They are right that there are cheaper ways to cool the planet. However I believe that they are seriously misjudging the costs of excess CO2. For instance they barely mentioned that excess CO2 results in ocean acidification. How important is that? Well for a start we're threatening the survival of every animal that depends on calcium carbonate shells (think corals and shellfish), and then threatening the rest of the ecosystem within which they live.<p>Injecting sulfur dioxide into the atmosphere to cool us down (and create more acid rain) helps our temperature, but doesn't help with the next pollution disaster. Or the one after that.<p>Spending about 2% of current world domestic product on CO2 mitigation seems perfectly justifiable to me.
SO2 injection is just such a bad idea it is scary there are still people talking about it. The problem is even if you ignore the nasty side effects, as CO2 carries on rising you have to inject more and more SO2 every year to counter the increased CO2 (especially combined with SO2's much lower residence time in the atmosphere being in the range of a few days/weeks instead of CO2 being measured in centuries).<p>It would also tie us in - if SO2 stopped being injected into the atmosphere for any reason, all the CO2 is still there and as the SO2 washes out of the atmosphere within a few weeks, the CO2 warming would come back full force immediately - the longer SO2 had been used to offset CO2, the worse it would be.<p>Offsetting CO2 warming with a seperate cooling effect is risky, the real solutions are either reduction of CO2 output - and even that might not be enough for many countries/cities because of the decades of delays in any meaningful action that have been engineered since we knew what was happening - or some technology to pull CO2 out of the atmosphere.
<i>Our question, at noted above, is what is the cheapest, fastest way to quickly cool the Earth.</i><p>Yes, this approach will cool it! But what will happen next? The Earth is a very complex system and its behavior might be impossible to predict with certainty. The more we mess with it and pretend to be all-knowing gods the worse it will get.
I agree with the analysis. The only thing I'd add (that they don't mention) is that the Earth's temperature will fluctuate wildly on it's own regardless of our CO2 output. We know this. The Earth dropped into an Ice Age and then got hot enough to thaw that ice age out all before we were even here. So the issue really has to do with finding ways to manipulate the Earth's temperature both ways, not just cool it down.
The Copenhagen Consensus deals pretty much entirely with this question:<p><a href="http://www.ted.com/index.php/talks/bjorn_lomborg_sets_global_priorities.html" rel="nofollow">http://www.ted.com/index.php/talks/bjorn_lomborg_sets_global...</a>