Our remaining 'carbon budget' means we can emit about 1400 gigatonnes of carbon dioxide and still have a 50-50 chance of staying below 2 degrees, scientists at the Intergovernmental Panel on Climate Change say.
But as well as asking whether the world's emissions cuts will be enough, we might also ask whether they will be fair. Two new studies examine how to divide the remaining carbon budget fairly, and show the answer depends on how you define fairness.
Staying within our carbon budget means cutting emissions 5.5 per cent per year, a paper in Nature Climate Change finds, if action starts without delay. That's dauntingly and perhaps even impossibly rapid.
Large one-off emissions cuts have happened in the past. For instance, the US cut emissions nearly 7 per cent in 2009 as a combined result of a shift from coal to gas-fired power and the impact of the global credit crunch.
But sustained cuts are much more difficult. France managed 2 per cent per year during its 30-year dash for nuclear electricity, when it went from 1 per cent nuclear-powered to 80 per cent.
So 5.5 per cent is a big challenge. But for now let's simply assume it's possible and move on to look at how to divide the budget fairly.
Dividing up the budget fairly
One way to divide the remaining budget would to divide it according to today's shares of total emissions. That would give China 24 per cent and the US 14 per cent of what's left, for example.
Another option would be to just give each person the same right to emit, regardless of nationality.
The chart below shows what these options would mean for different countries.
For instance the US (red line, far right) has very high emissions per person (per capita). If it got a generous 14 per cent of the remaining budget, matching how much it currently emits, it would have to cut its emissions by 2 per cent per year (faint red box at the bottom of the line).
This would be much slower than the 5.5 per cent global average (black horizontal line, below).
If US citizens were only allowed to emit at the same rate as everyone else, it gets a lot harder for them. US mitigation rates would need to climb to more than 15 per cent per year (solid red box). The heavy red box shows a middle ground between the two options.
Figure 1: Mitigation rates for a range of countries based on their current share of global emissions ("inertia", faint boxes) through to shares based on equal per capita emissions ("equity", solid boxes).
It's a similar story for all countries with above-average per capita emissions. This includes China (light blue) which is to the right of the global per capita average (black arrow). On the other side of this pivot, countries like India and Brazil have below-average per capita emissions. They would benefit from an equal per-capita approach to burden sharing.
The paper points out that unfeasibly high mitigation rates might be achievable if the burden is shared via carbon trading. This would allow the likes of the US and even China to pay for emissions reductions in third countries.
Other approaches to burden sharing
Supplementary information published alongside the main journal article considers a huge range of other ways to share out a remaining carbon budget.
For instance, China has become the workshop of the world. This leads to the common argument that emissions savings in Europe have been achieved by outsourcing heavy industry and the associated carbon emissions to China.
So let's say that responsibility for the emissions associated with your latest flatscreen TV is handed to the UK, rather than the Chinese factory where it was made.
This turns out to makes a surprisingly small difference to the mitigation rates required in China (green versus red bars, below centre), though it does make things quite a bit harder for Europe (far left red bar).
Figure 2: Mitigation rates to limit warming to 2 degrees with effort sharing based on territorial emissions or emissions attributed to the country that consumes the goods responsible.
Another common argument is that developed nations should take responsibility for their high-carbon path to industrialisation during the 19th and 20th centuries. To see the impact of this, we can divide up future mitigation efforts in proportion to the amounts of carbon each country has emitted since 1750 (red bars, below) or 1972 (green bars).
Figure 3: Mitigation rates based on including carbon emitted since 2013 (blue), 1972 (green) or 1750 (red).
Tying future efforts to historical emissions makes mitigation rates for the developed world extremely high, rising from between 3 and 6 per cent to over 9 per cent for Europe and over 14 per cent for North America.
In contrast this effort sharing approach makes much less of a difference to the efforts required of poorer and middle income nations. Chinese mitigation rates would fall by a relatively smaller increment from around 9 to 7 per cent if emissions since 1750 are accounted for.
Similarly, the impact of factoring in countries' wealth is "moderate, but not large", the paper says. Richer nations would end up contributing a little more, and poorer nations a little less to the global climate effort but the differences amount to a couple of per cent in the mitigation rates required.
One potential criticism of the study is that it looks at the impact of each factor in isolation. The change in effort shares when including GDP versus ignoring it might be small. But if you add up the impact of population, wealth, historical responsibility and emissions exported overseas the impact would be much more pronounced and it would present a much more favourable settlement to less developed nations.
A fairer deal for poorer nations?
That's where a second study on how to fairly share out climate action comes in. The 'Climate Fairshares' project comes from the Stockholm Environment Institute and Friends of the Earth.
It is based on an "extremely ambitious" level of climate mitigation effort where emissions peak immediately and reduce by a maximum rate of 6.1 per cent per year, a fair bit faster than the 5.5 per cent considered in the first study.
This highly ambitious effort is divided according to a combination of historical responsibility for emissions since 1850 and current ability to pay for action, measured as per capita wealth above a threshold set at the poverty line.
The Fairshares project then further divides climate effort into domestic emissions reductions and reductions overseas paid for by wealthier nations. The result is that the likes of the UK and US would be expected to cut emissions at home by 7 to 10 per cent, a similar rate to the first study found.
But at the same time they would be expected to pay hundreds of billions of dollars for emissions cuts overseas. The UK, for instance, would have to cut domestic emissions by up to 79 per cent in 2030 against 1990 levels, much higher than its existing 80 per cent by 2050 target. And it would have to contribute $US65 billion to mitigation overseas.
These overseas transfers would amount to $US60 per person per year for the UK and more than $US150 per person per year in the US. China and India, with the help of this international climate aid, would then be expected to cut emissions by between 1.7 and 4.3 per cent per year.
The problem for both studies is that they are based on a top-down theoretical assessment of the action required to limit warming to a particular level, rather than a bottom up look at what is politically and economically feasible.
Two commentaries published over the weekend said this is an "abstract" and "naive" approach that might hinder climate negotiations. Far better, they argue, to let nations make their pledges in New York according to what they themselves deem achievable.
Irrespective of what the best approach is in the abstract, that is our current political reality, whether it will put us on track for a two degree target or not.
Originally published on Carbon Brief. Reproduced with permission.