Climate change is often described as a stock-flow problem. The rise in global temperature depends on the concentration – or ‘stock’- of GHGs in the atmosphere. Over the years, human-induced emissions have become significantly more than what the natural processes (mostly forests and oceans) can remove from the atmosphere.
So, the total stock of GHG in the atmosphere has increased, leading to global warming.
To limit global warming, we need to stop the atmospheric stock of GHGs from increasing. For this, additional GHG emissions – the ‘flow’- must reduce significantly. net zero is a state where ‘the flow’ of GHGs becomes zero, which is to say that no additional emissions are added to the atmosphere.