Beware Proxy Problems and Correlation Cons

I receive email notifications of new posts on the Realclimate blog, a forum for discussion of the science of climate change, run by real climate scientists! Usually there is one post every few days, so I was slightly surprised to be notified about a second post last Friday (20th).

I was stunned when I saw that the second post was on the topic of the release of internal emails from the Climate Research Unit (CRU) at the University of East Anglia (UEA) in Norwich, England.

A single passage in tens of megs of emails has become the focus of the mudslinging. The Realclimate guys have this to say:

“No doubt, instances of cherry-picked and poorly-worded ‘gotcha’ phrases will be pulled out of context. One example is worth mentioning quickly. Phil Jones in discussing the presentation of temperature reconstructions stated that ‘I’ve just completed Mike’s Nature trick of adding in the real temps to each series for the last 20 years (ie from 1981 onwards) and from 1961 for Keith’s to hide the decline.’ The paper in question is the Mann, Bradley and Hughes (1998) Nature paper on the original multiproxy temperature reconstruction, and the ‘trick’ is just to plot the instrumental records along with reconstruction so that the context of the recent warming is clear. Scientists often use the term ‘trick’ to refer to a ‘a good way to deal with a problem’, rather than something that is ‘secret’, and so there is nothing problematic in this at all. As for the ‘decline’, it is well known that Keith Briffa’s maximum latewood tree ring density proxy diverges from the temperature records after 1960 (this is more commonly known as the ‘divergence problem’–see e.g. the recent discussion in this paper) and has been discussed in the literature since Briffa et al in Nature in 1998 (Nature, 391, 678-682). Those authors have always recommend [sic] not using the post 1960 part of their reconstruction, and so while ‘hiding’ is probably a poor choice of words (since it is ‘hidden’ in plain sight), not using the data in the plot is completely appropriate, as is further research to understand why this happens.”

I’m afraid the explanation of the use of the word “trick” makes me squirm! And to say the data were manipulated “to hide the decline” is unfortunate to say the least. The Guardian notes that “[t]he scientists [sic] who allegedly sent it [the ‘trick’ email] declined to comment on the email.” Well, if you ask me, they ought to be commenting, PDQ.

The ought to comment, because it’s important to get to the bottom of the issue. I have no doubt we are seeing spectacular climate change. What bothers me, though, is how little we know about past climates.

A couple of weeks ago I mentioned my puzzlement that the scientists are now saying that temperatures were considerably (3-4C) higher than at present during the last interglacial. Then, last week, I read this in the Telegraph:

“Louise Sime, lead of the British Antarctic Survey study, looked at ice cores to see how temperatures changed during periods of high carbon dioxide[.]

She found that during the last period of high CO2, 125,000 years ago [125 kya], temperatures were up to 6C higher than present day levels.

Such a hike in temperature could lead to a rise in sea levels of between 4 to 6 metres over hundreds of years as the ice sheets melt.

‘We didn’t expect to see such warm temperatures, and we don’t yet know in detail what caused them. But they indicate that Antarctica’s climate may have undergone rapid shifts during past periods of high CO2.’

Dr Sime said the study suggests that current high levels of CO2 could also cause a rise in temperature. She said further research could predict the affect on sea level rise.

‘If we can pin down how much warmer temperatures were in Antarctica and Greenland at this time, then we can test predictions of how melting of the large ice sheets may contribute to sea level rise.’ “

It might be worth pointing out that the “high CO2” 125 kya was nowhere near as high as it is now – 300ppm tops, compared to ~390ppm today (and the other greenhouse gases [GHGs] we’ve emitted make the present situation even worse).

The point is that if the climate system is more sensitive to elevated CO2 levels than we think, we have to revise our targets, as I’ve pointed out before.

If the more recent Medieval Warm Period (MWP) and Little Ice Age (LIA) were real events then we need to find out exactly what caused them. I suspect changes must have been triggered in the ocean circulation. Maybe we simply haven’t yet been cooking the planet long enough to disturb the system, or maybe, as I suggested before, the continued warming counteracts the planet’s normal negative feedback response to a period of warming.

What bothers me about the CRU leak is that it makes no sense at all to me to use a proxy for the temperature record when you actually have an instrumental record (or can even construct a record from historical documents). The instrumental record should be used to determine which proxies are valid for dates earlier than you have records for. It sounds as if the link between one possible proxy (Briffa’s tree-rings) and temperature doesn’t hold, so that particular proxy should simply be discarded altogether, not just for the period from 1961.

Lots of proxies have been used to reconstruct past temperatures, which is clearly a seriously complex and difficult exercise. Maybe the scientists need to explain a little more clearly exactly what these “temperature” series tell us.

The CRU hack controversy is a bit of a shame because it’s completely overshadowed the earlier Realclimate post on Friday. A Problem of Multiplicity currently has just 28 comments compared to 913 and counting for The CRU Hack, but in fact makes a much more important point.

If I interpret it correctly, A Problem of Multiplicity basically points out that if you compare enough sets of data you’re bound to find some correlations. This is not entirely disconnected, of course, from the problem of reconstructing past temperature records, though the area of research being criticised is the persistent attempt (often associated with a global warming scepticism agenda) to identify possible effects of solar cycles on climate.

I remembered A Problem of Multiplicity when I read Ben Goldacre’s Bad Science column in yesterday’s Guardian. It’s obvious, now Realclimate has pointed it out, that just by chance any drug is going to be associated with some side-effects. What’s needed is to use this initial detection as a hypothesis, and examine an entirely different set of patients to see if a statistically significant correlation is found. Tricky business. Maybe the NHS (or other organisation representing patients, not Big Pharma) should provide a website detailing exactly how possible side-effects have been determined. Because if you worried about everything listed on the leaflet in the packet you’d never take anything.

Back to climate. I don’t envy the scientists their job in trying to get their message across. I’m beginning to suspect the message needs to be a lot simpler. Especially when they’re up against this sort of thing from David Bellamy:

“I’m sceptical about man-made climate change. There’s absolutely no proof that carbon dioxide will kill us all. It’s not a poison, it’s the most important gas in the world. Carbon dioxide is an airborne fertiliser. How can farmers grow increasing amounts of food without a rise in CO2?”

Quite easily. Plant growth is rarely limited by CO2 availability, since they have adapted to the level that’s been in the atmosphere for the past 20 million years or so. Much better ways of improving plant growth are to improve the availability of other factors, e.g. water and mineral nutrients.

As I see it, responsible citizens have a choice. They can either accept the scientific consensus or they can delve deeply into the science themselves. I’m afraid I don’t see a lot of middle ground.

If people do decide to get to grips with the science they won’t be unduly alarmed by the dumb things scientists sometimes do. Just like the rest of us. Let alone the myriad mistakes regularly made, just as a random example, by economic policy-makers, their political masters and, of course, bank executives.

Informed responsible citizens will also realise that science is never the finished article, but continually evolving. Quite interesting really.

Unfortunately, as the science gradually changes, so must policy. And it seems to be becoming fairly clear that our targets for safe CO2 (and other GHG) levels are far too optimistic.