Tree rings: Chainsaws at dawn

Tree rings were thought to prove global warming – now climate-change deniers say they show the reverse. Both views are flawed, says Holly Williams

A A A

As every amateur naturalist knows, trees provide a record of their own history, in the pattern of rings seen in a horizontal cross-section. Trees growing at the same time will show similar patterns, and each year is distinctive enough to allow those who study tree rings – dendrochronologists – to date the rings, establishing chronologies stretching back thousands of years.

But trees tell us more than just when they were growing – they also reveal what the climate was like. Rings are formed as the tree grows, adding layers of new wood beneath its bark. How thick that layer – and how wide the resultant ring – depends on various factors, but most importantly the climate. A warm, wet summer will result in a wide ring; a cold summer or drought will produce a narrow ring.

Trees can therefore function as archives, invaluable sources of information for climatologists – and for those attempting to prove or disprove climate change.

And that's where it gets controversial. It turns out not all trees hold a reliable record of temperature. But as temperature-related data is hot property in the climate change debate, scientists who suggest that some tree data may not be helpful risk being accused of hiding important findings.

Last month, Queen's University Belfast was ordered by the UK Information Commissioner's Office to hand over data from 40 years of research into Irish tree rings to Doug Keenan. A City banker turned climate analyst – and climate change denier – Keenan believes the Irish tree-rings could provide evidence that there was a "medieval warm period" 1,000 years ago. If this were true, it would disrupt the notion that warming during the 20th century is unique and man-made.

Professor Mike Baillie, who collected much of the data, insists it is not suitable for use in the climate change debate. He explains why some trees – including those he studied – do not give clear, reliable temperature data. "Not all tree rings are the same. We live in a temperate climate, which means there's a limited amount of climate information you can get out of studying our trees – the climate they grew in is a balance of temperature and rainfall, it isn't extreme enough to allow the reconstruction of a single climatic variable. You get some information, but it appears to be mostly to do with rainfall – not temperature." Baillie adds that if you want accurate climate reconstructions, you have to go to places where the trees are responding to a controlling variable, such as summer temperature.

"You can extract climate information from trees – mostly pines – from more extreme environments, like northern Scandinavia and Siberia; trees from high altitudes and high latitudes where the summer warmth was very important." His tree rings just don't tell us anything very concrete about temperature, let alone confirm a medieval warm period.

It's not the first time that tree rings have been at the centre of a climate change furore. Tree ring data was at the heart of "climategate" last November, when hacked emails from the Climate Research Unit at the University of East Anglia allegedly revealed a cover-up.

Head of the unit, Phil Jones, famously wrote of "hiding the decline". But this was in reference to tree ring records, not global temperatures. The "decline" referred to the fact that recorded temperatures from reliable tree rings, broadly in line with thermometers since records began, started diverging around the 1940s.

Professor Keith Briffa, from UEA, explains that this "divergence" meant that "tree growth trends over the latter part of the 20th century, in areas where tree growth is known to be sensitive to temperature variations, were either stable or declined while measured temperatures increased".

This has led to climate scientists ditching recent tree ring data in favour of more accurate temperatures recorded by thermometers – a decision spun by sceptics as a general temperature-decline cover-up.

The real irony, however, is that it may even be because of climate change that tree rings stopped responding to temperature. Although no one has a definitive explanation, Baillie says that climate change has to be assumed to be a factor. "It is as if trees in areas where temperature was previously a controlling issue no longer regard it as such; they have enough temperature and hence their growth increasingly depends on some other variable. This could be regarded as worrying!"

Briffa says a lot more research needs to be done, but adds that "some people have proposed that erstwhile temperature-sensitive trees are becoming newly limited by lack of moisture as the climate warms".

Tree rings are undoubtedly a hugely valuable information source. "Not only should tree rings be used – they must be used if we are interested in recovering accurately-dated climate information year by year over long periods," insists Briffa. However, he adds that they must be carefully selected, from areas where a particular aspect of climate exerts a strong and unambiguous influence.

Not all tree rings lead to reliable climate reconstructions. And even some of those that used to, seem to have now – for mysterious and much-debated reasons – stopped responding to temperature. One thing seems certain though: sceptics are still likely to use trees to attempt to run rings round our perception of climate change.

News
Netherlands' goalkeeper Tim Krul fails to make a save from Costa Rica's midfielder Celso Borges during a penalty shoot-out in the quarter-final between Netherlands and Costa Rica during the 2014 FIFA World Cup
newsGoalkeepers suffer from 'gambler’s fallacy' during shoot-outs
News
people
Travel
travel
Arts and Entertainment
Sydney and Melbourne are locked in a row over giant milk crates
artCultural relations between Sydney and Melbourne soured by row over milk crate art instillation
PROMOTED VIDEO
Have you tried new the Independent Digital Edition apps?
Arts and Entertainment
Adèle Exarchopoulos and Léa Seydoux play teeneage lovers in the French erotic drama 'Blue Is The Warmest Colour' - The survey found four times as many women admitting to same-sex experiences than 20 years ago
filmBlue Is The Warmest Colour, Bojack Horseman and Hobbit on the way
News
Two giraffes pictured on Garsfontein Road, Centurion, South Africa.
i100
Environment
View from the Llanberis Track to the mountain lake Llyn
Du’r Arddu
environmentA large chunk of Mount Snowdon, in north Wales, is up for sale
News
Kenny Ireland, pictured in 2010.
peopleBenidorm, actor was just 68
News
i100
Arts and Entertainment
arts + ents
News
Morrissey pictured in 2013
people
News
people
News
A scene from the video shows students mock rioting
newsEnd-of-year leaver's YouTube film features playground gun massacre
News
i100
Life and Style
The director of Wall-E Andrew Stanton with Angus MacLane's Lego model
gadgetsDesign made in Pixar animator’s spare time could get retail release
Latest stories from i100
Have you tried new the Independent Digital Edition apps?
iJobs Job Widget
iJobs General

Financial Control Manager - Regulatory Reporting

£400 - £550 per day: Orgtel: Financial Control Manager - Regulatory Reporting ...

Lead Application Developer

£80000 - £90000 per annum + competitive: Progressive Recruitment: I am current...

Senior Networks Architect

£65000 per annum + 15% Pension, Health, Travel & Bonus: Progressive Recruitmen...

SAP BW/BO Consultant

£55000 - £65000 per annum + Benefits: Progressive Recruitment: SAP BW/BO CONSU...

Day In a Page

Dress the Gaza situation up all you like, but the truth hurts

Robert Fisk on Gaza conflict

Dress the situation up all you like, but the truth hurts
Save the tiger: Tiger, tiger burning less brightly as numbers plummet

Tiger, tiger burning less brightly

When William Blake wrote his famous poem there were probably more than 100,000 tigers in the wild. These days they probably number around 3,200
5 News's Andy Bell retraces his grandfather's steps on the First World War battlefields

In grandfather's footsteps

5 News's political editor Andy Bell only knows his grandfather from the compelling diary he kept during WWI. But when he returned to the killing fields where Edwin Vaughan suffered so much, his ancestor came to life
Lifestyle guru Martha Stewart reveals she has flying robot ... to take photos of her farm

Martha Stewart has flying robot

The lifestyle guru used the drone to get a bird's eye view her 153-acre farm in Bedford, New York
Former Labour minister Meg Hillier has demanded 'pootling lanes' for women cyclists

Do women cyclists need 'pootling lanes'?

Simon Usborne (who's more of a hurtler) explains why winning the space race is key to happy riding
A tale of two presidents: George W Bush downs his paintbrush to pen father’s life story

A tale of two presidents

George W Bush downs his paintbrush to pen father’s life story
Restaurateur Mitch Tonks has given the Great Western Pullman dining car a makeover

The dining car makes a comeback

Restaurateur Mitch Tonks has given the Great Western Pullman dining car a makeover
Gallery rage: How are institutions tackling the discomfort of overcrowding this summer?

Gallery rage

How are institutions tackling the discomfort of overcrowding this summer?
Louis van Gaal has £500,000 video surveillance system installed to monitor Manchester United players

Eye on the prize

Louis van Gaal has £500,000 video surveillance system installed to monitor Manchester United players
Women's rugby: Tamara Taylor adds fuel to the ire in quest to land World Cup

Women's rugby

Tamara Taylor adds fuel to the ire in quest to land World Cup
Save the tiger: The day America’s love of backyard tigers led to a horrific bloodbath

The day America’s love of backyard tigers led to a horrific bloodbath

With only six per cent of the US population of these amazing big cats held in zoos, the Zanesville incident in 2011 was inevitable
Samuel Beckett's biographer reveals secrets of the writer's time as a French Resistance spy

How Samuel Beckett became a French Resistance spy

As this year's Samuel Beckett festival opens in Enniskillen, James Knowlson, recalls how the Irish writer risked his life for liberty and narrowly escaped capture by the Gestapo
We will remember them: relatives still honour those who fought in the Great War

We will remember them

Relatives still honour those who fought in the Great War
Star Wars Episode VII is being shot on film - and now Kodak is launching a last-ditch bid to keep celluloid alive

Kodak's last-ditch bid to keep celluloid alive

Director J J Abrams and a few digital refuseniks shoot movies on film. Simon Usborne wonders what the fuss is about
Once stilted and melodramatic, Hollywood is giving acting in video games a makeover

Acting in video games gets a makeover

David Crookes meets two of the genre's most popular voices