The wave of national net zero CO2 and greenhouse gas emission targets could, if fully implemented, reduce best estimates of projected global average temperature increase to 2.0-2.4°C by 2100 and could bring achievement of the Paris Agreement within reach. 131 countries are discussing, have announced, or have adopted net zero targets, which together cover 72% of global emissions. Together, these net zero targets could significantly lower projected global warming compared to currently implemented policies (2.9 to 3.2°C) or to the pledges submitted to the Paris Agreement (2.4 to 2.9°C). MainAnalyses of current promises and actions by countries to limit climate change have concluded that they are by far insufficient to meet the goal of the Paris Agreement to limit global temperature increase to well below 2°C, while pursuing efforts to limit it to 1.5°C (refs. [1][2][3][4][5] ). Specific policies employed to reduce emissions in line with the Paris Agreement may differ from country to country. However, a chief requirement of collective action is codified in Article 4.1 of the Paris Agreement (Article 4.1, UNFCCC, 2015): emissions need to peak as soon as possible and anthropogenic emissions must be balanced by removals in the second half of the century. In other words, global emissions must reach a net zero level. The idea to include a global net zero target in the Paris Agreement was put forward in the run up to its adoption in 2015. 7,8 Pathways to meet this goal have been evaluated in the literature subsequently 9 .

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This article tests fairness justifications offered in 168 nationally determined contributions (NDCs) to the 2015 Paris Agreement against the touchstone of principles of international environmental law. It finds that while many NDCs refer to elements and indicators that are backed by principles of international law in determining fair shares (sustainable development, special circumstances, common but differentiated responsibilities and equity), some NDCs justify their contributions on the basis of indicators not backed by such principles (indicators including small share of global emissions (for states that are not LDCs and SIDSs), least cost pathways, and emissions per GDP). These insights are used to select a sub-set of approaches to the quantification of national fair share emissions targets among approaches previously surveyed in the literature. This leads to the exclusion of approaches based on cost and grandfathering. Next, the principles of harm prevention and precaution, and the normative pillars of the climate change regime, including its objective, 'progression', and 'highest possible ambition,' and the norms relating to human rights, are engaged to argue for further narrowing the range of national fair shares such that the sum of individual contributions is collectively compatible with the Paris Agreement's long-term temperature goal. This leads to the finding that developed states have a Paris temperature goal compatible emission level in 2030 that is net-negative. Of the G20 states, only India and Indonesia can temporarily increase their emissions relative to 2010, only India relative to today. Around half the G20 states have increased emissions over the 2010s, and those decreasing emissions have done so too slowly. Key policy insights:. States' fairness justifications for their contributions to the Paris Agreement should be scrutinized for compatibility with widely-accepted principles of international environmental law as well as the normative pillars of the climate change regime. . Fair share ranges consistent with international environmental law principles offer a benchmark for existing and new nationally determined contributions under the Paris Agreement, for peer-to-peer comparisons, and to feed into the global stocktakes. . Such fair share ranges can inform climate litigation in which the adequacy of national contributions, and thus a state's fair share, is at issue.

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Looking at policies instead of promises shows that global climate targets may be missed by a large margin