THE CHANGING CLIMATE OF NORTHAMPTON

The first detailed study of temperature trends at Pitsford Hall weather station has confirmed that the county’s temperatures are generally on the increase. Over the period 1931 to 2010, mean annual temperatures increased by 0.1C. However, superimposed on this long term trend are considerable fluctuations ranging from year to year and from decade to decade. Warm decades have generally followed cold decades from the 1940s to the 1980s. However, since the 1990s temperatures appeared to have stabilised and, even more recently, there is even a suggestion that they have begun to fall. Nevertheless, temperatures are now 0.3C on average higher than they were in 1971.

The cause of the 0.1C rise in temperature is most probably down to the increase in greenhouse gases in the atmosphere, most notably C0₂. However, the contribution of other natural factors cannot be entirely discounted. The decadal variations in temperatures closely correspond to cycles of solar activity. However, underlying the 11-year cycle of solar activity are also longer term changes. Since the 1900s, the number of sunspots recorded at each solar maximum has increased and this may itself contribute the long term rise observed in the record since 1931.

Northampton’s Changing Climate

The 1940s saw temperatures generally below today’s long term average with the winter of 1946-7 being particularly cold. In fact, the 1946-7 winter was second only to that of 1962-3 for being one of the coldest on record with an average temperature of 0.8C.  February 1947 recorded an average minimum temperature of -4.4C, compared to today’s 2.2C.

The 1940s were followed by a decade of steady warming culminating in the very warm summer of 1959 which saw temperatures in Northampton average 18.7C, compared with 16.6C recorded in the summer of 2010. September 1959 was one of the driest months on record with just 0.6mm of rain and daytime maximum temperatures in July and August regularly exceeded 33C. 

The warmth of the 1950s ended in 1960 and was followed by a succession of cooler years. The period from 1962 to 1966 was particularly cold, with the winter of 1962-3 being the coldest on record with an average temperature of just -0.5C. On one night in January 1963 temperatures in Northampton fell to -15.7C. There was a brief respite in 1967, but 1968 and 1969 both saw below average temperatures.

The exceptionally cold decade of the 1960s ran on into the early 1970s. However, by 1976 temperatures were generally on the rise. 1976 had a noticeably dry and hot summer marked by frequent wildfires. The highest June temperature ever recorded in Northampton (34C) occurred that year. The next few years did, however, see much cooler temperatures although the trend of steady warming returned to the late 1980s.

During the 1990s, temperatures in Northampton became a little more stable fluctuating above and below the long term average. There were some notably cold years including 1991 and warmer years such as 1997.  The trend in the early years of the new century was for these annual fluctuations to become less extreme and for annual temperatures to settle at around 0.2C above the long term average. However, records from the past few years appears to suggest that we could be entering a new colder phase. Temperatures have been falling consistently since 2007 with 2010 being the coldest year since 1965.

Dendrochronology

Site preparation for a new junior school within the grounds of Pitsford Hall in April 2011 provided a unique opportunity to use dendrochronology to analyse past climate records and substantiate the local temperature record. Several trees needed to be felled, some of which were planted around the start of the local temperature record in the 1930s. The photograph shows the annual tree rings on one of these trees. The larger growth rings of more recent decades (from 1970) are clearly wider than those of earlier years reflecting the gradual warming trend. Furthermore, the dense pattern of rings of the 1960s shows perfectly the poor summers around this time. Nearer the centre, wider rings reflect the warm years of the late 1950s.

Approximate dates & corresponding tree rings on a recently felled tree at Pitsford Hall. The decadal fluctuation between warmer and colder years is clearly seen.

Temperature Trends - click on graphs to enlarge into a new window.

Article © Pitsford Hall weather station, 2011.

Review of 2011 and 2012 in the context of climate change in Northampton.