What if global warming were natural? ...

Is a question that David Appell put, and that was picked up by William at Stoat.

There are two simple rules that govern such questions. The first is that it is easy to construct a question that has no meaningful answer. While the concept of global warming is reasonably clear, natural is so vague that it makes the question "What if global warming were natural" fodder for stupid answers, which appear to be in plentiful supply.

The second rule is that to ask a meaningful question requires knowing a lot about the answer. So let us proceed to break down the situation.

There is one simple answer which is that if there are other factors which overwhelm what people are doing to the climate system, then our understanding of the effect of greenhouse gases, global albedo and many other things is not just off, but drastically wrong. This is quite the claim and there better be a strong proof before anyone asserts any such thing without expecting to be laughed out of the house.

The next step is to differentiate between the various possible things that could lead to global warming and are not controlled by man. For example:

Can global warming be attributed to changes in solar irradiance? (Answer: some part can, but the measured changes in solar irradiance are not sufficient, especially over the last thirty years when direct satellite measurements of solar irradiance are available).

Well then, are there indirect mechanisms which could amplify the observed change in solar irradiance? (Answer some have been proposed, but they have not held up. This includes various cosmic ray dreams)

We can also rule out effects due to orbital changes over the last 150 years, or we have to show that our knowlege of the orbit of the earth is zilch.

OK, then what is left are processes inherent to the Earth system itself. See the simple answer above.

UPDATE: The last comment at Quark Soup pretty much nails it, but I had this up already