With this in mind, the three original scientists behind the curve – Michael Mann of Pennsylvania State University, Raymond Bradley of the University of Massachusetts Amherst and Malcolm Hughes of the University of Arizona – have, together with colleagues, carried out further temperature reconstructions using proxy data. The results, they say, "extend previous conclusions that recent Northern Hemisphere surface temperature increases are likely anomalous in a long-term context".

Instrumental climate records aren’t available for dates before the mid-19th century so scientists must turn to data from "proxies" such as tree-rings, corals, and ice cores. In this latest study, the team used an expanded set of proxy data, recently updated instrumental data, and two complementary methods to reconstruct temperatures for much of the last 2000 years.

"We find that modern warming is anomalous in the context of at least the past 1300 years, and probably considerably longer," Michael Mann told environmentalresearchweb. "Thus, the 'human' fingerprint is detectable in the changes we've seen in climate over recent decades. It tells us that modern climate change is unprecedented now in a very long timeframe. Only human impacts can explain the unprecedented changes we are seeing today."

Previous analysis had indicated that recent northern hemisphere temperatures were anomalous compared to the last 1000 years. The 1300 year figure stands whether or not tree-ring data are included. With the inclusion of tree-ring data, which is arguably less reliable, recent warming is anomalous compared to temperatures even further back – for the last 1700 years – although the team includes additional caveats for this analysis.

"The reconstructed amplitude of change over past centuries is greater than hitherto reported, with somewhat greater Mediaeval warmth in the Northern Hemisphere, albeit still not reaching recent levels," write the researchers in their paper in PNAS.

The team’s conclusions were much less definitive for the Southern Hemisphere and on a global scale, which they believe is because data for the south is much more sparsely available. A targeted effort to recover additional proxy records from the region could help reduce current uncertainties, the researchers say.

"We are in the process of developing not only hemispheric and global temperature reconstructions such as shown in the current paper, but also the detailed spatial patterns of climate change in past centuries," said Mann. "This can better inform our understanding of phenomena such as the El Nino/Southern Oscillation, and how it might be impacted itself by increasing greenhouse gas concentrations."