The administration of US President Joe Biden has pledged 2050 as its deadline for net-zero greenhouse-gas emissions. Earlier, China declared 2060 for its own net-zero date. A debate is under way in India, too, as in much of the rest of the world. In all, some 124 nations out of 202 surveyed in a report published last week have made net-zero pledges ahead of November’s world climate summit in Glasgow, UK (see go.nature.com/2puuzmh). Activities leading up to the United Nations summit include a ‘race to zero’ campaign to get the remaining countries on board by then.
But what does net zero actually mean? In a 2018 special report, the Intergovernmental Panel on Climate Change said that countries must bring carbon dioxide emissions to “net zero” by 2050 to keep global warming to within 1.5 °C of pre-industrial levels. But beyond that, there’s less agreement on which substances net zero applies to. This creates serious ambiguity, and countries and organizations are defining the phrase according to their own criteria. Agreeing on a definition is important for accountability but, most importantly of all, without it, the 2015 Paris climate agreement’s aim to limit warming to between 1.5 °C and 2 °C by mid-century might not be achieved.
Joeri Rogelj at Imperial College London and his colleagues have shown clearly how different definitions and pathways to net zero can have drastically differing outcomes (J. Rogelj et al. Nature 591, 365–368; 2021). For example, reducing CO2 emissions halts warming, but the CO2 that already exists in the atmosphere will remain for hundreds of years. At the same time, cuts to other greenhouse gases could affect warming faster. But eliminating these other gases is more complex than is cutting carbon.
The researchers also warn that the relative quantities of these emissions reductions will also affect the gases’ overall rates of decline. In some scenarios, temperatures might not start to decline, even with emissions reductions. The team rightly urges clarification on three policy areas: the scope of emissions reductions; their adequacy and fairness; and concrete steps towards achieving net zero. A 2050 or 2060 target becomes meaningless without interim milestones.
The Paris climate agreement applies to all greenhouse-gas emissions, but participating countries did not decide how emissions are to be reduced, nor how reductions would be measured. Agreement on both will be priorities for the Glasgow summit, but the information vacuum during the intervening six years has created a space for multiple interpretations and options. So, whereas the European Union’s pledge targets all such gases, China’s plan focuses only on CO2 emissions and does not include methane or nitrous oxide. And the US plan has yet to specify which gases are covered.
Corporations making net-zero pledges are being similarly flexible in their definitions. For some, net zero means removing greenhouse gases from all of their processes, including taking steps to offset historical emissions. But others — such as investment banks and fossil-fuel companies — will continue to invest in fossil fuels while pledging net-zero policies in other areas of their businesses. And this is rightly attracting criticism, not least from the inspirational school climate-strikes movement, Fridays for Future.