To construct vast underground complexes to shield against the effect of the sweltering heat. This was dropped because sooner or later the Earth would become a cauldron where nobody could live.
To launch nuclear missiles into the Van Allen radiation belt to blast holes in the belt to allow the carbon dioxide to escape the Earth’s atmosphere, but this proposal was dropped because putting massive holes in the belt would allow deadly cosmic rays to enter our atmosphere
Then came the third option which was to become known as alternative 3. This was the accepted alternative. It was decided that the only possibility of saving the human race from extinction was to bring together the World’s top scientists in every field of endeavour and to strive to colonise another planet so that small numbers of highly qualified personnel could start again to build up the human race. The best hope for this was to colonise Mars.