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  • Writer's pictureDr. Lee Anne Willson

What is a fair-share climate target?

What is a carbon budget? One option being offered to the Ames City Council by the Climate Action Plan consultants is a “Fair share / carbon budget” option. What does this mean? One approach to determining what needs to be done to keep climate change from disastrous levels is the computation of a carbon budget. I rather like this because I think it makes the problem easy to visualize.

In this illustration, the grey, blue and orange add up to the total amount that can be added to the atmosphere, from 1850 on, and still hope for a temperature increase below 1.5°C. The grey plus the blue areas represent all the carbon added to the atmosphere from 1850 to now, and the orange area is what we can use in the future. The blue square shows how much of the already-added carbon came from the United States. Computing the total budget: The first step is computing the total amount of carbon added to the atmosphere. For a discussion of this approach, see

  • Compared to the level of CO2 in the atmosphere in 1850, how much can we add without exceeding 1.5°C increase in the average temperature?

  • How much have we added since 1850?

  • How much do we have left?

Why 1850? A plot of the temperature anomaly vs. time, sometimes called the “hockey stick graph”, shows a rapid rise starting in the 1800s. If we look at the green points in this copy of the original northern hemisphere hockey stick graph of Mann, Bradley & Hughes 1999, we can see that in 1850 there is no sign of a temperature rise, but by 1900 it may be up some, and by 1950 it is well above the pre-1850 trend.

The latest assessment is available in preprint for ( The list of authors on this preprint is impressive and shows the international nature of the effort to understand and deal with the greenhouse gas excess. Their conclusion: “The remaining carbon budget for a 50% likelihood to limit global warming to 1.5°C, 1.7°C and 2°C has shrunk to 120 GtC (420 GtCO2), 210 GtC (770 GtCO2) and 350 GtC (1270 GtCO2) respectively, equivalent to 11, 20 and 32 years from the beginning of 2022, assuming 2021 emissions levels. “ In other words: To keep the temperature increase below 1.5°C we must keep the worldwide future addition of carbon to the atmosphere below 120 Gigatons of C (or, taking into account the oxygen in CO2, below 420 Gt of CO2). Computing how much time we have left The cited paragraph, discussed above, tells us we have 11 years before we will exceed the 1.5°C limit. That comes from dividing the total by the current rate of emission. However, if we can reduce our emissions now, we will have longer to reach the final solution. For example, if we assume a steady decline from current emission levels to zero, then we can double the length of time we have left – but only if we start effective action now. What is our ‘fair share’? The second step in the carbon budgeting exercise is figuring out what each country, state, city or individual has left to spend. That is where the decision-making involves values outside of simple calculation. A similar dilemma Imagine a family with three children, Pat, Jan and Kim. The parents reach the end of their life, and when all the accounts are settled, there is $300,000 left. How should that be distributed? Without knowing anything else, most of us would say, “$100,000 each, of course.” But there is additional information. Pat has been very successful in life; $100,000 will not make much difference to Pat. Kim has suffered misfortune – illness, a house fire, and loss of employment when the company shut down. Kim has limited resources, and $100,000 would make a great difference in Kim’s life. Should the estate still be divided equally? We might suggest that Pat give their $100,000 to the youngest, but that would be a decision by Pat; no outsider has moral standing to dictate that outcome. Here is some more information: During the lifetime of the parents, Pat was given $200,000 to start a successful business; this was a gift so Pat was never asked to pay it back. Jan neither borrowed nor accepted a large gift from the parents. Kim accepted a series of bailouts amounting to about $100,000. Now how do we divide it? We can revert to the original “100,000 each.” Or we can say “The total transfer of wealth from the parents to their children amounts to $600,000. So each child has a fair share of $200,000. Pat has already received that amount, so gets no more. Jan has received nothing, and gets $200,000. And Kim has already received $100,000, so gets another $100,000 to make a fair share.” Three answers for what goes to Pat, Jan and Kim:

  • 100k, 100k, 100k equal division of the estate

  • 0k, 100k, 100k based on need

  • 0k, 200k, 100k based on prior gifts

What is the right answer? A lot of hours in courts and estate planning have gone into resolving questions of this type! Now what? Computing our share of the carbon budget four ways: When it comes to the world carbon budget, we can similarly choose whether to divide up what is left into equal pieces; to take into account the different resources available to different countries; or to take into account how much carbon each country has already contributed to the atmosphere. Just as in my estate example, it is also generally true that the countries with more resources to deal with the problem are the same ones that have, in the past, contributed more to the carbon in the atmosphere. The simplest approach is to take the remaining carbon budget, divide by the number of people, and get a per-capita budget. That comes out to about 53 tons/person remaining in the budget. Then the budget for a city, state, or country is the per capita budget times the number of people. (The average person in Ames is emitting about 18 tons/year, so by this approach and at current emission rates Ames would use up its budget in less than 3 years.) A country that is currently emitting above the world average then has less time left to reach zero carbon, and one that is emitting less has more time left to reach zero. Some countries are emitting a lot now, but have not contributed as much historically, and may not have the resources to move quickly. Making the countries with the biggest problems and fewest resources have the shortest deadlines does look like a recipe for failure, so another approach is to say “all countries need to reach zero by the same date” and “all countries need to reduce by the same amount by 2030.” That is what most countries are assuming as they negotiate their climate action plans in meetings like COP26. That seems fair, but it overlooks that the countries with the most resources to use to solve the problems – the ones with the biggest GDPs in most cases – are also mostly the ones who, in the past, emitted the biggest share of the carbon already in the atmosphere. This approach essentially taxes poor countries for past emissions from rich ones. A third approach is to look at the GDP of each country and assign the share of mitigation according to how wealthy countries are. A country with an average per capita annual income of $50,000 would be expected to spend five times as much on mitigation as one with an average per capita income of $10,000. This still ignores the past history – we are dealing with a form of garbage, and asking everyone to pitch in to deal with it, not taking into account that most of the garbage came from a few entities. The fourth approach starts with an inventory of who has contributed how much CO2 or carbon to the atmosphere in the past. These contributions are added up, then compared with a fair division of the total from the beginning, based on population or GDP or some combination of those factors. When this is done, it turns out that the US has put in 25% of the excess carbon in the atmosphere, while its share of the total budget since 1850 is closer to 13%. We have already spent nearly twice our share! In “Fair share” assignments based on this approach, we are asked to pay back some of that excess by helping poorer countries (who have not spent their share) to reduce their emissions of CO2.

Figure from How much is our fair share? Just as in my inheritance example, when we look at “how much does the US need to do?” there are different answers depending on what we consider to be fair, and what we take into account. If we only look forward, and per person, then the US share of the carbon budget = remaining carbon budget times the fraction of the world population that lives in the US. Or, we can also look backward, and take into account past emission; then our fair share of what is left = total emission budget times (fraction of the world population that lives in the US) minus what we have already emitted.

The second calculation results in us having a negative remaining budget. How can we manage that? Without technology to remove a lot of carbon from the atmosphere rapidly, the only way is for the US to pay other countries to emit less carbon, after reducing our own emissions as quickly as we can. People are also investing in carbon removal technologies, but that will take time, and may never scale up to the degree needed to solve this problem. I didn’t provide numbers using the GDP to distribute the costs of mitigation efforts. It’s a more complicated approach, particularly since a lot of those “costs” pay for themselves in energy savings over time, so one needs to negotiate how to count costs that are also investments. So which answer is fair? Which answer is reasonable? Which answer is achievable? These answers are obviously not cut and dried, and why community discussion about them is so important. I hope this blog post will help you get a deeper understanding of what people might mean when they start talking about “carbon budgets” and “fair share.”

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