Solar geoengineering cannot be seen as a temporary measure

2023-05-23 06:30:31

Scientists demonstrate that using solar-modifying technologies to artificially reduce global average air temperature cannot be seen as a short-lived measure to combat global warming. In most scenarios of the latest IPCC report, they are expected to be used for more than a century.

Solar geoengineering encompasses all the technologies that aim to artificially lower the average global air temperature by returning more of the sun’s energy to space. Faced with the difficulty of respecting the Paris agreement which sets the objective of limiting global warming to 1.5 degrees by the end of the century, more and more publications are studying the modification of solar radiation, also called SRM for Solar radiation modification. On the other hand, they mainly focus on their potential as well as their side effects, but pay little attention to the time required for their deployment. A study, published in Earth System Dynamics and involving a CNRS-INSU laboratory, looked precisely at this point.

It should be remembered that three techniques are mainly the subject of discussion in the scientific literature. The first, called stratospheric aerosol injection (or SAI) involves increasing the number of small reflective particles in the stratosphere to deflect and reflect more of the sun’s rays back into space. Marine cloud brightening (MCB) is a technique that relies on scattering particles in the lower atmosphere over the oceans to make clouds brighter, which would increase the amount of reflected sunlight. And finally, the technique of thinning cirrus clouds (or CCT for Cirrus cloud thinning) consists in modifying the properties of ice clouds at high altitude to increase the transparency of the atmosphere to thermal radiation.

All of these methods could in theory halt global temperature rise very quickly, provided they are actively supported. They do not attack the cause of global warming, but are presented as being able to be used temporarily, while greenhouse gas emissions are reduced, and methods of eliminating and storing CO2, also called CDR for Carbon Dioxide Removal, become efficient enough that it is no longer necessary to resort to solar geo-engineering.

A duration ranging from 150 to 300 years to limit warming to 1.5 degrees

In this study, the researchers analyzed how long it would take to deploy SRM techniques to limit warming to 1.5 degrees. To do this, they examined 355 greenhouse gas emissions trajectories taken from the latest IPCC report for 2030, and extrapolated them to the year 2500. The study does not distinguish between the different existing SMR technological approaches. It also does not address questions of feasibility or environmental side effects of these techniques, of which there are many, nor does it offer potential implementation strategies and designs. Rather, it aims to provide a conceptual framework for exploring the duration of deployment of SRM techniques in a context where they would be used as a temporary measure.

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As a result, the scientists find that there is a wide range of potential deployment times ranging from zero to more than 470 years, and that in most scenarios deployment is expected to last between 150 and 300 years. They therefore refute the idea that SRM technologies are a short-lived measure and show that their deployment is likely to be a centuries-long commitment, with associated costs, risks and side effects. “Given that their effects will be mostly uncertain at the time of SRM initialization, a prediction of the duration of deployment seems unlikely, with possibilities ranging from decades to centuries. This is a knowledge gap that needs to be addressed before any SRM proposal is seriously considered.”write the authors of this study.

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