What would happen if the greenhouse gas levels doubled?

Answer 1

It depends on which greenhouse gas doubled its concentration.

Water vapour is the most plentiful greenhouse gas, but only remains in the atmosphere for a few days and precipitation will prevent its concentration from doubling.

Carbon dioxide levels have indeed increased by 35 per cent since pre-industrial times and the rate of increase in concentration is growing rapidly, so in the absence of any action at all, would certainly double. The present concentration is just over 380 parts per million (ppm), and doubling this to around 760 ppm would be disastrous. Long before this level was reached, the governments of the world would place our global economy on a war footing and mandate a zero-carbon economy no matter what the pain caused by this. Clearly, we never wish to get to the point that this action became necessary.

Methane levels have also increased since pre-industrial times but the rate of increase in concentration is growing more slowly. The disaster that could result from doubling atmospheric methane concentrations means that it could never be allowed to happen.

If greenhouse gas levels reach the point where there is a 'runaway greenhouse effect' because of positive feedback resulting from higher temperatures, then there will no longer be any action that we can take to prevent its continuance. We would simply have to accept the consequences.