There are three basic responses to geo-engineering ideas:
1. Noooooo. Mucking about with the planet got us into this mess.
2. Aaaahhhhh. You see all we need is a little ingenuity, and everything will be back to normal. Global warming will be solved, if it is even happening at all anyway, which I doubt.
3. OK, will that work then?
I am firmly in camp number three. There is no reason in principle to reject geo-engineering. The planet is not God, and if it decides to kill us we are entitled to disagree and fight back. Even if global warming were halted by reduction in greenhouse gas emissions alone, we would probably need geo-engineering technology sooner or later anyway - nature has the power to turn nasty without our help. But any project has some enormous hurdles - that the earth is basically very very big, and any solution has to do an awful lot, and without using monumental amounts of energy.
There have been 4 episodes so far, all, unfortunately a little underwhelming. They follow the usual Discovery channel formula of injecting tension by having some mechanical thing go wrong, or threaten to do so, to dramatic background music and interviews with anxious engineers. Gripping.
And what were the ideas? A wave-powered pump bringing nutrient rich deep ocean water closer to the surface. Dangerous mucking about with the ecosystem? Not really - much of the Pacific is like the Sahara desert. Deep water nutrients would make an oasis - they would create an ecosystem where there is currently next to nothing. Greater plankton growth would mean faster carbon sequestration, and some of this carbon would find its way to the ocean floor for a reasonably long time.
Did it work? Briefly. But despite falling apart after a few hours, the location of the pump a week later was found teeming with life. So it looks like this idea could work. It's not going to do enough to change the nature of the challenge significantly, but can make some difference at a reasonable cost.
Next up covering the icecaps with reflective material to lower the albedo and thereby prevent melting and keep the earth's albedo lower. This one kinda puzzled me a bit. Surely you'll get more albedo bang for your buck covering something that isn't alraedy white. What I remember of the programme was an hour spent struggling with the practicalities of airlifting big rolls of white stuff to the arctic and unrolling it. Would it work? No idea.
Third was scattering the light from the sun with lenses in space. Discovery is in its comfort zone here with high tech lenses, rockets and so on. How many 60cm lenses are we going to need? 16 trillion! That's a whole lotta lenses. Well maybe if we make them 1 micron (0.001mm) thick, they'll be light enough that we can launch lots into space. Cue an hour of testing rockets and guns that might launch squillions of extremely fragile lenses without shaking them to bits. What fun. Did the rocket work? No.
Clearly reducing the amount of light reaching the earth would reduce warming. A big lens in space - what could be simpler. But this idea involves a monumental space programme. How much, if it ever works at all, are 16,000,000,000,000 lenses going to cost to make and launch? Keep thinking, and lets not bank on this one.
Fourth was planting large numbers of trees by dropping seeds from an aircraft. Take a tree seed, in a lump of compost, wrap in a layer of wax, hard enough to break the ground surface, but which shatters on impact so doesn't impede the growth of the tree. Take 1000 of these in a cargo net under a helicopter and drop from a height and speed calculated to give a good spread pattern.
Again this was great fun, because the experiments involved aircraft, and dropping things out of them. The methods were ingenious, but I couldn't help wonder if - given the work that must be put into sourcing the seeds and assembling the wax canisters, and round tripping the helicopter (it would still take hundreds of trips to seed a square kilometre), whether it might not be easier to hire a few people with spades to plant the trees properly.
So we get to the dramatic final test of the idea. Did the seeds bury themselves in the ground at the right depth and the right distance apart from each other? Yes. Did they actually grow? No. Why not? Er, maybe the soil pH was wrong. Sigh. And maybe the guy with the spade would have known that.
Well that's all we've had so far. There's another 4 episodes, to come, and I hope they plan to end on a high note with some more promising ideas. Some geo-engineering ideas can be double-edged. If you lower the earth's temperature by controlling light or albedo, you do nothing for acidification of the oceans. So if it turned out that global warming wasn't a preeminent threat to us after all and was in fact no more serious than ocean acidification, then we may have to modify our strategy with this in mind. Much geo-engineering is diplomatically problematic. Whatever the global effect, there are likely to be local effects, that the locals may not like. Nonetheless, I think a common cause would be good for our diplomatic relations.
Yet I suggest this all has little significance yet for policy. Potential geo-engineering projects are much like potential technologies to save or generate energy. Some will work one day, some won't; we don't know yet which is which. Meanwhile we should apply the technologies and policy options we have. We are hardly at risk of doing more to combat climate change than might later prove to be necessary.