First, the data games: the data manipulation that has been most seized upon by bloggers involves the choice of which sources of temperature data should be used to reflect climate trends after 1960. Because thermometer-based measurements of the climate are only about 150 years old (and are quite spotty for much of that time), when scientists set out to construct long-term estimates of temperature trends, they use what are called “proxies,” such as tree-ring measurements that ostensibly reveal the temperatures that the tree experienced as it grew. As it happens, the tree-ring proxies match up with the thermometer measurements up until about 1960, when there is a “divergence” between the two sets of data. The tree rings indicate a global cooling after 1960, while the thermometer data indicates a sharp warming.
The CRU scientists decided to simply stop using the inconveniently non-warming tree-ring data after 1960, and splice the modern thermometer-based temperature readings instead, using statistical methods to smooth out and conceal the transition. In one email, this is discussed as a “trick” developed by Michael Mann, one of the creators of the infamous climate “hockey stick chart,” that would “hide the decline” shown by the tree rings and emphasize the recent spike in thermometer data, preserving the sanctity of the hockey stick. One problem with this is, if the tree rings don’t accurately reflect temperatures since 1960, why should we believe they accurately reflected temperatures in the past? If temperatures could diverge now, couldn’t they have equally diverged in the medieval warm period of 1,000 years ago? If so, current temperatures could be historically unremarkable, cutting away one of the key rationales for blaming human greenhouse gas emissions for recent climate changes.
There’s also the well-known problem in the thermometer record of an upward bias due to increasing urbanization around weather stations. Which is right, the trees, or the thermometers? Perhaps neither.
In another data manipulation discussion, one of the CRU researchers discusses changing the (arbitrary) baseline that is used to define “average temperature,” but is discouraged from doing so, as a less arbitrary baseline would reduce the appearance of global warming. About all we can say now is that it’s unclear that the public has been shown accurate reconstructions of historic temperatures, nor been given the context to understand whether recent climate changes are unusual or caused by human activity.That set of tangles alone should serve as a warning to Congresscritters willing to spend umpty-zillion tax dollars to "fix" the climate.