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Everyone is talking about emissions budgets – what are they and what do they mean for your country?

Our CO2 emissions are causing global heating. If we want to stop global warming at a given temperature level, we can emit only a limited amount of CO2. That’s our emissions budget. I explained it here at RealClimate a couple of years ago:

First of all – what the heck is an “emissions budget” for CO2? Behind this concept is the fact that the amount of global warming that is reached before temperatures stabilise depends (to good approximation) on the cumulative emissions of CO2, i.e. the grand total that humanity has emitted. That is because any additional amount of CO2 in the atmosphere will remain there for a very long time (to the extent that our emissions this century will like prevent the next Ice Age due to begin 50 000 years from now). That is quite different from many atmospheric pollutants that we are used to, for example smog. When you put filters on dirty power stations, the smog will disappear. When you do this ten years later, you just have to stand the smog for a further ten years before it goes away. Not so with CO2 and global warming. If you keep emitting CO2 for another ten years, CO2 levels in the atmosphere will increase further for another ten years, and then stay higher for centuries to come. Limiting global warming to a given level (like 1.5 °C) will require more and more rapid (and thus costly) emissions reductions with every year of delay, and simply become unattainable at some point.

In her recent speech at the French National Assembly, Greta Thunberg rightly made the emissions budget her central issue.

https://www.youtube.com/watch?v=J1yimNdqhqE

So let’s look at how the emissions budget concept can be used to guide policy on future emissions trajectories for countries.

Step 1: The temperature goal

First we need to determine at what level we want to stop global warming. That’s quite simple because it has already been agreed in 2015 by all nations in the Paris Agreement. That has taken decades of discussion and negotiations, ever since nations agreed in 1992 at the Rio Earth Summit to stabilize greenhouse gas concentrations “at a level that would prevent dangerous anthropogenic (human induced) interference with the climate system.” In Paris a consensus was finally reached on “limiting global temperature increase to well below 2 degrees Celsius, while pursuing efforts to limit the increase to 1.5 degrees”.

Last year, the IPCC special report Global Warming of 1.5 °C (in short SR15) detailed strong reasons for why limiting to 1.5 °C would be much more sensible than to 2 °C.

Step 2: The global CO2 budget

Once the temperature limit has been agreed, we need to know the corresponding CO2 budget. That is a question for science, and the IPCC SR15 answers that, including the uncertainties as always is a hallmark of good science. The following shows the budget table from IPCC.

The uncertainties to a large extent result from the fact that CO2 is the main but not the only cause of human-caused climate change, so the CO2 budget depends on how we will deal with non-CO2 climate forcings such as aerosol pollution. There are also different methodologies to estimate the CO2 budget. A thorough analysis of the uncertainties is found in a recent paper by Rogelj et al. in Nature. The bottom line, as one of the co-authors (Elmar Kriegler) told me, is that the SR15 estimates in the table above are still the best we have.

Some have argued that the uncertainties make the budget approach a poor guidance for policy. I disagree. First of all, practically all of politics operates under high levels of uncertainty about the outcome of policy decisions; that is inevitable. In fact it is rare that politics has a clear guidance like the well-established linear relationship between cumulative emissions and global temperature. Those who criticise using this as policy guidance must come up with a better guidance providing less uncertainty, then we can discuss.

Second, some of the uncertainty is captured by the probabilities for reaching a certain temperature limit, shown in the table, so society can simply decide what level of risk of overshooting a temperature level they are willing to take.

And finally, all policy is to a large extent learning by doing. You start with the best scientific advice now (especially since we cannot afford to wait any longer), and if we know more in ten years time we can adjust policy then. Given that climate change is largely irreversible it is best to err on the safe side, i.e. the uncertainty, if anything, is a reason to apply the precautionary principle and reduce emissions fast.

Greta has argued in her speech to use 67% probability for staying below 1.5 °C, i.e. a 420 Gt budget from the start of 2018. Subtract 2 years of emissions, i.e. 80 Gt, then we’re left with 340 Gt from the start of next year. That is 8.5 years of current emissions – or 17 years until zero emissions in case of a linear rampdown.

If you translate the “efforts to limit the increase to 1.5 degrees” promised by all nations in the Paris Agreement as a policy that gives us 50:50 chance to actually achieve that goal, that’s 500 Gt from the start of next year. That’s 12.5 years of current emissions, or 25 years for a linear rampdown, i.e. halving emissions in 12.5 years to reach zero by the end of 2044.

But beware of using those end dates instead of budgets, because it is not the end date but the cumulative emissions that count! A simple illustration: if you don’t achieve reductions in the next ten years but keep emissions constant, and reduce linearly after that, the result is that you have to reach zero ten years earlier! See the next figure.

This is why one should not attach much value to politicians setting targets like “zero emissions in 2050”. It is immediate actions for fast reductions which count, such as actually halving emissions by 2030. Many politicians either do not understand this – or they do not want to understand this, because it is so much simpler to promise things for the distant future rather than to act now. Greta asked the pertinent question in her Paris speech:

”What I would like to ask all of those who question our so called ‘opinions’, or think that we are extreme, is: Do you have a different budget for at least a reasonable chance of staying below a 1,5° of warming? Is there another, secret Intergovernmental Panel on Climate Change?”

Step 3 Computing a budget for your country

How do you divide up the remaining budget amongst humankind? This is a crucial step since most climate policy is made at the national level. Yet, this is not a scientific question but one of climate justice. Who gets how much?

I can’t solve this question but I am going to propose a starting point, based on the idea that a principle of fair distribution needs to be universal and simple. The most simple one clearly is an equal per capita distribution. Anyone who wants more of the budget than someone else would need to provide a good reason. There could be many reasons – cold countries might claim they need more emissions for heating, hot countries for air conditioning, large countries for transport over long distances, developing countries to eradicate poverty, rich countries because they are already developed.

A tricky question is: at what point in time do you distribute the budget? This is important because rich countries are eating up the remaining CO2 cake much faster than poor countries. I would propose: from the time of the Paris Agreement, i.e. from the start of 2016. Of course developing nations will argue (and have argued) for a much earlier start date, to account for the historic emissions of developed nations. That may be justified but has the practical problem that the remaining budget for countries with large per-capita emissions is then already zero or rather: overdrawn.

So just to be practical, let’s take 2016. So you compute the remaining global budget at the start of 2016 by adding 80 Gt to the IPCC budget table numbers shown above. Then you multiply that number by the population of your country, divide by the global population, and then subtract the emissions of your country from the start of 2016 until now. I have done this for Germany (in a German blog post) using a global budget of 800 Gt from the table, just to be generous, for a 67% chance to stay below 1.75 °C (my interpretation of “well below 2 °C”). The result was a remaining emissions budget of 7.3 Gt from the start of 2019 or 6.5 Gt from the start of 2020. That is 8 more years at current emissions. The next figure shows a linear reduction trajectory compatible with this budget.

I’m not so concerned about the exact numbers, given the uncertainties discussed above. But there are at least three important conclusions from the budget approach.

First of all, nations with high per-capita emissions need to recude faster than others, based on the limited budget and simple justice considerations. If some reduce faster and some slower, even if every nation reduces linearly global emissions will not decline linearly, but more rapidly at first from the reductions of wealthy nations, with a longer tail of emissions from developing nations reaching zero later.

Second, even with generous assumptions (like an 800 rather than 420 Gt global budget and a 2016 start date for dividing up the cake) emissions from developed nations need to drop much faster than almost all politicians think, in order to honour the Paris agreement.

Third, it is not some end date that counts but rather very rapid reductions starting right now. The end date is a moving target – every year we wait we lose two years: the year we waited, and a year at the end because the required end year moves towards us.

Finally, unprecedented global cooperation is needed to tackle the climate crisis. This may involve deals that make the tight budgets more palatable to countries with high per-capita emissions – for example they might find partners with low emissions and negotiate to use some of their budget in exchange for technological and financial support in climate adaptation and mitigation.

Reference

Messner et al. 2010: The budget approach: A framework for a global transformation toward a low-carbon economy (PDF)