By Rosamund Pearce

Map showing average annual growing season length for 1961-90

The growing season for crops and plants in England over the past decade is around a month longer than it was during 1961-90, say scientists at the UK’s Met Office.

The number of days where temperatures dipped low enough for a frost has also decreased in recent years.

The changes are a “double-edged sword” for UK crops, another scientist tells Carbon Brief, and shows that farmers need to be able to adapt their choices of crop to the UK’s changing climate.

England’s growing season

The growing season is the part of the year when conditions are warm enough for plants and crops to grow. The season starts when average daily temperatures for five days in a row rise above 5C. It ends once five consecutive days fall back below 5C.

For the UK, the growing season is longer in the warmer parts of the country, such as southwest England, and shorter in the chillier areas of northern England and Scotland.

Average annual growing season length for 1961-90. The darker the green shading, the longer the season. Source: Met Office.

In their analysis, the Met Office uses data from the Central England Temperature (CET) record, which covers a roughly triangular area between the three points of London, Bristol and Lancashire. With temperature data back to 1659, the CET is the longest instrumental record in the world.

In the late 19th century, central England’s growing season was around 244 days. About a century later, between 1961 and 1990, this had increased by about a week, the Met Office says.

More recently, however, the growing season has lengthened even more, says Dr Mark McCarthy, head of the Met Office’s National Climate Information Centre:

“For the most recent 10 years between 2006 and 2015, the average growing season has been 29 days longer – at 280 days – when compared with the period between 1961 and 1990.”

A longer growing season is a “double-edged sword” for England’s farmers, says Prof Andy Challinor, professor of climate impacts at the University of Leeds, who wasn’t involved in the research. He explains to Carbon Brief:

“Whilst there is more time in which crops can grow, existing varieties will tend to mature more quickly, meaning the crops will tend to be smaller.”

How different crops respond to a longer growing season will also depend on a range of other factors, says McCarthy:

“[A]s with many things, the overall outcomes [for crops] will be dependent on a host of other factors, too, such as type of plant, what type of life cycle it has, length of day, amount of rain and sun, etc.”

Decreasing frosts

In addition to lengthening growing seasons in England, the UK as a whole is also seeing fewer frosts, the Met Office says.

Between 2006 and 2015, the average number of days of frost per year was 17% lower than during 1961-90.

These figures refer to “air frosts”, where the air around one metre above the ground dips to 0C or below. You can also get “ground frosts” when ice forms on surfaces, such as plants, car windscreens and pavements, that are at or below freezing. Ground frosts can occur even when it’s not quite cold enough for an air frost.

Average annual number of days with air frost for 1961-90. The darker the blue shading, the greater the number of frost days. Source: Met Office.

Even with rising average temperatures, the UK will still get cold spells that bring frosts, McCarthy says:

“Although we have fewer air frost days on average than we experienced a few decades ago, the numbers can still fluctuate from year to year. Many people will remember vividly the cold spells during 2010 and 2013.”

While fewer frosts is good for most farmers, it’s only one part of how climate change is affecting UK crops, says Challinor:

“Extreme weather, such as droughts and floods, is on the rise and poses a challenge to crops. So, the Met Office analysis is telling us that we need to continually adapt to our changing climate through our choice of crops and varieties.”

Increasing temperatures

Longer growing seasons and fewer days of frost are indicative of the warming that the UK has experienced over recent decades, says McCarthy:

“The overall rise in the growing season length is consistent with the observed warming across the UK of just under 1C since the 1960s.”

But year-to-year variations in the weather also play a role, he says:

“For example, a late cold snap in an otherwise mild winter, or an early cold spell in an otherwise mild autumn, could affect the growing season more than, say, the accumulation of days of air frost through a winter season, so we do need to look across decades for the slower signals of change.”

Both growing season and frost days are influenced by variations in a natural fluctuation known as the North Atlantic Oscillation (NAO), the Met Office says. When the NAO is negative, the UK tends to get a cold, dry weather from the east – and hence more frost days and a shorter growing season. When it’s positive, the UK gets more wet and windy weather from the west, reducing frost days and lengthening the growing season.

The NAO can flip-flop from one year to the next, either suppressing or amplifying the underlying warming from climate change, says Prof Adam Scaife, head of long-range forecasting at the Met Office. He tells Carbon Brief:

“In terms of magnitude, for a given winter, the impact of the NAO is comparable to or greater than climate warming so far, which is why winters like 2010 can buck the trend. However, the NAO is much more variable and changes sign from year to year whereas the climate trend is present every year.”

In the chart below, for example, you can see how central England (black line) and global (red) temperatures have changed since 1850.

Global and Central England temperatures since 1850, relative to 1901-200 Global (red line) and Central England (black line) temperatures since 1850, relative to 1901-2000. Source: Climate Lab Book.

Although the CET shows much larger variations from year to year, it tracks the longer term trends in global temperature, says Dr Ed Hawkins, associate professor at the University of Reading, who also wasn’t involved in the Met Office research. He tells Carbon Brief:

“Central England represents just 1/20,000th of the planet’s surface area, but its temperature has increased in a similar way to global temperatures, and about 20% faster than the global average.”

And even with the UK’s famously variable weather, the long-term trend of rising temperatures is obvious, Hawkins says:

“We can be confident that the main reason for the increase in Central England temperatures is the same as for the global average, namely increased emissions of greenhouse gases.”
Main image: Dirt track and wheat field at sunset, West Yorkshire, England, UK.

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