By Philip Eden
The Central England Temperature series, collated by the late Professor Gordon Manley, extends back to 1659 and enables us to place recent changes into an historical context. The decadal statistics for July show no underlying trend for most of the last 340 years, but there is a clear warming process evident in the most recent 40 years.
Ten-year averages were mostly below 16°C from the 1660s to the 1720s and from the 1810s to the 1910s. They were consistently above 16°C between the 1730s and 1800s, and also from the 1920s onwards (with the exception of the 1960s). The warmest two decades during the entire period were the 1990s and 2000s with a mean temperature for both decades of 16.9°C.
The warming trend during the last 40 years is amplified by the fact that the 1960s constituted an abnormally cold decade with a mean temperature of 15.4°C. Only the 1690s were colder. The coolness of the 1960s was apparent in eight months of the year, and was attributed by many climatologists to the extensive sea-ice around eastern Greenland between 1962 and 1970. If the jump from 15.4°C in the 1960s to 16.9°C in the 2000s does not sound very big, the change may also be illustrated by the fact that seven Julys in the 1960s were colder than the coldest July in the 2000s.
Rainfall figures representative of England and Wales are available from 1766. There has been a marked decline in July rainfall over the last 300 years; this downward trend accelerated between 1975 and 2006, though it now appears to have been reversed. In the early part of the 19th century July was actually the wettest month of the year but by the 1990s it was the driest. The wettest decade was the 1800s with a monthly average of 92mm which contrasts with 55mm during the 1990s. If we describe any July with over 100mm as very wet, there were 12 very wet Julys in the first half of the 20th century, only five in the second half, but we have had four since 2007.
Sunshine recording began in England and Wales in the 1870s. The 10-year averages mirror the temperature changes with a marked increase in sunshine hours during the last 55 years, from 174 hours in the 1960s to 206 hours in the 1990s.
By Philip Eden