The cause of the so-called “little ice age”, when frost fairs were held on the Thames and there was a run of cold winters, has puzzled scientists. These uncertainties have given American sceptics opportunities to claim that global warming is nothing more than a natural variation in the climate.
However, new research by the Royal Astronomical Society shows that the period between the 16th and 19th centuries may have had cold winters but it also suffered scorching summers, so it was not an ice age at all.
At most the average temperature during this period was half a degree centigrade lower across the Northern Hemisphere, whereas during actual ice ages it was down 8C. Close examination of the data also reveals that not all the winters were cold, so the natural variability of the climate continued throughout the period.
The causes of the slight cooling were a combination of lower solar activity, volcanic eruptions and manmade changes to the landscape. Solar activity is again on the wane now, but this will not save us from rises in temperature. Greenhouse gases that have been pumped into the atmosphere in the last 160 years have already pushed the temperature up by 1C and it will continue to rise.
While a frost fair on the Thames is probably now a most unlikely occurrence, the scientists point out that the fairs stopped in the first place not because the weather got warmer but because London Bridge was replaced in 1825. Until then the low arches and large supports trapped ice as it flowed down the river – allowing the river to freeze over upstream.