(Posted by S2)BPSDBInspired by a comment by guthrie, I thought I’d take a look at the seasons here in Scotland. Are they changing as guthrie claims, or is this just natural variability being taken out of context?
It would seem that he is correct, according to the Scotland & Northern Ireland Forum for Environmental Research (henceforth referred to as SNIFFER).
In their publication An online handbook of climate trends across Scotland, they state:
- All regions have seen an increase of more than four weeks in the length of the growing season since 1961.
- The increase in the length of the growing season is part of a clear trend in all regions.
- The West has shown the greatest increase and the North the least.
- The greatest increases in the length of the growing season are in coastal areas and the Shetland Islands where the season has extended by two months, or more.
They also point out that
- The length of the growing season has changed very little since 1961 in some of the more mountainous areas.
- A few upland areas show a reduction in the length of the growing season of up to eight days.
(A caveat: the study only uses data up to 2004, so it doesn’t verify guthrie’s claim that Spring has arrived in February for the last three years).
So what is the reason for the longer growing period?
No surprise here – it is closely correlated with temperature. Scotland is getting warmer. And the report points out that for the period of the study (1961 to 2004) temperature trends are higher in Spring and Autumn (Fall) than the other two seasons – i.e. at the start and end of the growing season.
What other changes have there been?
Given the amount of snow we’ve had in the last few days it is interesting to see that we have seen a 32% reduction in the number of days of snow cover. By far the biggest change, though, is Autumn snow cover which has declined by over 70% during the period. Winter and Spring cover is around 30% down – but due to the high interannual variability SNIFFER don’t attribute any statistical significance to these two seasons.
Since Scotland has suffered from serious flooding quite a lot in recent years (my part of the country was hit twice this year, in September and November) I wondered if there were any statistical analyses on scottish floods – but I have not found any. I guess that there probably isn’t a significant trend at the moment.
I do know that the Highlands Council (a dour lot, they are unique in still using Gaelic on their website) decided in 2007 to divert cash from winter road treatment to flood alleviation, in recognition of the fact that the winters are getting warmer and flooding seems to be on the increase in their neck of the woods.
However we have seen changes in precipitation during the study period – on average we are now 20% wetter than we used to be.
There are regional and particularly temporal differences – Winter precipitation has increased by 58% overall, but Northern Scotland has seen an increase of 70%. In some areas of the west Highlands and the Hebrides winter precipitation has more than doubled since 1961.
(Changes in the other three seasons are much lower, and are not statistically significant).
SNIFFER don’t make any.🙂
This makes sense – Scotland is tiny on a global scale, so what happens here is not representative of the world as a whole (and vice versa). In the Hadley GCM03 model Scotland occupies just 2 grid boxes, which is far too few to resolve (e.g.) our many mountainous areas.
SNIFFER do compare their findings to the output of the regional climate model, with a much smaller resolution of 50 km (as against the GCM resolution of 300 km). In general they are a good fit, but there are some areas that don’t quite match up. In particular:
- On the growing season changes, the UKCIP02 scenarios suggest the increase will be greater in the east than in the west, but the information from observations shows the opposite pattern.
- The models agree that winter precipitation should be heavier, but suggest that summer precipitation should be less – and that isn’t what we’ve seen so far.
For me, the take home message is that regional/local changes can be large. A 70% reduction in Autumn snow cover or a 60% increase in winter precipitation is a lot. If you are living in an area where resources are already stretched, then even a small change can take you past your breaking point.
On a lighter note – the Met Office have a severe weather warning out for most of Scotland, Wales and South-West England: Widespread Icy Roads & Heavy Snow.
I can’t actually remember the last time I saw a white Christmas.🙂
If you celebrate, then have a good Christmas – and if you have one, may your God go with you.
All graphs from An online handbook of climate trends across Scotland
All other images from The BBC
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