This sort of nonsense is staggering in its naivety. It conveniently ignores the fact that any increase in fuel consumption, particularly on a long haul flight, may mean a reduction in payload to carry the extra fuel burnt. This payload would then have to be carried on another aircraft, burning yet more fuel and the domino effect would almost certainly mean a larger overall increase in CO2 emissions, surely outweighing any of the so called benefits from reflection of radiation by cirrus clouds.
Put any longhaul aircraft more than 4000 feet off its optimum altitude and the increase in burn is going to be closer to 10% than the 3.9% 'weighted mean'. Most 747s, 777s and 340s in ISA conditions are likely to be close to optimum cruise altitude at FL330 after a MTOW departure; they are also likely to get up to FL350 or metric equivalent within three hours of departure.
At lighter weights, on shorter (ex Europe to East Coast USA, Middle East, India, Pakistan etc.), they like to get to FLs above 370 straight away, therefore inflicting at least a 6000 foot deviation from the optimum.
Still, as we are in the U.K, according to experts, probably reaping the seeds of global warming, because we're actually having a summer, they must be right..................