Net zero: why is it necessary?

The science of ‘carbon budgets’

Climate science is clear that to a close approximation, the eventual extent of global warming is proportional to the total amount of carbon dioxide that human activities add to the atmosphere.

So, in order to stabilise climate change, CO2 emissions need to fall to zero. The longer it takes to do so, the more the climate will change. Emissions of other greenhouse gases also need to be constrained. In the Paris Agreement, governments agreed to keep global warming ‘well below’ 2 degrees Celsius, and to ‘make efforts’ to keep it below 1.5ºC. The Intergovernmental Panel on Climate Change (IPCC) released a report in October 2018 on the 1.5ºC target; it concluded that global emissions need to reach net zero around mid-century to give a reasonable chance of limiting warming to 1.5ºC.

Why ‘net zero’?

In many sectors of the economy, technologies exist that can bring emissions to zero. In electricity, it can be done using renewable and nuclear generation. A transport system that runs on electricity or hydrogen, well-insulated homes and industrial processes based on electricity rather than gas can all help to bring sectoral emissions to absolute zero.

However, in industries such as aviation the technological options are limited; in agriculture too, it is highly unlikely that emissions will be brought to zero. Therefore some emissions from these sectors will likely remain; and in order to offset these, an equivalent amount of CO2 will need to be taken out of the atmosphere – negative emissions. Thus the target becomes ‘net zero’ for the economy as a whole. The term ‘carbon neutrality’ is also used.

Sometimes a net zero target is expressed in terms of greenhouse gas emissions overall, sometimes of CO2 only. The UK Climate Change Act now expresses its net zero emissions target by 2050 in terms of greenhouse gases overall.