The problems with mirrors...

Well if you’ve read my previous post, and seen the size of the shades involved you’ll see the obvious problem. 4 million km²  is 300,000 km² (the size of the UK) bigger than the surface area of THE MOON! The sheer magnitude of engineering here is incredible and would take years to construct let alone the cost, which would, I assume, be in the billions of dollars (couldn’t find an exact figure for this).

A shade area of ∼2 million km2 (at the L1 point) would be needed to offset the current radiative imbalance of 1.6 W m−2 then with atmospheric CO2  rising by 2 ppm year−1, a surface area of ∼36,000 km2 would need to be added each year (Lenton and Vaughan, 2011). This equates to ∼155,000 launches per year of 800,000 space flyers per launch (Angel 2006). Thus, the area of shades or reflectors in space would need to increase significantly year on year to keep pace with the current rate of increase in radiative forcing (Lenton and Vaughan, 2011).

However taking this massive piece of engineering out of the equation, in practice, the cancellation would be imperfect because the radiative forcing to be counteracted would not be perfectly known.

Models predict that the meridional temperature gradient would tend to be reduced, leaving either excess cooling in the tropics (Lunt et al. 2008) or excess warming in the high latitudes (Govindasamy et al. 2003)

Model studies have predicted that the main side effect would be a slowing of the hydrological cycle with up to a 2% decrease in global mean precipitation (Govindasamy and Caldeira 2000; Govindasamy et al. 2002; Bala et al. 2008) Reduced hydrological cycling in the tropics is consistent with reductions in shortwave radiative forcing being biased to the tropics, whereas an equivalent increase in longwave forcing is more evenly distributed.

In the terrestrial biosphere, decreasing incident solar radiation would have direct effects on photosynthesis. Existing simulations (using the IBIS vegetation model) suggest the CO2 fertilisation effect dominates over reduced solar radiation, in affecting global net primary productivity (NPP) in a geoengineered climate (Govindasamy et al. 2002; Naik et al. 2003). NPP is predicted to decline in tropical evergreen and boreal forests but increase in temperate deciduous forests, grasslands and deserts (Naik et al. 2003).

As yet there are no global modelling studies of the impact of reducing incident solar radiation on the marine biosphere. However, complex relationships (incorporating a number of feedbacks process) exist between physical, chemical and biological processes in the ocean (Lenton and Vaughan, 2011).

There are more problems to be overcome before this scheme can be put into place; changing of the amount of solar radiation that reaches the earth is going to be risky. Unfortunately the effects can only at the moment be modelled and it will be very difficult to do a ‘test run.’

Sorry for the long post, but there are many issues involved which I have tried to cover!