Posted on 18 September 2010 by John Cook

A new paper has just been published employing a new technique for reconstructing past temperatures (Kellerhals 2010). It uses Ammonium concentration from an ice core in tropical South America (the eastern Bolivian Andes) as a proxy for temperature. This enables them to build a temperature record going back 1600 years in a region which has had little proxy data available until now. They find a distinguishable Medieval Warm Period and Little Ice Age in the record. Nevertheless, they also find the last few decades show unprecedented warmth over the last 1600 years.

Figure 1: Reconstructed tropical South American temperature anomalies (normalized to the 1961–1990AD average) for the last 1600 years (red curve, smoothed with a 39?year Gaussian filter). The shaded region envelops the ±2 standard deviation uncertainty as derived from the validation period. Poor core quality precluded any chemical analysis for the time interval between 1580 and 1640 AD.

Note that Figure 1 shows only the proxy record from the ice core - no instrumental data is included. Of course, the usual caveat applies when looking at a single proxy record - this is a temperature record for a single location.To get a better feel for past climate, you need to look at proxy records from a range of locations.

When we combine all the various temperature records, we find the same result: modern temperatures are significantly warmer than medieval temperatures. This is demonstrated in Moberg's reconstruction of Northern Hemisphere temperature (which happens to bear a striking resemblance to the South American proxy record).

Figure 2: Northern Hemisphere Temperature Reconstruction by Moberg et al. (2005) shown in blue, Instrumental Northern Hemisphere Temperatures from Hadley shown in Red. Thanks to Robert Way for providing this graph.