Royal Astronomical Society | 2017 Apr 04
The whole concept of the ‘Little Ice Age’ is ‘misleading,’ as the changes were small-scale, seasonal and insignificant compared with present-day global warming, a group of solar and climate scientists argue.
- Graphs informing the analysis of extreme temperatures in the Central England Temperature (CET) thermometer record. Part (b) shows the lowest monthly average in each winter whereas part (c) shows the hottest monthly average in each summer. In both cases blue shows lower temperatures, and red shows higher temperatures than the long-term average. The cold winter months match up very well with the years in which frost fairs were held (vertical mauve lines) or years when the Thames was reported as frozen solid (vertical orange lines). However these years are not usually also associated with colder summers, unless there was a large volcanic eruption (measured from the sulphates that it deposited in polar ice sheets) such as Tambora in 1815. The top panel (a) shows the level of solar activity as seen in sunspot numbers (from telescopic observations and deduced from Carbon-14 stored in tree rings). It can be seen that, contrary to common claims, the Thames did not freeze more often during the Maunder minimum (c.1660-1710). Thames freezing events ceased after the demolition of the old London bridge in 1825 and the installation of the embankments, completed in 1870 (both dates marked with black lines): the faster flow meant that the river no longer froze, even when temperatures fell to values that had previously caused freezing. Credit: M. Lockwood. Click for a full size image
Explanations for the cooling to Earth’s climate, thought to have occurred between the 16th and 19th centuries, include low solar activity, volcanic eruptions, human changes to land use and natural climatological change.
But in a new paper in Astronomy & Geophysics, the house journal of the Royal Astronomical Society, Professor Mike Lockwood, of the University of Reading, and his collaborators, note that the temperature shift was smaller than that seen in recent decades resulting from the emission of greenhouse gases, and that although low solar activity may have been one driving factor, it certainly was not the only one.
Professor Lockwood said: “Commentators frequently refer to the Little Ice Age in discussions on climate change. We wanted to carry out a comprehensive study to see just how reliable the evidence is for a cooler climate, how big an impact it really had and how strong the evidence for a solar cause really was.
“On the whole the Little Ice Age was a manageable downturn in climate concentrated in particular regions, even though places like the UK had a larger fraction of cold winters. Our research suggests that there is no single explanation for this, that warm summers continued much as they do today and that not all winters were cold.” ...
Much more dramatic variations can result from large volcanic eruptions. Samalas, a volcano which erupted in 1257 in what is now Indonesia, ejected large amounts of dust into the atmosphere, causing a temporary cooling effect. The years between 1570 and 1730, corresponding to the coldest part of the LIA, also saw continuous lower level volcanic activity that may have suppressed temperatures. Volcanic eruptions undoubtedly cause both cold winters and cold summers. One of the clearest examples was the Tambora eruption of July 1815, which caused the next year to be called "the year without a summer".
Professor Lockwood said: "This study provides little solace for the future, as we face the challenge of global warming. Solar activity appears to be declining at present, but any cooling effect that results will be more than offset by the effect of rising carbon dioxide emissions, and provides us with no excuse for inaction."
Frost Fairs, Sunspots and the Little Ice Age - Mike Lockwood et al