Are ideas to cool the planet realistic?

The deliberate large-scale manipulation of the Earth’s environment, called geoengineering, could be one way to cool the Earth or help reduce levels of carbon dioxide in the atmosphere.
But scientists are aware that these technologies are in very early stages of development and remain untested on a global scale.
Although there are great risks in deliberately interfering with nature to cool the planet, some researchers say that if the concentrations of carbon in the atmosphere reach a critical stage, geoengineering might become the only way to take control of our climate.
On the other hand, others worry that having the technology to "reverse" climate change could be seen as a get-out-of-jail-free card and that more effort should be put on existing ways of reducing emissions.
Steve Rayner of the Oxford Geoengineering Programme, UK, says that there is no easy answer, but it would be "irresponsible for us not to explore the potential to understand the technologies as best we can".
"Throughout human history the technologies of one generation created problems for the next. We have to find some way to deal with that; it’s part of the evolution of human society," he adds.
Geoengineering refers to the deliberate large-scale manipulation of the Earth’s environment to counteract climate change.
There are essentially two ways of doing this.
The first is called Solar Radiation Management (SRM) and involves reflecting more of the Sun’s rays away from the planet back into space.
One proposed method of SRM involves putting sulphur aerosols into the high reaches of the atmosphere.
This mimics what occasionally occurs in nature when a powerful volcano erupts. For example, the Mount Pinatubo eruption in 1991 injected huge volumes of sulphur into the stratosphere. The particles produced in subsequent reactions cooled the planet by about 0.5C over the next two years by reflecting sunlight back out to space.
Using SRM would only address the symptoms and does not tackle the issue of rising concentrations of carbon dioxide (CO2).
That is what the second option would aim to address by removing the CO2 already present. A number of ways to do this have been proposed; these approaches are known as Carbon Dioxide Removal (CDR).
This would tackle the root cause of the problem, but Prof Rayner says, it would be very slow to have any effect and would require extensive financial investment.