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| Source: Wry smiles |
Just like sunglasses, the purpose of the sunshade is to reduce incoming solar radiation, but instead from the top of the atmosphere to counterbalance warming induced by increasing greenhouse gases! Thin refractive mirrors reflect incoming solar radiation away from Earth, reducing the amount of radiation reaching Earth and warming the climate.
The 'Sun Shade World' was an idea proposed by Early in 1989. He proposed the implementation of a sunshade, composed of tens of thousands of metre-sized small spacecrafts, comprised of a thin refractive screen, at the Lagrange Point (L1) between Earth and the Sun, which would be designed to reduce incoming solar radiation to Earth (Lunt et al. 2008). Lagrange points are "positions where the gravitational pull of two large masses precisely equals the centripetal force required for a small object to move with them" (NASA, 2012). L1 in particular is the point directly between the Sun and the Earth, and is approximately 1.5km away from Earth (Washington University) with the same annual orbit of Earth- 365 days.
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| Source: NASA |
Interestingly, the world has been in a reduced solar radiation and high carbon dioxide level environment before in our geological past of the Cambrian era! This means the Earth has been in a similar climatic situation before, and it was actually an era of an evolution boom, with a warming climate and rising sea level (National Geographic, 2014).
However, reducing solar radiation does not dampen the effects of increased carbon concentrations in the atmosphere, and the forcings of radiation from increased carbon dioxide differs from solar forcings (Govindasamy & Caldeira, 2000). Firstly, carbon dioxide, and other greenhouse gases trap heat all day, all night, and all year round. Solar radiation, in contrast, is more attributed to daylight hours and is expected seasonally, and most abundantly towards the equator (Govindasamy & Caldeira, 2000), so it cannot completely characterise our pre-industrial world.
A Community Climate Model was produced by National Centre of Atmospheric Research (NCAR) which simulated three different global scenarios: pre-industrial, doubled carbon dioxide emissions, and finally, doubled carbon dioxide emissions with reduced solar radiation- i.e. geoengineering. From the three scenarios, the sunshade geoengineered response, cooled the climate the greatest by 1.88 Kelvin and particularly in the equatorial regions, but could intensify carbon dioxide's impact on stratospheric temperatures through enhancing stratospheric cooling (Murphy & Mitchell, 1995) which could lead to damage of the ozone layer (Houghton et al. 1990)!
Other issues of the Sunshade world, is it causes a reduction of intensity in the hydrological cycle such as a decrease in precipitation- particularly in the tropics, which has a web of social and economic issues attached, and in the Arctic sea ice melt would increase (Lunt et al. 2008). However most importantly, a dominant issue, and this has been highlighted above, is that carbon concentrations in the atmosphere will continue to increase. Therefore any atmospheric carbon effects will not be mitigated through the adoption of a Sunshade world. For example, ocean acidification will not be stopped, and the impacts this has for marine ecosystems is humongous, and has a domino effect on the ocean's future capabilities as a carbon store.
Also, lastly and foremost, is should this 'World' be adopted, what if a) it fails or b) we decide it needs replacing or no longer want to use it; this will cause a rapid increase in global warming, and as carbon concentrations were still increasing during the geoengineered time, this could have hugely adverse effects on the world, in terms of re-adapting to an anthropogenically intensified climate.
The 'Sunshade World' remains a proposal for the time being, and the list of uncertainties suggest if or when it is deployed, is a long way off, as does the economic expense of a few trillion dollars to spare. The only way this scheme could work effectively, is if carbon mitigation strategies are employed harmoniously, which could help reduce the rapid climate change potential, should the Sunshade be removed.
What do you think?
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Great post Sarah! I was wondering what you thought about the sunshade and primary production. If there's less solar radiation and less rainfall, would forests and plants be as production? In turn wouldn't this lead to further concentrations of CO2 in the atmosphere?
ReplyDeleteHi Charlie - thank you for your comment! Well within high to mid latitude countries, the impact on precipitation is quite small, and it is in the tropics where a reduction in precipitation is most noticeable. However, the impact on primary production is not as detrimental as you might think! Reduced evaporation (as a result of less solar radiation) means that there is an increase in soil moisture, which in turn means that impacts on primary production are not as negative. (Lunt et al. explains a bit further: www.ggy.bris.ac.uk/personal/AndyRidgwell/pubs/modern_mirrors_0.5.pdf)
ReplyDeleteIn fact in a world with increased carbon dioxide emissions (a sunshade world would result in this) net primary productivity is thought to increase, and reduced incoming sunlight is considered to have negligible effects (See: www.ocw.mit.edu/courses/civil-and-environmental-engineering/1-018j-ecology-i-the-earth-system-fall-2009/projects/MIT1_018JF09_sw_paper1.pdf)
The wonderful aspect of the Sunshade World design was that it can maintain the world as we live in it today. Solar radiation schemes are not aimed to reduced carbon emissions in anyway but to prevent the world from heating up, therefore the increased carbon emissions from forest and plant productivity would have to be displaced by carbon mitigation strategies, which is what science suggest should continue even when geoengineering has been implemented.
Hope this helps!