Spring Temperatures & First Flowering Dates In The UK
By Paul Homewood
My attention was drawn to the above paper today which was originally published in 2010. As the title infers, it investigated first flowering dates in the UK, and came to the conclusion that flowering has been getting earlier, as temperatures in February to April have increased.
This is the Abstract:
Widespread concerns about global biodiversity loss have led to a growing demand for indices of biodiversity status. Today, climate change is among the most serious threats to global biodiversity. Although many studies have revealed phenological responses to climate change, no long-term community-level indices have been developed. We derived a 250-year index of first flowering dates for 405 plant species in the UK for assessing the impact of climate change on plant communities. The estimated community-level index in the most recent 25 years was 2.2–12.7 days earlier than any other consecutive 25-year period since 1760. The index was closely correlated with February–April mean Central England Temperature, with flowering 5.0 days earlier for every 1°C increase in temperature. The index was relatively sensitive to the number of species, not records per species, included in the model. Our results demonstrate how multi-species, multiple-site phenological events can be integrated to obtain indices showing trends for each species and across species. This index should play an important role in monitoring the impact of climate change on biodiversity. Furthermore, this approach can be extended to incorporate data from other taxa and countries for evaluating cross-taxa and cross-country phenological responses to climate change.
The key graph, which tells the story, is this one:
The median (red line) and 95% credible intervals (grey area) of the estimated community-level index (day of the year) showing a temporal change in the timing of first flowering shared by 405 plant species observed throughout the UK. The black line indicates the mean for every 25 years and the dotted line that for the most recent 25 years. The years without estimates indicate those without any observation records (1766, 1813, 1814 and 1817).
As the index value falls, this indicates earlier flowering.
We can see how flowering has moved forwards in the last 25 years. But what is clear is that we are not seeing a simple, straight trend.
In particular, there is very little difference between the current period and that of 1910 to 1934. In between, there is a period of much later flowering. It is also noticeable that flowering was relatively early during the first 25-year period in the 18thC, followed by a persistent trend to later flowering up to the mid 19thC (although error margins are much larger in those earlier years)..
In fact, there should be very little surprise about this, as HH Lamb wrote about the subject in 1982, in “Climate, History and The Modern World”:
Lamb was clear that growing seasons were significantly shorter in the 1960s and 70s, than they were a few decades earlier, largely due to colder springs.
But note the comment about Oxford, that onset of spring was 16 days earlier compared to the 1920-50 period. This is a much bigger difference than the one showing in the Amano paper – they suggest something like only six days.
This surely reinforces the contention that there is actually very little difference between now and that earlier 20thC era.