Have any of the AGW people ever said just what is the optimal average world temperature and why that temperature is optimal??
The AGWers don't have a specific optimum global average temperature, but they do have a sort of optimum temperature band. This forms the basis of the idea, widely used by (and specifically invented for) AGW promoting politicians, that the global average temperature must be limited to not increasing by more than 2 degrees C above pre-industrial temperature levels. The 2 degree C figure usually appears in statements made by politicians at the various international climate change summits that seem to take place every year.
The story about where the 2 deg C figure comes from, which was invented by some German climate scientists back in the mid-1990s, is given in this link:
"The story of the two-degree target began in the German Advisory Council on Global Change (WBGU). Administration politicians had asked the council for climate protection guidelines, and the scientists under Schellnhuber's leadership came up with a strikingly simple idea. "We looked at the history of the climate since the rise of homo sapiens," Schellnhuber recalls. "This showed us that average global temperatures in the last 130,000 years were no more than two degrees higher than before the beginning of the industrial revolution. To be on the safe side, we came up with a rule of thumb stating that it would be better not to depart from this field of experience in human evolution. Otherwise we would be treading on terra incognita.""
So from that the above it looks like pro-AGW climate scientists are assigning an optimum temperature band to what they think the homo sapiens species has already experienced in its history. The 2 deg C rise figure would go outside this optimum band. It's an application of the precautionary principle.
As far as I'm aware climate scientists don't think a 'catastrophe' would occur if the limit is exceeded (for example Schellnhuber, who invented the limit, doesn't think that) but Green-leaning politicians often treat it as though it is a catastrophic limit.
The Green lobby, it goes without saying, claim that an apocalypse would occur if the 2 deg C limit is exceeded. For example the Stop Climate Chaos Coalition in the UK (a coalition of about 100 Green-leaning NGOs) have this on their webpage:
"But with a rise of 2 degrees C or more, southern Europe will suffer serious drought every decade; billions of people will not have enough water; 550 million will go hungry; 3 million will die from malnutrition.
In the UK coastal flooding will impact up to 170 million people. And many plant, bird and butterfly species will be consigned to the history books."
The above extract also gives the biggest Greenie numerical howler I think I've ever seen. They're claiming 170 million people in the UK would be affected by coastal flooding when the current total population of the UK is something like 60 million.