|The 'Greenhouse Effect' is real.||a) True|
That is not, strictly speaking, correct.
The greenhouse effect was an analogy used for classroom purposes to help people understand how the atmosphere traps heat. In fact it is a highly deceptive analogy, as the atmosphere acts like a greenhouse only in the crudest way. Greenhouses use a solid barrier (the glass roof) to prevent heat loss by convection yet, lacking such a barrier, convection accounts for about half the heat loss from Earth's surface.
The correct term for so-called ‘heat trapping’ gasses such as water vapour and, to a much lesser extent, carbon dioxide (CO2), is 'infrared (IR) absorbing gases'. The presence of such gasses in the atmosphere helps to moderate temperatures, especially at night.* Without IR absorbing gases, Earth would be much colder; according to computer-generated estimates, from -18°C to -22°C. Note that this range of computer estimates is equal to half the warming since the last Ice Age.
IR absorbing gases are transparent to Sunlight so allow incoming light from the Sun to pass through Earth's atmosphere, warming the surface. Heat is then radiated from the surface into the atmosphere and IR absorbing gases absorb a portion of this heat. At a certain point, the excited molecules of water vapour, CO2 and other IR absorbing gases re-radiate the heat in all directions, some of it returning to the surface and the planet warms in response.
However, the warming effect of CO2 does not continue indefinitely – as the atmosphere becomes ‘saturated with CO2 from a temperature perspective, greater amounts of the gas result in lesser and lesser amounts of warming. It is like painting a window black – the first coat blocks most of the light and subsequent coats have less and less impact. Dr. Richard S. Lindzen, Alfred P. Sloan Professor of Meteorology, Department of Earth, Atmospheric & Planetary Sciences, Massachusetts Institute of Technology demonstrates that we are already 3/4 of the way to a final equilibrium temperature that a doubling of CO2 from pre-industrial levels would cause; yet CO2 appears to have risen (new data suggests it may be far less) only 33% in this period.
* That is why the danger of nighttime frost is greater when the skies are clear than when skies are cloudy. This is caused solely by water vapor. Deserts experience very high daytime and very low nighttime temperatures because there is virtually no water vapor. CO2 is not a factor.