Myth: Past emissions will stay in the atmosphere forever

Even if we stopped all greenhouse gas emissions, the CO₂ and methane we have already released would stay in the atmosphere forever, so the planet is still doomed. 

Fact:

This is not true. The oceans and land absorb about 60 percent of the CO₂ we emit each year, so if we reduced our emissions to zero, these would become giant CO₂ sponges. Scientists estimate that it would take about 50 years for our cumulative CO₂ emissions to fall by 60 percent, and 200 years for them to fall by 80 percent. [Unfortunately, it would take thousands of years for them to fall back to pre-industrial levels. But this is not necessary.] 

Methane, which accounts for about a third of our greenhouse gas emissions, would be even easier to eliminate, as methane molecules only stay in the atmosphere for 12 years, by which time it is largely converted to CO₂. Methane levels would start falling very quickly if we reduced our emissions to zero (or to somewhere thereabouts).

The news is even better when it comes to global temperature. Climate scientists estimate that this will stabilise with three to five years after we reach net zero.