GWPF Traps RS: The Greenhouse Effect is Real?

In response to a suggestion by Sir Paul Nurse, the President of the Royal Society, the Global Warming Policy Foundation has invited five climate scientists and Fellows of the Royal Society to discuss the current state of climate science and its wider implications. This is a good initiative by GWPF in principle, but GWPF misses the opportunity to really challenge the CO2 alarmism of RS by uncritically accepting its basic postulate:

The starting point for the discussion as expressed by GWPF under "Matters where we agree with the dominant scientific establishment and can quantify the outcome" as the following proposition

1. The greenhouse effect is real and CO2 is a greenhouse gas.

This is an unfortunate unscientific statement which will make a scientific discussion meaningless, because "the greenhouse effect" is not well specified or described in scientific terms, see PS below. 

If "the greenhouse effect" was well specified, it would be absurd to add that in addition it "is real". If it is well specified, then it is real. If it is not specified, claiming it to be real is meaningless. 

Further, since "the greenhouse effect" is not well specified, the term "greenhouse gas" lacks meaning.

The question is why GWPF is so eager to express agreement with RS by stating 1? Is it to trap RS into a discussion?

PS If you wonder what "the greenhouse effect" is, you can ponder one of the most common definitions:

• The greenhouse effect is what keeps the Earth warmer than it would otherwise be (without the effect).

How much warmer and why is however not specified.