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A presentation of Geoengineering - focus on Solar Radiation Management “SRM” (and some recent business aspects)

Technical

 
Author: Mounia Mostefaoui, PhD Student in Climate Physics and Philosophy of Sciences at La Sorbonne Université, introduces geoengineering to us. Still unknown to a large audience, this concept is yet under increasing scrutiny with the hope that it could help reverse climate change.

In this paper, we will provide a general overview on geoengineering, by first defining this quite innovative concept with a focus on Solar Radiation Management.
We will then highlight recent business and operational aspects of Solar Radiation Management. Finally, the impacts of SRM on biodiversity and associated controversies will be presented.


Definition of Geoengineering
 
Geoengineering refers to emerging technologies that could manipulate the environment and partially offset some of the impacts of climate change. 
 
It includes two categories: 
·      One is the carbon dioxide removal (CDR), which aims to remove carbon dioxide from the atmosphere and reduce the accumulation of carbon dioxide in the atmosphere (this method is expensive and relatively slow). 
·      The other is called solar radiation management (SRM) or ‘solar geoengineering’ and refers to a simple principle of albedo modification that seeks to reflect a small fraction of sunlight back into space to cool the planet. We will consider this technique in the present article.

A brief introduction to Solar Radiation Management 
 
The principles of Solar Radiation Management are related to the climate system. Solar Radiation Management is based on the manipulation of the Earth energy balance between incoming radiation from the sun - essentially short-wave ultraviolet and visible light that heats the Earth - and out-going thermal infrared radiation in the long-wave which cools the Earth. This balance controls the Earth’s mean surface temperature (Fig. 0). About a third of the incoming solar radiation on average is reflected by clouds, by ice caps and bright surface (it’s what we call the “albedo”). 
The incoming solar irradiance[1] mostly passes through the atmosphere and reaches the Earth’s surface, where some irradiance is reflected, and the main part is absorbed by the surface that is warmed this way. A part of the outgoing infrared radiation emitted by the Earth’s surface is absorbed by the greenhouse gases of the Earth’s atmosphere (essentially water vapor and CO2) and by clouds, decreasing this way the thermal radiation escaping to space, and therefore warming the atmosphere and the Earth’s surface.
 After phenomena of absorption and re-emission within the atmosphere, in the end, on average 60% of the infrared radiation emitted by the Earth surface leaves the atmosphere. The outgoing thermal infrared radiation increases as the surface temperature increases whereas the incoming solar radiation is unchanged, creating a feedback (the temperatures of the surface and atmosphere increase until the outgoing and incoming radiation are stabilized at equilibrium). 
In 2007, a report by IPCC (the Intergovernmental Panel on Climate Change) showed that if radiation stream is perturbed by 1% (which corresponds to a radiative forcing of 2.35 W/m2), then the surface temperature will change by about 1.8°C.

[1] The flux of solar energy averaged over the whole globe is 342 W/m2.

Schematic of the global average energy budget of the Earth’s atmosphere. Yellow indicates solar radiation; red indicates heat radiation and green indicates the transfer of heat by evaporation/condensation of water vapor and other surface processes. The width of the arrow indicates the magnitude of the flux of radiation and the numbers indicate annual average values. At the top of the atmosphere, the net absorbed solar radiation is balanced by the heat emitted to space. Taken from Kiehl & Trenberth (1997).




 More precisely, Solar Radiation Management can be technically implemented by injecting stratospheric aerosols that could cool the planet thanks to tiny reflective particles, such as sulfate aerosols into the upper atmosphere. These particles modify the albedo by scattering a small fraction of sunlight back into space, acting as a thin sunshade for the ground beneath. Examples of natural volcanic eruptions demonstrate that reducing sunlight reduces global average temperatures. For instance, when Mount Pinatubo erupted in 1991, the earth cooled by 0.5° Celsius for more than a year. 



In early 2020, an acceleration for the development of SRM occurred in the United States of America, the leading country to date for research in this field. Four million dollars were allocated for technical improvements and proof of concept by the United States Government (Congress) to the NOAA (National Oceanic and Atmospheric Administration) for technical improvements and proof of concept of operational deployment of Solar Radiation Management.
            The bill “H.R.5519 - Atmospheric Climate Intervention Research Act” enacted by the Senate and House of Representatives of the United States of America in Congress assembled defining the scope of the technical implementation of the technical setting up of Solar Radiation Management in the United States of America was introduced on December 19, 2019 during the 116th Congress. The exact stated aim of this bill is “to improve measurement and assessment capabilities for understanding proposed atmospheric interventions in Earth’s climate, including, as a priority, the effects of proposed interventions in the stratosphere and in cloud-aerosol processes.” 
 
Impacts of SRM on biodiversity and controversies
 
The models including Solar Radiation Management to date do not provide precise quantitative assessments for the risks regarding the potential irreversible impacts on biodiversity. Solar Radiation Managements techniques are sometimes compared with the genome engineering also named CRISPR gene editing. Solar Radiation Management techniques can also be used as a possible military weapon to change the climate of a given country and requires much care before it can be operationally implemented. Many experts believe that to date Solar Radiation Management is a risky solution with still a lot of associated uncertainties. At the current state of research, it is considered to be implemented only if nations don’t succeed in mitigating global greenhouse gas emissions early enough.
The researcher David Keith, one leading figure of geoengineering, usually states that whatever the controversies are, research must be undertaken to assess more precisely the risks versus the possible positive outcomes of this set of techniques and we agree with that view given the uncertainties associated with the success of global warming mitigation and adaptation strategies.
            In this paper, we provided a global picture on geoengineering, by first defining this concept with a focus on Solar Radiation Management. We also highlighted recent business and operational aspects of Solar Radiation Management, as well as the non-negligible impacts of SRM on biodiversity and associated controversies.
 
Whilst we must remain focus on tackling the primary causes of climate change, we cannot afford to discard the possibility of adapting to its consequences. Should we fail to reduce anthropogenic greenhouse gas emissions early enough to avoid disastrous global and local effects, geoengineering might be a still risky Plan B as we definitely do not have any Planet B! 
 
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