By Environment Correspondent Alister Doyle

DOHA (Reuters) - Cheap, short-cut ideas to cool the planet such as shading sunlight are failing to win support from U.N. delegates looking to improve on the slow progress made by existing technologies.

Many scientists say the proposed solutions, known as geo-engineering, are little understood and might have side effects more damaging than global warming, which is projected to cause more floods, heatwaves, droughts and rising sea levels.

"Let's first use what we know," said Christiana Figueres, head of the U.N. Climate Change Secretariat, dismissing suggestions that it was time to try geo-engineering to halt a rise in greenhouse gas emissions.

"There are so many proven technologies we know exist that are tried and true that have not been used to their maximum potential," she told Reuters. "To begin with, the simplest is energy efficiency."

Geo-engineering options include adding sun-reflecting chemicals to the upper atmosphere to mimic the effect of big volcanic eruptions that mask the sun, or fertilizing the oceans to promote the growth of algae that soak up carbon from the air.

Among other ideas, a giant mirror could be placed in space to block some sunlight or sea spray could be injected into the air to create clouds whose white tops would reflect sunlight.

"Let's face it, geo-engineering has a lot of unknowns," Rajendra Pachauri, chairman of the U.N.'s panel of climate scientists, told Reuters on the sidelines of U.N.-led climate change talks among 200 nations in Doha from November 26-Dec 7.

"How can you go into an area where you don't know anything?" he said. The Intergovernmental Panel on Climate Change (IPCC) is examining geo-engineering in depth for the first time as part of a major report due in 2013 and 2014.

Still, one study by U.S. scientists in August indicated that planes or airships could carry a million tonnes a year of sun-dimming sulfate materials high into the atmosphere for an affordable price tag of below $5 billion.

CHEAPER

That would be far cheaper than policies to cut world greenhouse gas emissions, estimated to cost between $200 billion and $2 trillion a year by 2030, they wrote in the journal Environmental Research Letters.

"If you are looking at solutions you could look at solar energy," said Mira Mehrishi, head of India's delegation in Doha. "It's a little premature to start looking at geo-engineering." Continued...