Why the Little Ice Age ended in the middle of the 19th century
The fact that the Alpine glaciers grew and then retreated during this so-called Little Ice Age was a natural process. This has now been proven by PSI researchers using ice cores. Until now, it was assumed that industrial soot from the middle of the 19th century had triggered the glacier melt at that time.
The reasons that contributed to the Little Ice Age, to the cooling of the Earth are now known. The findings were published in October 2018 in the scientific journal The Cryosphere. In the first half of the 19th century, a series of large volcanic eruptions in the tropics led to a temporary global cooling of the Earth's climate.
In popular scientific accounts, images of Alpine glaciers from the 1850s are often used as a comparison to visualize man-made climate change. However, this is wrong, researchers have now proven using data from ice cores. The scientists led by Michael Sigl of the Paul Scherrer Institute PSI analyzed the air composition archived in the different ice depths and, in particular, the amount of industrial soot.
They thus established the first uninterrupted data series for Central Europe on the amount of industrial soot in the atmosphere for the period from the 1740s to the present.
Volcanic eruptions in the tropics
These data clearly show that industrial soot can hardly be responsible for the melting of Alpine glaciers at that time, which occurred mainly between 1850 and 1875. "By 1875, about 80 percent of the glacier retreat at that time had already been completed," Sigl said. But it wasn't until 1875 that the amount of industrial soot in Central Europe exceeded the amount naturally present in the atmosphere. "Only in the last 20 percent of the retreat could soot possibly have had an influence," Sigl clarifies.
The first half of the 19th century was characterized by several large volcanic eruptions in the tropics, whose ejected sulfur particles led to a temporary global cooling. During this final cold phase of the so-called Little Ice Age, the Alpine glaciers once again grew strongly until the middle of the 19th century. Until now, it was thought that their decline from the 1860s onward was also due to the onset of industrialization. But the PSI results now clearly refute this theory:
It was (initially) merely a decline to the previous, undisturbed glacier extent.
1850 is not suitable as a reference year for climate models
"The question of when human influence on the climate begins remains open," Sigl says. And that onset, this study shows, is not necessarily a suitable reference for climate models because of other factors. Sigl estimates that the 1750s are more suitable as a pre-industrial reference time, that is, a time before the Little Ice Age. Even now, whenever the sparse data from past centuries allows, 1750 has been adopted as a reference year when climate models need to compare data from the pre-industrial period with those after the onset of industrialization. "That makes sense, because we now see clearly in our data that the climate in the mid-19th century was not the primal one."
Future climate models take into account soot data
In model calculations on climate change, the time course of the amount of soot in the atmosphere is also included as one of many variables. "Until now, however, modelers have used an estimate of the respective amount of soot," Sigl said. For the 19th century in particular, this is based only on rough estimates of the individual industrial nations based on energy consumption at the time. For the second half of the 19th century, a linear increase in the amount of soot in the atmosphere has been assumed so far, Sigl said. That this does not correspond to reality can now be proven thanks to the ice core investigations by Sigl and his co-researchers.
The researchers therefore advocate that experimental soot data be included in future model calculations. These models, in turn, form an important part of the report that the IPCC, known as the Intergovernmental Panel on Climate Change, issues about every seven years.
"In the IPCC report, the model calculations that mathematically reproduce the climate since 1850 have a central role," emphasizes Margit Schwikowski, head of the project under which the research was conducted. "With our research, we have now contributed to the fact that the scientific groups that create such climate models will be able to draw on experimental data in the field of industrial soot."
