Thursday, 9 January 2020

Sun Shades to Cool the Earth?

Introduction

Given the present inadequate global response to reducing greenhouse gas (GHG) emissions, some people are considering geoengineering options to ameliorate the effects of the climate changes which we have begun to observe.  Many worry that deliberate intervention in the Earth's climate system is a bad idea since the unexpected effects could be dangerous and in addition the usual fossil fuel polluters might use this solution as a "get out of gaol" card.  The top priority should be drastic GHG reduction, but it seems prudent to explore the feasibility of geoengineering methods as backup.  Hence this page briefly reviews the idea of reducing global warming by a space-based approach of solar flux reduction.   See [1] or [2] for an introduction to geoengineering. 

Solar Flux Control 
The idea of controlling solar radiation to limit climate change effects is not new (e.g. see [3]).  Terrestrial methods of geoengineering may be cheaper, but this space-based approach has potential advantages:

  • once installed, it might be possible to adjust the attenuation fairly rapidly
  • it might offer some ability for varying the solar radiation depending on latitude.  
The figure below contains a simple 2D model of flux attenuator located near the L1 Lagrange point, a distance (L in the figure) about 1.5 million km from the Earth on the left hand side.  At this location the gravitational forces towards the Earth and the Sun are balanced, providing a pseudo-stable "parking spot".   The sun's disc is shown in the right;   it subtends an angle of about half a degree from the Earth.  We assume some type of mirror is used to block solar radiation that would otherwise fall on the Earth's surface.  (Other options such as a large thin Fresnel lens to disperse sunlight have also been proposed.)  

Fig 1:  2D Model of Solar Flux Control (not to scale) 

For realistic mirror dimensions, no part of the Earth would be fully in shadow from the mirror (i.e. the umbra doesn't extend to the earth).  An observer on Earth would not notice any change in the Sun, unless equipped with a pin-hole camera or similar. Assume that the mirror's position  can be adjusted in the vertical direction shown in Fig 1.  This allows some scope in varying the flux versus latitude on the Earth.   For example a simple evaluation using the 2D model above, with a 1000 km mirror, offset by 3000km, gives the Figure 2 reduction in flux versus position on the Earth.  In this example case the mirror is located "below" the Earth-Sun midpoint so for regions near the North Pole, no part of the Sun's disc is obstructed and the attenuation is zero.   


Fig. 2: Example of Solar Flux Reduction 
Previous studies of similar sun shades have concluded that only a few percent reduction in solar radiation might counter the GHG heating effects and have suggested mirrors of similar dimensions.  

Feasibility  
Radiation will exert a force on the mirror (solar sails use this  effect) and for (say) a 1000 km square mirror this is large (e.g. almost 8 million Newtons). However this should not pose a major problem as the structure can be placed on the Sun side of the L1 Lagrange point such that the radiation force balances the net gravitational force.  Of course second-order effects would require an active positioning system, probably provided by many small thrusters.  

The main challenge appears to be manufacture and transport of the mirror(s).  For a mirror of 1 gram per square meter, the area mentioned above requires 1 million tonnes of mirror.  Fabrication on the Earth  doesn't appear to be likely given estimates of the total mass launched into space are considerably less.   Manufacture in space seems more promising but is probably a long term project compared to the time available.  For example the moon contains a large percentage of Aluminium that might form the bulk of the reflecting material.  Some asteroids could also provide useful raw materials, such as Nickel.  If significant amounts of Aluminium could be mined and smelted on the Lunar surface, it could be used to make very thin reflecting sheets (possibly unrolled and assembled at L1) together with structural elements required for the large fleet of positioning cubesats.  Presumably the required micro-electronic components could be made on Earth, with highly automated assembly of the cubesats on the Moon and possible launch via a Space Gun

Conclusions 

Control of solar flux seems feasible but is a long term project.  All the more reason to make rapid cuts in our greenhouse gas emissions and avoid the need for these extreme measures.