Yale Environment 360

August 4, 2023
By Yale School of Environment

Was the Medieval Era Warmer Than Now? New Tree Ring Study Offers Insight

A new study of tree rings in Scandinavia is helping to resolve a longstanding question about the Medieval Warm Period.

Past studies of tree rings had indicated that the Medieval period was as warm or warmer than today, but climate models found otherwise. Climate deniers have long pointed to the tree-ring data to suggest the current period of warming is not without precedent.

The new tree-ring study, which used more precise methods than prior research, agrees with the climate models.

For the study, researchers gathered data from 188 Scots pines, both living and dead, in Sweden and Finland. While past studies largely looked only at the width and density of rings, the new study analyzed tree rings at a microscopic level, examining 50 million tree cells to infer changes in temperature — cell walls tend to grow thicker in hotter weather, which is why a darker layer of each ring forms in late summer.

Researchers used the tree ring data to reconstruct summer temperatures, finding that Scandinavia is now warmer than at any point in the past 1,200 years. The findings, published in Nature, line up with the climate models, highlighting the impact of human-caused warming.

“This means that there are now two independent accounts of the regional climate that both find lower temperatures during the Medieval, providing new evidence that this phase was not as warm as previously thought,” lead author Jesper Björklund, of the Swiss Federal Institute for Forest, Snow, and Landscape Research, said in a statement. “Instead, both show that the current warming is unprecedented, at least in the past millennium.”