"Objection: The surface temperature record is full of assumptions, corrections, differing equipment and station settings, changing technology, varying altitudes, and more. It is not possible to claim we know what the “global average temperature” is, much less determine any trend. The IPCC graphs only say what the scientists want them to say.
Answer: There is actually some truth to the part about the difficulties; scientists have overcome many of them in turning the hundreds of thousands of measurements taken in many different ways and over a span of more than a dozen decades into a single globally averaged trend.
But this is the nature of science — no one said it was easy. It’s taken the scientific community a long time to finally come out and say that what we have been observing for 100 years is in fact exactly what it looks like. All other possible explanations (for example, the Urban Heat Island effect) have been investigated, the data has been examined and re-examined, reviewed and re-reviewed, and the conclusion has become unassailable...."
http://grist.org/climate-energy/the-temperature-record-is-simply-unreliable/
And another one:
http://www.skepticalscience.com/surface-temperature-measurements.htm