What do the hacked CRU emails tell us?

Earlier this week, the servers at the University of East Anglia were illegally hacked. Emails dating back to 1996 were stolen and leaked onto the web. Phil Jones, the director of the Climate Research Unit, has confirmed the emails are not forgeries although there is over 60Mb worth of material - they can't guarantee all of it is genuine. What does it all mean? Michelle Malkin labels it the global warming scandal of the century (of course the century is only 9 years old but even 'scandal of the decade' would be no mean feat). James Delingpole at the UK Telegraph claims the emails are the final nail in the coffin of 'Anthropogenic Global Warming'? So just what do these emails tell us?

Some of the emails must be embarrassing for the authors. One email responds in poor taste to the death of a well known skeptic. There's scathing discussion of skeptics such as Steve McIntyre and Roger Pielke, including imaginings of violence. However, the crucial question is whether these emails reveal that climate data has been falsified. The most quoted email is from Phil Jones in 1999 discussing paleo-data used to reconstruct past temperatures (emphasis mine):

"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."

What do the suggestive "tricks" and "hiding the decline" mean? Is this evidence of a nefarious climate conspiracy? "Mike's Nature trick" refers to the paper Global-scale temperature patterns and climate forcing over the past six centuries (Mann 1998), published in Nature by lead author Michael Mann. The "trick" is the technique of plotting recent instrumental data along with the reconstructed data. This places recent global warming trends in the context of temperature changes over longer time scales.

The "decline" refers to the "divergence problem". This is where tree ring proxies diverge from modern instrumental temperature records after 1960. The divergence problem is discussed as early as 1998, suggesting a change in the sensitivity of tree growth to temperature in recent decades (Briffa 1998). It is also examined more recently in Wilmking 2008 which explores techniques in eliminating the divergence problem. So when you look at Phil Jone's email in the context of the science discussed, it is not the schemings of a climate conspiracy but technical discussions of data handling techniques available in the peer reviewed literature.

In the skeptic blogosphere, there is a disproportionate preoccupation with one small aspect of climate science - proxy record reconstructions of past climate (or even worse, ad hominem attacks on the scientists who perform these proxy reconstructions). This serves to distract from the physical realities currently being observed. Humans are raising CO2 levels. We're observing an enhanced greenhouse effect. The planet is still accumulating heat. What are the consequences of our climate's energy imbalance? Sea levels rise is accelerating. Greenland ice loss is accelerating. Arctic ice loss is accelerating. Globally, glacier ice loss is acceleratingAntarctic ice loss is accelerating.

When you read through the many global warming skeptic arguments, a pattern emerges. Each skeptic argument misleads by focusing on one small piece of the puzzle while ignoring the broader picture. To focus on a few suggestive emails while ignoring the wealth of empirical evidence for manmade global warming is yet another repeat of this tactic.

Posted by John Cook on Sunday, 22 November, 2009

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