Swiss research suggests current global warming is unprecedented in the last millennium.

A study by the Swiss Federal Institute for Forest, Snow and Landscape Research (WSL) states it is "probably beyond the natural temperature variations of the last 1,200 years".

The new WSL study, unveiled this week and published in the journal Nature, puts into perspective the importance of the hottest period of the Middle Ages, which was not as scorching as previously thought. On the contrary, it shows that "current warming is unprecedented", at least in the Scandinavian region and Finland.

The researchers point out that this discovery underlines the role played by greenhouse gas emissions in temperature variations. Warmer than ever

Using dendroscience, the scientists measured the walls of 50 million wood cells from 188 living and dead Scots pine trees in Scandinavia and Finland. All the rings observed cover a period of 1,170 years..

The Middle Ages and the centuries that followed were tumultuous. They saw not only a cold phase, the Little Ice Age, but also its opposite, the "medieval climatic optimum", a period during which it is assumed that the weather was abnormally warm, according to the researchers. To date, physics has provided no explanation for medieval periods of exceptional warmth. Analysing tree rings

The technique employed involved measuring the thickness of wood cell walls in tree rings. "Each cell in each ring records the climate in which it was formed. By analysing hundreds or even thousands of cells per ring, we obtain exceptionally precise climatic information"," says Jesper Björklund, lead author of the study and researcher at WSL.

The scientists reconstructed summer temperatures in the regions under consideration and compared them with both regional climate model simulations and previous reconstructions based on tree-ring density.