Why we have to reduce carbon emissions to zero

A budget of global CO₂ emissions

Research in the past decade has shown clearly that we have only a limited amount of CO₂ emissions available in order to reach a specific climate target. This amount is determined by the climate sensitivity and the capacity of forests and oceans to take up CO₂ from the atmosphere, i.e., the strength of the carbon sinks. The climate sensitivity describes how much the Earth warms when the CO₂ concentration in the atmosphere increases. The strength of the carbon sinks determines how much atmospheric CO₂ increases for a given amount of emissions.

We can obtain a zeroth order estimate of this permissible budget by assuming that the forests and oceans continue to absorb around half of the CO₂ emissions (as they used to in the past) and that the Earth gets about three degrees Celsius warmer when CO₂ concentrations in the atmosphere double compared to pre-industrial times. With a number of simplifying assumptions (see box), one arrives for the two-degree target at a permissible budget of around 800 gigatonnes of carbon (Gt C). This is 800 billion tonnes C, or around 3,000 billion tonnes of CO₂. Simulations using state-of-the-art climate models that also allow for feedback mechanisms between the climate and the carbon cycle result in a very similar number. We can envision these permissible emissions as a pie that we are allowed to eat, but must last until the end of the century. At first glance the pie seems to be sufficient – in fact it is actually twice as large as it would be if these CO₂ sinks had not helped us with the removal of atmospheric CO₂.

More than half already eaten up

But if we consider how much CO₂ humankind has emitted already in the past 150 years by burning fossil fuels and cutting down forests, then unfortunately less than half of the pie is left. In concrete terms, we have already consumed more than 500 Gt C from this budget, leaving us only around 300 Gt C [1]. If we start from today’s emissions (around 10 Gt C per year) and their trends, then we will have used up the entire budget in less than three decades [2]. After this, we can allow no net additional CO₂into the atmosphere, otherwise we greatly increase the probability that we will overshoot the two-degree target.

This means that from the second half of this century onward, global CO₂emissions have to be reduced to zero. As a result, we should not focus on achieving a low CO₂ future, we need to achieve a CO₂-free future.

Which reduction rate is realistic?

A rapid reduction path would make it possible for future generations to still emit CO₂, but such rapid reduction paths are very difficult to achieve. A strong lock-in effect results from our heavy investment in a global fossil-driven infrastructure over the past decades – and we are still investing in it today. When we consider that the average running time of fossil fuel-fired power plants is several decades, it is clear that it is not easy for new energy sources to crack this market, even if prices are becoming competitive (see Tony Patt’s blog). In addition, wind turbines, solar panels and the entire associated infrastructure have to be built and installed first, which requires building new factories, training new specialists and making substantial investments. This takes time. Judging from the speed with which new technologies diffused into market in the past, it will be very difficult to achieve a reduction rate of more than six percent per year.

Vision zero

Even if we follow this optimistic reduction path, we will still have exhausted the global emission budget before the end of the century. When we allocate emission rights to individual countries, we also need to take into consideration that developed countries have proportionally less emissions available in the future due to their past emissions [3]. And if we do not manage to reverse the global CO₂ emissions trend soon, the limits on the reduction rates implies that we can achieve the two-degree target only with negative emissions, i.e., we would need a net removal of CO₂from the atmosphere. While this is technologically possible today [5], it is very expensive and it remains unclear where the CO₂ would be stored.

Conclusion

For Switzerland, I see the need to achieve a full decarbonisation of our society in the second half of this century [4].We have nearly 50 years to implement this process; but it is a long journey and one that goes beyond 2030, as it has to continue until emissions are reduced to zero.

Acknowledgements: I thank Professor Reto Knutti for his valuable input.