Records of the river Thames in London having frozen go back to as far as 1063, and the event – for it did become an ‘event’ featuring ‘frost fairs’ – occurred not infrequently between about 1500 and 1820. After old London Bridge was replaced in 1831, ice no longer accumulated against its supports, allowing the river to flow more freely, and the Thames has not frozen since. The period from 1400 to about 1800 is now called the Little Ice Age.

According to the Milanković curve described in Chapter 3, we should have been experiencing a slow cooling trend for the past several thousand years, and Figure 8.5 shows exactly that. There are gentle variations on that slow trend; the historical records show that England, Iceland, Greenland and western Europe enjoyed a warm period in mediaeval times. There are records of crops being grown in places in which by 1500 it had become too cold for farming. Climate scientists suggest that the change from the warmer mediaeval period to the cooler Little Ice Age is just the sort of slow climate change to be expected from the changes in the Sun’s intensity, such as the changes in the sunspot cycle.