(Based on the recent paper Below Zero. Story also published on Medium.)
Atmospheric CO2 concentration is currently around 420 parts per million (ppm), far above the range of 180 ppm to 280 ppm during the last million years.
And it will continue to grow: even the fastest, most ambitious climate-optimal transition will increase CO2 concentration to at least 430 ppm. This is because we need energy for building renewable energy infrastructure, which has to be supplied by current fossil power during the transition. Any delays in building renewables will further increase atmospheric CO2 concentration and, thus, the risks of triggering a climate tipping cascade.
The ongoing anthropogenic geoengineering experiment of massive CO2 emissions into the atmosphere was not intended to heat the climate but to provide cheap and abundant energy. It, however, not only destabilizes the climate but also comes with a general feeling of human superiority, dominance, and detachment from the rest of nature. This additionally leads to mass extinction, resource exploitation, waste, and pollution. The experiment is failing catastrophically because the unintended side effects destabilize our life support system.
It is not yet too late to navigate back into a stable Earth system state.
However, this will require actions far beyond current political and societal ambitions. For stabilizing the climate in the long run, it is not only necessary to halt emissions, aiming to stay below 1.5°C heating. But it is furthermore inevitable to reduce radiative forcing to prevent triggering a tipping cascade. Reverting anthropogenic CO2 emissions is the most direct way of cleaning up our past damages, as it meddles with the carbon cycle we pushed out of balance in the first place. All other geoengineering proposals for radiative forcing management (for example, ocean fertilization, artificial aerosols, enhanced weathering, etc.) interfere with other Earth systems and geochemical cycles, thus bearing high risks for further unintended side effects.
We will need to remove all cumulative emissions since 1988: the point in time when atmospheric CO2 concentration first crossed 350 ppm, proposed as the safe long-term climate threshold. Restoring natural ecosystems is essential to taking up CO2, relieving the pressure on the biosphere, and changing our mindset. However, it will alone be insufficient to clean up our emissions in time. As most anthropogenic emissions had been released technically (power plants, car engines, aircrafts,…), most clean-up will also need to happen technically with below-zero emissions. Technologies for direct air capture and safe, permanent storage are emerging; we need to prepare them to scale up.
At least 400 Gt of pure carbon (i.e. 1500 Gt of CO2) must be permanently removed from the atmosphere. This is as much carbon as there had been concrete in use in society in 2015. In other words, all concrete structures — buildings, bridges, foundations, etc. — in use in 2015 weigh combined as much as the carbon we need to remove to stabilize the climate. If the phase-out of fossil burning is further delayed or we still think we can allow ourselves “hard-to-avoid” emissions, we will have to remove and store even more. To keep this already gargantuan task at a minimum, an immediate, fast, and complete transition to 100% renewable energy is vital!
To better illustrate the task ahead of us: Would we pile up 400 Gt carbon rubble in a cone (45° slope angle, ~1 t/m³ bulk density similar to coal)? It would be a mountain more than 7 000 m high. Mount Carbon of Anthropa, the human “continent”, would be the second highest among the now eight summits. And every day we wait, it will grow taller. I’m convinced we can rise to this challenge, and we will have to!
