* Dimming sunlight, fertilising seas said too uncertain
* U.N. favours focus on known technologies
By Environment Correspondent Alister Doyle
DOHA, Dec 2 (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 fertilising 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 Nov. 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 sulphate materials high into the atmosphere for an affordable price tag of below $5 billion.
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."
"There's a lot of scepticism" about geo-engineering, said Artur Runge-Metzger of the European Commission. "Research is necessary to see if it could be viable in one way or other."
U.N. negotiations on slowing global warming have been running since a U.N. Climate Convention was agreed in 1992.
One problem is that adding sulphates - a form of pollution - to the air would not slow an acidification of the oceans since concentrations of greenhouse gases led by carbon dioxide in the atmosphere would keep building up.
Some carbon dioxide, absorbed into the oceans, reacts to form carbonic acid. That erodes the ability of creatures from clams or mussels to lobsters and crabs to build their protective shells. In turn, that could disrupt marine food chains.
"You might temporarily delay the warming but you are certainly not going to help the oceans at all," said Jean-Pascal van Ypersele, a vice-chair of the IPCC, of using sulphates. "Ocean acidification is a real emerging issue."
A mask of pollution might help some crops by reducing heat stress but it might have other side-effects, for instance, by disrupting Monsoon patterns. That could bring disputes between countries that benefited and others that suffered.
And Van Ypersele said that, if geo-engineering went wrong and needed to be shut down after a few years, there would be a big, damaging jump in temperatures.