Update On Thermal Growing Seasons
By Paul Homewood
Last week, I showed this graph from DECC, analysing thermal growing seasons.
Top recap, the data is from CET and according to DECC:
The last report published by DECC only took us up to 2012, so I asked them if the data was still being maintained.
It is pleasing to report that I got the following very prompt and full reply:
As far as I can make out this data used to be updated every January but stopped when the person who did the work left the department about 3 years ago and it seems to have been completely forgotten. I think it is an interesting dataset which is also reasonably long term and I might do an update when all the 2016 data is in. In the meantime the growing season lengths in 2013, 2014 & 2015 (which are all provisional as they were obtained manually) appear to be 228 days, 336 days and 303 days respectively. 2013 is well below the 1961-90 average (252 days) because of the very cold spring of that year, 2014 is slightly above the previous highest value of 330 days (in 2000) due to the absence of any sustained cold spell in the first few months of that year; it was not until early December that the first 5 day cold spell appeared. 2015 only exceeded 300 days because of the freakishly warm December in 2015, otherwise it would have been fairly average. The initial data for 2016 suggests a possible return to normality depending on what happens in the autumn.
I see that you recently posted up an article about this dataset on your blog. Regarding Dorian’s comment suggesting a lack of correlation between DECC’s growing season curve and a 20th century UK mean temperature graph from another post, I don’t think that is the case. The latter graph includes January & February, months which don’t normally figure in growing season statistics, though it can happen in February. Met office data show these 2 months were on a downward trend up to about the 1950s with only a slight recovery up to about 1980. Spring & autumn on the other hand warmed up throughout the 20th century apart from a dip in the 1960s & 70s. These trends appear to have been captured by DECC’s growing season curve. On NeilC’s comment criticizing the use of temperature as the sole criterion, I am not a plant expert so I can only speak for myself but it seemed to be good enough for HH Lamb who knew a thing or two about climate variation (Lamb used 6 degrees as the threshold compared to DECC’s 5 degrees according to the blog’s extract from his book). The other factors mentioned in the comment undoubtedly have an influence, especially in greenhouses, but I would suggest that at the start and end of the growing season in the UK natural environment temperature is the critical factor.
Climate change statistics
Just to pick up on one of Paul’s points, the 1981-2010 average is 270 days, implying a growing season of roughly mid Feb to mid Nov. DECC state that the increase since 1980 is largely due to the early onset of spring, which would indicate warmer Februarys, rather than warmer springs.
This would appear to correlate with the Met Office temperature record for February, showing a sharp uptick from the mid 1980s. It is clear that this is predominantly due to the lack of any really cold months since 1986, and that the warm months are not getting any warmer.