Penn State researchers have called into question the common practice of using tree rings to reconstruct past climate events, showing that tree rings are sometimes completely absent for very cold years.
The new findings, presented in Nature Geoscience, are based on a comparison of tree ring temperature reconstructions and simulations of past temperature changes. “We know these tree rings capture most temperature changes quite well,” said Michael Mann, director of the Penn State Earth System Science Center. “But the problem appears to be in their response to the intense short-term cooling that occurs following a very large volcanic eruption.”
Tree rings are used as proxies for climate because trees create unique rings each year that reflect the weather conditions in that growing season. For reconstructing climate conditions, tree-ring researchers seek trees growing at the extremes of their growth range (either at the tree line caused by elevation or at the boreal tree line). For these trees, growth is almost entirely controlled by temperature, rather than precipitation, soil nutrients or sunlight, supposedly yielding a good proxy record of surface temperature changes.
“The problem is that these trees are so close to the threshold for growth, that if the temperature drops just a couple of degrees, there is little or no growth and a loss of sensitivity to any further cooling. In extreme cases, there may be no growth ring at all,” said Mann. “If no ring was formed in a given year, that creates a further complication, introducing an error in the chronology established by counting rings back in time.”
The inconsistency showed up most dramatically when Mann compared temperature reconstructions from actual tree ring data with temperature estimates from climate models driven by past volcanic eruptions. “The response to the three largest tropical eruptions – AD 1258/1259, 1452/1453 and the 1809/1815 eruptions – is sharply reduced in the reconstruction,” he noted.
Following the 1258 eruption, the climate model simulations predict a drop of 3.5 degrees Fahrenheit, but the tree ring-based reconstruction shows only about a 1 degree Fahrenheit dip and the dip occurs several years too late. The other large eruptions showed the same type of discrepancy.
The potential absence of rings in the first 1 – 3 years following eruption further degrades the temperature reconstruction. Because tree ring information is averaged across many locations to obtain a representative estimate of northern hemisphere temperature, tree ring records with and without missing rings for a given year are merged, indicating reduced and delayed apparent cooling.
“Scientists look at the past response of the climate to natural factors like volcanoes to better understand how sensitive Earth’s climate might be to the human impact of increasing greenhouse gas concentrations,” said Mann. “Our findings suggest that past studies using tree ring data to infer this sensitivity have likely underestimated it.”
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