What are the questions?
So now, let’s consider what questions we have to answer to confirm this hypothesis — or better, what observations might disprove it.
Is there observable warming?
This is the obvious first step: if global warming is an issue, there has to be global warming. There seems to be general agreement that there has been effective warming, at least over the 300 or so years since the thermometer was invented and there was some agreement on temperature scales. On the other hand, the distribution of thermometers around the world was very sparse at first, and surprisingly sparse now; estimates of the global average surface temperature (GAST) still depend on statistical processes being applied to the measurements that exist. As a result, the estimates of GAST do vary from source to source — as we’d expect.
There is an additional confounding factor, though: those measurements themselves can be affected by systematic error, which is to say errors in the process by which the original measurements are made in a way that biases the results. As Anthony Watts and others have shown, siting changes — things like an air conditioner blowing its warm exhaust on a measurement station, or an asphalt parking lot being built where there had been an open field — have made a number of the surface stations in the United States questionable.
Is the Earth’s climate changing as a result?
Looked at narrowly, of course, this is a tautology: if the temperature is increasing, the climate is necessarily changing. The implication is that it’s changing in harmful ways. Again, there is certainly some agreement that the Earth’s overall climate has changed over the same 300 year interval. That 300-400 year interval since the invention of effective thermometers starts roughly at the depth of what is sometimes called the Little Ice Age, a period in which winters were significantly colder and harder.
There is less agreement about the magnitude of the change, and about how warm the preceding period, which is called the Medieval Warm Period or Little Climatic Optimum, really was.