Brian Henderson: More to climate change than global warming

For some in regions which are notoriously cool and damp – like ourselves – that might have painted a picture of slightly warmer summers which, to be frank, many of us probably wouldn’t really object to the temperature rising by a degree or two.

Most people who have been following the story a bit more closely though will, of course, be aware that the rise in average temperatures is exactly that – and the extremes cover the ‘head in the oven and feet in the freezer’ type of average with different parts of the world showing swings in different directions at different times of year.

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But, writing this at the beginning of this week as the clear up and sort out was still underway after Storm Arwen, the message that climate change is also likely to deliver not only more sudden swings in the weather but also for these to be more extreme in either their effect, extent or duration has also been pretty well driven home.

Further afield, the torrential floods across many of the grain growing areas of Australia have been another example of this phenomenon in recent weeks, with a country which is more prone to suffering from parched conditions and drought seeing some real extremes of rainfall at harvest time – enough indeed to have had a considerable knock-on effect in the wheat market.

So, in recent years scientists have spent a lot of time analysing events such as floods, heatwaves, droughts and storms, logging these ‘extreme event’ attributions. Many such occurrences which would previously have been likely be a one-in-a-hundred or one-in-two-hundred year event seem to be happening every few decades, or even years.

This has been pretty widely recognised not only by scientists, but also in the media and public imagination – and I guess it’s beginning to dawn on us that we’ve all got our own personal experiences of weather which we could link to the more abstract concept of climate change.

From studies of heatwaves in Sweden and droughts in South Africa, to flooding in Bangladesh and hurricanes in the Caribbean, the results have pointed to mounting evidence that human activity is raising the risk of some types of extreme weather, especially those linked to heat such as the conditions which have seen a huge increase in wildfires across the globe in recent years.

An analysis of extreme-weather showed that of 122 studies that looked at extreme heat around the world, 92% found that climate change made the event, or trend more likely or more severe. For 81 studies looking at rainfall or flooding, 58% found human activity had made the event more likely or more severe. For the 69 drought events studied, the figure was 65%.

So, with farming standing in the front line, our feeling that weather events are becoming more extreme in both their extent and intensity have been pretty well backed up by science.

However, some recently published research has taken the effects of differences in weather events one step further and highlighted yet another effect – that of changes within long-standing weather patterns which can have a highly significant effect, not only on the overall climate but also on the health and yields of crops growing under these differing weather patterns.

Shifts in more distant weather patterns – such as monsoon failures in the Far East – have had a considerable effect on some countries in that particular area to feed their populations and prompted famine in past decades might be the more obvious and better publicised case.

But new research based on UK weather statistics highlighted that even here subtle changes in patterns have had a significant effect on grain output.

After analysing 124 years of crop yields obtained from long term experiments, researchers working at the Rothamsted crop research centre in England found that weather conditions typical of the last 30 years had significantly limited harvests of winter wheat and spring barley.

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Looking at monthly summaries of daily temperatures, rainfall and sunshine hours for one site, they identified 10 distinct ‘clusters’ of yearly weather patterns that occurred from 1892 to 2016.

The generalised trend picked up from the work showed that the annual weather cluster most dominant after 1991 generally had higher summer temperatures and more intense winter rainfall distributions but with a particularly dry June. It was this weather pattern which was also associated with the lowest cereal yields.

Unsurprisingly, though, given their differing requirements, hay yields from pasture under the same weather pattern were not impacted to the same extent though.

The scientists from Rothamsted used data collected from the weather station and crop experiments first set up at their Hertfordshire institute in the 19th Century and are now claiming that their analysis of weather patterns and associations with crop yields is more relevant and realistic than solely looking at the effect of average temperature.

They found that five annual weather patterns which occurred frequently during much of the 20th Century and which had been conducive to good cereal yields had not recurred in more recent years, and, according to lead author, Dr John Addy: “This provides unambiguous evidence of climate change in several more dimensions than mean temperature alone.”

He said this highlighted the fact that climate change was about much more than just increasing temperature: “The long-term trend of annual mean temperature is only one aspect of recent climate change. Other changes in climate, seen in changes to within-year weather patterns relevant to crop production, have also occurred since the late-19th Century.”

Understanding simultaneous changes across multiple weather variables provides an understanding of how the climate has changed for agriculture, and potentially how this influences yield, he pointed out.

The team used complicated statistical analysis to provide insight into the different patterns of variation in multiple weather variables during a year, the variation in these patterns amongst years, and how the frequency of the different patterns has changed, particularly in the last 25 years.

The frequency of occurrence of years within each weather cluster varied considerably during the study period – but researchers did notice that the late 20th and early 21st Century had distinctly different distributions across clusters to earlier in the 20th Century.

Three clusters (‘Cold winter and early-spring’; ‘Cold August to September’; ‘Cool and dry March’) contained 64% of years during the 20th Century, whilst one cluster, ‘High temperatures, drier June’ contained 63% of the 21st Century years included in the analysis.

The data for the study came from the Rothamsted Meteorological Station, which had recorded daily rainfall, temperature and sunlight since 1892, and Rothamsted’s famous long-term experiments with winter wheat, spring barley and pasture grasses, which began even earlier in the mid-19th Century.

Analysis of the annual yields of winter wheat, spring barley, and grassland across the different weather clusters showed that, on average, years in clusters typical of the 20th Century climate provided greater off-take of cereals than those from the early-21st Century.

Years with a cooler winter and early-spring provided the greatest winter wheat yields, whilst drier years with a warm early-summer provided the highest spring barley harvests.

So, that effectively means that winter wheat and spring barley yields were greater under typical 20th Century than 21st Century climate conditions – so we might need to be asking ourselves of we should be building such ‘drifts’ in weather patterns into our plans to ensure greater resilience in future varieties and cropping systems.