There have been a lot of claims as to what the East Anglia emails are supposed to document; all of these claims are pretty much humbug, but I find the reasons why have not been presented very clearly either by scientists or by reporters.

The only claim pertaining to an actual scientific investigation is that of manipulating or hiding data, and the relevant quote from one of the emails is:

“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.”

Let me explain: There are two fundamentally different types of data that can be used to study climatic trends. Type one are direct instrumental measurements, of which we have tons for recent times, but going back further in time (say, more than one or two centuries), they get sparse. Direct measurements leave no doubt whatsoever about the occurrence of recent global warming.

Type two are indirect data (we call them proxy data or climate reconstructions). Here, a physical quantity that is affected in some way by temperature and preserved in some sort of long-lasting record is measured. Examples include ocean sediments, layers of ice in polar ice caps and other glaciers, or the now-infamous tree rings (and many more). As the term ‘reconstruction’ already implies, the relationships between temperatures and the measured physical quantities are not always straightforward, and methods are not always foolproof (but subject to constant improvements). Having said that, some of these indirect climate reconstructions are more reliable than others, but scientists do not underplay these uncertainties in publications such as the IPCC reports.

“Decline”: The decline in the email excerpt refers to the apparent decline in temperatures reconstructed from tree rings. Not measured temperatures. The latter clearly show global warming. So why use tree rings at all? Apparently, the discordance between tree-ring-reconstructed and measured temperatures only affects the time after about 1960. The reasons for this are, I believe, not known (but this is irrelevant for this discussion). Before c. 1960, tree-ring and measured temperatures are in agreement, which is why tree-ring data are occasionally used to corroborate data from earlier time periods, for which not as many instrumental measurements are available.

“Trick”: The trick here refers to a way of overcoming the known issue of the unreliability of tree ring reconstructions after c. 1960, namely by replacing them by instrumentally measured, and hence intrinsically more reliable, data. The email is claimed to refer to the preparation of a graph for the cover of the WMO statement on the status of the global climate in 1999. (This can’t be deduced from the email in question, though.)

I followed up the references in the graph, and it turns out that only part of the data are from tree rings at all.

In short, no decline in temperatures has been hidden, but unreliable data have been replaced by more reliable ones. More importantly almost, the whole discussion is about one single graph. There is an ocean of data and the work of many thousands of scientists which unmistakably shows that global warming is occurring, and also that it is caused by human activities.

As to the rest of the usually quoted emails, they have little to do with science and I will leave commenting on them for another time.