The global average temperature
is not merely hard to measure, but harder to define then it might seem at first.
One measure of the Earth's temperature used by the IPCC is based on an average of land and ocean temperatures over a long period of time and looking for changes. This is not always simple, since the methods for measuring ocean temperatures have changed, so it is hard to say if a small change is due to a change in ocean temperatures or measuring methods. On land, if temperatures are measured in different places a different average temperature is only to be expected - and if the same spot has gone from being in a remote country area to the middle of a city build up around it, that might well change the average temperature as well.
Temperature measurements from orbital satellites are believed to reflect the temperatures of layers of the atmosphere rather than the ground - but these are part of the Earth as well, and show differences.
Even those scientists who object to the Kyoto protocols and any efforts to fight global warming which might impact the economy in any way generally agree some warming has occured, despite all measurement questions they might raise.
Source: The Satanic Gases by Patrick Michaels and Robert Balling.