(Part of the How to Talk to a Global Warming Skeptic guide)

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.

And while it is true that differing weather station locations, from proximity to lakes or rivers or elevation above sea level, probably make it impossible to arrive at a meaningful figure for global average surface temperature, that is not what we are really interested in. The investigation is focused on trends, not the absolute level. Often, as in this case, it is easier to determine how much a given property is changing than what its exact value is. If one station is near an airport at three feet above sea level and another is in a park at 3000 feet, it doesn’t really matter — they both show rising temperature, and that is the critical information.

So how do we finally know when all the reasoning is reasonable and the corrections correct? One good way is to cross check your conclusion against other completely unrelated data sets. In this case, all the other available indicators of global temperature trends unanimously agree. Go ahead, put aside the direct surface temperature measurements — global warming is also indicated by:

All of these completely independent analyses of widely varied aspects of the climate system lead to the same conclusion: the Earth is undergoing a rapid and substantial warming trend. Looks like the folks at NASA and CRU know what they are doing after all.