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Comment by johnloeber | Hacker News Reader

The issue with large-effect, untested ideas like this one is that while it might seem like an appropriate solution for this specific problem, it might carry enormous side-effects that we aren't really aware of yet (and that may be difficult to reverse).

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But climate models will tell us exactly what will happen, so there's no risk of anything going rong! :-P

I'm being snarky, and I both believe AGW is real and we should be taking strong specific action to alleviate it (building nuclear plants as the only proven-viable replacement for base-load coal, investing in solar and storage, shifting taxes from income to carbon emissions and putting on carbon-equivalent tariffs to export domestic policy world-wide), but it bugs me a little how people (not necessarily you personally) can on the one hand say, "The science is settled! The models are telling us in detail how the Earth's climate system is going to be affected by the additional 0.5% effective insolation due to GHGs!" and then turn around and say "We mustn't even think about geo-engineering because we can't possibly predict what effects it will have!"

Anthropogenic GHGs are nothing but uncontrolled geo-engineering, and either we can predict their effects accurately or not (spoiler: we can't). If we can, then we can predict the effects of other forms of geo-engineering comparably accurately.

This is a bit of a hobby-horse for me because I think the public puts far too much credence in the detailed predictions of climate models, and it's going to go badly for us when those detailed predictions turn out to be not so good.

> Anthropogenic GHGs are nothing but uncontrolled geo-engineering, and either we can predict their effects accurately or not (spoiler: we can't). If we can, then we can predict the effects of other forms of geo-engineering comparably accurately.

Without addressing the first sentence or its "spoiler" (preserved only for context), the second doesn't follow: our ability (or inability) to predict the effects of GHG emissions in the ranges relevant to evaluating the effect of actual and reasonably-possible human actions with regard to them do not really have much bearing on, say, our ability (or inability) to project the impact of changes to (for example) solar energy input that are outside of past experience.

Actually, in terms of side-effects, I think this is preferable to most other solutions. You can turn this one on/off very quickly (or shoot it down for a more permanent off setting if the control mechanisms get stuck).

That's not so easily doable with dumping iron into oceans, etc.

I was coming to add this exact thought! A solution like the umbrella could be activated and easily deactivated at will, since it's a physical object that could change positions.

The other solutions, such as putting chemicals into the sky or sea, cannot be reversed. Better hope they work correctly the first time!

Actually, the half-life of sulfur aerosols in the stratosphere is on the order of a few months. We would have to badly overestimate the amount required in order to put so much up there that waiting for it to fall out would be painful. And the proposal is not to put it up there all at once, but to do so gradually.

Nature puts a whole bunch of sulfur up there every couple of hundred years or so, through large volcanic eruptions, so we have a pretty good idea what the effects are. Besides global cooling, which of course would be the point, the major side effect is more colorful sunsets.

The most telling objection I've heard to the use of sulfur aerosols is that they do nothing for ocean acidification.

Sulfur aerosols attack the ozone layer (using anthropogenic chlorine) - yay. And if we're not careful with horizontal distribution, the cooling of the troposphere below and the warming of the stratosphere by the absorbtion will be different in different places, and there will be changes in pole-equator (or smaller) temperature gradients, and this in turn will change circulation patterns, and boom, some other kind of climate change! (This happens for volcanism for example, because most volcanoes are at the equator.) I somehow doubt we can really control the distribution well, and long-term it might distribute itself differently due to changed circulation. Hm, I haven't seen model runs for this kind of thing, might be interesting. (I have papers for the base facts somewhere though. Most knowledge is from the well-studied Pinatubo eruption.)

Plus, what happens if we stop, for any reason? This could only be a temporary solution.

the problem is if by the time you turn i off you have somehow tilted a situation off equilibrium (i.e. some previously underrated insect species disappears and you only notice when plant X stops growing).

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