Wednesday, 23 March 2016

On the plant-growing season in UK

In The Guardian today is an article titled "Plant-growing season in UK now a month longer than in 1990". Earlier springtimes are said to be responsible for the overwhelming part of the extension. So, it looks like good news for gardeners.

True, but it's also very very old news in terms of science. Scientists have been tracking the increase in England since 1980...

(Source: Thermal growing season in central England)

Scotland shows much the same upward curve. Though there it appears to have started more gradually, and twenty years earlier in 1960. Possibly the difference is partly also due to their lesser urban heat-islands, and less light-pollution at night...

(Source: An online handbook of climate trends across Scotland)

However, don't go taking today's 'news' literally and merrily planting your potatoes in mid-February. Take a look at the savage ups-and-downs of the actual recorded growing season lengths on those graphs, rather than at the computer-smoothed statistical trend line. Those jaggies reflect what gardeners know all too well, living on an island so sensitive to the warmth-giving powers of the North Atlantic Drift (aka 'The Gulf Stream'). We know that an essentially random tweak of an Atlantic ocean current, or a twist in a vortex of Arctic winds, can often decide the arrival date of spring. As H.W. Linderholm concluded, after surveying all the scientific literature on the topic to 2005...

"... several authors have found strong associations between large scale weather phenomena (e.g. the NAO [North Atlantic Oscillation]) and growing season variability, suggesting that global warming may not be the only explanation. Also, other factors like land use change may have been of importance. It must be kept in mind that the observed changes in GSL [growing season length] are not uniform; while increased GSL has been observed in low-to-mid latitudes, the GSL seems to decrease at some high latitude and altitude sites." ("Growing Season Changes in the Last Century", Agricultural and Forest Meteorology, March 2006).

How does it affect the local wildlife? Well, our hardy wildlife has evolved to be highly adaptable to the variable timing of the arrival of spring. It thus seems unlikely that such natural variability will phase them, especially given how long it's been happening — the graph above shows it happening long before the Industrial Revolution. In our own time, a few extra years (per decade) in which there's an earlier arrival of a warm spring, and/or an extended Indian summer in Autumn, would seem to me to benefit almost all species.

Much the same can be said of the apparent trend toward slightly wetter and slightly milder winters in the UK, as that's also broadly beneficial since it prevents drought (at the cost of a little localised temporary flooding). According to the peer-reviewed papers, the rainier winters are due to natural multi-decadal variability that has been going up and down — with no significant trend-pattern — for the last 150 years. We're currently in a decade-long wet patch, after a run of dry years when there were alarmist headlines that the UK was now living in "permanent drought". But we'll come out of it and dry off again. Like the growing season shifts, such rolling multi-year shifts from wet to dry seem to be due to essentially random tweaks of an Atlantic ocean current, which then does or doesn't funnel winter storms toward the British Isles.

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