One proposal is a massive reforestation campaign used as a possible method of slowing the buildup of carbon dioxide in the atmosphere. So too was fertilizing vast areas of the ocean with iron to stimulate the growth of carbon dioxide-absorbing phytoplankton.
Another option for mitigating global warming would be to try to control our planet’s radiation balance by limiting the amount of incoming radiation from the Sun. This could be done by increasing the reflectivity of the Earth — its albedo. Calculations show that an increase in planetary albedo of just 0.5 percent is adequate to halve the effect of a carbon dioxide doubling.
Several schemes suggested involved tossing additional dust — or possibly soot — into the stratosphere or very low stratosphere to screen out sunlight. Such dust might be delivered to the stratosphere by various means, including being fired with large rifles or rockets.
Another scheme envisioned placing thousands of large mirrors in Earth orbit to reflect incoming sunlight. Alternatively, billions of aluminized, hydrogen-filled balloons would be lofted into the stratosphere to provide a reflective screen.
Also, the NAS study team thought that vast arrays of pulsed lasers at mountain altitudes could be used to focus intense infrared beams into the atmosphere. The laser beams are on a mission to selectively destroy chlorofluorocarbon molecules in the atmosphere through the process of multiphoton dissociation.
None of these schemes apparently seemed practical in 1992; but maybe it’s time to revisit them?