH7 has 16% variance in common with average SH summer (December to February) temperatures and 49% on decadal timescales, markedly poorer than the reconstructed NH series. The coldest year of the millennium over the NH is ad 1601, the coldest decade 1691–1700 and the seventeenth is the coldest century. A Principal Components Analysis (PCA) is performed on yearly values for the 17 reconstructions over the period ad 1660–1970. The correlation between PC1 and NH10 is 0.92, even though PC1 explains only 13.6% of the total variance of all 17 series. Similar PCA is performed on thousand-year-long General Circulation Model (GCM) data from the Geophysical Fluid Dynamics Laboratory (GFDL) and the Hadley Centre (HADCM2), sampling these for the same locations and seasons as the proxy data. For GFDL, the correlation between its PC1 and its NH10 is 0.89, while for HADCM2 the PCs group markedly differently. Cross-spectral analyses are performed on the proxy data and the GFDL model data at two different frequency bands (0.02 and 0.03 cycles per year). Both analyses suggest that there is no large-scale coherency in the series on these timescales. This implies that if the proxy data are meaningful, it should be relatively straightforward to detect a coherent near-global anthropogenic signal in surface temperature data.

Two centuries of limited variability in subtropical North Atlantic thermocline ventilation

“…Low ∆ 14 C variability along the south shore is also seen during the end of the Little Ice Age from 1800 to 1870, when SSTs are low 22 and NAO variability is weak 21 . In summary, surface waters to the south of the Bermuda exhibit a significant atmospheric 14 C imprint and minimal changes through time in the mixing rate with older subsurface waters, despite significant changes to NAO behaviour 21 and mean hemispheric climate 16 (Fig. 2c,d).…”

Space Storms and Space Weather Hazards

“…Two of the recent reconstructed temperature series have been extensively discussed, namely the temperature series by Jones et al (1998) and Mann et al (1998Mann et al ( , 1999. These curves have been derived using different methods.…”

Grain Prices in Cairo and Europe in the Middle Ages

“…10 The temperature series used is a northern hemisphere temperature reconstruction by Jones, Briffa, Barnett, and Tett (1998). This series combines evidence from tree rings, ice cores, and historical documents, and is often quoted by climatologists, e.g., Berger (2002, p. 4).…”

“…Table 5 reports correlations between wheat price fluctuations in Cairo and temperature. The wheat price series has been detrended by calculating Jones et al (1998). Ã Significant at the 5 percent level.…”

“…The chronology of the Toussoun series differs from the Popper series and has been adjusted to the latter. Tessmperature: Jones et al (1998). Grain prices are residuals from a linear time trend, in logs.…”