Increasingly and with varying degrees of optimism, more and more opponents of Assad advocate a no-fly zone in Syria. It would be great if it saved civilian lives. But probably its capacity to do so has been exaggerated, and certainly it isn't nearly enough to bring substantial protection to Syrians. To avoid its being touted as some sort of 'solution', its limitations need keeping in mind.
What sort of zone is at all likely? Certainly a no-fly zone over all of Syria would save many lives; it might even help end the conflict. But no such zone is suggested by any prominent advocates, and for good reason. It would, in the attempt, be more costly than the West would ever countenance. Syrian air defences are not state-of-the-art but they are substantial and in large measure located near populated areas. Taking them out would certainly mean civilian deaths. But the main problem with any such proposal is political, not military.
Such a zone would rightly be seen as a fatal blow to Syria's sovereign government. (Indeed that would be the best thing about it!) It could never pass muster as a mere implementation of some alleged 'duty to protect'. Russia would veto its endorsement in the UN, and without that endorsement there is no reasonable chance the West would go ahead with it. Indeed Russia's ability and its unknown degree of willingness to provide Syria with advanced means of air defense would alone make such a proposal too radical for the West to contemplate. The remote possibility of confrontations between Western and Russian air assets in Syrian skies would seal the death of such ideas. It is now clear that Russia would at the very least not tolerate a no-fly zone anywhere near its base in Tartous.
So the live option is a no-fly zone over part of Syria. This will accomplish little.
There is a fundamental difference between a limited zone and the no-fly zones of Iraq, Libya - or even what some hoped for in the earlier stages of the Syrian conflict. In Libya and Iraq, the zones were to function, or did function, as a stage in the overthrow of the régime. In Libya's case this was less than explicit only not to offend Russian sensibilities. In Iraq it was always understood, and indeed the source of complaints, that humanitarianism was not the sole motivation: the US and the UK were bent on Saddam Hussein's removal. In Syria today, removal is at most something contemplated as a long-term possibility, and then with considerable reluctance.
The difference has to do with the factor not present in Iraq or in the Libya prior to Gaddafi's fall. In Syria today, of course, the West is virtually panicked about strong Islamist elements. This concern isn't confined to ISIS, and its extension to Jabhat al Nusra has to do with more than its 'official Al Qaeda' label. It's not just that the West is appalled by ISIS' conduct. It's also that it doesn't trust any Islamist elements in the Syrian revolution, which means that for all practical purposes it doesn't trust the revolution itself. Groups like Ahrar al Sham, despite their attempts at rapprochement with the West, will never gain Western confidence: who knows what these guys are really up to? And that produces a kind of chain reaction. If you can't trust the radical Islamists, you can't trust the moderate ones, because they're too chummy with the radicals, and you can't even trust the secularists, because they're too chummy with the moderates. Long story short, by now the West has no enthusiasm for the fall of the régime, so that a no-fly zone would actually be what it pretended to be in earlier cases: a humanitarian gesture. But the West is far more interested in fighting ISIS than in humanitarianism.
Anyway, as a gesture, how humane is it?
It will do no great harm to Assad's air capacity; his air force would escape damage because it would not confront advanced US aviation. So that air capacity would still be fully available for attacks on civilians outside the zone. Outside the zone, in other words, the slaughter from the air would continue.
What about inside the zone? Bear in mind that the Syrian armed forces have operated without air cover for over half a century; their air force was entirely outclassed by Israel's. Today it would be far, far easier for the Syrian army and its allies to accomplish most of their objectives inside the zone.
Their prime objective would be what it has always been, to terrorize civilians in rebel-controlled areas: beyond that its operations have been largely defensive. It would be an extreme of wishful thinking to suppose that Syria, with its very extensive artillery and rocket resources, would spare civilians in a no-fly zone. Such terror operations wouldn't require anything remotely like the exposed troop formations it fielded against Israel, and these operations would be supplemented by militias, operating in built-up areas, utterly beyond the reach of a mission whose sole purpose was to deny them air cover. Indeed since no one is in any case threatening régime forces from the air, the denial would mean nothing.
Note too that the infamous barrel bombs were themselves a novel improvisation. It is impossible to believe that the sadistic ingenuity of the régime would be hard-pressed to find some equally devastating means of inflicting agony on civilians, one that did not require delivery from the air. Indeed not much ingenuity seems to be required. Even in Aleppo, well within any proposed no-fly zone, the régime has attacked bakeries with artillery. Every inch of the no-fly zone would be within the reach of régime missiles, rockets or field guns.
You might say that if a no-fly zone proved inadequate, further measures would be taken. But this is exactly why a no-fly zone encounters such opposition in the first place. Every new measure would encounter enormous political obstacles and - given years have past and a no-fly zone itself is still only a possibility - there is no good reason to suppose that further measures would in fact be implemented. And since there is no real political desire to overthrow the régime, the governments implementing the no-fly zone won't have the will to extend it at Assad's expense.
In short the limited no-fly zone proposals seem little more than comfort for those unwilling to countenance a real attempt to destroy the Assad régime. Only such an attempt will end its murder of innocents. It would require an unlimited no-fly zone, not a limited one.