Nature Geosci. http://doi.org/hp5 (2012)

Credit: © ISTOCKPHOTO.COM/THINKSTOCK

According to research, tree-ring records have a blind spot for cooling after large volcanic eruptions.

Most tree-ring records come from the poleward or high-altitude edges of forests, where the trees are most sensitive to changing temperatures. Michael Mann, of Pennsylvania State University, and colleagues note that if temperatures drop suddenly, leading to a growing season of less than a few weeks, these trees might grow so slowly that the rings would be too small to see. This could lead to the coldest years being skipped over in the climate record. At the same time, the aerosols released by volcanoes cause more sunlight to be scattered and hence create more favourable growing conditions. This can make the record look warmer than it really was. Models taking these effects into account better replicate the tree-ring data, the researchers show.

Together these effects seem to explain why tree-ring records tend to never show more than 1 °C of cooling, when models predict cooling of 2 °C or more after gigantic eruptions such as the one that occurred in AD 1258–1259.