With the close of the hottest decade in recorded history, and with climate impacts steadily worsening along with their human and ecological toll, the urgent need to keep warming well below 2°C – and to aim to keep warming below 1.5°C – has only become clearer.

Those mitigation objectives, agreed by all signatories to the Paris Agreement, imply that the available carbon budget is finite and rapidly diminishing. The recent IPCC Special Report on the impacts of global warming of 1.5°C above pre-industrial levels (SR1.5: IPCC 2018) concludes that the available budgets are as shown in Table 1 below.

An urgent decarbonization transition is needed. And despite how stark these figures are, they may be overly optimistic. They define an ‘acceptable’ level of warming as a level that is still significantly higher than today’s warming, which is already bringing devastation to many communities, from the Arctic to the Amazon to Australia. They assume that it is acceptable to pursue a future course in which there is a one-in-three risk of exceeding the specified threshold, or even a one-in-two chance. They are derived from mixed ocean-atmosphere general circulation models of the climate that do not account for certain feedbacks that could lead to much more warming, especially in the long term (Rogelj et al. 2019).
Still, the effort even to stay within such an arguably inflated carbon budget requires society to make deliberate and morally justified choices about its use. This report presents information on the use of the carbon budget, in the past, at present and in the future. Its focus is on the stark inequalities in income and emissions across the global population, and what they may imply for practical, feasible, politically acceptable and morally equitable options for the future.