Copenhagen: Just five years ago, anyone who talked of easing Earth`s climate crisis by fertilising the seas with iron, scattering particles in the stratosphere to reflect sunlight or building a sunshade in space courted ridicule.
Today, such advocates -- "geo-engineers" -- are getting a respectable hearing.
Their ideas are still beyond the scientific pale, for they remain contested as risky for the environment and laden with unknowns about cost, practicality and legality.
But mainstream scientists who once dismissed these projects are now looking at them closely.
And some grudgingly accept that at least some concepts are worth exploring as a possible "Plan B" -- a last-resort option if political efforts to tackle global warming fail and catastrophe looms.
Plan A hangs in the balance at the December 7-18 UN talks in Copenhagen, where 194 nations are called to craft a post-2012 treaty to slash greenhouse-gas emissions. Related article: Copenhagen climate talks
But the negotiations are hideously complex, a Gordian knot of interlinked issues, national interests and economic stakes.
The plodding, consensus-driven process is being far outstripped by the surge of fossil-fuel emissions, placing Earth on course for as much as 3.5 degrees Celsius (6.3 degrees Fahrenheit) of warming over pre-industrial times, way over a 2.0 C (3.6 F) threshold widely considered safe.
"A lot of people don`t like to say that Plan A is not working," Jip Lenstra, a senior scientist at the Energy Research Centre of the Netherlands, told AFP at the Copenhagen talks.
"They know that, but they don`t want to say that aloud because it`s very frustrating and it`s not the right signal at the right time.
"If you are working quite hard to make a success of Plan A, and somebody says that we should look at Plan B if Plan A is not successful, that`s not a good strategy."
Geo-engineering broke new ground this year with an assessment of its options by Britain`s Royal Society, one of the temples of science.
A 12-member panel found that some geo-engineering techniques could have "serious unintended and detrimental effects on many people and ecosystems."
But they cautiously said some schemes were technically feasible and could be a useful fallback tool to help the switch to a low-carbon economy provided safety worries and doubts about affordability were answered.