'Belette' commented that I hadn't been consistent in my definition of 'global warming'*. I had asked for a 'Fermi explanation' showing why it exists, but that requires an understood and agreed definition of what it is. OK. There are three ideas locked up in that term, as commonly used and understood. First, that warming is not a mere accident. 'Global warming' does not mean simply that the earth is hotter than it was some time ago. It means that there is some underlying cause or reason why it is getting warmer, and that as long as the cause exists, the warming will go on. Second, that the underlying cause or reason is the cumulative increase in carbon dioxide in the atmosphere, caused by industrialisation. This is properly called 'anthropogenic' global warming, but it usually has that meaning even without the qualification. Third, that the warming will if unchecked lead to to disaster. This should properly be called 'catastrophic anthropogenic global warming'.
The proposition I am sceptical about is there "that increasing atmospheric concentrations of greenhouse gases, notably carbon dioxide, will have catastrophic consequences for mankind and for the planet". Obvious I don't deny that the cumulative increase in carbon dioxide is causing long-term global temperature to rise. But no one has explained clearly to me why this should matter.
*Vallicella has just reminded me that he has some definitions here.