Climate skeptics – future’s visible foe

Last week, I posted a blog – Climate change: history’s invisible foe – about a paper published in Science journal that links temperature changes with human upheavals over the last 2500 years. An individual calling themselves ‘Boballab’ commented on this post raising important issues. Today, I will try to allay Boballab’s concerns.

Boballab: I noticed that you neatly skipped over the divergence problem in tree ring proxies:

That’s right, there’s no reference to a ‘divergence problem’ in my piece. However, there’s a good reason for this, which the paper’s lead author, Dr Ulf Buentgen, puts much better than I can, “We did not observe evidence of the so-called ‘divergence issue’ in our data,” he says.


“Two roads diverged in a yellow wood” – or did they?

Not that the ‘divergence problem’ isn’t important: it’s a phenomenon that’s been openly debated by scientists all over the world since the 1990s and is also a darling of those who argue that climate change is not happening.

The ‘divergence problem’ is the observation that, while tree ring widths matched thermometer temperatures closely from 1880 to 1960, there has been a drop in tree ring widths in northern forests and a rise in temperatures since 1960. Climate change sceptics argue that, if tree rings and temperatures have diverged since 1960, then we cannot trust tree rings to calculate past temperatures.

However, sceptics sometimes miss an important 2004 study led by Dr Edward Cook of the Lamont-Doherty Earth Observatory, USA. It showed that tree ring records closely matched temperature observations at least as far back as 960 AD. This means that the divergence since 1960 is unprecedented in over 1000 years.

So is the ‘divergence problem’ actually our fault? Are trees not growing like they used to because we’ve created an unpleasant environment that trees don’t like? Many studies internationally suggest that this is the case; pointing fingers at global warming, acid rain, sulphur dioxide emissions and ozone depletion.

Boballab: You state that Tree Ring data up till 1900 deviates no more than 2° from average but you do not state exactly what type of data you are talking about for the late 20th century.

All the data used in the study to measure temperature and precipitation were derived from tree rings.

Boballab: You also over simplified what makes tree rings wider. Not only does temperature changes, change width so does: Precipitation, CO2 concentration, Available Sunlight, Local Animal Behavior (ie did a bear do his business on the trees roots) and forest fires.

Buentgen et al’s paper discussed both precipitation and temperature. I focused purely on the temperature signature because I felt it to be the most compelling and news-worthy. For precipitation, 7284 samples were used, gathered from three sites across France and Germany. The data from these were averaged, so any signals from local variations – like sunlight, forest fires and the odd proverbial bear – would be ironed out.

For temperature, over 1500 samples were used from two different tree species across a variety of sites in the Austrian Alps and surrounding area. Again data were averaged to remove local signals.

I confess I did make a mistake in last week’s blog; I suggested that the temperature data came from oak. However, temperature data actually came from European larch and stone pine: the precipitation data came from oak.

Again, my thanks to Boballab for the comments, I learned a lot from the challenge.

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