London – The famous Little Ice Age, when Londoners could roast oxen on the frozen River Thames, may not have been so icy after all – and “little” might indeed have been the appropriate description.
New research suggests that the climate change at the time might have been small-scale, seasonal and insignificant compared to the changes now happening around the world.
A study in the Astronomy & Geophysics journal, published by the Royal Astronomical Society, suggests that the temperature change between the 16th and 19th centuries was smaller than that seen in the last half century or so as a consequence of greenhouse gas emissions in response to human fossil fuel burning.
“Commentators frequently refer to the Little Ice Age in discussions on climate change” says Mike Lockwood, professor of space environment physics at the University of Reading in the UK.
“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.”
At the heart of a long and complex issue lies an argument about the sun’s influence on planetary weather − in particular, the cycle of sunspots.
Researchers have in the past associated climatic change and cosmic radiation, and others have linked very low solar activity to periods of extended cold with the implication that more vigorous solar activity could be responsible for global warming.
But at least two studies have questioned the scale of this influence or absolved the sunspots from any additional role in the recent inexorable rise in planetary temperatures.
Compared to the Ice Ages of prehistory, which can be linked to slow changes in the planet’s orbit around the sun, the Little Ice Age, the researchers say, was “a very short-lived and very puny climate and social perturbation”, and the label itself was introduced into the scientific literature only in 1939.
Professor Lockwood and his co-authors searched the historical records and paintings of the era – the diarist John Evelyn recorded the frost fairs on the Thames and Pieter Bruegel the Elder painted winter landscapes – and examined records of the low sunspot numbers linked to the frost fair phenomenon.
They also compared historic temperature records – the Central England Temperature records date from 1659 and are the oldest of all − and evidence preserved in ice cores, tree rings, coral growths, and counts of fossil shellfish and insects − all sensitive to climate shifts.
And they found that, through the 16th to 19th centuries, average temperatures may have fallen by 0.5 °C. The comparable fall during the Ice Age that ended 12,000 years ago was about 8°C.
They also noted that once the old London Bridge was demolished in 1825, the flow of the Thames accelerated and prevented much further freezing − that is, the frost fairs were more probably linked to hydrology than astronomy.
And the temperatures were not necessarily cooler overall. In 1701, the lowest point of the Little Ice Age, the July temperature that year was the tenth hottest on record. The second hottest June on record happened in 1676, a year bounded by very cold winters.
The researchers also detected the signature of volcanic activity, eruptions that discharge large volumes of dust in the upper atmosphere are linked to low planetary temperatures, and the 1815 Tambora eruption in what is now Indonesia was followed in Europe by the notorious “year without a summer” in 1816.
In all, the centuries may not have been so cold, and sunspot activity may have been only one factor at work.
“This study provides little solace for the future, as we face the challenge of global warming,” Professor Lockwood says.
“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.”
Climate News Network