Ideas from the BBC program 'Five Ways to Save the World':
1) Deflect 2% of the sun's rays with a wall of glass 100 000 km in diameter, 3 microns thick! It would consist of many small glass panels. Construction would require 870 000 trips into space. This could be achieved by propelling a rocket from deep inside a mountain (thunderbirds style) using EMF; passing a current through a coil underneath an aluminium disk under the rocket produces a magnetic field which then lifts the disk and the rocket. Rocket accelerates up the mountain via the induced magnetic field.
2) Make clouds more reflective (thus reducing the amount of heat reaching the earth). San Francisco is protected from the sun by the consistent fog. Pump silver iodide into clouds to induce precipitation.
3) Propel sulphur rockets into the stratosphere. Idea derived from volcanic eruptions which spew sulphur high into the atmosphere, leading to a reduction in temperature. At this altitude (20-40 km) SO2 cools the planet, in the lower atmosphere it is a pollutant.
4) Increase the amount of phytoplankton in the oceans by adding nutrients (granules of nitrogen-rich urea). This leads to an increase in CO2 absorption.
5) Create artificial trees that contain sodium hydroxide. Fibres of sodium hydroxide absorb CO2 and convert to sodium carbonate.