1. Claiming that weather stations are placed in locations that produce warming
The main proponent of this view is Anthony Watts at his surfacestations.org site.
Watts uses photographs in an attempt to demonstrate his claim but two papers have been written analysing the US temperature record that completely demolish Watts' claims. Discussion of the papers can be found here and here.
2. Claiming that the stolen CRU emails prove that climate scientists have deliberately manipulated the temperature record to exaggerate the warming trend.
Here is one of many examples:
Perhaps the most infamous example of this comes from the "hide the decline" email. This email initially garnered widespread media attention, as well as significant disagreement over its implications. In our view, the email, as well as the contextual history behind it, appears to show several scientists eager to present a particular viewpoint-that anthropogenic emissions are largely responsible for global warming-even when the data showed something different.
The climate scientists were actually discussing a technical issue in dendrochronology (study of tree rings)called the divergence problem. See here and here for details. Climate deniers who mistakenly believe that the planet has been cooling over the last decade or so, quickly assumed that the scientists were trying to hide this, nonexistent, temperature decline.
There have been three enquiries resulting from the campaign of the deniers against the CRU scientists. All three have exonerated the scientists involved, see here, here and here. What is the denier response to this thorough debunking of their claims? You might expect that honest people would apologise in such circumstances, but that is not the denier way. The response to the exhonerations has been to claim that the people making the reports are part of the conspiracy, as demonstrated in this post, by Australia's foremost climate denier. Such people believe in conspiracies wrapped up in conspiracies, and people treat such twaddle seriously. Weird!
Stephan Lewandowsky has written an interesting article on conspiracy theories including the truthers and AGW deniers. The most interesting thing about Lewandowsky's article is the number of conspiracy nuts who have come out in the comments.
Dr Roy Spencer, runs the UAH satelite temperature record. Spencer is a well known climate skeptic. One of the major targets of the attacks on scientists and the temperature record is Phil Jones, who runs the HadleyCRU temperature record. At a recent Heartland Conference Spencer defended Jones in the following terms:
"He says he's not very organised. I'm not very organised myself," said Professor Spencer. "If you asked me to find original data from 20 years ago I'd have great difficulty too.
"We just didn't realise in those days how important and controversial this would all become - now it would just all be stored on computer. Phil Jones has been looking at climate records for a very long time. Frankly our data set agrees with his, so unless we are all making the same mistake we're not likely to find out anything new from the data anyway."
(Source here paragraphs 3 and 4 after the heading Cloud Effect)
The final two reports on the climategate "scandal" are now in, see here and here. There have now been five enquiries about the CRU email issue. All five have exonerated the scientists involved and pointed out the untruths and misrepresentations put around by the global warming denial industry. Will we see the climate deniers retract their untrue claims and apologise for their mistakes - I wouldn't hold my breath!
There have been many posts on the anniversary of the publishing of the stolen emails; for examples see posts here here.
The National Science Foundation (NSF)has conducted a very thorough review of the allegations about the behaviour of Michael Mann (of hockey stick fame). In the report that can be found here he was completely exonerated from any implication of impropriety or research misconduct.
For more evidence on the similarity of the various temperatrue records see this post.
3. Claiming that weather stations in colder locations have been deliberately removed from the temperature record
Watts and D’Aleo make the following claim in this document on page 9.
Around 1990, NOAA lost more than three-quarters of the climate measuring stations around the world.
It can be shown that country by country, they lost stations with a bias towards higher-latitude, higher-altitude and rural locations, all of which had a tendency to be cooler.
This claim has been widely debunked here, here, here, and here.
At this link Roy Spencer states:
... at face value, this plot seems to indicate that the rapid decrease in the number of stations included in the GHCN database in recent years has not caused a spurious warming trend in the Jones dataset — at least not since 1986.
All of these attacks on mainstream climate science show strong elements of paranoid conspiracy theorising. It is clear that the vast majority of people qualified to make a judgement accept that the planet is warming and that most of the warming in the last 50 years has been induced by human activities - anthroprogenic global warming (AGW). For evidence of this claim see this post. The making of paranoid claims that the climate scientists are conspiring to distort reality for left wing political aims, is one of the few responses available to those who want to deny the reality of AGW. The response to the official exhonerations of the CRU scientists shows that deniers not only believe in conspiracies but conspiracies wrapped up in conspiracies.
In more rational eras the crazy claims of the climate deniers would be laughed at by most people, who would then move on to more productive pursuits, but irrationality seems to be rife in the world at the moment. See here and here for some ideas on the current irrationality.
It is clear from an analysis of the climate data that the five main temperature data sets are quite similar, when properly analysed, see this post for details. The Hadley / CRU time series sits squarely in the middle of the range of results. If the CRU scientists had been deliberately manipulating the data to enhance the warming then the Hadley / CRU data would be an outlier. Clearly this is not the case.