Recent research by Esper et al (2002) suggest that the period from 1000 to 2000 BC had an extended sub-period of cooler temperatures compared to the previous 1000 year record of Mann et al (1999). The Esper et al record shows the well-known little ice age in the 1600s but also shows that there were some almost equally cool periods in the 12th, 13th, and 14th centuries which are not shown in the previous climate reconstruction records. Briffa et al provide an analysis and interpretation of these data and make comparisons with bore-hole derived temperatures from about 1500 onward. The bore-hole data show a protracted period of warming beginning about 1500 that accelerated in the 20th century. The Mann et al data, by contrast seemed to show the warming began near the end of the 19th century. More reconstructions of this period are needed in order to narrow the range of uncertainty on the amount of warming that might be attributable to anthropogenic causes.
References
Briffa, K. R., and T. J. Osborn, 2002: Blowing hot and cold. Science, 295, 2227-2228.
Esper, J., E. R. Cook, and F. H. Schweingruber, 2002: Low-frequency signals in long tree-ring chronologies for reconstructing past temperature variability. Science, 295, 2250-2253.
Mann, M. E., R. S. Bradley, and M. K. Hughes, 1999; Geophys. Res. Let., 26, 759.