Despite
looking like a pie in the sky (no pun intended) idea, there have been many
modelling studies to better understand the impact that sunshades in space would
have on the Earth’s climate. This is a solar radiation management (SRM) technique.
Objects
placed in solar orbit (Angel, 2006), or in Earth’s orbit (NAS, 1992) (Pearson
et al., 2006), could reflect away solar radiation from the Earth before
reaching the surface. A decrease in incoming solar radiation of 1.8% would be
required to offset a doubled pre-industrial concentration of CO2 (Govindasamy
and Caldeira, 2000) (Angel, 2006).
Now to
achieve the required reduction of incoming solar radiation a sunshade with a
total area of 4.7 million km2 consisting of multiple ‘flyers’ at the L1 Lagrange point (Angel, 2006).
(I’ll link the Wikipedia page concerning the definitions of Lagrange points at
the end of this post, as it is too long to explain here).
Altering
the size of the sunshade could vary the negative radiative forcing effect. Closer
to Earth NAS (1992) suggested placement of mirrors in orbit around Earth and
Pearson et al (2006) suggesting rings of either particles or multiple
spacecraft.
In the
literature, calculations are based on a static radiative imbalance, however as
Raupauch et al (2007) stated radiative imbalance is set to increase due to
observed upward trends in emissions.
If a
sunshade was sent into space today, (at the L1 point) to offset the current
radiative imbalance of 1.6 W m−2 , a shade area of ~2 million km2 would be needed. Given that atmospheric CO2 is rising at
2 ppm year−1, a surface area of ∼36, 000 km2 would need to be added each year.
Regionally
targeted shades have been proposed by (Caldeira and Wood, 2008). They state
that shades focused on 61◦ N or 71◦ N to 90◦ N, could protect tipping elements in the Arctic. They go on to say that from model simulations,
shades applied over 71–90◦ N would reduce incoming solar radiation by 21% and would be sufficient
to restore Arctic sea-ice to its pre-industrial state in a doubled CO2 world.
Overall,
Matthews and Caldeira (2007) made the point that after deployment,
reflectors in space could quickly (within ~5 years) counteract global warming.
~5
years to counteract global warming?….. Perfecting this could prove to be an
invaluable geoengineering technique. However it oculd seem too good be to true
as I’ll explain in a later post.
For
now…
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