How much more can be emitted?

Photo: Takeshi Kawai CC-BY-NC

Determining a limit for the level of greenhouse gases that nature can “cope with” is difficult for several reasons. It is uncertain how the climate system will react to emissions, and we know relatively little about how nature is affected when the climate changes.

Research has shown however that ecosystems and people are affected negatively even if the mean global temperature rises by just a degree or so above the pre-industrial level – which is the expected effect of the cumulative emissions to date.

With a temperature rise of less than one degree, species such as the Bengal tiger and the mountain gorilla are threatened, while a rise of 1–2°C affects coral reefs, coastal wetlands, the availability of food and water for people, etc.

The Paris Agreement sets a long-term temperature goal of holding the global average temperature increase to well below 2°C, and pursuing efforts to limit this to 1.5°C above pre-industrial levels.

In the fifth assessment report, published during 2013 and 2014, the IPCC has for the first time agreed a figure on how much carbon dioxide mankind can still emit while staying below a 2°C global temperature increase.

The IPCC report suggests that to have at least a 66 per cent chance of keeping to less than 2°C of warming, mankind must emit no more than 2,900 Gt of carbon dioxide, after excluding non-CO₂ forcing, during the rest of this century. The conclusion of the IPCC is that more than half of this budget has already been used up (1,630 to 2,150 Gt of CO₂ were already emitted by 2011). The world is currently emitting about 50 billion tonnes of greenhouse gases in carbon-dioxide-equivalents each year, which means that with current emission levels the emissions budget would be used up within 15 to 25 years.

	2°C		1.5°C
Likelihood to stay below target temperature	Carbon budget for the rest of this century (CO₂ only)	Year when budget will be used up assuming current annual emissions (50 GtC)	Carbon budget for the rest of this century (CO₂ only)	Year when budget will be used up assuming current annual emissions (50 GtC)
>90%	730 GtC	2026	548 GtC	2022
>66 %	1,000 GtC	2031	750 GtC	2026
>50%	1,212 GtC	2035	909 GtC	2029
>33%	1,567 GtC	2042	1,176 GtC	2035
>10%	3,567 GtC	2082	2675 GtC	2065

Source: >33%, >50% and >66% budgets developed from figures in IPCC AR5 WGI, Summary for Policy Makers, E.8 Climate Stabilization, Climate Change Commitment and Irreversibilty, >10% and >90% budgets developed from figures in presentation by Prof. Reto Knutti.