Can humans change their behaviors to fight against and anticipate the impact of changes that are affecting the planet? In 2009, 26 international researchers identified nine planetary boundaries (see figure 1): thresholds beyond which human activities could have a significant impact on the operation and stability of the planet, even threatening the future of humanity. One of these, the planetary boundary linked to climate, is based on maintaining levels of atmospheric CO2 below 350 ppm (parts per million, in volume of air molecules).
However, two years after the signing of the Paris climate agreement in 2015 (Cop21), which aimed to keep global warming at less than 2°C more than preindustrial temperatures by 2100 , the rate is still higher than 400 ppm. This rate was reached in 2013 and continues to increase!
For Jean-Denis Mathias, researcher at Irstea’s Clermont Ferrand center and main author of the article published in Nature-Scientific Reports , "limiting temperature increases to 2°C will not be enough to return to a CO2 level of less than 350 ppm, which would ensure the proper functioning of the planet. Furthermore, for every year that temperature control targets are not met, the deadline gets pushed back, which means that greater efforts are required in an even shorter amount of time (see figure 2)." The priority is therefore to focus quickly on the main cause of climate imbalance - CO2 levels - by reducing emissions linked to human activities and by implementing innovative, large-scale, carbon capture technologies. "By analyzing the problem from the perspective of CO2 concentrations, our work showed that although there is still a certain amount of leeway, this is disappearing quickly," continues the scientist.
Thinking of trajectories
As part of their study, the researchers chose not to focus on fixed objectives that should be achieved, but on trajectories, a set of potential strategies to be implemented until 2100 relating to the two climate change planetary boundaries:
1) maintaining CO2 levels below 550 ppm until 2100, this being the threshold above which the impact of climate change will have exceeded acceptable limits
2) returning to a CO2 level of 350 ppm by 2100, the threshold at which the impact of human activity on climate is minimal.
Scientists used the DICE  model used by GIEC experts, combining a representation of the economy and a representation of the carbon cycle and the impact of climate change. "Using this base, we applied a viability theory to study dynamic systems in an uncertain future. The theory is not based on finding a perfect solution (the fixed objective to be achieved) but rather on thinking in terms of meeting constraints at all times and policy decisions made in time in relation to the status of the systems."
Carbon neutrality and negative CO2 emissions
The results showed that the target can still be met, but the choice of options available to achieve it gets smaller over time (see figure 3). In order to maintain levels below 550 ppm until 2100, we need to reduce CO2 emissions by 33% by 2055. The longer it takes to implement the necessary policies, the greater the effort required, with emissions needing to be reduced by 46% in 2055 if we do not act before 2035. Moreover, in order to achieve a level of 350 ppm by 2100, carbon neutrality would have to be achieved before 2060, and 10% of CO2 would have to be removed from the atmosphere (negative emissions) using innovative geo-engineering techniques. It is now up to public decision-makers to act and decide on the direction of future climate policies and geo-engineering research programs.
For further information:
- Article. Global and climate change: 55 plans for action in the service of the environment
- Consult the Irstea Clermont-Ferrand Center page
 Reference period 1861-1880
 J.D. Mathias, J.M. Anderies, M.A. Janssen: On our rapidly shrinking capacity to comply with the planetary boundaries on climate change: in Scientific Reports 7:42061, February 2017
 The DICE (Dynamic Integrated Climate-Economy) model is an integrated evaluation model that takes into account the interaction of phenomena set out in the GIEC report. These phenomena relate to the global economy, the carbon cycle, the state of geophysical environments (climate, prevailing marine currents, etc.) and ecological ecosystems and their biodiversity.