Flatten-the-curve: Why total carbon emissions matter much more than ‘date of zero’

Executive Summary

The recent Intergovernmental Panel on Climate Change (IPCC) report on climate change painted a dire picture of global warming, especially climate shifts and variance across countries. It highlights the need to limit emissions, and if the ambition is to keep warming under 1.5° Celsius, the summary is the world needs to get to zero by 2050.

A number of countries have announced plans to reach “net-zero” by 2050 or thereabouts. There are several problems with these announcements. First, are these really zero, or ‘net’ through futuristic or unfair offsets or, even worse, accounting tricks? Offsets are unfair when they are based on ‘all carbon is equal’ even though abating carbon isn’t equal in cost. ‘Economic efficiency’ has a skew that favours high-emitters, who benefit from offsets out of low-emitters (poorer countries) with early-in-the-trajectory abatement. Second, even if one does reach zero, what is the shape of the trajectory to get there? Most countries are conspicuously quiet on the details. The shape would determine the cumulative emissions over time, which is what really matters. Third, are all countries expected to reach the target at roughly the same time? Not only is this unfair to low-emitters (who are invariably poorer, developing countries), coming to zero by 2050 would still mean many high-emitters would still emit well more than their “fair share” of emissions through most apportionments of a global carbon budget. This is after being generous and writing off all historical emissions, even though carbon dioxide lingers in the atmosphere for centuries.

Becoming zero is important, but “date of net-zero” is incomplete, lacking any measure of what the date should be. “Sooner is better” remains a motherhood statement. To address many of these issues, this paper presents a richer framework for both creating a yardstick for cross-country comparison as well as incentivizing countries to lower their cumulative emissions going forward. This yardstick is, by design, based on total emissions. It thus overcomes the limitations of the ‘date of zero’ approach, which ignores any front-loading of emissions. The area-under-the-curve approach, which also tells us the date of zero, is richer because it directly indicates if a country is behind schedule and likely to bust its budget. It also tells us how much time a country with low emissions has before it must peak emissions.

The framework uses a standardized trajectory for all countries that assume they need time, nominally 30 years, to reach zero. Low emitters also have some remaining carbon space so they can continue as-is for “N” years until they have to peak and still stay in budget. High emitters may not even have 30 years to linearly decline to zero and stay in budget. They solve for a negative “N”, meaning they should have begun their decline in the past to allow themselves 30 years to come to zero, or they must decline faster to stay in budget. To reach zero in thirty years (a 2050 target) means a country has no years left before the 30-year decline to zero (“N” = 0).

Growth in emissions from low emitters for a few years is inevitable and embedded in the framework. In 2019, the countries with per capita fossil CO2 emissions below the world average, with 60% of the world’s population, had only 22% of global fossil CO2 emissions. A billion people in sub-Saharan Africa (excluding South Africa) were responsible for only 1% of global emissions. Any model expecting them to immediately focus on “zero emissions” is naïve and discriminatory.

Importantly, this framework enables a system that incentivizes low-emissions countries to “flatten the curve” – to peak emissions later, but lower. This would enable them to have a combination of lower cumulative emissions or the same emissions at a lower cost (given low-carbon technologies are getting cheaper over time). Net-zero by a rigid date wouldn’t support this. It would also fail to push for any low-hanging fruit of early emissions reduction, which could give some countries limited carbon space to let small residual emissions continue for a few years.

If the world does bust its carbon budget, it will be because of a subset of high-emission countries. These countries should make global finance transfers that enable low emitters to flatten their curve. They should also pay down costs along the learning curve for abatement technologies, such as green hydrogen, carbon capture, etc., that are currently expensive. However, for most high-emission countries, relying on direct air carbon capture to stay within an appropriate carbon budget as per this framework will not be practical due to the scale involved. In some cases, they would need to remove 50 percent to 275 percent of current emissions annually. This is before considering the high cost of such solutions. These points emphasise that countries should first focus on decreasing emissions.

This paper also shows that carbon pricing, currently based on incremental emissions per ton, may not be directly compatible with a framework focused on cumulative emissions. It is also unclear if pricing alone will lead to sufficient reductions. When we consider issues of equity, the situation is unfair to poorer (low-emission) countries, whose emissions will only grow. Any framework with rising carbon prices is especially unfair.

Given we do need to get to zero globally, countries need to articulate their trajectories for achieving their targets. Offsets should be considered with strong caution, and with separate accounting. As this framework shows, the shape of the trajectory is very important, which ultimately determines the cumulative emissions (the area-under-the-curve). All countries should focus on any low-hanging fruit of early reductions, which for high emitters will become absolute reductions and for low emitters represent a reduction in emissions growth rate.