Net zero emissions are achieved when all emissions of greenhouse gases to the atmosphere, caused by all human activity, are balanced by the same amount removed from the atmosphere over a specified period. If multiple greenhouse gases (carbon dioxide, methane etc) are taken into consideration, the quantification of net zero emissions will depend on the climate metric chosen to compare emissions of different gases.
The term is important because achieving net zero on planetary scale will mean that our contribution to global warming will stop. The Paris Agreement underlines the need for net zero, requiring countries to ‘achieve a balance between anthropogenic emissions by sources and removals by sinks of greenhouse gases in the second half of this century’.
The ‘net’ in net zero is important because it will be very difficult to reduce all emissions on the timescale needed. As the world reduces its emissions we will likely need to massively scale up removals too. In order for net zero to be effective, it must be with high permanence, meaning that any greenhouse gas removals do not leak back into the atmosphere over a short or medium period (generally fewer than 1000 years).